LX200 Mirror Shift & Focusing Issues

       

Subject: Focus Shift During Long Sessions --part 1 of 3  

From: Alan Voetsch <critter12952yahoo.com> Date: Apr 2003

Abalos Fernando wrote:
> I am noticing the problem of the focus. I am quite sure that this
> problem is becoming worse and worse. The second half of the night
> everything is out of focus.

Over a four hour period many things are happening to your scope to cause the focus to change. The mirror will settle as the OTA sweeps across the sky, as it crosses the meridian it will "flop" over a bit. The mirror can rotate, shift, flop, all of which change focus. As the night wears on, the OTA will contract due to lowering temperatures, this will also move the focal point.

What can you do? Pray, use the mirror lock, force the mirror flop out before you start your exposure, achieve focus properly (CCW for 8-10" SCTs, CW for 12"), use temperature compensation, and go out and check your focus occasionally.

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Subject: Focus Shift During Long Sessions --part 2

From: Gene Horr <genehorrtexas.net>

The temperature drop is causing the tube to shrink. A very common problem with scopes not designed with temperature compensation. This problem is magnified (literally!) by the 5x secondary.

The expensive solution is to buy a focuser that once trained will keep sampling the temperature and make adjustments for this.

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Subject: Focus Shift During Long Sessions --part 3 of 3  

From: Mark de Regt <deregtearthlink.net>

As others have already replied, this is a problem with practically any scope (except perhaps those whose OTA is made of carbon fiber), and is principally a result of contraction of the aluminum OTA which changes the distance of the light path, which takes the scope out of focus. Other things might contribute to the problem, such as mirror flop.

The amount of adjustment which will need to be made is a function of, among other things, the focal ratio at which you are imaging (the faster the focal ratio, the less "depth of field" you have, requiring more frequent refocusing), and the amount the temperature drops in the course of a night's imaging. If you wish to get sleep, there are two ways I know of to deal with this problem completely: (i) use a temperature compensating focuser, of which there are several on the market, which will make precise changes as the temperature of the OTA changes, or (ii) scripting (I don't know how to do this), so that your sequence includes periodic autofocusing runs.

Since there are so few good imaging days where I live, and so many other things can go wrong in the course of an imaging session, I typically stay up to chaperone the setup, and refocus every so often, as I see the focus of images getting soft. I typically will refocus four or five times in the course of a night.

Subject: Image Shift Adjustment--Part 1 of 2  

From: Sam Lattuca <lattucamidwest.net>

When I got my 10" LX200 f/10 used some months ago, one of the first things that I found objectionable about it was the amount of image shift that I encountered when focusing. After delving through months of MAPUG-Astronomy Topical Archives and talking to other users, I knew that mine was definitely excessive. Using a 21mm objective I would get anywhere from 1/3 to 1/2 fov shift. I have been working on this problem and now have next to NO image shift at all I'm happy to say. I now can contribute what I think can be some worthwhile information to the group that some may find helpful.

If you encounter what you suspect is excessive image shift try this test. Set up your scope and elevate the end with the correcter up 30 degrees. Back the focus out fully CCW then undo the three screws holding the focuser assy in place. Now, while holding the focuser knob in your hand, allow the weight of the primary mirror to slide down the optical tube and time how long it takes to reach the back of the scope. Obviously, you dont want to offer resistance to its downward slide, except to keep it in control and guide it. Try timing it 2 or 3 times. It should slide effortlessly down the tube to the back of the scope in app 7 secs or less.

My experimentation has been that if it takes longer or has to be physically pulled backward then its much to stiff. The result is that if the mirrors sliding action of the baffle against the optic tube is too great, when you attempt focus you will induce lateral motion into the image. It can be seen by putting a piece of masking tape across the front of the scope(not on the corrector plate obviously) and while looking down the end at the primary mirror, run the focus back and forth. You can see the mirror stress sideways before movement occurs. The possibility of running the focuser rod back and forth to re-distribute the grease could help, but if the grease has aged and gotten stiff, disassembly and replacement of the grease is probably in order. After having gone through this now a number of times. The lesson is this, use a lightweight,dark grease that wont liquify or stiffen in temperature extremes. A little grease goes a long way. I found that merely applying a thin coat and then wiping most of that away with my hand was quite sufficient. I suggest that if re-lubricating is done that you perform the slide test above again. The primary mirror baffle should slide easily down the tube and should not require assistance at a 30 degree incline. Nor should it obviously go crashing down. It occurred to me that if there were too much slop in this arrangement that image shift could occur here also. I suspect that too much slop could be removed by the application of a thicker lubricant.

As to lubricants, the tolerances of my scope dictated a very thin grease and I settled on a dark Molytone grease. This grease is available in many places, particularly where VCR repair would be performed or electronic stores. I suspect that if lubrication is required that there may be some playing by ear required, per the tolerance of your particular scope.

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Subject: Image Shift Adjustment --part 2 of 2     

From: James Burrows <burrjawearthlink.net>

Jon Brewster wrote:
> I measured 81 arc sec image shift due to focusing from different directions.
> I'm not all that concerned, especially since it seems to be repeatable.
> Also, I'm hip to the need for coming to focus from CCW direction.
> I am curious what others have found. Is 81 arc sec in the middle of
> the pack or on the edge?

Another sample (8" LX200, new Aug 95, no fiddling w/baffle grease), mean of two readings of angular difference between focusing CW & CCW:
alt 2.5", azm 11" On this scope, the backlashes are: alt 67", azm 44"

You didn't say how you measured the focus shift; are you sure you didn't measure a combination of focus shift and backlash? My experimental design was, in LAND mode:

1) focus, last motion CCW,
2) center an object using GUIDE speed, backlash aware*,
3) record alt and az,
4) focus, last motion CW,
5) center same object using GUIDE speed, "
6) record & difference alt, az.

* To center w/backlash awareness, choose one button of N&S, say N, and one button of E&W, say E. When the object is considered centered, the last button pushed of N,S is N, _and_ the last button of E,W is E.

Being backlash aware when centering stars in ALTAZ mode is considerably more difficult. I had to write a program on the laptop to tell me which were the "last" buttons at center and GUIDE speed (they're different).

Subject: Mirror Stabilization Bolt --part 1 of 3   

From: Chris Vedeler

As I'm sure most of you know, a SCT focuses by moving the primary mirror up and down inside the OTA. The mirror is mounted inside an aluminum or steel mirror cell that rides on the central tube (where the light comes through) inside your scope which you can easily see. This mirror cell is moved by screwing up or down on the focus knob. As far as I know, there are only 2 points of contact with the mirror cell, the central tube and the focus screw. Since the central tube is a nicely machined match for the mirror cell hole, and they use special grease the mirror cell doesn't move from side to side much at all, but it still moves a tiny bit (giving a slight image shift). There is an inherent slop in the focus screw that can only be improved upon by machining a better focus screw. When you change direction with the focus knob, you have to make up a tiny bit of slop in the screw before the top of the teeth of the screw engage with the bottom teeth in the mirror cell or visa versa depending on the direction of the focus. If you have ever played with a bolt and a nut from the hardware store, you know that there is a tiny amount of play when the nut is screwed onto the bolt. The same thing is happening inside your SCT.

When your 10" or 12" SCT first arrived from Meade, the mirror cell was locked down to avoid problems during shipping. This is done by moving the mirror all the way to the back of the OTA and screwing in a 1/4" screw through the hole right next to the focus knob. When this is done, the mirror cell can not move at all (which is what you want when shipping in case UPS drops it). When you are focused using normal eyepieces and cameras, the mirror cell is actually about another inch or two deeper inside the OTA. The original shipping screw that came with your scope is not long enough to reach the mirror cell now. The mirror (inside the aluminum or steel mirror cell) is now "floating" on the central tube and is adjusted by a screw mechanism that is controlled by the focus knob. Given the design of this type of focusing mechanism, a little movement is inherent and unavoidable.

I got my idea for my mirror stabilizer from Randy Rubis. All I really changed from his design was adding a compression spring in between the wing nut and the spacer. In theory what you are doing is locking down the mirror cell just like you would for shipping, except the mirror is not moved all the way down to the bottom of the OTA. This requires a longer screw. It takes about a 2" or 2.5" screw just to reach the mirror from this hole. The reason you need to use a 6" screw is so you can clear the focus knob (i.e. about 3 .5" outside the OTA). It is necessary to have a nice tall spacer so that you can turn the wing nut above the end of the focus knob (imagine trying to screw a wing nut between the focus knob and the back of the OTA.)

I've tried to remove the backlash per Meade's instructions as well, and it really didn't do anything either. Unless it is really, really bad, I wouldn't bother. I put pictures on my webpage:

   <http://www.isomedia.com/homes/cvedeler/scope/mirrorlock.htm>
     Clicking this link should evoke a new browser window.

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Subject: Mirror Stabilization Bolt --part 2  

>Okay, I have some pretty simple questions. Sorry if these are rehashed questions, I just need some clarification.

>To make my mirror stabilized, I need the following?

>1) one (1) six inch long, 1/4 inch thick, 20 thread bolt.

>2) one (1) wing nut for the 1/4 inch bolt.

>3) two (2) one inch tall nylon spacers.

>4) one (1) three inch long, 1/4 inch internal diameter compression spring.

>

>Then, I assemble them like this?

>1) Take 6" bolt and thread on wing nut.

>2) Sleeve on one nylon spacer

>3) Sleeve on the compression spring

>4) Sleeve on the second nylon spacer

>5) Screw the entire assembly into the focus locking bolt hole one to

> two and a half turns. Is this correct?

You almost have it. One thing I did different. The spring goes between the wing nut and the nylon spacers, but I guess it would work your way too as long as you clear the focus knob. I've used just one 1" spacer with some success as the spring is anywhere from 3" to an inch depending on how compressed it is. I got the other spacer just in case. Ultimately the spacers are just a way to clear the wing nut past the focus knob.

> Okay, I'll assume this is correct. Now what?

> Do I focus as usual and then, when I've gotten as close as I can get,

> I screw in the bolt assembly inorder to stabilize and then make minor djustments?

Once it is installed (remember, don't thread it in more than a turn or two into the mirror cell!) with the spring uncompressed you focus as normal. Once you get close to focus, screw down the wing nut until you compress the spring to about one-half it's original size. This puts a gentle and controlled pressure on the mirror cell that keeps in place. Then do the final focus. You will find the focus behaves much more predictably, smoothly and stops on a dime (the way it should from Meade, but doesn't). If you move the focus a whole lot (add a barlow, different camera adapter, add focal reducer etc.), make sure you watch the spring, as you don't want to compress it all the way or you could damage your focus mechanism (just like you don't want to try and move the focus when the shipping screw is installed). The spring makes this a lot safer, but you still want to be careful.

> Or, do I just begin the night by screwing in the bolt in 1 to 2.5

> turns and let the spring take the tension away from the mirror when I focus?

If you aren't changing focus drastically, you can just leave it in all night. One thing though, I do remove it when I am packing the scope up for the night. It does stick out a ways (3" or 4" from the back of the OTA), and is directly connected to the mirror cell inside the OTA. I'd hate to bump it and do damage to the mirror or mirror cell!

> This brings me to my final question. Does this bolt thread into the

> locking bolt hole, or does it just slide into the hole?

The locking bolt hole is not threaded, it is just an access hole to get to the mirror cell female threads inside the OTA.

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Subject: Mirror Stabilization Bolt --part 3 of 3 (Mirror Locking Bolt's Effect on Collimation)  

From: Bruce Dickson <bdicksonmweb.co.za> Date: Sep 2003

I did some experimenting with a mirror locking bolt (as originally proposed by Chris Vedeler).

I've been doing some planetary imaging using a webcam, and based on comments by Thierry Legault, I decided to check my collimation just before I started up the camera.

I was quite surprised - although the mirror lock bolt kept my mirror static, it twisted so much that the collimation was rubbish. I found that I achieved far better planetary images without the "stabilised mirror". Although I tried, I wasn't able to achieve acceptable collimation with the bolt in place.

I've also found that the mirror "flop" was sufficiently repeatable that provided I _always_ focus in a clockwise direction, the object will return to the centre of my webcam's field of view, even at f/45. (The direction might is different for the 7", 8"and 10" LX200s.)

That's not to say stabilising isn't useful for _long_ exposures, it's just that for high resolution imagery on low contrast objects, perfect collimation is far more critical.

Subject: Reducing Mirror Shift and Improving Classic Focus Action   

From: Michael Hart, Date: June, 1998

BACKGROUND
I ran across my old notes concerning the Meade SCT focus mechanism and decided to post a few details related to recent inquiries on this subject. At one time while performing a few optical measurements, I decided to explore the Meade SCT focus system. I attached lasers to the front and rear of the slider tube and observed the moving mirror in action. I also checked several other SCTs (one a non-Meade) and found the amount of mirror shift was related to but not necessarily effected by slider and baffle tube clearances. Some scopes with greater clearances had less mirror shift. The Meade SCT focus knob mechanism is attached to a stainless steel threaded rod with a hole on the end which slides over a pin protruding from the focus arm. The focus arm threads onto the slider tube which sandwiches the tapered primary mirror between a neoprene impregnated cork gasket (toward the corrector and under the slider face) and a O-ring (toward the eyepiece end and under the focus arm). The rear casting uses a thin ring of silicone as a semi flexible stop.

THE 12" SCT FOCUSER DIFFERENCE
The 12" SCT uses a large spring between the focus arm and the rear casting to compensate for the weight of the larger mirror. This spring has just a bit more tension than the mirror weight, so the primary is actually lifted when the optical tube is vertical and more so when away from vertical. While it may seem the springs only purpose is to equalize focus action, the mechanical advantage of the focus mechanism is more than sufficient for this purpose. Those using the 12" for CCD may want to reverse the usual advice of finishing focus counter-clockwise.

CONTROLLING MOVING MIRROR IMAGE SHIFT
Another useful purpose for the spring is to help control image shift of the larger and heavier 12" mirror. It does this by preloading the slider tube to minimize the force needed to move the mirror with the offset focus arm. Since the focus arm is offset, any force applied tends to twist the mirror on the slider a bit, increasing image shift. If the slider is evenly lubricated and the lubricant sufficiently viscous, the attached primary mirror is moved along the baffle tube with little to no twisting, even with fairly large clearances. It is tempting to conclude minimal clearances between the baffle tube and slider is desirable, however, we need adequate clearance to accommodate a wide range of temperatures. Incidentally, at one time a SCT manufacturer tried a three screw and timing chain approach to moving the mirror which proved worse than today's mirror moving methods.

HOW THE FOCUS KNOB MECHANISM WORKS
The focus knob pushes and pulls on a threaded rod loosely held with a pin to the focus arm. The focus knob mechanism consists of a internally and externally threaded brass bushing using several nylon thrust washers under the front facing and behind the focus knob that rotates against an anodized aluminum retainer. All is secured by three Allen button head stainless steel machine screws to the rear casting. The focus rod is threaded into the brass bushing and secured with a spring washer and I believe a 4-40 Allen head machine screw about 1/2" long. The length of the machine screw and the length of the threaded rod determines the maximum amount of backfocus, limited by the machine screw stop or the retaining ring on the slider (visible through the corrector plate). The former machine screw on the threaded rod serves to prevent the threaded rod from separating from the threaded brass bushing when the focus knob is turned fully counter-clockwise. My slightly modified focus mechanism achieves 52.252 turns.

Stock focusers will probably achieve a bit less. The focus knob is bored out almost to the end to allow the threaded rod to move up inside the focus knob. The focus knob is of aluminum construction and has a slight lip at the end which contains the space for several (4) nylon thrust washers. The focus knob is threaded onto the external brass bushing threads along with the nylon thrust washers and secured by two set-screws. All is lubricated with a molybdenum based grease. Celestron uses a slight variation in controlling backlash consisting of double ball bearings and a floating bearing housing with a fixed focus rod.

ADJUSTING THE SCT FOCUSER BACKLASH
Focuser backlash should be close to zero. Excessive backlash results in delays in focus changes quite possibly mistaken for optical anomalies. The telescope doesn't "snap" into focus and the 12" may have a slight focus drift in a few seconds, depending on the position of the optical tube as a result of the added slider spring. An overly tight focus knob is difficult to use at high powers and is difficult to feel minute rotational changes. A well adjusted focus mechanism works very nicely. It is light, smooth and backlash free and minimizes or may even eliminate the need for add on Crayford focus accessories such as the JMI NGF-S or the Feathertouch focuser. For those in need of extra backfocus, clearance problems, or on a tight budget, a well adjusted focus mechanism will help. I am quite comfortable with a well adjusted SCT focuser for CCD work including it's stand-alone use with supplementary focus arm stabilization using a long 1/4" bolt and a knurled nut in place of the shorter shipping bolt. Focuser backlash is adjusted by tightening or loosening the focus knob onto the threaded brass bushing and securing the adjustment with two set-screws. In many respects, it is easier to adjust the backlash with the focusing mechanism removed, however, Meade advises a method to do this with the focuser mechanism installed. It is important that enough thrust washers are used under the focus knob or the knob will scrub the aluminum retainer. It is also important to evenly tighten the set-screws securing the focus knob so that this knob remains parallel to the aluminum retainer. Proper adjustment of the focus knob set-screws is easily done by viewing the focus knob at an angle, looking for focus knob wobble. Alternately loosen and tighten the two screws to remove focus knob wobble.

IMPROVING THE MEADE SCT FOCUSER FEEL AND ACTION
Though the Meade SCT focus mechanism is quite adequate when adjusted properly, the focus mechanism feel may be improved with a few simple modifications and $3.54 plus tax for the necessary parts. For those that are mechanically challenged, this is NOT a complex procedure.

Essentially, we are going to replace the nylon thrust washers with steel roller thrust bearings. Purchase the following parts at a bearing supply house (Berry Bearing or equivalent). (2) 1/2" ID roller thrust bearings (Torrington # NTA-815) (1) 1/2" ID 0.030" thrust washer (Torrington # TRA-815) Start by tipping the front of the scope slightly up from horizontal and turning the focus knob fully counter-clockwise, moving the mirror forward and extending the threaded focus rod forward. This could take as much as 40 turns, depending on the scope and initial mirror position. Remove the three focuser retainer machine screws and pull on the focus mechanism moving the mirror back. If you have significant mirror shift, now is the time redistribute the grease between the slider and the baffle tube by moving the mirror fully in and out multiple times. This may not cure your mirror shift problems, but can often help. This procedure is recommended by Meade and is described in detail in Ed Stewart's MAPUG Topical Archive.

On the 12", the use of a 1/4" X 1-1/4" bolt or the original shipping bolt in the shipping bolt hole will help keep the internal spring from moving the mirror forward. Remove the focus mechanism from the focus arm by sliding the focus rod off the end of the focus arm pin. Loosen the two focus knob set-screws and unscrew the focus knob from the threaded brass bushing. Slip the threaded brass bushing out of the focuser retainer and remove any nylon thrust washers from the threaded brass bushing. These will be replaced with the roller bearings. Pack both roller thrust bearings with a good all temperature grease such as LubriMatic LMX or you could use a bit of excess grease found around the threaded bushing. You may want to avoid some molybdenum based greases (often black in color) that are designed for high bearing pressure and exhibit high adhesive properties as they may stiffen the action too much. Slide the roller thrust bearing on the bushing and the bushing back into the focuser retainer. Slide the second thrust bearing onto the bearing retainer. Slide the 1/2" ID .030" thrust washer down over the threaded bushing. Use enough of the original nylon thrust bearings to clear the focus knob lip--probably four. I used a brass faucet washer of the same OD as the nylon thrust washers that was carefully reamed out and finished for bearing face service with 320, 400, 600 and polished with 1500 grit sandpaper. Final assembly an adjustments are as described in the second paragraph under Adjusting the SCT Focuser Backlash. The results are excellent control of backlash with a very smooth action that almost feels as if the focus knob is disconnected from the focus arm.

Subject: Forcing Mirror Flop in SCTs  

From: Alan Voetsch <critter12952yahoo.com> Date: Nov 2002

A couple days ago, this subject came up on the SBIG users group. Adam Block advised a technique to pretty much eliminate the dreaded mirror flop. This will not be that critical for visual use, but for those imaging, it will make a big difference. I have always had the problem of focus shift. I see it on my autoguider brightness readings, even though I use the Vedeler mirror lock-down system.

Here's what to do. If you're imaging to the East, move your SCT low towards the Eastern horizon. Use the focus knob and adjust focus in and out a little. Then simply adjust for sharpest focus and proceed to your target. If you're working to the West, just use the Western horizon. This should remove the potential for flopping until you pass the meridian.

I just picked up my latest batch of pictures and there is not a focusing problem among the six I've taken since I implemented this technique. Amazingly, as I've monitored the brightness readings, I've actually seen them improve during the exposure. Why? I have no idea, but I'm not arguing with success.

Subject: Mirror Flop Explained  

From: Roger Hamlett <ttelmahntlworld.com>

The central spring does _not_ push on the mirror, but instead pushes on the outer 'slide' tube, over the baffle tube. It's function is not really to get rid of mirror shift, but rather to minimize backlash in the focusers mechanism (a separate problem), and to keep the focussing action from being too heavy. The problem, comes about, because of the extremely small tolerances that are involved. You have a mirror assembly, supported on a tube, which itself slides over another tube. The outer tube than has an arm, going to a 'pin joint', on the end of the focusers adjustment rod. If you visualize the situation with the focusers arm below the baffle tube, the entire assembly (of tube, and mirror), will rock, pivoting on the focusers pin joint, till the outer tube rests on the inner tube, at it's front upper edge. If you add a pressure spring, the rocking is reversed, with the system resting on the rear upper edge instead. Repeat with the system inverted, and the rocking is in the opposite direction in each case. The actual motion in use, is very complex, forming a rolling action round the inner tube, with the one fixed point, and with the motion damped by the grease.

The way to reduce mirror shift, is to reduce the gap between the inner and outer tubes. The 'collet' system on the latter Meade scopes, serves to tighten the gap at the rear of the tube, and _reduces_ the problem (however because the front is still loose, there is still slight detectable shift). The Celestron scopes, are usually slightly 'better' than the unclamped Meade units (their tolerance on the tube sizes seems to be a little tighter - this is probably why Meade felt they had to introduce the clamp). The 'travel bolt' locking systems, work by adding a second fixed point onto the arm, applying some force to the entire system, and deliberately rocking it in one direction. Ideally, what is needed, is a tighter collar, both in front of the mirror, and behind it, but you then have the problem that anything in front of the mirror, may introduce either diffraction spikes, or if round, increase vignetting...

It is possible to make Maksutov's, and SCT's, with no detectable mirror shift at all. The problem is that the low profile linear bearings used, are very expensive.

It would be possible to make a design similar to the existing units, which would not add too much to the cost, and would give very good results, by replacing the screw at the front of the baffle tube with an adjustable 'collet', with an inner PTFE ring, and have a similar adjustment at the rear. Part of the setup of the scope would then be to adjust both collets till the motion is as tight as practical, without sticking. The 'downside', is that it adds another assembly operation (cost), and as the scope ages, it will become necessary to re-adjust the collets. I have a Maksutov, that uses such a system, and shows image shift, of only a couple of arc seconds. One other method, is to go away from the single focussing knob system. If instead, you had a knob that turned a ring gear, and three focusers shafts, each going to a separate arm behind the mirror, the system again becomes properly supported.

Subject: LX200: Ball Bearing Focuser Upgrade --part 1 of 2   

From: Paul Luckas <luckasoptusnet.com.au> Date: Jun 2001

I've just completed an upgrade to my LX200 focuser assembly using the method outlined by Michael Hart here in the Mapug Topical Archives. The thrust bearing and washers suggested by Michael work perfectly, and are easily available (even in Australia).

Assembly was very easy, and essentially involves replacing the standard plastic thrust washers on either end of the brass focuser sleave with needle bearings. I made one modification which was to have approximately 3mm taken off the internal end of the outer sleeve (the black bit that screws to the outside of the OTA) to allow the outer needle bearing and thrust washer to fit onto the 1/2 inch portion of the brass shaft (the slight extra thickness of the bearings meant that - at least on mine anyway - the thrust washer didn't quite make it onto this part of the shaft. More importantly, it avoids having the grub screws bite into the brass thread when you reassemble the focus knob, and seats them down onto the part of the shaft where they are meant to be. This tiny modification is not absolutely necessary, but anyone with a lathe (or friend who has one) will find it very easy to do if needed.

Warning, the needle bearing modification results in a very, very smooth focus action, and if you're used to friction you may find it a little disconcerting to begin with. You can actually very easily feel the weight of the mirror when focusing anti clockwise as opposed to clockwise.

I still have about 1/4 of turn of backlash - despite no observable movement in the focus knob assembly. The left over backlash is probably a result of less than perfect screw threads, or maybe associated with the several arc minute mirror shift that I have on my 10" LX200. Nevertheless, the focus action itself is much improved.

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Subject: LX200: Ball Bearing Focuser Upgrade Kit --part 2 of 2    

From: Pete Peterson <mapugpeterson-web.com>

Actually this "fix" is so good that we're selling them as the EZ Focus Kit for $30. It's detailed on the web:
   <http://www.petersonengineering.com/SkyDiv/ez_focus_kit.htm>

There are 3 basic causes for backlash. The EZ Focus Kit only addresses the major one, which is slop in the focus mechanism. With Meade's low cost approach to focusing using greased plastic washers for bearings you need to make regular adjustments to the focus mechanism, and even then when the focus is backlash free it's tight to turn. This retrofit kit has to be experienced to be appreciated.

The second cause is fit of the focus screw onto the 1/8" diameter pin linkage

on the back of the mirror mount. If the hole through the end of your focus screw is more than a few thousandths of an inch oversize you'll get a little backlash. This is uncommon, but I've heard of it happening.

The third cause for backlash is the mirror flop itself. If your mirror fits loosely over the baffle tube the first thing that happens when you reverse focus direction is that the off center force of the focuser "flops" the mirror to take up the slack between the mirror and the baffle tube before the mirror starts to actually move in the new direction. You'll know from the movement of your image as you change focus direction if this is going to be a problem. A small amount of Image movement at high magnification is normal and won't be much of a problem. But if there's significant "flop" you're going to have to address that separately. The special grease provided in the kit to lubricate the thrust bearings is the same grease used by Meade to coat their baffle tubes - so you do have the option of using the remaining grease from the EZ Focus installation to recoat your baffle tube - that's the best way to reduce mirror flop although it's another whole procedure not associated with the kit.

The bearing fix Michael Hart (described here in the MAPUG Topical Archives) is a very good one. With Mr. Hart's blessing we've put together a kit with all of the hard to find parts for this fix complete with detailed step by step photo illustrated installation instructions.

LUBRICATING THE SLIDER TUBE
Others may wish to re-lube the slider entirely, though I believe the need for this should be very rare and avoided. This requires the removal of the corrector plate retaining ring, corrector plate, and to fully complete the job, removal of the primary mirror from the baffle tube. Meade indexes the corrector plate (if yours is not, do this now), but does not index the secondary (index this as well now), so if your secondary was/is loose (rare), you will need to slightly loosen the secondary retainer on the inside of the corrector plate, install the corrector and spin secondary around while looking for the best optical results. A star test is the ultimate arbiter, but close planetary examination for contrast and detail may speed your work. To remove the primary, the baffle tube machine screw stop or retaining ring must be removed. The mirror is pulled straight up and tilted a bit to clear the front casting slots provided for this purpose. Great care must be taken at this stage to avoid scratching the front of the mirror. I believe the use of a pure silicone based lubricant is adequate such as laboratory high vacuum stop cock grease or pure silicone grease used on diving equipment. Another good alternative may be a wide temperature range grease such as LubriMatic LMX (a red grease). Molybdenum based greases (often black) may not be appropriate due to extreme adhesive like properties. I do not have experience with beeswax used by Doc G for his 5" SCT guidescope, but his purposes are a bit different than most. I strongly suspect the focus action would be quite stiff, especially in freezing weather. The grease is applied thinly and evenly onto the baffle tube. Assemble in reverse order and assure the 1/16" cork spacers around the perimeter of the corrector adequately center the corrector. If not, they can be replaced with common 1/8" cork gasket material compressed with a pliers just before insertion. You may need to pull down on the corrector plate to insert the last 2-3. Adequate cork gasketing helps maintain the corrector centering while minimizing distortion because the corrector can expand and contract between the paper gaskets while remaining supported and centered in the castings. I am quite please with the results obtained. My worse case image shift is 13 seconds in my 12" LX200 and very fine focus action.

Note: another kit and details are available at: <http://www.petealbrecht.com/focuser/focuser.htm>
and an additional info at: <http://www.g0poy.me.uk/Pages/Astro/Focusser/Focusser.htm>

Subject: Focus Bearing Modification (Classic) --part 1 of 5 

From: Philip Peake <mapugvogon.net> Date: July 2001

Some comments which may be of interest to people thinking of making this modification:

I decided to try out this modification mainly hoping it would remove some of the backlash in the focus control. Not being able to find a local distributor of the thrust bearings I decided to take the easy way out and order the kit. I have to say that ordering was easy, and I soon received my kit, which contains everything promised including some quite clear documentation on how to perform the modification.

The only part I have any issue with is the warning about using pliers and paper or cloth to hold the brass body of the focuser while unscrewing it from the aluminum knob.

The warning really isn't strong enough!

The brass used is very soft. Even holding the brass body with some padding, it is all too easy to distort it at the edges, particularly because the end being held is thin, and difficult to grip. Mine ended up with some small dents in the edge which were enough to raise bumps on the top and bottom of the flange against which the thrust bearing runs. When I assembled the bearings and tried turning it, it all moved easily but there were definite bumps to be felt while turning - this was the small, really difficult to see bumps raised on the brass flange. It took some time with a fine stone to remove these bumps.

Once that was done the focuser assembly turned rather smoothly and easily compared to its original state.

After re-assembly, I was a little disappointed. The movement was easier but now there was some "gritty" feel when focussing ... pulling the focuser assembly out again, it turned smoothly. Pushing the mirror assembly back and forth with the retaining bolt revealed a definite "gritty" feeling to that movement. I swept it back and forth a few times, and it got better, but I still won't call it really smooth.

The tension on the thrust bearings also makes a big difference, to little tension and the mechanism is sloppy, too much tension and the bearings begin to feel "gritty". After some thought I decided to see if I could make the focuser move even more smoothly by experimenting with different grease. Eventually, I found that I got best results by using a PTFE grease intended for lubricating guns (Tetra Gun Grease). This results in a really silky smooth focuser.

I also removed the focus screw, cleaned off the existing grease on that and used the Tetra gun grease to lubricate that thread, as well as the hole and bar on the mirror assembly to which it attaches.

This seems much smoother than using the Meade recommended grease. I can still feel some 'grittyness" as the mirror moves back and forth, but that is definitely not due to the focuser now.

This grease is not cheap, but I do suggest trying it out with this modification. My results are certainly better.

Also, if there is *any* feeling of bumps or grittiness in the focus assembly when not fitted to the telescope. Take a long hard look at the brass flange and see if there are any small areas of distortion caused when disassembling (hopefully, most people's focusers will come apart somewhat more easily than mine did !).

One final remark on the kit: the Allen key provided is too short to make removing the retaining screws easy. I ended up using one of my own, about an inch longer ... much easier.

--------------------------------

Subject: Focus Bearing Modification --part 2  

From: Mark de Regt <dmark18qwest.net>

I just wanted to say that my experience with the EZ Kit was polar opposite to Mr. Peake's. I found the warning about pliers on the brass to be strongly worded, and I was very careful; no damage done. I did not find the "gritty" feel unpleasant or unsatisfactory; quite the contrary, if it were any smoother, it would be downright disconcerting.

After the modification, the focuser is easy to work, even for the fine adjustments necessary for CCD imaging.

The one thing I agree with is that the Allen wrench is too short. I suggested to Mr. Peterson that he make the Allen wrench longer, and he is looking into that. For more info:

<http://www.petersonengineering.com/SkyDiv/ez_focus_kit.htm>
Note: should open a new browser window over this one.

--------------------------------

Subject: Focus Bearing Modification --part 3  

From: Pete Peterson <mapugpeterson-web.com>

Thank you again for your comments. For a relatively simple product, it's hard to believe that we had a full man-month of engineering invested in it before we released it. There were many fine details that needed to be addressed, and based upon inputs such as yours we continue to improve this product.

The long arm Allen wrenches are on order and will be substituted in the kits later this week when received. And as soon as you and I (working together) identified the source of your problem the kit instructions were revised to more strongly warn against damaging the brass focuser insert. We also explained in detail just why it's so important not to leave pliers marks on the brass focuser body.

Although we'd sold a good number of the EZ Focus Kits you were the first person and only person that I know of to encounter this problem. With 4 pages of detailed instructions you'd think we'd had covered everything.

--------------------------------

Subject: Focus Bearing Modification --part 4 

From: Stephen Hutson <sehutsonnet66.com>

Ron Columbo wrote:

>Before the mod there was a non-uniform resistance to the turning
>motion of the focuser. For example, it was harder to turn between 9
>o'clock to 12 o'clock in either direction, (assuming a line was
>scribed/taped onto the focuser knob). That non-uniform resistance
>still exists after the mod, and is even more noticeable since the
>knob turns easier in the smooth portions of knob turning.

That happened with me, too. Additionally, my focus knob is machined in such a way that it "wobbles" as it turns -- the threads going down the middle of the aluminum knob do so at a very slight, but noticeable, angle.

>As for the positives, the modification definitely resulted in lower
>turning forces necessary, and more importantly, virtually no
>backlash that I could observe. The latter result was one of my
>primary reasons for performing this mod.

Indeed! I can live with a somewhat poorly machined focus knob, now that my LX200 focuses almost as smoothly as my old Questar, which has one of the best focus feels of any telescope I have ever used.

--------------------------------

Subject: Focus Bearing Modification --part 5 of 5 

From: Nigel Burge <nigel.burgepobox.com>

I too had a slight amount of grittiness until I realized that the final locking down of the focuser knob setscrews is very critical. It took me a long time to get this exactly right -- not too loose so that there was backlash and not too tight so that it felt gritty.

Once it was right though, the feel was superb - one of the best mods I have ever made to my LX50 and certainly the most cost effective. I now have no use for my electric focuser. Michael Hart and Pete Peterson have come up trumps here for the mechanically challenged like me.

One point worth mentioning is that I had to smooth off the flange on the threaded brass part against which the bearing sits. Mine was very slightly rough with bumps which made the roller bearing rotate unevenly, and smoothing out the flange eliminated this.

Incidently, my focuser assembly unscrewed under finger pressure alone. It would appear that there is a very wide variation in how tight this is from only finger tight to very difficult to budge.

Subject: Removal of Classic 12" Focus Assembly  

From: Doc G, Date: Sep 2003

To remove the focus assembly on the 12" do the following:
Move the primary all the way back by turning the focus knob clockwise. Insert the shipping bolt to hold the mirror to the back. Remove the focus mechanism by removing the screws holding it in place. Turn the focus knob counterclockwise to free it. Slip it out of the latching mechanism on the back of the mirror.

Adjust it as you need to and replace it as you removed it.

Subject: Re-Attaching the 12" Classic Focuser Mechanism --part 1 of 3  

From: Doc G

Tugrul Ussakli wrote:
> After using my LX200 12" Classic with #1206 focuser successfully for 8
> years today I disassembled my focuser to replace it with a Robofocus
> unit and I ruined everything!!.
>
> While calibrating the focuser I saw that the focuser knob
> never stops in either direction. I then manually tried to
> focus the telescope by manually turning the focuser knob but
> it wasn't focusing and the knob kept on turning without
> moving the primary mirror at all.
>
> To see what's wrong, I removed first the set screw from the
> knob, unscrewed the knob out, then removed the 3 screws
> fixing the assembly to the rear cell. THAN I SAW THAT THE
> SMALL THREADED ROD BETWEEN THE MIRROR AND THE BRASS PART INSIDE
> THE FOCUSER KNOB FELL DOWN INSIDE THE REAR OF THE TELESCOPE.
>
> I live in Istanbul, Turkey and there is no Meade service
> nearby so I have to do the repair by myself. I guess the
> first thing to do is to remove the rear cell and find the rod
> and put it back to its place and re-assemble everything. But
> I don't know how to do.
>
> Can somebody tell me exact procedures, with drawings if
> possible, on how to remove the rear cell and how to put
> things back including the focuser assembly.

It is not necessary to remove the corrector or primary. Here is a technique that works. Using a long bolt, 1/4-20, pass it through the shipping bolt hole and into the mirror mount. Use this bolt to pull the mirror backward. At the same time, use a very long nosed pliers to manipulate the focus bolt through the focus mechanism hole. Or I do recall now that one time I had to use a long grabber. It is a flexible tube with little pins sticking out of the end that you can manipulate from a plunger on the tube. An essential tool for every well maintained shop. Not real easy to do, but well worth the struggle. Pull the mirror back until the focus bolt is cleared enough to reassemble the mechanism. Reassemble the focus mechanism. Release the shipping bolt and reseat the focus mechanism.

This is relatively easy to do. I have done it a few times. It is sort of like doing a gall bladder removal through the nose. Then do not unscrew the focuser without installing the shipping bolt again, and you will prevent this embarrassing problem.

----------------------------------------------------

Subject: Re-Attaching the 12" Classic Focuser Mechanism --part 2 of 3

From: Dennis Williams <drwicu812comcast.net>

Hey Tug --Been there-done that, no big problem. Use a magnet to reach down in there and pull it up to be accessible with needle nose pliers. You may have to play around with it to get the threaded rod to behave but you can do it. Don't force anything and you'll be fine. I used one of those extendable magnets that can be clipped to your shirt pocket.

----------------------------------------------------

Subject: Re-Attaching the 12" Classic Focuser Mechanism --part 3 of 3  

From: Tugrul Ussakli <tugrulussakli.net>

I managed to fix my problem with your help, what I did is :

1A) Tried to take the threaded rod inside the rear cell with a magnet but learned that since it is brass or similar there were no grab.

1) Removed all the focus assembly.

2) I put the OTA in almost upward position.

3) I took a plastic covered rope, about 1 foot long and bent it into half.

4) From the focuser hole I put the rope in, captured the rod coming out of the primary mirror and draw the mirror all the way in towards the rear cell, and with my other hand I took the red locking bolt and locked the mirror. This needs a bit strong force. There is a spring in LX200 12" that tries to force the primary towards the corrector.

5) I took the 6 screws out from the corrector plate flange.

6) Removed the flange.

7) Checked the marking on the corrector and the OTA. They were about 1" offset. (Maybe the corrector turned when I took the last screw out)

8) Put the OTA at little above horizontal.

9) Holding the secondary mirror housing I removed the corrector plate out and put it on a previously prepared soft and clean place and covered it with a clean nylon.

10) Then put the OTA all the way down, pointing the floor.

11) The missing threaded rod fell down by gravity.

12) Cleaned the interior carefully.

13) Put the corrector plate, and the flange back, screwed the allen bolts back. (Aligned the marks first)

14) Put the threaded rod into the rod coming out of the primary. (A single use glove is needed for this)

15) Greased the rod.

16) Screwed back all the focuser assembly. Removed the red shipping 1/4"-20 bolt.

17) Checked the backlash.

18) Cleaned the rear flange.

19) Assembled the Robofocus back.

20) Re-Calibrated the Robofocus and DONE.

THE NEXT TIME I WILL NEVER TAKE ANY SCREWS BACK FROM THE FOCUSER ASSEMBLY BEFORE TURNING THE FOCUSER KNOB ALL THE WAY IN AND FIXING THE PRIMARY WITH THE RED BOLT.

Subject: Stuck Focuser --LX200 Classic --part 1 of 3  

From: Keith Graham <kagcore.com> Date: Dec 2002

From: Matt Thomas
> I just got through installing a RoboFocus unit on my LX200 classic.
> I mistakenly ran the focuser all the way clockwise, and now it is stuck.
>
> The focus knob slipped when I attempted to back up the focus (the two
> set screws were not tight enough to hold on). I am not sure what to
> do to un-stick the mirror.
>
> If it matters, I had installed the Peterson Ez-focus modification a
> while back. Unfortunately, I don't even have the instructions anymore
> to walk me through disassembly of the focuser (although everything
> I've found says to move the focuser all the way CCW first).

By "stuck" do you mean that the focusing knob is so tight that it won't back out? Or does the focusing knob slip on the threaded shaft so the mirror will not back out?

When I put my Robofocus on, I had a similar experience. The instructions indicated that when the knob reached the end of focus, a clutch in the Robofocus mechanism would slip. Well, it did not, and the knob just kept turning.

Now, there is a little stop (like a very thin washer) on the threaded rod on the focusing mechanism. If you remove the focusing mechanism, you will see that the little stop is bent way out of shape. You can call Meade and they will send you a new one.

It does sound like you will be able to remove the focusing mechanism with no problem. Once you do that, you can examine it for further damage. If need be, you can get the whole mechanism from Meade for about $30.00.

----------------------------------

Subject: Stuck Focuser -LX200 Classic --part 2

From: Keith Graham <kagcore.com>

OK, you said that you rotated the knob CW. That means that the mirror is tight against the back of the scope. Now, try removing the focusing knob (loosen the set screws and rotate the knob CCW). If memory serves me correctly, the threaded rod has a hole in it on the end. If you pass a nail through that hole to use as a lever, you may be able to turn the threaded rod CCW to loosen the grip. GO EASY - do not just apply a heavy torque CCW. Try a little pressure a first to feel what is going on. If you can break that grip, continue the CCW movement a few turns. You should then be able to slip the threaded rod from the mirror backing plate. The end of that rod has a right angle bend that slips onto "hook" on the backing plate.

----------------------------------

Subject: Stuck Focuser -LX200 Classic --part 3 of 3 (Solved)

From: Matt Thomas

Ok, I was able to get it fixed. The RoboFocus had actually unscrewed the knob from the brass nut (when I tried to focus the other direction).

I was finally able to disassemble everything. Perhaps the assembly cooled down a bit from last night. Since I had the mirror in the fully back position, once I removed the mounting collar, I had to turn the focus screw forward so I could remove the assembly from the mirror pin.

Once I had it out, it was a simple job of just putting it back together properly. The threads on the brass nut were slightly damaged from the set screws on the focus knob, but I was still able to put the knob back on.

Subject: Mirror Shift Fix (Classic) --part 1 of 2   

From: Chris Heapy <Chrisheasynet.co.uk>

Philip Freeman wrote:

> I'm frustrated by my mirror shift and am looking for an

> opinion on whether regreasing is a good idea and what to expect if I do it.

> I have measured the shift at about 2 arc minutes:

> Eyepiece: 6.7mm > Apparent FOV: 50deg > FL Telescope: 2000mm

> Magnification: 298.5x > Actual FOV: 0.1675deg=10'3"

> shift is about 1/5 of the FOV or about 2'

> Did I do this calculation correctly?

> I have raked the focus knob all the way forward and back a few times, but it

> doesn't seem to be helping much.

I would say the degree of movement you describe is excessive, but not the worst example I have ever heard about (like, 1/5 FOV with 26mm EP!) You don't say how old the scope is, nor whether it's had heavy usage. Also, whether this mirror shift has developed slowly over a long period of time or was there from new. My own 10" LX200 had undetectable mirror-shift in a 7mm eyepiece when new, but soon developed a minor amount of shift (I'd guess about 20% of what you describe) within a couple of months which got no worse over 12 months or so. It was not detrimental to the scope's operation but it was irritating, so I tried various fixes. The focusing action was still very good - no backlash detectable and very smooth so I didn't bother pulling that apart. Like you I found racking the focusser from one end to the other did nothing to cure the problem so decided that a re-lube was in order. Removing the corrector plate is easy and necessary to gain access to the slider tube. This operation is fully described in the archives. I simply racked the mirror all the way backwards to expose as much of the baffle tube as necessary.

The lubricant I used was a pure silicone-based grease ('Stop-cock' grease for laboratory use). It's translucent, odorless, detergent-free, and my example came in a small tube like toothpaste. I'm confident that Edmund Scientific in the USA will have it in stock but I have not checked. Others have reported success with Teflon based greases, but I have a sample of a this and it smells so strongly (and awful!) that I really didn't want to put it in my scope -- I strongly suspected outgassing of solvent or vehicle for the Teflon. The silicone grease (about a 1/2" length of 1/4" cylinder, or around 1 ml) is simply put on the end of your finger and spread evenly all over the exposed baffle tube surface, but concentrate on the joint near to the end of the mirror slider.

Don't use a huge gobs of grease, it's neither necessary nor helpful. Rack the mirror forwards and backwards and then remove any excess grease that has been scraped forward to the end of the baffle tube with a tissue. Wash your hands afterwards before re-assembling the corrector plate (wearing thin white cotton gloves is good to prevent finger marks getting on the corrector, I got some from a sports shop (!) -- used by snooker referees believe it or not). On my scope this simple procedure essentially *eliminated* the mirror shift, I could barely detect any shift at all. This was done some 3 months ago with no change since (though I admit the scope has had less than normal usage due to poor weather). I should point out that if the mirror shift is due to physical wear of the baffle tube or mirror slider resulting in excessive clearance between the two, then it's likely that any improvement due to re-lubing will be very temporary. This condition *might* occur in older instruments if it has been used extensively with inadequate lubrication. I would say that repeating the procedure at 12 month intervals will prevent this problem. In fact, routine maintenance at 12 month intervals will prevent many of the common problems that crop up - disregard the 'If it 'ain't broke don't fix it' adage! It might be worth someone listing a maintenance schedule to check over the mechanics.

Subject: Mirror Flop-part 2 of 2 -- the Final Solution!

From: Chris Heapy <Chrisheasynet.co.uk>

Now showing at a website near you... if you love your LX200 don't look, there's gruesome pictures of an LX200 savagely attacked with a 5/8" drill. Hey, but it works.

<http://easyweb.easynet.co.uk/~chrish/lx_flop.htm>
Note should open a new browser window over this one.

Subject: LX200 (Classic): Loss of Focus -- Resolved   

From: Michael Hart

> I think I am missing something here. The only way a star can become

> de-focused is by:

> 1. The primary mirror shifts.

> 2. A loose secondary mirror shifts.

> 3. The eyepiece moves in the barrel.

> How can a loose dec. mechanism cause a change in the optical path?

There are two additional considerations. First, the optical tube may have loose retaining screws attaching it to the front and/or rear castings. The Dec axle shafts are part of a bracket attached by three machine screws in Meade and Celestron SCT's. The Meade shafts on the 8-12" are nominal 1" diameter. One of those screws retains a portion of the optical tube between the Dec shafts and rear casting. The rear casting extends inside and under optical tube and is tapped for the needed machine screws. To tighten this portion of the optical tube, you must tighten one screw on the Dec bracket. I believe this was the repair initiated and completed by Meade to correct focus problems in the original post.

Another is a loose corrector. The corrector on a LX200 floats between 6 or 8 cork spacers on the corrector edge with a fiber gasket underneath.

The corrector must float as the front casting expansion rate is considerably different. Heavier correctors such as the 12" have sufficient weight to compress 1/8" cork commonly used by Meade and Celestron for this purpose. Eventually, the cork may remain semi-compressed. When the telescope is moved from the north to the south on a polar mount or during a German Equatorial flip, the corrector may move about under the retaining ring slightly. This can go unnoticed for years. I have described elsewhere in detail how to permanently correct this anomaly.

In both of these instances, focus changes are indirectly related to where the telescope is pointed.

Subject: Grease for LX200 Baffle Tubes   

From: Chris Vedeler <cvedelerix.netcom.com> Date: Sept., 1998

I've been wanting to try putting thicker grease on the baffle tube in my LX200 for a while now. The idea is to take away a lot of image shift in the focus. For those of you who have never taken the front corrector plate off your LX200, the grease used is about like wheel bearing grease. Kind of sticky, but not the kind that really clings. I had in mind that really sticky and thick grease that I've felt on the focus rack of my Short Tube refractor. That stuff has the consistency of tree sap and is about as hard to wash off.

I called around to several local camera stores since I knew that this type of grease is used in the focusing and zoom mechanisms of many camera lenses. No one had anything like it, but one camera repair store recommended "marine wheel bearing grease". I went to a local boat shop and asked to feel the grease (yes, I did get a strange look). It turned out to be not a whole lot different than what the LX200 had already.

I decided to ask the head of the lab at work to see if he had any ideas. It turns out he had the perfect grease. It is a grease used in high temperature laboratory vacuum machines (VWR Scientific Hi-Temp-Vac Silicon-based Lubricant, CAT NO. 59340-000). Any off gassing would be intolerable for scientific research (which is good to keep it from filming up the mirror and inside of the corrector) and this stuff is like slippery tree sap.

I removed as much old grease as I could, and smeared this new grease all up and down the baffle tube. It took some major muscle to get the mirror cell back down to the point where the focusing rod could reconnect since this grease was so sticky. I felt that this was a little too sticky as it took some effort to move the focus knob. So, I removed the mirror and mirror cell again and added a layer of "Archer Lube Gel" a silicon grease I got at Radio Shack. The Lube Gel is a Silicon based grease as well, but not nearly as sticky.

The result of the combination is quite encouraging. The extremely thick grease seems to take up a great deal of the tolerance between the two baffle tubes (the cause of image shift) while still providing a layer of lubrication, while the thinner grease seems to add just enough slipperiness to make focusing easier.

Subject: Grease for Mirror Slider  

From: P. Burgardt, Date: March, 2000

A slight word of caution is in order here relative to using vacuum grease as a general lubricant. I have tried using Dow Corning vacuum grease for a variety of lubrication applications in my work and would be hesitant to recommend it for most things. The grease is highly viscous and is quite "sticky". It is surprising how bad a lubricant it can be. You sure could try it for lubricating the mirror slider and I'm sure it wouldn't hurt anything. However, I would anticipate it being just too "sticky" to work out really well. For a focuser it would probably be better but it still strikes me as probably not the best solution. There are other vacuum greases that have somewhat similar physical properties but which are better lubricants. Apiezon brand vacuum greases (they sell all sorts for various applications) tend to be better lubricants. Unfortunately, these can be pretty expensive relative to the Dow Corning stuff. Anyway IMHO, you might want to look further before going the Dow Corning route.

Subject: Hartman Focusing Aid --part 1 of 3 

From: John Ruthroff <johnjohnruthroff.com>

You can have any number of holes with any spacing you want, but for the greatest sensitivity small holes (to prevent blooming) spaced as far apart as possible work best. I knew a guy who has his initials drilled into his mask! It looked kinda neat when the 'scope was way out of focus, but it didn't really facilitate focusing that much.

It's easy enough, with a razor blade and a piece of cardboard and tape to experiment with this. I tend to agree with Chris that (for me at least) the best solution is small holes far apart. I use two 1-1/2" diameter holes which are opposite and right up against the edge of the corrector plate on my 10" LX200.

I've never found the *which way to turn* to be a big deal. If my initial turn is the wrong way, I just turn the other way and remember which way is correct. When I get close I start focusing very slowly as there isn't much to be gained by overshooting the focus point over and over again. With the above arrangement, and using the focus mode of Win245, when I get the image down to one (binned) pixel in size then I'm "there". Once I reach the one-pixel-square star, if I turn any further I go out of focus again, so experience has taught me that when I'm at the "square star" point that attempting to refine the focus any further by continually racking in-and-out of focus is a waste of time.

Subject: Hartman Mask Focus Aid URL --part 2

From: Richard Robinson

See my Hartman Mask Focus Aid at: <http://rao.150m.com/Focusaid.html>

------------------------------------------------------------------

Subject: Hartman Mask Focus Aid URL --part 3 of 3

From: Eric Schandall

Been using the three hole mask as per Paul Gitto's suggestions on his website:
    <http://cometman.com/Mask.html> Note: should open new browser window over this one.

This has been a great help for me and think that his site is very good. The only thing I do in addition is to use a Sharpening routine toward the end of the process when the exposures are shorter and also enlarge these sharpened images. Images are focused quickly and quite well for me at this stage. Perhaps there are more demanding focusing requirements but this is a good focus.
I use a JMI NGFS focuser and am using a Pictor 416XT. For eyepiece projection particularly the focuser works very well.

Subject: Meade Electric Focuser/Update    

From: Leroy Guatney <lwlguatneyusa.net> Date: Feb 2001

I had previously asked for advice on my #1206 electric focuser. I got another field test last night, and still had the problem. Having asked this list for advice, and I tried several different things. Last time, I had tested in my colder garage, so today, I brought the OTA/Fork assembly into the house, let it all warm up, and hooked up the power. This time, I pointed the OTA at the horizon.

Tinkering, I determined that my focus knob was much smoother after warming up. No surprise there. Even still, the controls would not completely turn the focus knob. It would reach a point and stop.

I reasoned and confirmed that the clutch nut at the gear and telescope end of the focus motor needed to be much more tighter than I could do by hand. After iteratively tightening, and testing several times, I finally reached a point where the focus motor could steadily turn the focus knob.

Works like a champ in "summertime" conditions. Now back to the field to see how much the cold affects things.

There was previous mention of the possibility that I may have gotten into a mode where the focus button did not work. I did not exhaustively check all submenu items, but I was able to switch throughout the five modes and check the main submenus (TELESCOPE, OBJECT LIBRARY) that I tend to operate in, and confirm that the FOCUS keys still function in those modes/menus.

I think previously, the clutch was loose enough that the motor would spin when it was just too cold to turn the knob and I would hear nothing because no gears turned (?).

Whatever the case, thanks again for the ideas, and maybe this update will help someone else.

Subject: Optimum LX200 Primary/Secondary Spacing    

From: Doc G

Matt Considine wrote:
> Does anyone know what the designed spacing is for the LX200
> 10" f/10 primary/secondary mirror combination? I recall reading
> somewhere that the optimum spacing assumed a 35mm camera
> and T-adapter - but nothing else - were put in the optical path.

A recent discussion had to do with the fact that you can with an SCT get a large amount of back focus when focusing on an object at infinity. This is very nice since you can then use a lot of tubing on the back and especially things like focal reducers. Focal reducers gobble up the back focus in a hurry.

However, when you study the optical design of the SCT, there is a design center for the primary to secondary spacing which gives the best nominal optical performance. At the time of the discussion, it was felt that the best primary to secondary spacing was achieved when the focal plane for an object at infinity was about 3 to 4 inches beyond the back of the telescope back plate.

At the time and still today we do not seem to have the details of the Meade designs in hand. We would need to have the exact curvature of the mirrors and the corrector plate to determine the most desirable spacing of the elements. Even then there are compromises in the design since not all aberrations come to their lowest values for the same spacing. It is clear that any optical design with purely spherical mirrors is a compromise that is popular mainly because spherical mirrors are the easiest to make.

But that all said, there is some satisfaction in knowing the Meade are very good "glass pushers" and that the exact spacing of the mirrors is not highly critical. This is fortunate, since so many users apply generic focal reducers to the SCT to shorten its focal length. This is not an ideal optical solution. Still the results they get are optically quite excellent.

For an in depth understanding of the optical problems with folded optical design I strongly recommend the book by Suiter, Testing Astronomical Telescopes, Wilmann-Bell, 1994 or the even more difficult book Telescope Optics by Rutten and van Venrooij, Wilmann-Bell, 1993.