LX200 Astrophotography --Page 2
(Guiding Issues)

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Subject: ETX as Guidescope --part 1 of 2

From: Donald Tabbutt <dona_ttabbutt.com>

Steve, The ETX works very well as a guide scope. The focal length makes it no dimmer than any other 90mm scope (central obstruction notwithstanding). It only makes the field of view smaller (than a scope with a shorter focal length) on a given CCD. And 90mm should be adequate aperture for guide stars down to mag 10 or 11 under dark skies.

I have an ETX piggybacked using the Losmandy "D" system on an 8" LX200.

Also, see the quoted mail message where I explained via private e-mail to a questioner how to better center the image in the guider. Quoted message follows:

I think maybe your guide star isn't really in the guider's field of view. Use the Pictor as an imager and flip between the images from the Pictor and the view through the 9mm Reticle eyepiece using the flip mirror. When the Pictor image is centered, move the eyepiece's reticle screws to center it on the star. Note the orientation of the eyepiece...always use that orientation in the future. Remember to switch back to the 26mm eyepiece that was parfocused per my previous post to focus the camera for imaging or guiding. Also remember to flip the mirror... this'll bite ya!

One last thing...cover the eyepiece after flipping the mirror. Extraneous light can enter the ETX through the eyepiece port even when the mirror is flipped to the rear port (the mirror flips down to the "floor").

I just photographed M13 this past weekend while guiding on mag 7.2 SAO 65481 with no guiding problems for a one hour exposure.


Subject: ETX as Guidescope --part 2 of 2    Top

From: Emery Hildebrand <emeryha_tearthlink.net>

> I would appreciate anyone who is currently using the ETX with an LX200 to
>share some knowledge and help.... I would be mounting the ETX on a 10" LX200.
>Some pictures I have seen have the ETX mounted on the LX200 with 6.5" rings.
>The outside diameter of the ETX is only about 4", why are such large rings used?
>Can you use smaller 5" rings?<

I don't have an ETX, but I do use a 2045 SCT as a guider for my 8" and 12" LX200s. I use the 6.5" Milburn rings. The extra diameter allows you to point far enough away from the object you are photographing to locate a suitably bright guide star. You can use smaller rings with an ETX, but the larger the rings, the more range you'll have. The larger rings will also come in handy if you upsize to a C5 guider, which would be a more suitable guide scope for a 10" working at f/10. With either, you should guide with at least a 2x barlow but a 3x would be better.

> From my estimation, once mounted the ETX would be about 10" off center
>from the center of the LX200 as compared to an OAG which is only about 1"
>offcenter. Is there any difference in a photograph taken at prime focus
>between the two setups given the large variance in the offcenter distance?<

The off center distance won't make any difference. You can accurately guide on stars at least 5 degrees away from the object you are photographing. Both Losmandy and Milburn rings will be stable enough so that flexure will not be a problem - as long as you tighten the alignment screws very tight. I use rubber caps made for wire shelving tips to keep the screws from marring the guide scope.

> I have read a lot about guidescopes having flexure problems. The ETX
>seems fairly light and the dovetail mounting looks fairly snug. Does the
>ETX flex at all or is this setup snug enough to eliminate this problem?<

If they are badly mounted they can be a nightmare, but the rings you're considering won't allow flexure. Some people worry about mirror shift during exposures, but I've never seen this happen in 30 years of astrophotography with SCTs. What I have seen people blame on flexure has always been caused by not using enough magnification in guiding.

>My 4" SCT has a 1,000mm focal length. Using a 9mm eyepiece and 2x barlow
>is barely adequate for guiding my 8" f/6.3.
>It isn't nearly enough magnification for guiding the 12" scope.<

The ETX has a 1,250mm f/l which with a 9mm and barlow just might be enough to guide your 10", but it will be close. The C5 has about the same focal length as the ETX, but it would be a better guide scope since the guide stars will be much brighter and easier to track.


Subject: ETX and Autoguider      Top

From: Don Tabbutt <dona_ttabbutt.com>

I use an ETX as a guidescope on my 8" LX200, along with a 208XT, and I have been experimenting to find a solution to two issues:

1. Mount the autoguider rigidly to the rear of the ETX.

2. Find an eyepiece that is parfocal to the autoguider using the ETX's built-in flip mirror.

The answer to (1) is fairly straightforward, and may apply to owners of 201XT and 216XT cameras as well. When you remove all screw-in adapters from the front of the camera, the threads that are left are T-threads. Into these screw in the front half only of the ETX T-adapter (model #64), then mount the camera/T-adapter to the rear of the ETX. This forms a very rigid mount with no flexure like that caused with the slip-in eyepiece adapter.

Once mounted this way, a means of focusing the autoguider and finding a guide star must be found. My experiments have found that using the ETX's right angle eyepiece holder with its 26mm eyepiece works pretty well. The locking eyepiece spacer ring that comes with the camera should be installed on the eyepiece such that it is about 3mm (1/8") from the eyepiece's stop (i.e. you can see 3mm of shiny metal between the spacer and the black part of the eyepiece barrel). This makes the eyepiece very nearly parfocal with the guider. If you can, image a star to fine tune the focus and readjust the spacer on the eyepiece. This combination also gives a one degree field of view, which is important when you can't see through the main telescope due to a camera being attached there (you can, of course see through a film camera's viewfinder, but that's bad at best).

That's it...works fine and lasts a long time. The only caveat is to remember to flip the mirror down after finding a guide star. That can (and will) bite you from time to time.


Subject: When Autoguiding, Does the Guidescope Have to be Pointing in same Area as Main Scope?    Top

From: John Mahony, Date: March 2005

>When using a separate guide scope and SBIG's STV unit as an autoguider,
> does it HAVE to be pointing in the same direction as the imaging scope?

One of the main reasons for using a guidescope rather than an off-axis guider is that you can aim it slightly different for a better choice of guide stars. But you don't want to go too far from the imaging frame, or errors from polar misalignment are not corrected as well. In the long run the image will rotate around the guide star. Actually, the rotation from polar misalignment will always be there, but the effect on individual star images depends on how far they are from the center of rotation, which will be at the guide star. So you want the guide star to be in or near the imaging frame. A guide scope also has an advantage here if there happens to be a good guide star in the middle of the imaging target, where you couldn't guide on it with an OAG (off-axix guider).

>I've noticed that tracking circumpolar stars is steadier than tracking the more equatorial stars.

That's only because the scope itself is not moving as much, so the tracking errors that need to be guided out are smaller (the RA motor is still running as fast, but the angular motion on the sky is less).

>The tracking is a lot better if tracking a star bright enough to average 3 or more exposures per second.

I don't think you get much advantage from such fast corrections, since the mount itself has a hard time reacting this fast. But if the guide star is reasonably bright, guiding is better because the guiding camera "reads" it more accurately. If you push for short exposures (for fast corrections) on a dim star, the noise in the image will cause larger errors in the guide star centroid location algorithm in the autoguiding software.

>I can imagine that pointing a guidescope at all odd angles
> would confuse the tracking software, (?) but perhaps orienting
> the guidescope at 90 deg. angles from the photo-scope would work?

No, you want to stay close to the imaging frame.


Subject: Guide Scope Focal Length Calculations    Top

From: Greg Hartke <ghartkea_tclark.net>

Rich Michaels wrote:
> Maybe that's the way to go. But the problem is that I would need a
> hell of a focal length since I prefer to shoot at f10 (3048mm) and
> f/6.3 (over 2000mm) on the LX200 12" My 500mm scope is too small and
> my ETX might work (if I tear it apart)

I do have one germane comment that may help you with your guidescope ruminations. No matter what the focal length of your scope, the seeing (particularly when considering long exposure photos) will prevent you from getting star images smaller than a couple of arcseconds or so. Hence any guider resolution greater than this is actually wasted because you won't be able to see finer detail than this in your photos anyway. We can certainly calculate the shortest focal length required for a guidescope to have an accuracy of at least 1 arcsecond per pixel:

To a good approximation, the image scale at prime focus is:
       I = 1/F

in units of radians per unit length where F is the focal length of the scope. (The length units here are whatever units you use to express F.) In degrees per unit length, this is:
       I = (180 degrees)/(pi*F) ,

and in arcseconds per unit length, it's:
       I = (2.063 x 10**5 arcsecond)/F .

The pixel size (S) of the 201XT is 10 microns or:
      S = 10**-5 m = 10**-2 mm.

We then want an image scale of:
       I = (1 arcsecond) / S
         = (1 arcsecond) / (10**-2 mm)
         = 100 arcsecond/mm.

       100 arcsecond/mm = (2.063 x 10**5 arcsecond)/F .

Solving for F:
       F = 2.063 * 10**3 mm.

So any guide scope with a focal length of around 2000 mm will give you a resolution of very close to 1 arcsecond per pixel on the 201XT. As I said, I would expect you could do handily with a resolution of 1.5 to 2 arcseconds/pixel which would reduce you're requirements to, say, 1000 to 1500 mm focal length. My 80 mm guidescope has F = 912 mm which gives a resolution of 2.26 arseconds/pixel for the 201XT. I don't think I'd want to go with resolution any worse than that with my 10" f/6.3. (I have some nice round stars on 90 minute exposures that indicate that the focal length of my guidescope is adequate.) What's the focal length of the 90 mm ETX? I seem to recall that it's 1250 mm. That would seem to be spot on your requirements so it might be the way to go.

Thus any guide scope with a focal length of around 2000 mm yields a resolution of very close to 1 arcsecond per pixel on the 201XT. As previously mentioned, I expect we could do handily with a resolution of 1.5 to 2 arcseconds/pixel which would reduce the focal length requirements to, say, 1000 to 1500 mm focal length. My 80 mm guidescope has F = 912 mm which gives a resolution of 2.26 arseconds/pixel for the 201XT. I don't think I'd want to go with resolution any worse than this with my 10" f/6.3. (I have some nice round stars on 90 minute exposures that indicate that the focal length of my guidescope is adequate.)

So now we know the minimum focal length necessary for adequate guiding. I would suggest the user choose nothing less than an 80 mm aperture for a guidescope. The 201XT will be able to guide on any star you can see in the guidescope and larger apertures will, of course, equate to more stars from which to choose. I use an 80 mm guidescope and wish I had more aperture. Then again, I *don't* wish I had more weight to put on the mount!

After mounting the guidescope above the main OTA, it will be obvious that it must be counterbalanced. I (and most others) recommend the flexibility of a 2-D counterweight set for the Meade SCTs. Scott Losmandy (of Hollywood General Machining) makes a version that works very well and is used by many of us.

Focusing the 201XT is not the easiest task in the world and of course it's imperative to have good focus to insure the guider properly locks onto the guide star. My 201XT locks onto and guides on stars very easily; I've successfully guided on stars that would really be too faint for me to guide on manually. (BTW, I use an 80 mm f/11.4 Celestron refractor as a guidescope and never use an exposure time of greater than 10 seconds on the 201XT and that only for the absolutely faintest stars under rather dewy conditions. Without dew, the longest exposure time I ever need is 5 seconds for the faintest star I can see in the guidescope.) Note that a guide star that's too bright can't be used - it saturates the CCD. For my guidescope, any star brighter than about magnitude 2 is too bright to use for guiding. With my equipment, stars of approximately magnitude 2 to 3 or so can be easily used for guiding with an exposure time of 0.1 seconds and I always use a star of approximately this brightness to calibrate the 201XT. (More on calibration below.) Anyway, I focus by cheating: I have a Meade 9 mm illuminated reticle EP that is well known to be very close to parfocal with the 201XT. I've never been able to consistently improve the focus I obtain using the autoguider at the same focal position as the 9 mm reticle EP. Since I seem to be able to guide on stars which are quite faint (and for exposures in excess of an hour), I would suggest that my focus seems to be perfectly adequate. I understand the 201XT does not have the most complex algorithms on the market so I would expect that it requires star images that are well focused.

As mentioned above, I always calibrate the 201XT motion using a moderately bright star. Experience has shown that the calibration procedure works best this way: I found it fails with alarming regularity on faint stars and succeeds with virtually uniform success with stars bright enough to use a 0.1 sec exposure with a brightness reading in the 60 to 90 range. Because of this, I index the 201XT in the guidescope so that I rarely have to recalibrate - I can calibrate and go through an entire roll of film over many nights without having to recalibrate. Again, I cheat. <G> Against the advice of others, I mount the 201XT in a diagonal on the guidescope. (I find this makes it much easier to find and center guide stars when the guider is swapped out with the reticle EP. This can, however, increase the likelihood of differential flexure. I'll discuss this below.) Whenever I place the 201XT in the diagonal, I always make sure that the 201XT is aligned parallel to a portion of the backplate of the diagonal. In this way, the guider is effectively indexed so that I can always replace it in exactly the same orientation obviating the need to continuously recalibrate. This is very sneaky and works great.

A serious problem for any astrophotographer using a guidescope is differential flexure. I have several techniques to maintain rigidity. First, I have the guidescope focuser locked for rigidity. Secondly, I've drilled and tapped the diagonal *and* the mounting ring on the guidescope for 2 extra 6-32 knurled screws (that I got from mcmaster.com) at each coupling so that I don't have a single set screw holding any tube in the optical path in place. Instead, all tubes are tightly held in place by a trio of set screws set 120 degrees apart to effectively eliminate any chance of flexure in the system. I do the same thing for all couplings in the camera optical path on the main scope. It was traumatic to drill and tap my 2" TV diagonal, but necessary if I wanted good results! I would expect this to be unnecessary with the 2" AP diagonal which uses a brass locking ring.

Another source of potential flexure in the guiding system comes from stresses induced by the power and signal cables running to the guider. I capture the cables and run them from the control panel, along the mount, down the side of the guidescope, and to the guider using ratcheting adjustable quick clips that I found in the electrical section of the hardware store. These clips have adhesive pads on the bottom allowing me to stick them where I need them. I used duct tape before I found these but of course the clips are much neater.

In addition, much consideration must be given to the mounting of the guidescope to insure there is no flexure there. Many recommend (as do I) Losmandy components here. The dovetail and rings attach very rigidly on the main scope OTA. When I'm shooting, I make darn sure I tighten the rings very smartly on the guidescope without worrying about marring the finish on the OTA. The proper mantra for using a guidescope is tight, tighter, tightest! Don't use rings that are very large compared to the size of your guide scope OTA. I use 108 mm rings for my 80 mm guidescope.

For mass-market SCT drivers, the major source of potential differential flexure comes from the moving primary mirror. Meade SCTs of 10" and greater aperture actually have provision for locking the main mirror in place which can, with care, eliminate this major source of flexure in the optical path. You literally cannot use a guidescope for exposures over 15 or 20 minutes with an SCT (at least a Meade SCT) without the ability to lock the primary rigidly in place.

If you go to Chris Vedeler's web site at

you'll find that he has a nice discussion with accompanying pictures that should easily allow you to properly put together the bits you need to lock the mirror down. The basic parts are a 6" long 1/4"x20 bolt with some nylon spacers and a wing nut. Chris also details how to spring load the focuser for a better feel. This isn't suitable for astrophotography but works nicely for casual observing. For my own use while photographing without an aftermarket focuser, I found it very clumsy to use the wing nut etc. when trying to tweak the focus with a Spectra SureSharp. (Check focus, loosen wing nut, change focus a tiny amount, retighten wing nut while trying to hold the head of the bold with fingers. Check focus, find it still isn't right, repeat ad nauseum.) I bought a couple of 1/4"x20 knurled nuts from McMaster-Carr. (mcmaster.com, part no. 91833A134, $3.27 each, although maybe you can find them locally. I tried but couldn't.) One I permanently attached to the top of the 6" long 1/4"x20 bolt using JB Weld and the other takes the place of the wing nut I once used that is tightened against the nylon spacers and locks the mirror in place. This makes it *much* easier to lock the mirror consistently.

All of this allows me to take prime focus exposures up to 90 minutes with a pretty reasonable probability of success. I'd guess I get approx. 60-70% (say, 2 out of 3) success rate at the moment with exposures this long. That's actually pretty good with this equipment - SCTs are notorious for making guidescope operation extremely difficult with long exposures. BTW, the likelihood of a good exposure does indeed improve with shorter durations. I'm still refining my techniques and equipment and expect to get my success rate still higher: I have reason to expect it to be at least 80% or so for these long exposures by the time I'm done.

I actually find the 201XT to be very easy to use. I had no real trouble right out of the box - my only real challenge has been to systematically refine my techniques to eliminate sources of flexure now that longer exposures are very practical. Once done, the above techniques (locking the primary mirror in place, triply locking in place all connections between tubes in the optical paths, and capturing cables to remove strain) will serve to eliminate most of the rest.

If you have trouble remembering the mode tree for the 201XT, I would suggest that there are at least a couple of mode diagrams on the web that are easier to use than that in the 201XT instruction manual.

I printed this out and placed it in a clear plastic vinyl slip cover for protection. With a little experience you'll probably never need it again (I didn't after about 2 sessions) but it's still handy to have around. Guido also has some other interesting info on the 201XT on his website that you might like to read. It's worth noting that he's working with an OAG (not a Lumicon GEG, either) on his LX200 and is doing very well with his 201XT. Go figure.

Using a guidescope with an SCT is definitely a challenge but it can be done. If the weather ever improves here, I'm going to try some shots using an AP Barlow in the optical path to increase the image scale when shooting certain galaxies. I'll probably add a Barlow to the guidescope, too, to effectively get guider resolution of around 1 arc second per pixel. I'm curious to see if this can be made to work because of the additional potential flexure problems with the Barlows in the paths. With everything triply locked in place, I think it has a fair chance of success. I'll start with some bright planetaries like M57 etc. which record nicely in only 30 minutes or so. (I'm shooting with Elite Chrome 200 pushed 1 stop and a 48 mm Tokai LPS filter.)

Another common mass-market SCT problem: My scope (a 10" f/6.3 LX200) originally exhibited a bit of retrograde motion in declination before I went to the 201XT. I was able to work around it when I was guiding manually (I always drift align so the Dec. corrections are small and rare anyway) but I knew an autoguider would never be happy about it. I pulled the Dec. drive, just generally tightening and tweaking and redistributing grease, and the problem went away.

My impression is that a lot of problems with the 201XT come from some users inability to focus and calibrate consistently. If you can beg, borrow, or steal a Meade 9 mm reticle EP, the focus problem would be solved in one swell foop (sic). If you can borrow the Meade 9mm, you can make a parfocal EP with one in your possession and you're good to go. (Heck, buy one. You'll need an illuminated reticle EP anyway.) The inconsistency with the calibration procedure was a real irritation for a while 'til I found a way to hugely reduce the necessary frequency of the procedure and to do it so that I need only calibrate on nice bright stars. Notice how I solved both of these problems: I cheated. Fie for shame!

As an aside, any astrophotographer will tell you that one of the key elements of a good shot is good focus. This is very difficult to achieve and absolutely critical. I use a Spectra SureSharp knife-edge focuser but this is no longer available. My opinion: Knife-edge is the best way to go. I suggest looking at Richard Shell's new Stiletto Mark 2 knife-edge focuser at <http://www.stellar-international.com/>.

This is most of the experience I've gained with developing good techniques for the use of a guidescope with a Meade SCT. OAGs work well but have their own difficulties. Some OAG devotees will tell you it's not possible to use guidescopes with mass market SCTs to take long exposures. Wrong. It's demonstrably practical. Everyone assumes that trailed stars in guided long exposures with mass market SCTs comes from uncontrolled movement of the primary mirror and so they say they can't do anything about it. In my experience, properly locking the primary is important but it's not the only piece of the puzzle. It's also absolutely vital to eliminate potential flexure in all of the couplings in both optical paths and eliminate strain on the cables to the guider. Only in this way is it possible to get good results for long exposure photography with an SCT and guidescope.

Bottom line: You can't be lazy and expect good results. You have to work for 'em. It's very hard to get good results in astrophotography without working very hard to learn the craft. The results, however, can be very rewarding. Just don't expect great results too quickly. On the other hand, my first shots were 10 to 20 minute manually guided exposures which were almost uniformly good but I was a very experienced amateur astronomer before starting to use my LX200 for photography. It got *really* hard when I started trying longer exposures.


Subject: Guidescope Selection Criteria     Top

From: Greg Hartke <ghartkea_tclark.net> Date: June, 2000

On June, 2000 <WCannistraa_tcs.com> wrote:

> Greg: BTW I was thinking of using an ETX90 as the guidescope for my
> LX50 mounted using Milburn rings. I believe it to have enough focal
> length. The only thing that worries me is that its focal ratio is f/15
> which makes its light gathering ability relatively slow, so most
> guidestars may be almost too dim without having jack up the 201XT
> exposure time significantly. Any advice about this particular choice
> of scope for use as a guidescope?

Mr. Cannistra raised an interesting point here about focal ratios in this private post that I think deserves a public comment. I recall Dick Greiner also commented recently on this (a post which I applauded) so I thought I'd also add a few comments.

Many fine astrophotographers use the ETX as a guide scope. Ralph Pass is one, as is Dick Greiner. Check their web pages. Dick in particular has a long (and very useful) exposition about adapting the ETX for use as a guidescope. Actually, I suppose you'd find this at the MAPUG web site since his page is linked there. I think Ralph's is also, thru the "Proud Owners" links.

Many people are confused about the role of focal ratio, calling large focal ratio scopes "slow" and small focal ratio scopes "fast". This nomenclature is a carryover from cameras. Camera lenses are of fixed focal length and vary the focal ratio by changing the aperture. For this reason, a higher focal ratio for a particular camera lens is called "slow" because the aperture is smaller, restricting the amount of light reaching the film plane. Conversely, when small focal ratios are selected, the aperture of the lens is maxed out or nearly so and the lens is in a "fast" mode.

This really isn't germane to telescopes, which are of fixed aperture. The focal length of the telescope determines the image scale at the prime focus. Now, the image scale does indeed determine the surface brightness of extended objects. Why? A longer effective focal length gives a larger image than a scope of shorter effective focal length. For 2 scopes of the same aperture, the same amount of light is gathered when observing an extended object but the scope of longer effective focal length spreads the light over a larger area, yielding a lower surface brightness. If you arrange the two scopes (using EPs, etc.) to give an image at the same image scale, the surface brightness of the 2 images would be identical except for whatever transmission losses were incurred thru the interposed lenses.

For looking at stars, the focal ratio doesn't matter at all. Since stars are point sources, the focal length (hence focal ratio) has nothing to do with the brightness of the star. This is determined solely by the aperture. The only effect of the larger focal length (hence higher focal ratio) is to narrow the field of view which somewhat limits the number of stars available for guiding. But this is really a non-issue here. Dick and Ralph both think the ETX (with suitable modifications) is a pretty darned good guidescope and these guys are very experienced. Trust 'em in this. The ETX is a nice compact little package and is probably even lighter than my 80 mm refractor.

Actually, if you're planning on doing CCD work, you should be very darned pleased using a guidescope. CCDs don't need nearly the length of exposure that film does. Using the outlined techniques for a guidescope, my success rate with exposures in the 20 to 30 minute range is *way* up there. It's on the really long exposures where the probability of a passing cloud, or dew, or leaving something not quite tightened properly brings the success rate down a bit. With 20 to 30 minute exposures, I always expect to get a good image. Shooting is a piece of cake in this exposure range!


Subject: Guidescope for LX200 URLs      Top

From: Chris Vedeler <cvedelera_tix.netcom.com>

I go into some detail about using a 80mm guide scope on my 10" LX200 on my webpage. Check it out at:
   <http://www.isomedia.com/homes/cvedeler/space.htm> A new browser window should open over this one.

Then go to the article about using a guide scope vs. off axis guiding. If you have any questions about my set up please feel free to e-mail me.

Losmandy's URL for mounting rings:
   <http://www.company7.com/losmandy/>  A new browser window should open over this one.


Subject: Guide scope vs. Off-Axis Choice Revisited     Top

From: Michael Hart

> From: Chris Vedeler
> As many of you know, I have been a proponent of using a guide scope with my
> LX200 for almost a year now. In the past week I have been doing some very
> precise and controlled tests with my setup in my yard to see if I can track
> down the flexure and or mirror slop that is causing oblong stars in many of
> my shots, and I think I am coming to a realization. A separate guide scope
> on an SCT will not give consistent good guiding over 30 minutes. The best
> guiding I have been able to consistently achieve over 60 minutes is about
> 10 to 15 arc seconds. This is with the guide scope very securely mounted and
> the mirror cell locked tight. Where as this may be fine for many people
> (10to 15 arc seconds is just enough to make the stars slightly oblong), I
> guess my standards have improved to where I am finding this to be quite
> objectionable.
> I haven't totally given up as I am wondering if a better guide scope
> mounting system than my home made setup would improve the performance. I'm
> not optimistic; however, as my setup seems quite ridged. I am wondering if
> what I am experiencing is the mirror movement of the LX200 even though it is
> quite snugly locked down via the shipping bolt. After seeing how Meade
> mounts the mirror in the LX200 last weekend I would expect some mirror
> movement within the mount itself. Perhaps the cure would be to redesign
> the mirror mount so that it more securely holds the mirror.
> There is no question that using a guide scope with an autoguider is MUCH
> easier than using an off axis guider with an autoguider. But I am slowly
> loosing ground in my position that guide scopes are the way to go with
> SCT's as my demand for more precise guiding increases. As a fall back, I have
> purchased all the necessary parts for my GEG to guide with it, but I dread
> returning to the pain of getting my autoguider to work with it.
> Chris Vedeler <http://www.isomedia.com/homes/cvedeler/space.htm>

I believe using a guidescope with a SCT is limited by the folded SCT design, fast primary mirror and baffle tube support methods. Chris Vedeler's words stated above reflect much of my experience with this subject. I have quoted Chris Vedeler's entire comments because they are an important contribution to this discussion.

I have found as mirror size goes up, guiding dependability tends to go down even when using a refractor suitable for guiding in heavy rings on a heavy Losmandy mount to minimize scope/scope flexure. I use my 5" SCT whose mirror is wedged to the baffle tube from time to time to guide a shift and combine CCD exposure. This is a very powerful combination for CCD. In this case, any guidescope anomalies are addressed by my the imaging CCD software. It is one thing to use a guidescope for a fast CCD image and quite another for a two hour exposure.

For long exposure emulsion (over 1 hour) I have been less than satisfied with the results. With my 12" and locked down mirror, dependability starts going downhill in about 20 minutes, even with my permanent mounted, fully balanced scope that is at thermal equilibrium. With my 12" mirror wedged to the baffle tube, I can get dependable results for about 30 minutes. With smaller SCT's, the dependability seems to improve.

I believe part of the problem is ambient temperature changes during the exposure in many locations. When the ambient temperature is stable to a few degrees or less, my results improve, but they still need further improvement to meet my personal requirements.

Here are a few thoughts from my earlier posts that relate to this topic that I have copied to provide a more complete picture:

Much of image shift occurs at the rear portion of the slider tube as a result of the focus arm rocking the primary mirror on the baffle tube as you apply force to it through the focus knob. I used a laser to determine this. The baffle tube is precisely press fit into the rear casting. Using bearing assemblies on both sides of the mirror does help with image shift, however less spectacular results are obtained with long exposures. I believe the fundamental problem is expansion and contraction of the optical tube and flexure of the baffle tube produced by the heavy mirror which results in star elongation. Invar rods used in Schmidt cameras will help control optical tube anomalies, but the baffle tube flexure is difficult to control. Worse, the fast primary mirror causes any flexure to be amplified by 5X or so (less in a native f/6.3 scope) by the secondary mirror.

>From another post: As you observed, the SCT primary mirror back is dished and tapers is designed to be self-supporting from it's much thicker center. This mirror will not support itself in a conventional mirror cell. Semi-permanent fixing of the primary is quite easy, just wedge the slider to the baffle tube with shim stock. You will need to bore holes in the rear casting or remove the metal tube to wedge the baffle tube behind the mirror, of course.

Temporary fixing should be done around the back of the primary mirror slider so as to not alter the mirror's figure. Anything other than very light pressure on the back of the primary mirror will cause it to become astigmatic because the SCT primary mirror is as described above, not full thickness, tapered and self-supporting from the center.

I suggest you try the semi-permanent fixing method first. I believe you will obtain the results I described in a previous post. Since you already have your scope apart, why not see for yourself? Carefully wedge the baffle tube to the slider, take several long exposures of an hour or more. Then, carefully gauge the results you obtain with a high power loupe or better still, use a good high-res negative scanner for comparison on your computer monitor.<--end of quoted text

For anyone interested in serious guidescope use on a SCT for long exposures, this post is not meant to discourage, rather to help interested parties explore new directions. For those getting started in emulsion or CCD imaging, I recommend using a guidescope as a tool for perfecting techniques. Once more, there is no reason not to continue using your guidescope as long as your satisfied with the results.


Subject: Guidescope vs. Off-Axis Guiding    Top

From: Philip Perkins <philipa_tastrocruise.com>

Chris and I have shared many interesting exchanges on this over the years! :-) There are bound to be different viewpoints on this because it is very human to be different. But I think an awful lot depends on one's starting point. If one started out using a guidescope and had never before used an off-axis guider, then I agree that one's first experience might seem painful. However, start out using an off-axis guider, and one quickly grows into it. Using an off-axis guider is simply a series of natural and logical steps -- there is nothing actually hard about it. I really mean it when I say that using the GEG is both easy and enjoyable.

The message I'm trying to convey is really a simple one: any new user whose ultimate ambition is to achieve high quality, high resolution photographs of the night sky using an f/10 SCT should be advised as early as possible that using a guidescope will not work. It may be that in progressing towards that ultimate goal, all one wants in the meantime is low resolution web images, and in that case a guidescope may be acceptable as an interim solution. But even in this there is a problem, because the user that starts out using a guidescope is going to find it harder moving to an OAG than one who starts out with an OAG. Such a user will probably feel inclined to try all sorts of experimentation to try to get his guidescope to work and in the process may waste months or even years in abortive efforts, because at the end of the day it will not work, and there are simple laws of physics that determine why it will not work. If that new user had started out using an OAG, then instead of all the wasted effort, he would have achieved real results that both he and his friends would be proud of.

There are enough hurdles to this thing as it is without putting me off new users by telling them that they can mount guidescopes on their SCTs... frankly I want these people to be successful - - the more of us in this club the better. I understand the discussion about the niceties of using a guidescope compared to an OAG, but for me that discussion is irrelevant, because a guidescope is incompatible with systems that have moving optical components. It doesn't matter if it's nicer if it doesn't work.

Now that we have this thing called the Internet it is very easy to avoid reinventing the wheel. One can easily check out any number of web sites of those who have already been there and done it. Pick any web site of a proven, accomplished astrophotographer who uses (or has used) an SCT, and mail them. I'll bet one thing - they wouldn't dream of putting a guidescope on their SCT (though I am sure they will put it much more nicely than that OAGs are not an oddball thing used by only a few cranks out there.. OAGs are used by literally thousands.. and amongst this number are all of the most accomplished and experienced SCT astrophotographers. Some of those who are most fanatical about quality even use OAGs with refractors!

Editor's Note: See the article on Thermal Effects under Michael Hart's section. Note should open a new browser window over this one.


Subject: SCT Guidescope Issues   Top

From: Philip Perkins <philipa_tastrocruise.com>

For the very short exposure times I think that you may achieve better results with no guiding at all. I'll explain below. The prerequisites here are good polar alignment and well trained PEC. The most accurate method of polar aligning the LX200 is described here: <http://www.astrocruise.com/polarnew.htm> The article is quite verbose because it's aimed at the new user, however there is a summary of the procedure in a 'field document'. You may achieve very good PEC training by using the ST-4 to do the training. I will send you a short description of the procedure by private mail if you wish.

I'll try to describe the problem of using one moving mirror system to guide another moving mirror system. Although the tube assemblies of both telescopes are (hopefully) rigidly coupled, the optical axes of the two instruments are essentially free-floating, within small limits. The degree of this free-float may seem tiny from a normal visual perspective, but it is in fact quite massive from the perspective of the 40-50 micron stars that one is trying to achieve with an average SCT. No matter how rigidly the physical assemblies are coupled, the optical axis of each telescope is essentially free to 'do its own thing'. Within 20 degrees or so of the meridian it is quite possible to have the two axes moving in different directions.

For example, the ETX image may be moving towards the south, whereas the LX200 image may be moving towards the north - hence the ST-4 will pull the mount in the exact opposite direction that it should do to counteract the error in the imaging instrument. There really is no predictability about the way that SCT mirrors move as the mount moves (I like to think of the mirrors 'creaking' in one direction or another, because it is very rarely a sudden jump). All that can safely be predicted is that most problems will occur in the region of the meridian, and that the longer the exposure, the more likely it is that the mirrors will move sufficiently to spoil the image. By the very nature of this predictability, you will certainly get some images that seem to come out looking OK at low resolution (although virtually never would one be able to attain 40-50 micron stars consistent with precision guiding). But it's really 'pot luck' as to which images are completely spoiled and which come out looking reasonable. You may even get a run of images that look OK, but the problem will always be there and you will always get a high proportion of images that are spoiled, even at low resolution. Even the 'nice round stars' that may seem to be indicative of success will in truth almost certainly be bloated well beyond the size attainable with precision guiding. Unfortunately using one free-floating optical axis to guide another free-floating optical axis is just not a viable guiding system.

A fair proportion of the time such a system will actually be acting as an 'anti-guider' - in other words you would attain better results by using no guiding system at all. Tracking accuracy certainly isn't going to be optimum, but it may well be as good as you obtain in your best 'guided' images, and what's most important is that the results will be consistent - whatever error you see will be entirely repeatable and you will not have to contend with a high proportion of the images being completely spoiled. I agree with you that one of the weak points of the SCT design is in the guiding of planetary objects that need exposure times of more than a few seconds. OAGs don't work when the object being imaged is moving relative to the stars, and one is left requiring some form of external guider. However in this case I would strongly recommend using a refractor as the guidescope, since it has fixed optical components. If you can keep to very short exposure times I believe that you will achieve higher success with this method. However the real answer is that the SCT is not the preferred instrument for imaging faint comets and asteroids -- a refractor guiding another refractor would be the real answer.

Editor's Note: See the article on Thermal Effects under Michael Hart's section. Note should open a new browser window over this one.


Subject: Guidescope Mounting and C-5 Mirror Operation URL     Top

From: Doc G

I have added to my web site two major articles on mounting of a C-5 to an LX200 as a guidescope and opening the C-5 to check for mirror flop. They are the result of about a year of trying to find a way to FIRMLY mount a guide scope and still allow for some adjustability. The same techniques are applicable to the ETX or other guiders.

They are under the topic: "Attachments"   I hope these articles provide some useful information. Note: a new browser page should open over this one.


Subject: Guiding w/Guidescope      Top

From: Doc G

More info is available at my website:
   Note: A blank browser window should open over this one.

The following is a long note on guiding and using two or more tubes for the guider, the imager and or a piggy back camera. The several possible These thoughts on guiding are for cases where a camera (imager) is used piggyback and the main tube is used for guiding, the main tube is used for imaging and a separate tube (telescope) fastened to it is used for guiding or some such combination. This does not preclude using several imagers on several tubes with a guider on another tube all simultaneously.

The advantage of using a separate guider is apparent to anyone who has ever used an off axis guider. The separate guider tube can be of fast focal ratio so as to get a bright guide image, its axis can adjustable and so be moved a bit from that of the imaging tube so as to center the guide star on the guider chip and an independent, stand alone guider chip can be used in its independent mode. Assume that the guider finds a star and is working correctly. This insures that the guide tube, is pointed at a star and locked onto it. The simplest case to think about is when the telescope is perfectly aligned in the polar mode. Also assume for this simple case that the atmosphere causes no distortion of the celestial sphere. This assumption is not correct, so this issue will be discussed later.

For the ideal case described, the declination is fixed and the telescope needs only to track the R.A. perfectly. There will be no derotation required and a derotator is not needed. If the telescope moves precisely in R.A. no guider correction is required either. Now assume that the telescope does not move precisely in R.A. Then the R.A. drive needs to make corrections. This has been done manually in the past and now, with a guider, can be done automatically. If there is no distortion of the celestial sphere by the atmosphere, then the guider tube can be pointed at a declination other than that of the imaging tube and guiding will still be perfect. This is true since the undistorted celestial sphere in its entirety moves with the same angular rate in R.A. Also note that the above assumes that the guide tube and the imaging tube are rigidly held together with respect to each other. Considerable care must be exercised to insure mechanical rigidity of the tubes.

A counter example is given by suggesting that this scheme will not work with the guide tube pointed at the pole star. And sure enough it won't. So what is wrong? It is this. The effective length of the guider telescope is longer when it is pointed at a guide star that is moving the greatest linear amount for a given angular motion of the celestial sphere. This linear amount is proportional to the sine of the angle from the pole to the declination being guided upon.

Thus when pointing the guide telescope at the pole star. The motion of the star is nearly zero and the guider fails. To get the best accuracy with the above conditions, the guider should be pointed to a declination at 90 degrees to the pole. Then, the guider will be locked to the celestial sphere where the linear motion of the guide star is great and the guiding will be accurate and all will be well.

Even if the polar alignment is perfect and the guider is perfect there is a complication. That is the atmosphere. Except at the zenith, the atmosphere distorts the position of the stars in the celestial sphere as it is seen from the telescope position. Thus as a star moves from near the horizon to a position higher in the sky and again toward the horizon, its motion is not perfectly regular in angular rate of R.A. nor does it maintain exactly the same declination. The Meade telescope has a first order correction for this effect in its computer. This is why the correct latitude and longitude must be entered into the computer to insure accurate pointing.

Thus it is clear that the guide tube cannot be pointed anywhere other than near the position of imaging tube. If they are close together, they both see the same atmospheric distortion and the guider will make the required R.A. and declination corrections. A long as the angular area encompassed by the imager is not too great, the field imaged will not be distorted enough to cause a problem. Usually, imaging is done well away from the horizon because of light pollution anyway so the atmospheric distortion is usually not too great a consideration.

Since we have assumed polar mounting, there will be only a slight star field distortion, including twisting and rotation, do to atmospheric conditions. This distortion is usually masked by "seeing defects" in the atmosphere in any case. In summary, polar mounting eliminates the principle tracking defects and is thus very attractive for imaging. Several tubes can be pointed from the same platform and all will function well with a single guider. A separate but important consideration is the focal length (more importantly, the effective focal length) of the guider tube compared to that of the imaging tube. The guider cannot point more accurately than the angle subtended by one pixel within the guider image and may not be that good.

Thus, the focal length of the guider tube should be similar to that of the imaging tube. It is usually recommended that the guider be at least 1/2 the focal length of the imager. For example, I use a C5 (fl=1300) for my 10 inch f6.3 (fl=1200) or my 12 inch f10 (fl=3000). I feel this is an adequately long guider focal length. Any shorter focal length imaging lens such as a typical piggy back camera might use is more easily guided of course. Now, what about the Alt/Azm mounting for which a derotator is necessary. The amount of derotation required depends upon the focal length of the lens or telescope and the length of the exposure required. If guiding is required, generally derotation is as well.

A guider located on a separate tube from the imaging tube will, within the limits described above, keep the imaging tube pointed to the correct R.A. and declination. That is all there is to the pointing question I believe. The rate of derotation is a very complex function of the R.A. and declination to which the telescope is pointed. At the pole for example, the rate is 360 degrees per 24 hours. At the ecliptic it is zero. And that is not taking into account atmospheric distortion effects. However, the Meade derotator is designed to take care of all the calculations and turn the derotator at the correct rate as long as it knows where the telescope is pointing. I am told that the derotator does this operation very well. (Note that I have not use one.)

Note also that the telescope must be leveled and the correct location entered so that the telescope knows exactly where the pole star is. This is because the derotation is calculated on the basis of the known R.A. and declination. So, the setup cannot be more "sloppy" than that required for polar alignment. I believe that the derotator can be used to derotate an image on any imaging tube mounted on the same pointed platform as long as it knows the exact R.A. and declination.

In summary, if the guider points the telescope to the correct R.A. and declination and tracks a guide star accurately and the telescope computer knows the value of the pointing position, it will calculate the correct rate of derotation. I believe that the guider tube and the imaging tube need only to be aligned to the accuracies and with the rigidity required for the polar guiding case. But conversely, they need to be set with these accuracies and not less. Considerations for atmospheric distortions and cetera are the same for either setup.

A few final thoughts, if you please.

I believe that accurate setup in either polar or alt/azm mode is required for precise imaging and that neither is much simpler than the other to achieve the same accuracy. In one case you need the wedge and in the other the derotator. One case is a well known solution the other, currently, somewhat unknown. I still think that for a permanently mounted telescope the polar mount is the least problematic. That is because it is a simple, well understood and versatile solution to imaging guiding. I have a derotator on order. Thus when it becomes available I will have the "true" facts upon which to make a judgment. (See "Derotator" topic)

Finally (at last), when using a piggy back camera, I know of no way a derotator can be mounted to the camera without very special adapters and/or special lens mounts.


Subject: Pros & Cons: Flip mirror, OAG, Guidescope, Parfocal Tube    Top

From: Gene Horr <genehorra_tswbell.net>

I have tried flip mirrors, OAGs, guide scopes, and parfocal tubes. Here's my comments:

Flip Mirror

Pro - Good image, can help with focusing attachments otherwise you get flexure.
Con - Eats up back focus, require thread-to-thread limited to 1.25" EP.


Pro - Uses less back focus than a flip mirror.
Con - Bad images, especially with a focal reducer


Pro - Doesn't use up back focus, more flexibility with image scale than with a flip mirror, you don't have to worry about parfocal light path.
Con - Requires rigid mounting (Losmandy), expen$ive, more weight on your mount.

Parfocal Tube

Pro - Straight light path, slightly better image than a flip mirror, can help with focusing.
Con - A non-flexing solution (Takahashi) is very expensive, changing parts can throw off alignment.

Personally, I prefer the flip mirror but can't use it often due to lack of back focus. I use the guide scope for bright objects and the parfocal arrangement for the very dim ones. The parfocal arrangement uses some expensive parts to cut back on flexure. Money could be saved by using the 1.25" "snout", but I don't recommend it. Even with a compression ring EP holder the camera still has movement.

There may be a simple and free solution. Just recalibrate the computer on a star near the object you are trying to image. I have found that this works even at 3,000 mm focal length. No Digital Setting Circles/Goto system can slew back and forth to objects over 90 degrees apart and maintain exact accuracy. Yes, it may put objects in a low power EP, or somewhere on a CCD chip when you have a very low FL, but if you try this with a longer FL it is chancy whether or not the object will be on the chip at all.


Subject: Economical Classic Autoguiding?    Top

From: John Mahony, Date: Jan 2005

>>what's the most economical setup for autoguiding the LX200 Classic?

>"Economical" is a broad word. The most economical is something like a Pictor 201, but that's a piece of junk.

You can autoguide much more easily and cheaply with the same chip as the 201 by buying an old Connectix Quickcam (the original parallel port B/W version- I think this was the very first webcam ever made) on eBay or Astromart for about $20. You'll need to open the case to remove the lens and IR filter, clip one wire (get rid of anti-blooming for better sensitivity). Unlike newer webcams, this cam can be used as is for long exposure for better sensitivity. Then get a webcam/1.25" adapter for $20 from <http://www.webcaddy.com.au>, and then find the freeware "guider.exe" on the web. The camera has the same chip as the 201, and the software will let you see the guidestar image on screen for easier operation, which you can't do with the 201. Total cost with shipping would be about $50. Occasionally you'll find the webcam already converted, with adapter, on Astromart for about that price.

Other newer webcams, including those modified for long exposure, can be used with the freeware "Guidedog", and then you'll also have a great planet cam, if you get the right webcam. Price (without long-exposure mod) is about $160 from Scopetronix or Adirondack. Even without the long exposure mod, the newer chips are so much better than the old 201/quickcam chip that it will be about as sensitive.

Webcams modded for long exposure can be gotten for about the price of a 201, from SAC Imaging, and should be vastly superior to the 201. I believe they should work with Guidedog, but I'd check on that first (QCUIAG would be a good place to ask).


Subject: Meade Off-Axis Guider Adjustments    Top

From: Ted Agos <lestera_tziplink.net>

Just a quick note on my modifications that makes it easier to align the prism. First take the prism holder out (making a note of it's position in the slot) and remove the prism, on the side of the slot away from the viewing face, just back of the central hole, drill and tap for a 4-40 set screw. Reassemble the prism in the slot, lightly tighten the hold-down setscrew, then carefully pry the prism face up about 1/32" to 1/16". Reassemble to the guider body, if everything is aligned, then the new set screw should tilt the prism up as adjustments are made (IE the prism would be to low to start) the adjustments can be made by only removing the eyepiece.


Subject: Finding Stars for Off-Axis Guiding     Top

From: Kevin Wigell <kwemaila_ttwcny.rr.com> Date: Feb 2002

Pat Wicker wrote:
> Does anyone have any neat ideas on locating stars for off-axis guiding? I
> have tried to do it a couple of times and can never seem to find any I can
> see well enough to track.

Pat, here's what I do. It takes a little advance work and planning but pays off when you are trying to find a guide star. Also, you will need some sort of planetarium software. I use TheSky.

First, find out the radius, in arc-minutes, of the circle described by your OAG when you rotate it around your subject. Do this by exactly centering a fairly bright star in the viewfinder of your camera or CCD (mounted on the OAG). Note the coordinates on the LX200 keypad. Set your OAG so that the guiding port is directly north or south of the star. Use of a reticule eyepiece in the guiding port is best here. Then move the scope until the same star is centered in the guiding eyepiece. Again note the coordinates. The difference between the first coordinates and the second (it should just be in declination) is the radius of the circle of your guiding port. It is best to repeat this process several times and average the results.

Also, some OAG's guiding ports are adjustable so that you can vary the radius of the guiding port circle. In this case, you will need to do the above steps at minimum and maximum radius to end up with the radii of two circles. My OAG is the Taurus Tracker III, and the diameters of the circles at prime focus of my 10" f/10 LX200 are 32 arc-minutes for the inner circle and 40 arc-minutes for the outer circle.

Now go to your planetarium software. In TheSky, you can set "Field of View Indicators", which will show circles or rectangles of various sizes superimposed on the sky. Set the circle or circles you measured with your OAG using the Field of View Indicators in the program. I labeled the circles "OAG - Inner" and "OAG - Outer".

The hard work is now done. Using your planetarium software before your imaging session, you can center the object you wish to image, and any potential guide stars will appear on or near your circle (if you only have one circle) or within the two circles (if you have an inner and outer guiding circle).


Subject: Lumicon GEG Experiences     Top

From: Philip Perkins

I decided to give my GEG a try at f/5.5. I found that I needed to put a total of three shims (of electrical insulating tape) under the front edge of the prism housing recess. This was to ensure that guide stars were readily visible throughout the range of axial movement of the OA tube.

Firstly, the quality of the guide star images didn't seem at all bad - if anything, better (crisper) than at f/6.5. Secondly, the quality of the primary image was excellent. I had the film processed today - one of M42 and 1975 (60 mins) and one of the Horsehead and 2024 (90 mins) - on hypered PJM-2, deep sky filter and guiding by ST-4. They are the best pictures I have yet taken of these objects. The nice thing about f/5.5 is that (apart from the increased speed) you can group some very nice things together in the same frame. Also you don't need the 3.5" OA guide tube extension.

I thought I might put a post on MAPUG about this, since there's been quite a bit of interest on the GEG. BTW, my scope is LX200 10" f/10 - I am only using 35mm. There is more vignetting than at f/6.5 - it's a graduated cut off at the corners that extends further into the frame than at f/6.5 (at f/6.5 it's absolutely minimal). However the vignetting is of no consequence once the normal amount of cropping and enlargement has been done. Frankly, I'm extremely impressed with the GEG, and f/5.5 is wonderful.

> I have used the GEG at the f/5.5 setting (actual f/ratio was higher on my scope, I
> recall f/6) Most problematic was the distortion through the guiding eyepiece. It
> was severe enough that my autoguider couldn't be used, even with shimming the
> prism. The prism is just too close to the focal reducer, perhaps if the prism
> was a little lower in the light cone.... I went back to the f/6.5 position, which I
> really was happy with. The light cone decreases from the 2" stated below, so
> vignetting increased, but I haven't measured it. More coma seemed apparent,
> but I was not fully polar aligned, which may have contributed to star elongation.
> The following reflects what is based on my experience on 12" SCT:
> GEG and focal reducer 2" image circle with medium format = 3.14 sq. in.
> GEG and focal reducer 2 " image circle with 35 mm camera = 1.28 sq. in.
> T- adapter and SCT focal reducer 15/16" image circle = .69 sq. in.
> T- adapter and SCT focal reducer 13/16" optimal printable image circle = .52 sq. in.
> You can see the advantage of the GEG with 35mm film, every part of the frame
> is fully illuminated, 2-1/2 times more area covered.


Subject: Lumicon GEG Report URL  

From: Philip Perkins <philipa_tastrocruise.com>

I have posted a thorough testing of the Lumicon Giant Eazy Guider (off-axis guider) on my home page. Please find the report in the Articles section of: <http://www.astrocruise.com> A new browser window should open over this one.


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