LX200 Collimation --Page 2

       

Subject: Laser Collimators for SCTs --part 1 of 5   

From: Doc G

A considerable number of persons have posted me about their problems with the use of a laser collimators to collimate their SCT telescopes. There are a number of caveats that have to be applied when trying to do this operation.

The posts have related mainly to erratic results when the tube holding the collimator is a tube which screws directly onto the back of the telescope or another such as the 2" eyepiece tube on a JMI or other Crayford type focuser. The underlying issue is holding the laser in the exactly correct position when attempting collimation. There is a certain amount of sag in the tubes holding the laser device with all methods. The tubes might not even fit tightly enough to insure that they are parallel to the optical axis of the telescope. A burr or bubble of paint might skew the tube position.

Unfortunately, an essential requirement for collimation is that the laser beam be exactly lined up with the center of the optical axis of the telescope. If the laser holding tube sags or is skewed in any way, the laser beam will not line up correctly. This effect is quite obvious with most focusers. Even a tube screwed directly to the back of the telescope might not be perfectly aligned with the optical axis of the telescope. The alignment required when making optical adjustments is very critical and might not be attainable in some cases.

When the laser beam is not perfectly aligned, the collimation obtained will not be correct. This is a serious problem. While some have reported good results, many have reported varying results when the laser device is removed and reinserted or varied results when the holding screws are tightened more or less. Sometimes just tightening the tube or the holding screws a bit seems to change the results significantly.

I do not have an answer to these problems. I am simply warning users that great care has to be effected in order for the laser method to be reliable and consistent. It is quite attractive to have such a device but it may not work well in all cases. What I do know is that the real star method, as recommended in all optical books, always works. It should be used as a final check and it must be the final arbiter of correct collimation.

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Subject: Laser Collimators for SCTs --part 2  

Richard Seavey wrote:
>Doc, Why do you believe that the telescope optical tube mechanical axis is
>coincident with the optical axis? I thought that the optical axis of the
>primary mirror was not well defined and was unstable with respect to the
>tube mechanical axis. What am I missing?

Ah!! You are very perceptive. I have been waiting for someone to mention this issue. It is true that the primary might not be mounted so it exactly lines up with the mechanical tube. It might be off center or it might be tilted. You are absolutely right. It might even saunter around a bit as it is moved to focus the telescope.

But if this is the case with an SCT there is nothing you can do about it short of dismantling the telescope and lining it up with a careful optical method. This would be possible. I have not tried it on a Meade SCT. Then you could use the back plate to adjust everything else. Generally there is nothing you can do about this sort of misalignment so we have either to do nothing, or make the assumption that the primary is Ok and proceed from there.

If the primary is really badly mounted, the telescope probably will not collimate well but will instead be compensated by moving the secondary. I would call such an SCT not too good. A very few will have telescopes, like the standard CAS which has primary adjustments. I have stated in my articles, that we have to start with some assumptions about the quality of mounting of the primary. It seems that so many of the Meade SCTs do indeed reach very good collimation, so I think, and hope, it is safe to assume that they do a rather good job of setting up the primary.

So you have indeed found the fly in the ointment as it were. I wish you had a good solution for this problem. Even better, I wish I had a solution.:-) I do not.

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Subject: Laser Collimators for SCTs --part 3  

Paul Goelz wrote:
> >At this point I am suggesting that the tube used to hold the laser be screwed
> >very firmly to the back plate of the telescope. I am also making an experimental
> >mounting tube with a generous flanged foot and a very tight fit to the laser
> >device to insure that the tube is mounted exactly parallel to the telescope's
> >optical tube (the optical axis of the telescope)
> > Doc G

> This has always bothered me.... If you screw the collimator to the back of
> the scope, aren't you at the mercy of the orthogonality of the rear
> surface of that port? Does anyone take care in manufacturing so that the
> surface is orthogonal? And of course, as soon as you screw anything else
> to it to mount an eyepiece, all bets are off!
> My vote would be to use the 2" version and put it in whatever will
> eventually hold your eyepiece.

I have promised I would not comment further about "the C word." But this post is about the telescope tube itself and not about "c."

You are quite right, also, about the question of orthogonality of the back plate of the telescope with respect to the primary mirror. But what else can we get our hands on easily. If we do not want to dismantled the OTA, we have to use the back plate and assume that the manufacturer has mounted the mirror and the slider tube orthogonal. If this is not done, all bets are indeed off. We would have to rebuild the OTA.

But to the second point, I would not vote with you this time for the following reason. Perfect alignment of the OTA itself is a property of the OTA and has nothing to do with the focuser, external tubes, eyepieces or the like that you fasten to the back of the telescope. All of that stuff is simply to look at the aerial image that the OTA has formed at the opening in the back of the OTA.

So, if you use a twisted or sagging collection of stuff on the back and then try to make your OTA perfect through this stuff, it will not be so. It will reflect the defects of the attached accessories and will thus be inherently maladjusted.

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Subject: Laser Collimators/Optical Adjustments --part 4  

From: Doc G

Paul Goelz wrote:

>>So, if you use a twisted or sagging collection of stuff on the back and then try
>>to make your OTA perfect through this stuff, it will not be so. It will reflect
>>the defects of the attached accessories and will thus be inherently maladjusted.
>But, but, but.... isn't the idea to take all this sagging mess into
>consideration by keeping it in place while you tweak the "C" thing? In
>other words, what's the point of doing the "C" thing if you then add the
>sagging stuff afterwards?

No, No. No! It is perfection of the image that the telescope forms that is the issue. The perfection of the aerial image (called the real image) that the optical system generates at the opening at the back of the optical tube is dependent only on the quality and alignment of the primary and secondary mirror. (and in an SCT the corrector plate) (A focal reducer might be a part of that optical system, but that is a secondary issue.) No amount of twisting and turning the additional tubes will fix a maladjusted OTA. (Optical elements can fix symmetrical aberrations as described below)

It is the job of the visual astronomer to look at the aerial image with a magnifier of some sort (the eyepiece). The eyepiece may affect the total image quality, but it has nothing whatever to do with the quality of the image that the telescope forms. There are some minor issues, like an eyepiece might have a curved focal field that matches the curved field of the OTA or not, but these are other issues not related to the basic quality of the image that the telescope optics forms.

When additional optical elements are added to the telescope, they would ideally be designed to minimize or even fix aberrations in the original optics. But it is a rare telescope that has additional optical elements designed in this way. (Some high quality refractors do. Probably the most famous example if this sort of "fix" is Hubbell, where spherical aberration was fixed with added optical elements)

When one puts a film or CCD at the point where the OTA forms the real image, you capture the real image. Again the fact of the photo surface being placed correctly has no effect whatever on the quality of the real image formed by the telescope itself.

The telescope does not know, nor does it care, if you are going to look at the image through a magnifying glass or a microscope or intercept it with a film or a chip. Normally adjusting the primary and secondary are the only controls one has over the quality of the aerial (real) image in an SCT. (or possibly the corrector plate if it is flawed.)

This might at first sound a bit philosophical, but it is not. It is basic to understanding optical systems and things like cause and effect. Again, I am sorry to go on so long and to sound so darn pedantic, but science has some immutable principles and the above example is one of them.

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Subject: Laser Collimator for SCTs --part 5 of 5    

From: Howie Glatter <howieglattermindspring.com>

Whether we collimate a SCT with a star test, or with a laser collimator, we use a collimated beam of light (parallel rays), and arrange things so that the beam is directed along the telescope's optical axis. Using a real or artificial star, we do this by placing the star in the center of the telescope's field of view. When using a laser collimator, we must also arrange things so that the beam travels along the telescope's optical axis.

With a Cassegrain, this can be done by checking the centering and angular alignment of the telescope back fittings. This includes the threads machined on the telescope's back casting, the axis of the threads or inside diameter of any additional adapter, focuser or holder threaded onto the back casting, and the alignment of the laser collimator, itself, with the final cylindrical I.D. of whatever adapter, or combination thereof, is being used during collimation.

Significant flexure of these parts under their own cantilevered weight is a variable misalignment, which is an even worse problem As far as I can see, the solution is to check and repair as necessary all telescope back misalignments, and minimize flexure by using shorter, lighter and stronger components for collimating.

It's not necessary to use a long adapter tube with a side cutout to observe the reflected central beam on the collimator face. In most cases the collimator face, with the reflected central beam impact visible, may be seen from the front of the telescope, by double reflection in the primary and secondary. This may be the best way to view the collimator face, as you can adjust the secondary screws while you are viewing the beam impact. If you view from the front of the telescope, you can use a short and inexpensive 2" to SCT thread adapter.

To check the back alignment, you must first check the centrality of the back opening with respect to the tube. Place a collimator or eyepiece in the holder and slide it so that some of it's outside diameter is showing, before clamping it. Extend a reference backwards from the main tube's outer surface using a straightedge held parallel to the tube's axis. You will have to use a strip of material of uniform thickness as a spacer between the straightedge and tube sheet metal so that the straight edge clears the back casting. Then measure all around between the straightedge and collimator O.D. It should be uniform. The secondary should similarly be checked for centrality by measuring all around between the secondary mount and the inside of the front tube casting. Hopefully, the secondary will be centered in it's mount ; if not, this must be corrected or compensated for. Then, the back's angular alignment can be checked by seeing if the laser beam strikes the center of the secondary.

Using a single beam collimator, the beam centering on the secondary must be judged by eye (viewed by reflection in the primary). Using a holographic collimator, you can see if the target pattern is centered on the secondary.

Subject: Laser Collimation Eyepiece for SCT?  

From: John Mahony <jmmahonyhotmail.com>

>From: Oscar
>Has anyone ever heard or used a Laser Collimation Eyepiece for the LX200 ?
>I found a website that shows the different types of Laser Collimation
>eyepieces. They look very impressive. <http://www.digitecoptical.com/>

First, the collimation eyepieces are a completely separate product from the laser collimator. And while a reticle consisting of a bunch of concentric circles could be used to judge collimation, you could just as easily go closer to focus, which exaggerates any asymmetry. If the image is too small to see easily, use higher power. If your scope's mount is difficult to use at high power, just remember that with the concentric rings in the collimating EP, the out-of-focus star image has to be perfectly centered to judge by the circles.

The laser collimator looks like it might actually work, but not as well as a conventional star test. It has the same problems the other "alternate collimation methods" have. For most SCTs, the mirror will shift so much when aimed horizontally that the collimation will be worthless. At one of my club's public events in August, there were still a few visitors around when Mars rose around midnight. Since we didn't expect our visitors to stick around until 3 am to get a good view, we all immediately aimed our scopes at Mars, near the horizon. My collimation was suddenly terrible. I fixed it with the tube nearly horizontal, but then later when I aimed it back at higher objects, it was terrible again. Maybe the mirror lock in the GPS scopes will reduce that problem, but for other SCTs, collimating near the horizon is worthless.

A common reason I hear for why people want to be able to do a daytime collimation with a near target is too avoid bad seeing. But collimating better than the seeing allows is also pointless. It won't help to have perfect collimation in bad seeing. Seeing limits collimation and viewing in the same way, and the point of collimation is to produce a cleaner star image- measured directly just as you view it. Put those together and it means that collimating better than can be measured through the seeing doesn't give you a significant advantage.

Finally, a daytime collimation on a near target means that the mirror is on a different part of the baffle tube. Unless the baffle tube is perfectly aligned, the mirror will be slightly displaced to the side when it is moved back for a close focus, further interfering with collimation.

I used to think that the reason so many people put so much effort into finding alternate ways to collimate is that they had been misled by experiences with Newtonians, where a full collimation is complicated enough to be intimidating to a beginner. But I see so much of this that I'm starting to think that the real reason is that collimating an SCT is _too_ easy, removing some of the mystique of their high tech hobby.

Subject: Using the Kendrick Laser Collimator --part 1 of 2  

From: Doug LePage <xuxekapcn.net>

----Original Message-----
Tim deBortoli wrote:
Yesterday I finally had the chance to try out my Kendrick Laser Collimator on my LX50 10". It was snowing out so I was limited to my basement and the maximum distance from scope to artificial star was approx. 51-52 ".

I first tweaked collimation on the artificial star until I was convinced it was perfect. I centred the star in a 12mm reticle. Replaced the visual back with the laser and followed the instructions. After the procedure I rechecked the diffraction ring pattern on the artificial star and found that collimation was off. The rings were off-centre towards the bottom of the field of view. Once again I tweaked my collimation and repeated the process only to get the same results.

I had the OTA on its Superwedge and on the tripod. The OTA was balanced. The laser was firmly attatched.

My first question is why isn't it working - it seems many people are gettting good results with this device and Kendrick typically makes quality products.

My second question - why go through the process of centering the laser's halo of light on the artificial star? If the scope is perfectly colllimated using the star then why not attach the laser, turn it on and mark on the spot where the return beam strikes the face of the collimator. Would this not be where it will have to return to in the future if you are to be perfectly collimated again?

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Tim, It isn't perfectly collimated until you get the beam's halo centered over the artificial star. Merely centering the star in your reticle does not collimate the scope, you are centering the laser along the scopes optical axis by doing this. I gotta tell you that I didn't achieve really good collimation until I did several things that I will describe for you.

I had no luck with the close focus distance of 50' I use 150-200' I guess that it is difficult when it's snowing outside....it's 83 here in Miami. I shoot it down the street to a vacant house. You should wait till the thaw it will be worth your while.

Take the scope off the wedge per Jim's instructions on his web site
     <http://www.kendrick-ai.com/> (Note: should open a new browser window over this one.)
and place the OTA on a table or other sturdy support... get the weight off the forks. People say there is a flexure problem with the OTA ... Doc G could explain this better. I was ready to pull my hair out until I had the patience to do it slowly and methodically after many e-mail's to Jim K. MOST IMPORTANTLY!! I don't know how long you've had your laser but originally Jim didn't include a paper target. If you have one disregard this part, but I use a box with a halogen bulb inside and a sheet metal front painted with FLAT WHITE Krylon or similar product and small hole drilled in it. I could never see the halo on tin foil, which was the original instruction. If you follow this you should see a dramatic improvement...I sure did.

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Subject: Using the Kendrick Laser Collimator --part 2 of 2    

From: Doug LePage <xuxekapcn.net>

Tim deBortoli wrote:
How exactly do you centre the artificial star/target in the scope with the OTA lying on a table or whatever -especially if the target is 150' away? And how do you know when the halo is focused well enough and when it is centred on the target? My finder scope wasn't useful for this at 50'. I can't see that a pair of 10x50 binocs. are going to allow you to accurately centre the halo.

Tim,
I just walked back and forth until I got it right but an assistant would be helpful for this. I used small sand bags and duct tape to hold it in position but I heard others say that they made a wooden cradle. Bottom line is that you really need to see (up close) what is going on and the best way is the walking.

Subject: Laser SCT Collimator? --Stay with Star Test   

From: <Carailwayaol.com>

I have a 12" LX200 that I haul around in my pickup and I collimate it every time I setup. It only takes a few seconds, and I can't think of any reason to use a Laser collimator. Unless there is something drastically wrong with the SCT there is only one element to adjust, the secondary. A quick look at the defocused blur circle and a tweak if necessary, then with higher power take a look at the diffraction pattern, if the seeing is steady enough center the Airy disk in the diffraction rings. If seeing is bad then seeing will probably be the limiting factor. But always go for as perfect as you can get so when there are moments of good seeing you will be ready for it. After collimating a few times you will learn the tricks of the trade and it's so quick you will wonder why you ever looked through a misaligned scope.

Editor's note: a number of members wrote to agree that the star testing method was superior to any of the lasers, including the Kendrick.

Subject: Collimation On Axis of Symmetry? --1 of 5  

From: Michael Sparks <sparksmdzoomnet.net>

I purchased a Kendrick SCT rear cell version laser collimator. I followed the collimation instructions precisely. The final position of the reflected beam is about 0.25" from the center if the grid. A friend of mine who also owns a 12" LX200 also purchased the Kendrick's SCT collimator. He however purchased the NGF-S version. After several attempts to collimate his LX200 using the laser collimator attached to the NGF-S he discovered that his view through the eyepiece was very fuzzy. The only way he could improve the clarity through the eyepiece was to continue making random adjustments to the secondary mirror until the target was very clear. After this adjustment the reflected beam was not even on the grid. Only some of the diffraction rings could be seen near the edge of the grid. Unfortunately we live about 100 miles apart so we haven't been able to get together to swap collimators. We have discussed this problem extensively on the phone and reviewed Doc G's and Ed Stewart's archives.

The question we keep coming up with is, "When performing a collimation are you attempting to collimate with respect to position of the eyepiece/CCD or attempting to put the light cone and focal point right dead center of the optical axis?" In other words, with an NGF-S, flip mirror and CCD hanging off the back of the rear cell it is difficult to determine if the CCD chip is orthogonal to the optical axis. It probably is not due to flexing. In fact by attaching my laser collimator to the NGF-S via the SCT adaptor I could get the reflected beam to project anywhere I wanted to on the grid just by adjusting the three set screws that attach the NGF-S to the rear cell.

If the purpose of collimation is to line up the light cone and focal point to coincide with the optical axis, how can you verify that the eyepiece/CCD is orthogonal to the optical axis, or does being orthogonal to the focal point important? If the purpose is to place the light cone and focal point orthogonal to the eyepiece/CCD position, then is it better to collimate from the end of the optical train where the eyepiece/CCD would be positioned and would include the flex caused by the weight of the train?

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Subject: Collimation On Axis of Symmetry? --part 2   

From: Doc G

I have now worked with three laser collimators. These include the Kendrick, the Glatter and the LaserMax. (one paid for, one a gift and one on loan)

The LaserMax instructions say right up front that it is designed to collimate Newtonian telescopes and not SCTs. It produces a central spot and a grid similar to one of the others. It is well built and offers similar abilities to the others.

I have seriously tried all three of them on two different LX SCT instruments. The results vary. I believe this is because of the inability to adjust the collimator to be parallel with the optical axis of the telescope in a reliable manner. I no longer believe that laser collimators can be expected to align an optical system with two movable optical surfaces, especially when one of them is not adjustable.

I have not been able to verify collimation except with a real star and have not gotten consistent results with any of them. I now believe that results are sporadic and an illusion based on luck in some cases.

I now believe that the traditional way for aligning the optics is to use a real star with the telescope focused at infinity. This is the traditional tried and true way to do this job. It is described in all books about the optical properties of telescopes. Even then, with one adjustment on a two element telescope the results are a compensated optical path and not necessarily a well collimated telescope.

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Subject: Collimation On Axis of Symmetry? --part 3  

From: Michael Sparks <sparksmdzoomnet.net>

Doc, I apologize for burning bandwidth here but I read your site. I think you missed my question of which the answer is not clearly defined in any archive I have reviewed.

What I am trying to learn or understand has nothing to do with using lasers as collimating devices. I apologize if I did not make that clear in my original post. What I am trying to learn is when collimating a scope is the intent to align the light cone/focal point with the optical axis so that it is coincident to the optical axis, or is the intent to align the light cone/focal point to be orthogonal to the resting position of the eyepiece/CCD which due to flexure probably is not orthogonal to the optical axis. Or is the intent something totally different as far as the geometry of collimation goes?

My thought which is totally speculation is that it is more important to collimate so that the light cone/focal point is orthogonal to the resting position of the eyepiece/CCD. This way the light striking one side of the CCD has traveled the same distance as the light striking the opposite side of the CCD. Or does it? Because of the convex surface of the secondary mirror maybe the geometry of the light path is more complex than I have assumed.

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Subject: Collimation On Axis of Symmetry? --part 4  

From: Doc G

Ah Yes! I now understand your question. It is a very good one. The issue of collimation which is discussed extensively in telescope optics books requires that the optical elements each or which has an axis of symmetry be aligned with each other. Collimation has nothing to do with the nature of the receptor, be it film, CCD or an eyepiece. An eyepiece is generally used to inspect the real image field of course.

Many things can go wrong. If the optical elements are made in such a way that they are not symmetrical about a single axis there will be serious aberrations. You depend on the glass pushers to do this job well. Many of them do a fine job.

Assuming you have good glass (mirrors) many things can go wrong with the way they are mounted. They might not all be centered on the same axis and/or they may be tilted with respect to the desired optical axis (which is also the mechanical axis of the optical system). Thus when aligning perfect optical elements, you have to move them laterally until the centers of curvature are on one axis and then you have to tilt them so that their axes are on the set optical axis. Not only parallel, but coincident.

Generally the owner of the telescope, especially an SCT, has no control over the centering of the primary nor the tilt of the primary. It has to be taken for granted that the primary is well set in the optical tube and centered with respect to the corrector plate. The only adjustment possible is the position of the secondary. If everything else was lined up, the owner can move the secondary in such a way as to point it along the optical axis of the primary.

It is impossible to get perfect collimation unless the primary has been well aligned by the manufacturer. Generally, the exact position of the secondary in the lateral direction will be changed when the secondary is tilted. This limits the perfection of the collimation. The best that can be done is to "compensate" the alignment to give as good a star image as possible. Such a "compensated" alignment may be near perfect collimation if the owner is lucky enough to have a telescope with good optics that were correctly installed.

The key words here are to adjust the secondary until the real star image looks as good as possible. Criteria for "good" images are in many books on optics. I recommend Suiter or another books on modern telescope optical design for the details.

I do hope that this answers some of your questions. As to the issue of the film or CCD plane, it is also required that they be orthogonal to the optical axis of the telescope. This is usually quite easy to do compared to the original collimation process. The image will be most perfect on the optical axis of course. There will still be coma and curvature of field even with excellent optics. Some designs are better than others in this respect.

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Subject: Collimation On Axis of Symmetry? --part 5 of 5    

From: Tom Wideman <twidemanearthlink.net>

I use the Kendrick laser for the NGF-S. I cannot speak to the very specific question you ask; however, I'll just point out that Jim Kendrick has updated the instructions for use of his collimators, and I think they're a little more help now. Jim pointed them to me recently while I was discussing another issue with him, and I think the instructions have improved.

To reiterate what you already know, the main problem is that the mechanical axis and the optical axis may not be the same.

I'd recommend against your friend making random adjustments. I used the target, then also used the target to do a regular "star" collimation, and made fine adjustments kind of using both methods from there. I initially had the problem Jim mentions (see the new instructions) in which reiteration worsens collimation. I spent some time switching between methods, finally got what I thought was the best collimation, and marked the laser face with the paper ring. I'll point out that this may be somewhat compromised by collimation at the shortest focus distance instead of on a real star at a realistic focus range as Doc mentions.

I've found that the Kendrick unit takes a little apprehension out of the collimation on-site, and adds convenience, but I'm still playing with it. My overall impression of methodologies is probably the same as Doc's, but the laser adds convenience, perhaps at the cost of "perfect" collimation. In fairness, I must also add that I haven't had enough experience with it to speak conclusively, I'm just sharing my opinions of the experiences I've had so far. All things being equal, I think I would rather have spent the money on an Everbrite diagonal. ;) I do not say this in any way negatively about Jim's product, his quality and support are excellent and I give Kendrick my highest recommendation; I just am not convinced yet that the laser methodology delivers the bang for the buck that I'd hoped for.

Subject: Collimation Using Two Telescopes? --part 1 of 3   

From: Bill Keicher <wekeichercomcast.net>

Bill Keicher wrote:
> Some of us have more than one telescope. Has anyone ever used
> one telescope to collimate a second telescope?

> One telescope ("the collimator") could be setup with a high
> magnification ratio, say 400X, and an attenuated, low power laser could
> be run into the telescope eyepiece and the laser beam would be expanded
> to fill the main aperture with laser light. Assuming that this telescope
> was already collimated and focused at infinity, this telescope (the
> collimator) would produce an artificial star available to collimate the
> second telescope. The second telescope aperture would have to be smaller
> than (or equal to) the telescope aperture used to produce the artificial
> star for proper collimation. The telescope adjustment could be made
> inside with the telescope apertures adjacent to each other. One would
> have to be careful in using a neutral density filter to attenuate the
> laser since looking into the scope being collimated would be like looking
> directly into the laser beam.

> The main disadvantage is that the telescope used as the collimator must be
> collimated and focused at infinity. Could this process "bootstrap"
> the collimation of both telescopes? Anyone ever try someting like this?

William McCarthy wrote:
>>If you have a laser why not just point at a plane white surface not too far away.
>>The reflected image will still be a coherent parallel beam i.e. an artificial star.

William, If you point a laser beam from a laser pointer at a diffuse, white surface it will produce a spherical wavefront after it reflects off of that surface and you will have to focus the telescope on the white surface (say, 150' away). You would be forming a magnified image of a laser spot that is a few millimeters in diameter. You can't collimate a telescope using this technique because the laser spot is too big and the source is only 150' away. If you point a laser at a specular surface (a mirror) then it will just return a parallel, narrow beam back to you. Illuminating a small curved, specular surface, for example a ball bearing, with a laser (or the sun) is a step in the right direction but it is still only an rough approximation to a star. A parallel beam as large or larger than the aperture of the telescope being collimated is necessary. The laser beam has to be expanded. If you run the laser beam into the eyepiece of a telescope focused at infinity then the light coming from the telescope will have a near planar (or flat) wavefront. Stars produce plane wavefronts so using a laser this way is as close as you can get to simulating a star. The advantage of this technique is that you should be able to do the collimation inside and atmospheric turbulence won't be a factor.

How do you produce a large parallel laser beam? If the laser's beamwidth is 2 mm in diameter, then you would need an angular magnification of at least 100X to fill a 200 mm telescope aperture. For an F/10, 200 mm telescope, a 20 mm eyepiece would be needed. The laser should be centered on the eyepiece and aligned with the optical axis of the telescope. If the telescope has a central obscuration (secondary mirror), then the laser beam coming out of the telescope will have a hole in it. This large beam could be put right into the telescope (a few feet away) to be collimated. Once again, caution should be taken since looking into the telescope being collimated this way is equivalent to looking directly into the laser so the laser beam intensity should be reduced with a neutral density filter on the eyepiece and by using a magnification greater than the minimum (say 400X instead of 100X on the collimator). A video camera could be used to view the diffraction pattern to make this technique safe.

By the way, simply running a laser pointer into the eyepiece of an SCT (magnification same as above) and looking at the "donut" beam reflecting off of a white surface a few feet away will tell you if you are in need of collimation. An SCT badly out of collimation will produce an asymmetric "donut". This technique would apply to any telescope with a central obscuration. This is a very easy test to perform. Crude focus (at infinity) can also be gauged by measuring the diameter of the donut. The diameter of the donut should be identical to the telescope's aperture and should not change appreciably over a short distance.

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Subject: Collimation Using Two Telescopes? --part 2

From: John Mahony <jmmahonyhotmail.com>

I've run into too many cases of newbies convinced that the standard collimation method is difficult, and looking too hard for alternatives. Recently I heard from one who made the mistake of buying Kendrick's SCT laser collimator. The most basic understanding of how a scope forms an image immediately shows that Kendrick's method has nothing to do with collimation. Spending $185 on a worthless product can take the fun out of experimentation in a hurry.

Your method avoids two of the counterarguments against daytime collimation. Yours has the focus set at infinity. The others have the focus set very close, so the primary is moved far back. So if the baffle tube is not perfectly straight, the mirror will move laterally a bit as you move it forward to focus at infinity for night-time use. Also, it is claimed that daytime collimation avoids bad seeing, but you can easily get turbulence across the distances normally used in daytime collimation. In your method, the two scopes can be literally face-to-face.

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Subject: Collimation Using Two Telescopes? --part 3 of 3   

From: Bill Keicher <wekeichercomcast.net>

An SCT primary mirror shouldn't shift if locked and not subjected to shock. The primary mirror should remain in place regardless of gravity induced torques caused by changing the position of the telescope. By the way, the indoor collimation technique could be used to carefully measure how much the primary mirror shifts as it is locked in position.

Subject: Collimation Screws for f/6.3    

From: Tom Wideman <twidemanearthlink.net>

I have a 10" f/6.3 LX200. The collimation screws for this are very long (3.25") and there is minimal clearance between the secondary housing and the end cap (or solar filter) -- about 0.15" to be roughly precise (a new term).

I checked McMaster-Carr <http://www.mcmaster.com/> as someone suggested, and found the following item:
(Stock# 91833A118) 18-8 SS Round Knurled Thumb Nut 6-32 Screw Size,
Partially Threaded, 3/4" Head Dia, $ 3.17 Each.

This knurled nut has a "T" profile with the top of the "T" 3/4" across and 1/8" thick, and with the stem (the vertical part of the "T") 1/4" long and 1/4" across. It is tapped 6-32 in the stem portion.

Since the stem, being 1/4" long (total height 3/8"), was too high to clear the end cap, I trimmed the stem portion down to about 3/32".

I used a piece of 6-32 stainless steel all-thread, trimmed to the proper length, for the collimation screw, and secured the nut onto the all-thread with Lock-Tite.

I set up an artificial star and collimated the scope to see how the thumb nuts worked. The thumb nut profile is quite close to the secondary, so gripping isn't easy (not really problematic, but I don't think it would be possible with gloves). However, I think I still prefer it to dropping my Allen wrench on the corrector plate. I feel it will be a worthwhile change.

If the clearance is higher (1/2" or so) on the f/10 as Andy indicated, this technique could be used on that model without having to trim the thumb nuts down (the screw is also much shorter).

Thanks again to whoever provided the URL for McMaster-Carr -- besides the thumb nuts, I got some teflon for my binocular mount, and an in-line GFCI (plugs into an outlet, I can plug my cord into that and protect myself -- thanks to whoever recommended that, by the way) and some other tidbits that I don't think I could find locally.

Subject: Collimation Thumbscrew (Bob's Nobs) Replacement Procedure 

From: Allan Keller

I ordered and received three replacement collimation thumbscrews from Bob's Nobs at:
   <http://hometown.aol.com/rkmorrow/myhomepage/>

to replace the Allen head collimation screws in my 10" f/10 Meade LX200. I developed the following simple procedure to prevent damage to the scope and to ensure that the collimation did not become so far off that I would have trouble collimating the scope after the screws were changed.

1. Prior to removing any screws, be sure that your scope is collimated. If it is not collimated then take the time to get it reasonably close. Collimation procedures can be found at here in the MAPUG Topical Archive. Just type "collimate" in the search engine on the homepage to be directed straight to the relevant page(s).

2. Once you have collimated your scope, use a low power eyepiece with a large field of view, focus and center a bright star in the eyepiece's field of view. Leave the low power eyepiece in place until you are finished exchanging all three screws.

3. After you have centered the above star in the eyepiece, carefully center the same star in the crosshair's of your finderscope. Double check do be sure that both images are centered. THIS IS IMPORTANT! If you do not do this step, you may waste a lot of time collimating your scope after changing a screw.

4. You may now begin the process of removing and replacing the collimation screws with the new thumbscrews. REMOVE ONE SCREW AT A TIME! If you remove more than one screw at a time, you risk having the secondary falling out of it's holder and landing on your primary mirror. Once the stock screw is removed, carefully insert the thumbscrew into the hole and turn clockwise until the threads have engaged. Continue turning until the thumb screw just begins to feel snug.

5. Check to be sure that the bright star is still centered in your finder scope. Next, looking into your low power eyepiece, begin to tighten the new thumbscrew until the star is centered in the eyepiece. Double check until you have the star centered in both scopes. This will return your scope to a state of near collimation and help ensure that you don't end up with your collimation totally lost.

6. Repeat the procedure above for the next two screws.

The new collimation thumbscrews are of high quality stainless steel with knurled edges. They work very well. No more lost Allen wrenches in the dark! I highly recommend them.

Subject: Bob's Nobs Collimation Screws -- part 1 of 11  

From: Bill Arnett <billnineplanets.org>

Yesterday I installed a set of "Bob's Nobs" on my 12" LX200. I wish all modifications went so easily! But now I'm a little unsure about how much force to apply to the knobs ("Nobs"). The resistance they offer seems to be very nonlinear. For most of their range they turn very easily, so easily that I feel like they're doing nothing. But then they abruptly get pretty stiff and a quarter of a turn after I felt like any more force would not be a good idea. So there seems to me to be a very small range in which to work. I managed to collimate OK by repeatedly tightening one and loosening the others. With the original screws and an Allen wrench I was probably using a lot more force and driving the screws past the point that seems inadvisable with the Nobs. So, am I right that I shouldn't turn them very hard?

(BTW, I have an awful time trying to collimate. I can get close but the in focus image of the diffraction rings dances around so much that I can't see if it's really concentric. I suppose I'll just have to wait for better seeing.)

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Subject: Bob's Nobs Collimation Screws -- part 2 

From: Gene Horr <genehorrtexas.net>

Bill Arnett wrote:
> But now I'm a little unsure about how much
> force to apply to the knobs ("Nobs" :-). The resistance they offer seems to
> be very nonlinear. For most of their range they turn very easily, so easily
> that I feel like they're doing nothing. But then they abruptly get pretty
> stiff and a quarter of a turn after I felt like any more force would not be
> a good idea.

This is exactly how it should work. You're actually doing it correctly now. There is a central pivot point against which the mirror cell should rest. So your adjustments should always be loosen two and tighten one.

Very, very few SCTs are shipped with the collimation tightened up. Whether this is on purpose or not I have no idea. But because of this many people (myself included for many years) thought that the "loose" condition was normal.

IMO you should tighten them down just to where you feel the resistance. Past that point and you may start applying stress to the mirror.

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Subject: Bob's Nobs Collimation Screws -- part 3 

From: Greg Hartke <ghartkeclark.net>

Bill Arnett wrote:
> (BTW, I have an awful time trying to collimate. I can get close but
> the in focus image of the diffraction rings dances around so much that
> I can't see if it's really concentric. I suppose I'll just have to
> wait for better seeing.)

I had the same trouble and hardly even trusted my ability to collimate on a star - I could never use sufficient magnification to really believe that I had decent collimation. Then I learned of the joys of artificial-star collimation. It turns out that it's *really* easy to collimate at high mags (I use 500x which is all I can get with the EPs etc. that I have) using any spherical metal object placed in the sun at a reasonable distance. I went to the hardware store and found a polished metal knob of approx. 1.75" diameter that works fine. I put it in the sun (on a stake with a cardboard backing) maybe 150 to 200 feet away and have at it. It's so easy you'll wonder why you ever bothered to try it with a real star. A smaller target would probably let you place it closer but the distance mentioned above is easy for me. Try it!

Editor's Note: also see "Collimation Method Using Artificial Star" on Star Testing Page.

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Subject: Bob's Nobs Collimation Screws -- part 4

From: Greg Hartke <ghartkeclark.net>

Bill Arnett wrote:
> ghartkeclark.net wrote:
> >... I put it in the sun (on a stake
> > with a cardboard backing) maybe 150 to 200 feet...
> > Doesn't it have to be well past the point at which the scope focuses
> at "infinity"? I'm sure one of our optics gurus can figure out the
> minimum distance given the focal length and f ratio.

Not when collimating at or very near focus. What you'll find when the target is not at infinity is that the diffraction rings are not the same inside and outside of focus - as I recall you're seeing an image that's spherically aberrated. Fortunately that doesn't mean anything when you're collimating with the image right on the optical axis. The rings will still be concentric as long as the target is centered. As a practical matter, you need only be far enough away from the target for the scope to focus. I like to get the target a fair distance away anyway but I've known users to collimate on a target much closer than what I use.

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Subject: Bob's Nobs Collimation Screws -- part 5 

From: Philip Freeman <Philip.FreemanMW.Boeing.com>

Since this question keeps popping up, maybe this item info should be stored in the Topical Archives.

For the f/10 telescopes: McMaster-Carr (www.mcmaster.com). Part number: 91746A128.

"#6-32 Size x 1/2" Long 1/8" Height 3/8" Diameter 18-8 Stainless Steel Knurled Head Flat Point Thumb Screws". It was about $10 for three screws after shipping. Might need some spacers, or trim the length of the screw down a bit.

For the f/6.3 telescopes: Bob's Knobs seems to be the best choice. There are some unusual aspects of the 6.3 screws (long and slender, rounded underside) that makes finding a replacement difficult. If you want to go the DIY route, by some thumb screw heads and Locktite them onto some allthread. This information is in the Topical Archives.

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Subject: Bob's Nobs Collimation Screws Clearance -- part 6

From: Bob Morrow <RKMorrowaol.com>

I noticed there are some questions about clearance between Bob's Nobs and the dust cover of some of the Meade scopes. The 10" f/10, 12", and 16" SCTs have no clearance problems, but there are some issues associated with the 8" f/10, 8" f/6.3, and 10" f/6.3 scopes. These three models have a dust cover that essentially rests against the secondary housing when it's installed.

The instructions supplied with the Nobs suggest "contouring" the cover by pressing on the inside to bulge it a bit to obtain clearance. This is actually very easy to do, and you can make the cover flat again just by pressing the outside of it against a flat surface. I was able to get plenty of clearance on my 10" f/6.3 by doing this. For the 8" scopes, I include a set of stick-on bumpers that can be attached to the inside flat of the dust cover against the rim. These allow the cover to sit a bit further away from the end of the OTA to provide clearance for the Nobs. Bumpers really aren't needed if you're willing to contour the cover instead. Nobs for both f/10 and f/6.3 8" scopes are low-profile to minimize their height above the secondary housing while providing enough surface to grip them for adjustment.

BTW, Nobs for each Meade scope model are different, and the threaded part for f/6.3 models is much longer than that used in f/10 scopes. That's because the secondary mirror is much closer to the primary so a steeper (shorter focal length) light cone can be obtained between the secondary mirror and eyepiece. One scope I haven't yet tested is the new LX90 with its plastic dust cover. Of course, that cover can't be contoured. I'm awaiting word from buyers of Nobs for this scope on how the dust cover fits with Nobs installed. I'd be happy to answer any questions directly, either on the list or via private e-mail.

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Subject: Bob's Knobs Collimation Procedure --part 7   

From: Rod Mollise <RMOLLISEaol.com>

Mark writes: Should I just snug them all down, then gently collimate, being careful not to take any one out very much? Later, when I collimate again, should the first step be to tighten, until snug?

Mark: No. Snug em down to start with as Bob says in his instructions. Thereafter, to collimate, always _tighten_ a screw. Only when you can't tighten it easily should you switch to its opposite number(s). Loosen the opposite screw or screws slightly, and you will be able to continue in the proper direction with the original screw. If you do this, you'll never have a problem with any screw being loose.

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Subject: Bob's Knobs Collimation Procedure --part 8

From: Mike Dodd <mikemdodd.com>

Andy Heath wrote:
>...is there anything I can do during daylight that may give me a head start?

Yes. Check out Bob's Knobs Web site at:
<http://hometown.aol.com/rkmorrow/myhomepage/> and look toward the bottom of the page for a link to the installation instructions (148K PDF file). This has directions for a preliminary collimation by looking in the corrector lens of the scope. Photos are included, so you know what to look for. I performed this when I bought my Nobs, and it got pretty close.

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Subject: Bob's Knobs Collimation Procedure --part 9 (disassembled secondary photos)

From: Anthony Kroes <akroesvenomtech.com>

I put mine 'very' finger tight and have no problems with collimation. I have Bob's Knobs on both my 8" and 12" LX200's (both classics). I have had the secondary assembly on the 8" completely apart to see how it ticks and the collimation screws go into an aluminum plate that is glued onto the back of the 3/4" thick secondary.

I am not an engineer nor an optician, but it would seem to be that it would be pretty difficult to stress something that thick using such a short radius (center pivot to collimation screw) when only using finger pressure, but someone here please correct me if you can!

Check out the pics (click them to see a larger version) of the disassembled 8" secondary at:
   <http://www.cdo-astro.com/Telescopes/Lx200/Secondary/Secondary.html>

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Subject: Bob's Knobs Collimation Procedure --part 10 

From: Roger Hamlett <ttelmahntlworld.com>

Anthony Kroes wrote:
> I am not an engineer nor an optician, but it would seem to be that it
> would be pretty difficult to stress something that thick using such a
> short radius (center pivot to collimation screw) when only using finger
> pressure, but someone here please correct me if you can!

The question is how much distortion is necessary? If you look at something like the floor of your house, it is normal for the construction codes to specify the required beam sizes, to accept a deflection in the centre, of perhaps 1/300th the span under full load. If you put a small chair in the middle of a room, the distortion is so small that it is normally undetectable. However if this floor, was a mirror, the distortion made by even such a single object, would be enough to take a perfectly 'figured' set of optics, to a point where it would probably be no longer acceptable. A distortion of just 0.1um, would be very significant. Now when you 'tighten' a bolt, the increase in force you feel as it tightens, occurs as materials distort. The friction you feel, is a result of the bolt itself going into compression, the surfaces of the threads carrying this force, and generating friction.

If you work on optics, just having a small hair, under the centre of a piece of 2" thick glass, not only lifts the glass (as you'd expect), but also bends the glass itself...

_Gentle_ finger tight (perhaps a few oz/in of torque), is what the unit is designed to have. If it feels 'tight', it is almost certainly too tight. The real 'pity', is that Celestron (or Meade) don't specify a torque for the bolts, so that they can be set to the same level that was used when the scope was originally adjusted. I have seen a Meade scope, where the bolts were tightened to a firm finger tight, and there was a visible 'pinched' effect on the optics, when collimating.

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Subject: Bob's Knobs Collimation Procedure (caution)-- part 11 of 11   

From: Dennis Williams <drwicu812comcast.net>

Bob's Knobs are truly wonderful to use compared to the allen wrenches ...what were they thinking? Small wrenches in the dark... A word of warning though, snug is plenty tight. The plastic secondary cell is not very thick and can easily be cracked from overtightening with the allen wrench or fingers on Bob's Knobs. Been there and done that. Got a new cell from Meade, because I cracked all three screw holes. I cut apart the cracked cell and was amazed to see such a small cross sectioned area around the screw holes. I took it apart after about six months of not being able to keep my scope collimated and found the cracks...they are very hard to see, need a magnifier to notice. The plastic is Delrin (I think) which is a good tough easily machined thermoplastic...a very good choice but just too thin in the critical places.

I hate to think how many scopes are out there with cracks in the secondary cell and cause the owners to doubt their knowledge/ability of collimation procedures (as I did). The worst is not fully utilizing fine telescopes. Just be careful about overtightening when the temperature is low.

Subject: Field Flattener Requires Re-Collimation?  

From: Doc G

> Dr. Michael Blaber wrote:
> I have a question about whether it is necessary or expected to
> recollimate when using a field flattener. I star-tested and
> collimated my LX200 and it looked superb. However, after
> installing the Meade F/6.3 field flattener, and looking at the out of
> focus diffraction pattern, it was clearly no longer collimated.
> Should this be expected? Can the field flattener affect the
> collimation like this? Is expected the you will need to
> recollimate upon installing the field flattener?

I believe your result is not unexpected. You must consider that the application of a focal reducer/flattener is an extreme change in the telescope optics. The base telescope has carefully designed optics, we hope, that give a focal length of say 1500 to 3000 mm. The focal reducer changes this focal length by a factor of one and one half to two times. Thus the "strength" of the optical system is at least half dominated by the optics of the reducer.

This means that the optics of the reducer are as important as that of the prime telescope optics. This gives one pause when adding a reducer. The reducer can re-introduce all of the aberrations that have been designed out of the telescope in the first place.

The very best telescopes have a reducer specifically designed for the optical system to which they are being applied. Helter skelter application of an optical element of such strength to an OTA is chancy at best. I would experiment with placement of the reducer. It may well be that the reducer you are using is not perfectly centered or has de-centered elements or any of many other problems. You might also try another sample of the reducer.

My comments are simply to warn those who use either reducers or extenders (Barlows) that they are greatly modifying the characteristics of their optical systems and should do so with caution.