LX200 Collimation -- Page 1

       

 

Subject: LX200 7" Maksutov: Collimation Issues --part 1 of 2 

From: Robin Casady <rcasadyscruznet.com>

Leigh Daniels wrote:
>With all this talk of how much collimation has improved performance, I
>started looking more critically at my new LX200 7" Maksutov. When I look
>at the inside-focus and outside-focus rings generated by looking at
>Arcturus or Vega, they look quite symmetrical. I tried Robin's daytime
>look-down-the-front test and things looked pretty good there, too. When I
>have a star in focus, however, the diffraction rings around the Airy disk
>are significantly brighter on one side than on the other. This is not the
>result of bad seeing.

>I suspect that the scope requires some minor collimation but the LX200
>manual says no collimation is required for the 7" Mak, so I have no
>instructions on how to collimate my scope. The ring holding the corrector
>plate has three large-head Allen screws. Next to each of these screws is
>a tiny, countersunk, headless Allen screw.

>Now, my questions:
>1. Why would Meade say no collimation is required for the Mak?

The Mak should hold collimation better than the SCTs and it is a bit tricky to collimate. It is easy to go too far and get lost (not know which way to go to correct). I speculate that they figured there would be more frustration generated by people trying to do it themselves then there would by some having to send the scope to Meade.

>2. Could the imbalance in brightness of the in-focus diffraction rings be
>the result of bad collimation? If not, what might be the cause and the cure?

I could be. There might be other causes, but it could be collimation. did you remove the star diagonal? Did you rotate the eyepiece to see if the bright area rotated? Did you try another eyepiece? Were you using at least 300x? If yours shows an Airy disk with rings, that is pretty good. If it is out of collimation that is causing this it is very slight and would need only a very small adjustment.

>3. Can I *safely* collimate the scope myself and if so, does anyone know which screws to use for collimating?

What is safe? You could make it worse and not be able to get it as close as it is. Then you would have to send it back to the factory and hope they got it right.

Because my only other option was to return the scope to Meade, John Piper told me how to collimate it. It is very much like collimating a refractor. There are three sets of push/pull screws around the edge of the corrector. Each set has two screws, a capped screw and a set screw. When both are tight, the corrector cannot move. To make an adjustment you must loosen one and tighten the other. Make very small changes. In your case, make them as small as you can.

ALWAYS KEEP TRACK OF WHAT CHANGES YOU MAKE. You should always be able to undo what you have done in case it gets worse. If you lose track and get way off, you may have to use the sight-down-the-front technique to get you close to collimation.

If I remember correctly, the area that is dark relates to the part of the corrector that is too close to the eyepiece. I could have that backwards, and remember that the image is flipped. Best to see which direction causes improvement and which makes it worse. It is easy to get confused. Take notes.

The capped screw pulls the corrector towards the eyepiece. The headless set screw pushes the corrector away from the eyepiece.

>4. Would I follow essentially the same procedure as the one outlined in
>the manual for the SCTs or is a special procedure required for the Mak?

There are similarities but it is not the same.

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Subject: Daytime Collimation Method for Maksutovs --part 2 of 2  

From: Robin Casady <rcasadyscruznet.com>

Bob Middleton wrote:
>Is there a better way to reach good collimation besides just eyeballing the
>doughnut of the out-of-focus star image. I know there are commercial aids
>for Newtonians, but what about aids for a 10" SCT? (LX200) Other tricks or tips?

There is a daytime method that works for Maksutovs, and I assume would work for SCTs.

Look down the front of the scope from a little ways away, perhaps about 10 ft. The exact distance is determined by what you see. Right around the secondary you should see a dark ring. Move closer to the scope until the dark ring is lined up with and almost completely obscured by the secondary. It is best to set up a white card with a 1/8" hole at this position so that you can come to this position easily after making an adjustment, and the reflected image in the scope is white. You should see: the secondary, a narrow dark ring, a white ring, a dark ring, a white ring.

The first dark ring around the secondary is used to insure that your eye is looking straight into the scope. If it is not concentric with the secondary, move the card with the viewing hole until it is lined up. If the first white ring or second dark ring are thicker on one side than they are on the other side, you need to adjust collimation.

This technique was demonstrated to me at Chabot Observatory's ATM workshop by Kevin Medlock. It seems to work pretty well on the 7" Maksutov. It may not give you the ultimate collimation on the faster Act's, but it should get it close so you can do fine tweaking on a star.

One thing I found helpful was to set up a white card 8.5 x 11" or larger with an 1/8" sighting hole in the center. I used a photo light stand and clamp to hold it vertically in place. This reflected white into the scope to give the best contrast between bands and it allowed me to find the proper eye position easily and quickly between adjustments. BTW, the mirror position should be where it will be focused on a star in your normal viewing setup.

Subject: Collimating an SCT with a Cheshire?  

From: Rod Mollise <RMOLLISEaol.com> Date: Jan 2002

<spectrumsoftteleviso.com> writes:
I'd be curious to find out how many folks collimate their SCTs with a Cheshire instead of doing a star test, or in addition to a star test. As you know, conditions often make it difficult to perform an accurate star test-based collimation, although you can usually get pretty close. Would a Cheshire give me better collimation?

No. Not often. Usually a Cheshire yields miscollimation...both due to the nature of SCT optics and to the fact that it's quite normal for the system not to be _quite_ concentric with regard to the rear port.

However, I've discovered a rather painless way to collimate your scope indoors. Just get a hold of a string of Xmas tree lights. You know, the modern kind with the small, clear colored bulbs. I found that one of these bulbs placed a decent distance away (not critical) produced nice diffraction rings in an SCT! Experiment with different colors. You might want to pose the bulb of choice in front of a black piece of cardboard. It might also be helpful to dim the thing down a bit with a dimmer or variac. Works amazingly well. No fuss, no muss.

Subject: Basic Collimation Technique  

From: George Malyj

>>>>... when the seeing is good and the scope has reached thermal equilibrium, take that extra couple minutes to further tweak the collimation screws so that the high-power (>250x) Airy disk of an *in-focus* star is at its centered best...<<<<

>>It sounds simple enough, but trying to make adjustments and ensure they are visible on an in-focus star does not sound so simple.<<

It's actually fairly easy once you've done it the first time and know what you're looking for.

Basically you need a good seeing night (so that the Airy disk isn't boiling) and at least 35-40X per inch of aperture. I use a 6mm Clave Plossl yielding approx. 420X on my 10" f/10, and usually select a star of 2nd or 3rd magnitude that is at least 60 degrees above the horizon. Starting with a medium-power eyepiece, go ahead and first do the out-of-focus centering of the "hole-in-the-donut", confirm with the high-power eyepiece, then rack the high-power eyepiece into best focus finishing with a counterclockwise turn of the focus knob (to prevent image shift). If the seeing is steady enough, you should be able to see at least one bright ring and one or more additional fainter ones around the central point of light. Move the star around in the eyepiece field of view from center to edges and note how the orientation of the Airy rings relative to the center disk changes. What you strive to end up with is perfectly concentric rings around the center Airy disk when the star is in the very center of your high-power field of view. Start with an extremely small nudge on a single collimation screw to see what changes. You'll note that the star will move and will need to be recentered in the field of view before making any additional changes. Under near-perfect seeing conditions, you may even be able to go one step further. The rings could be as centered as best you can tell, but the center disk seems slightly smeared or elongated in one direction; the final adjustment would be to make the central Airy disk itself as round as possible. Don't be discouraged if don't get a perfect disk, as a combination of excellent unstrained optics, ideal seeing, and a scope at ambient temperature all need to exist at same time.

Anyway, once you've done this, pop in one of your low-medium power eyepieces and go check out globular and open star clusters, and Jupiter or Saturn if high enough in the sky. It makes a difference. And if both seeing and transparency are superb that night, you'll really appreciate what your scope is capable of showing you.

Subject: New Collimation Page Ready 

From: Sylvain Weiller <sweillerfree.fr> Date: Nov 2003

I have just finished the translation of my French page on collimation.
It's at: <http://sweiller.free.fr/collimation.html>

I hope you will enjoy it. I had many good comments. People telling me it helped them much in that hard known enterprise. I hope it will help you too getting better images more easier than before.

Subject: Advanced SCT Optical Adjustments 

From: Michael Hart

PREFACE
SCT optics are often criticized as mediocre as compared to refractors and other less complex optical designs such as a Newtonian. This reputation is due, in part, to the rather complex optical design which if not properly adjusted or figured, may inhibit ultimate performance. It is often assumed that if the image is not so good, the optics and/or the SCT design is at fault. This is unfortunate because the SCT is arguably a very compact, readily available, reasonably priced and versatile design with an enormous selection of accessories.

Serial optical manufacturing methods have the potential of producing consistently good optics. It is quite possible those whom own what may be described as less than excellent optics actually do have good optics whose full potential may be realized with a bit of adjusting. I should point out that there are vastly different words used to describe an observation using the same telescope. Some will use superlatives while others see the change more like the difference between 97 cents and a dollar.

Sometimes, a SCT with rather good optical potential does nor realize it's full potential in the field. This may be so because the telescope is not adjusted properly both mechanically and optically. Specifically, the focuser, corrector orientation, corrector positioning, secondary orientation, collimation and very rarely, a loose primary can result in less than optimal performance. As a result, the user may conclude they have poor optics, which may not be the case. Of course, another telescope where the optics are properly adjusted will likely perform better, confirming (in error) the user's conclusion about the original optics. Then of course, excellent optics look bad before the telescope is at thermal equilibrium and/or during average-poor seeing.

In addition, the SCT optical potential is sometimes criticized because inside and outside diffraction patterns are not identical. This is often so because the tester and/or the collimation & optical orientations were done using the SCT focus knob. Those that have removed most of their image shift through procedures I described elsewhere are likely to see close to identical inside and outside diffraction patterns. Those with a higher amount of image shift can still enjoy good optical performance with a few workarounds.

BACKGROUND
For precision optical alignments and repeatability in a SCT, it is best to avoid using the SCT focus knob for inside and outside focus changes or at the minimum, finish focus in the optimal direction described later to pre-load the focus mechanism. Use this same focus procedure for viewing and imaging as well. An even better method is to roughly set focus in counterclockwise direction to pre-load the focuser. Those with the 12" should pre-load the focus mechanism clockwise due to the mirror spring. Use a reticle eyepiece with a barrel extension (if needed) secured by double screws to adjust inside and outside focus distances. The JMI NGF-S is likely to have insufficient travel for these purposes. I use a special SCT threaded accessory manual focuser for this purpose.

For optimal diffraction patterns as well as precision optical orientation adjustments, do not use a focal reducer or diagonal. A focal reducer is likely to effect the sharpness of diffraction rings and visibility of the airy disk and a diagonal is likely to be a bit out of collimation. It is conceivable, though somewhat rare, that the secondary of a SCT may become loose. It is also conceivable the corrector may become loose as well, though this may not be as obvious. If the SCT has optics where the secondary is orientation sensitive, the full potential of the scope will not be realized if the orientation is not restored to the factory position. However, improper secondary orientation may not severely effect viewing, especially under typical seeing.

It is assumed the reader has good knowledge and experience with SCT collimation procedures, which are not covered in detail here. Those that attempt star testing with the help of a textbook will find real diffraction patterns will not match the examples contained in most textbooks. Experience and good seeing are most important. I repeat, EXPERIENCE AND GOOD SEEING ARE MOST IMPORTANT, especially at this stage.

It is possible to improve upon factory corrector/secondary index settings, though I believe this requires a VERY experienced observer and excellent seeing to do so as the differences are very subtle. It is also possible the corrector may move under a normally tight retaining ring if the cork spacers have become permanently compressed. The symptoms are a slight change in collimation which is likely to go unnoticed and slight corrector movement under the corrector retaining ring. This may result a gradual loss of the corrector index position. Meade usually marks the corrector orientation. However, a few may not be marked, so the procedure below will restore factory orientation of the corrector.

If the reader is not experienced with collimation procedures and star testing, it is recommended the secondary/corrector orientation be left at or near the factory position. I have recommended rather coarse (10 degree) adjustments which, in the eyepiece, are adequately fine.

LOOSE SECONDARY REPAIR WITHOUT CORRECTOR REMOVAL
The standard procedure for correcting a loose secondary is to remove the corrector and tighten the secondary retainer while holding the secondary cell on the outside. It is possible to tighten a loose secondary WITHOUT corrector removal. This is useful for secondary orientation adjusting described later. Here's how:

To prevent accidental primary mirror damage, point the optical tube horizontally (in case the secondary falls out). To tighten, firmly grasp the secondary cell and pull AWAY from the corrector while simultaneously turning the secondary cell clockwise.

To loosen the secondary cell, pull AWAY from the corrector while simultaneously turning the secondary cell counterclockwise This will method will be useful for adjusting the secondary indexing or adjusting the secondary orientation as described below.

SYMPTOMS OF INDEXING ERRORS IN THE CORRECTOR & SECONDARY
Adjust collimation and star-test under excellent seeing without a diagonal or focal reducer. Check inside and outside diffraction rings, Are they up to your expectations? Check planetary views. Do they seen to lack contrast and punch? Check focus. Does the telescope seems to focus a bit softly? If you have some or all of the above symptoms, you may have an optical set whose secondary is orientation sensitive and/or a corrector that has moved from the from the factory orientation.

USING THE STAR TEST TO ADJUST THE CORRECTOR AND SECONDARY
With the corrector, a good place to start is to remove the corrector retaining bolts. Use the procedures for adjusting the secondary orientation while turning the corrector in the front casting. You will likely need to remove two or three cork spacers from top corrector edge to allow corrector turning.

Since Meade doesn't index the secondary, and I'll bet you didn't either, you will need determine the secondary's orientation to your optics. Do this on a night of excellent seeing. You may need to patiently await for several months for a really good night, but the following just can't be done unless seeing is quite good, such as well after midnight and just before dawn. It is possible to equal or exceed original factory secondary orientation placement using the star test as the arbiter.

You will want an assistant who can rotate the secondary 10 degrees or so at a time while you observe the diffraction patterns. You will likely need to tweak rough collimation when the secondary is rotated. You will want a fast and efficient method for rough collimation to speed things along. Here's how:

Using a straight-through reticle (no diagonal), outside defocus a centered star (move the mirror forward by turning the focus knob counterclockwise). This will enable you to see the shadow of the Allen wrench and your finger. Use your finger and point to the elongated diffraction rings. Have your assistant place the Allen wrench on one of the three screws with the handle sticking straight out nearest your finger. Tighten (turn the screw clockwise) to move the airy disk (and the diffraction rings) toward the shadow of the wrench. Recenter the star in the reticle.

When you find a location that looks good, have your assistant mark it. Continue on- you will likely see other areas that are good. Have your assistant mark them. Now, have your assistant go back to the marked locations. Compare them against each other. Pick the best one and tighten the secondary and place an indexing mark for future reference.

POTENTIAL RESULTS
If you have completed all the above checks and adjustments, you are likely to want to see the results. Pick a time of excellent seeing with the scope at thermal equilibrium. Try a bit of planetary observing. Don't be surprised if your planetary views are very good. You will know this is so when under good seeing, higher powers display more details. Under good seeing, I observe planets at 800X and more on my 12". If the seeing supports higher powers, the aperture of a SCT really brightens the image. The airy disc is clearly visible with circular diffraction rings that look close to identical both inside and outside of focus. It is likely that stars seem to snap into focus at pinpoints that easily allow reticle powers of 800-1200X, useful for excellent PEC programming while providing sharp stars at lower powers for manually guiding.

Subject: Maintaining SCT Collimation 

From: Michael Hart

BACKGROUND
Reports from this list indicate that some need to tweak collimation frequently while others do not. At first, this sounds like a paradox. However, my notes tend to indicate that some may want and need to tweak collimation more frequently than others depending on how the scope is used and under what conditions it is used. However, after several thousand hours of CCD and film imaging on SCTs (and other scopes), I have found collimation to be one of the lessor hurtles to produce good images. It is important to put efforts in the areas likely to require attention (such as mount stability, alignment, focus, and guiding) and minimize efforts worrying about other adjustments that have less an impact on the final results so that imaging does not become needlessly difficult.

For general astrophotography and CCD imaging, critical collimation does not severely effect image results until one starts to approach the theoretical resolution of the optics. Typical seeing of 4 arc-seconds experienced by most does not allow one to record details at the theoretical resolution of the optics. As a result, critical collimation does not result in visibly better images because image blurring caused by seeing hides slight collimation errors and other optical anomalies.

I must say that with 1 arc-second seeing, any optical anomalies really start to become evident. Field flatness and other aberrations are now apparent to the experienced imager. That mega-special accessory now produces or worsens distortions if it is not optimally corrected for the position of the spherical primary SCT mirror. Good collimation is noticeable under excellent seeing.

**NOTE: I want to caution those that are checking collimation with a dew cap in place that if the cap is slightly askew, even a slight blockage of the clear aperture will result in flattened diffraction rings on one side.

COLLIMATION FOR VISUAL WORK
Visual work requires different considerations. Visually, the eye and brain act as a signal processor combining several images, discarding the bad portions of the image while retaining the better portions, then assembling a composite image. With observing experience, the human signal processor is programmed to see more detail while discarding what is not relevant. Likely, most of us have been to star parties where an experienced observer sees things we cannot. We may not be able to easily surmise whether the experienced observer is exaggerating or is really seeing what he reports until we gain similar observing experience.

Here, excellent optics will help with nice visual images under good seeing, but somewhat less so under typical 4 arc-second seeing experienced by most. Often, Cassini's division is used to "test" the collimation and resolution. However, Cassini's division is an example high S/N ratio (contrast), not resolution, because we aren't really resolving it, even though we believe we see it, so the results may be somewhat misleading.

COLLIMATION REQUIREMENTS FOR GENERAL IMAGING
Quite frankly, imaging is often less demanding of optical quality than visual work because using amateur equipment, the signal is diluted with noise of atmospheric blurring, reddening produced by absorption of shorter wavelengths (extinction), and a host of other factors. In this example, the benefit of precision collimation and diffraction limited optics is buried in the noise produced by other sources.

COLLIMATION REQUIREMENTS FOR HIGH RESOLUTION IMAGING
When we are imaging resolvable extended objects such as the moon and planets, it is possible to produce images with an apparent resolution exceeding the theoretical limits of our optics. In this case, precise collimation increases the signal to noise ratio (contrast) resulting in a image that appears to be more resolved, when in fact it is not really.

The moon is a great object for high resolution astrophotography because it's contrast and brightness provide the high S/N ratios we need to record fine details at or better than the theoretical resolution of our optics. This is why the best lunar images are not taken during a full moon where the lack of shadows reduces contrast and resulting S/N ratios. Some of the best lunar images have been and are made on small telescopes of 16" or less, many are with SCTs not noted for high Strehl ratios. These scopes are looking through relatively small columns of air resulting in images less likely to be exhibit less contiguous blurring at typical amateur locations as compared to a larger aperture telescope.

With the case of SCTs, the secondary obstruction and subsequent contrast losses are small as compared to the overall S/N ratio. However, good collimation increases the S/N ratio of the individual point sources that make up the extended object and thus apparent resolution. In this case, a small collimation error can produce enough loss in S/N ratio that apparent resolution is reduced by 30-60% or more. In fact, the resolution has not decreased as much as it would seem (as determined by viewing close double stars for comparison), but the loss of contrast makes it appear as so.

CAUSES FOR SCT COLLIMATION LOSS
It would seem that maintaining good to excellent collimation is a good idea. Newtonians often loose collimation as the weight of the diagonal increases deflection of the spider vanes when pointing away from the zenith. Newtonian primary mirrors may also move a bit in their cell. A counter weighted secondary and/or thicker spider vanes helps.

SCTs such as the LX200 that tend to loose collimation for mechanical reasons that are quite correctable by the user. I often transport my LX200 and collimation remains generally quite good. A SCT can be prone to excessive collimation losses from a number of sources such as corrector movement in the front casting, excessive secondary cell clearances in the corrector, excessive primary mirror tilting (image shift) excessively loose secondary collimation screws and differential optical tube expansion/ contraction, optical tube looseness, as well as a loose primary mirror. If all the described sources are controlled, collimation in a SCT remains good without frequent adjusting.

PREVENTING EXCESSIVE COLLIMATION LOSS
Corrector movement (more common in larger SCTs) is controlled by adding thicker cork shims to the corrector edge, compressing the cork just before insertion. This allows even corrector expansion but stops gravity from shifting the corrector under the corrector retaining ring as the optical tube is moved across the sky.

Excessive secondary cell clearance in the corrector is controlled by adding masking tape to the secondary cell circumference for a snug fit. This also helps prevent secondary loosening and movement during temperature changes.

Excessive primary mirror tilting (image shift) is minimized with a somewhat slippery and viscous lubricant. I have been testing a new lubricant that appears better than anything I've used to date. The image shift on my 12" LX200 was reduced to a few arc-seconds in cold and warm weather- not enough to effect collimation except for the most severe imaging requirements.
Excessive loose secondary collimation screws is minimized by tightening all three completely, then adjusting collimation by loosening the desired screws. This helps to prevent the secondary mirror from moving slightly during thermal and mechanical stresses.

Differential tube expansion is controlled by waiting to start an image until temperatures are stable and/or the use of gentle heat may help. The tube may expand a bit on one side due to differences in cooling rates. This causes collimation loss, focus loss, and guiding errors when using a guidescopes. I have observed this anomaly in the Land Mode. Invar rods will control tube expansion, but are not used in commercial or any known amateur SCTs (other than Schmidt cameras) to my knowledge.

Optical tube looseness should be rare in Meade SCTs because more recent models (as reported by John Downs) are cemented to the castings. Loose primary mirrors should also be rare. Meade uses a cork gasket that allows for expansion/contraction and a rubber O-ring (or silicone rubber) to grip and hold the mirror on the baffle tube. My observations in the Land Mode indicate very little image shift and collimation loss result from the Meade methods of securing the primary mirror to the slider tube. Attempts to support the primary in the thinned upper sections as often done with full thickness mirrors are likely to result in astigmatism and should be avoided.

CONCLUSION
Good SCT collimation is always desirable and should be quite easy to maintain in most SCTs in good mechanical adjustment. Those that find they must constantly adjust collimation may want to review a few of the items described above. Those with critical imaging requirements may want to check collimation more frequently; however, only slight collimation adjustments should be required.

Subject: Collimation Demostration Webpage  

From: Robert Preston

Mssr. Legault has a page that provides wonderfully clear demonstrations of the effects of different amounts of miscollimation on lunar and planetary images:
   <http://perso.club-internet.fr/legault/collim.html>
    Note: should open new browser window over this one.

There is essentially no difference between the images on that page that simulate perfect collimation and those that simulate slight miscollimation (as evidenced by slightly asymmetric in-focus diffraction patterns at high mag.) So I'm not going to worry about this non-problem, personally. But Legault says that it is his experience that a large fraction of amateurs who use reflecting telescopes are using BADLY miscollimated scopes, and suffering severe image degradation as a consequence. That's unfortunate, if true, since it's so simple to fix the collimation with in-and-out-of focus star testing. It's in the LX200 manual, even. It seems very worthwhile (and extremely simple) to check the diffraction pattern at every observing session, and fix it if it's badly off.

Subject: Collimate With or Without the Diagonal? --part 1 of 7  

From: Richard Shell, Date: Jan 2003

The basic problem is that manufacturers (and we) assume a diagonal mirror needs to be placed at an exact 45 degree angle for perfect centering. And, that assumption is true. However, the proper angle is only relative to the mirror's location. If you just hold up a diagonal and look at it, just trace the path from where the center of the eyepiece is to where the center of path of the exit attachment tube is. It is at that point where the mirror should be placed for perfect collimation. And, if you look at where the mirror is actually placed in the diagonal, you can easily see that it rests at a 45 degree angle, but at a significant distance below the point of intersection. This distance varies from manufacturer to manufacturer depending upon their design and the thickness of the mirror itself, but none of the mirrors even come close to that ideal point of intersection.

So basically, one has two choices to fix the problem: 1) leave the mirror at a 45 degree angle but move it upwards to the proper point, or 2) change the angle of the mirror to compensate for the overshoot distance. I am sure someone who still remembers high school trig could formally develop a formula relating this distance to angle change, but it's really not necessary.

The problem is that diagonal manufacturers do neither - and for a number of reasons: moving the mirror upwards requires making it smaller to accommodate the original diagonal housing. In this case the field of view is reduced. The other alternative is to redesign the housing and make it significantly larger to accommodate the same size mirror. And, since many manufacturers have been using the same dies and molds for decades, there is not much chance of that happening.

On the other hand, placing the mirror in the same offset position but at an angle would center the image and preserve the field of view. But here again, most diagonals were designed with mirror guides cut into the mold that allow the mirror to be simply plopped in place with no movement during assembly. Unfortunately, the mold makers all assumed a 45 degree angle. And probably for a good reason: diagonal mirrors themselves come in various thickness ranging from 1/4" Pyrex to 1/8" god-knows-what. And since they change mirror suppliers at will, it would be difficult for them to commit to a certain mirror specification when designing a mold.

In developing the Series I focuser, I tried all sorts of solutions and ran into all of the problems above. The fact of the matter is that diagonal manufacturers do not pay this problem much attention -- mainly because in visual use no one notices that there is a problem. And visual use is probably 95% of the diagonal market. And of the 95%, probably 95% of that are beginner scopes with 1/4 wave mirrors.

In the end, however, if I were a conscious and informed diagonal user, I would much more worry about the flatness of the mirrors rather than mis-positioning of the mirror as far as diagonal quality is concerned. If you are to take full advantage of excellent optics manufactured to 1/15 or 1/20 wave, it makes no sense at all to simply plop in a diagonal with a 1/4 or 1/8 wave diagonal mirror. But unfortunately, those are usually the types of diagonals included 'free' along with an otherwise excellent telescope.

I hope this explains diagonals a bit and explains why anyone in their right mind does not use a diagonal when imaging.

When collimating a scope, just keep in mind that (in a SCT) all one is doing is aligning the secondary mirror with the primary mirror to have a perfect diffraction pattern. The moral is: (1) nothing one can do with a diagonal can overcome this misalignment. Just remember the Hubble fix. It required addition of corrective optics, not just realignment. For a diagonal to overcome initial miscollimation, its surface would need to be optically designed to reverse the damage. And the common 'flat' surfaces of diagonals are not going to do this; (2) using a diagonal during the collimation procedure can only add to the problem, not reduce it. In practical terms, however, it makes little difference if one collimates with or without the diagonal in place, as long as the primary and secondary mirrors are in proper alignment; (3) there is a difference between optics being collimated and images being on center at the exit of the final eyepiece holder. A bad diagonal (even a cheap one) will not alter the collimation of the optics significantly, rather shift the entire collimated image off center at the exit point;

(4) to check the degree of shift, simply put an astrometric or crosshair eyepiece in the diagonal and center a star. Then, remove the diagonal and insert the same eyepiece. Refocus, and you should be able to measure the difference in position. (Don't be surprised if the star is not even within the field of view!)

STELLAR TECHNOLOGIES INTERNATIONAL <http://www.stellar-international.com/>

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Subject: Collimate With or Without the Diagonal? --part 2  

From: Eugene Lanning

It would appear to me that what we can learn from this thread is
  a) Diagonals do not necessarily have the mirror at 45 degrees
  b) Diagonals do not have the mirror center at the centerline of the eyepiece holder and the eyepiece
  c) No to diagonals are probably identical, due to manufacturing tolerances.

Therefore even a) and b) are generalizations
  d) The offset in using a diagonal is on the order of a couple of arcminutes (undocumented)
  e) Collminable diagonals are not commonplace
  f) No information on the diagonal optical properties is readily available... what is the surface tolerance of the standard issue Meade diagonal 1/10 wave?, 1/20 wave?, what percent of light is transmitted? Indeed, what *are* the appropriate performance measures vs. what are just advertising ploys?

It would seem that we have treated the diagonal as a convenience item, and ignored it as an optical item. People spend a lot of time to learn/master their eyepieces, and spend a lot of money on them too. Then we place a diagonal, of which we know little, in front of it and go onward.

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Subject: Collimate With or Without the Diagonal? --part 3  

From: Don Tabbutt <dontabbutt.com>

I've followed this thread for a while, and here's my take: if the scope is not collimated on the optical axis, then it is not collimated. That's why for critical collimation you use high power and a reticle eyepiece to make sure the star is at the center of the optical axis, and keep recentering it every time you tweak a collimation screw.

If you were to collimate the scope through a diagonal that is several minutes off-axis, then in effect you have miscollimated. I would never collimate any telescope through any diagonal.

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Subject: Collimate With or Without the Diagonal? --part 4

From: Taras Hnatyshyn

From: Charles Crapuchettes:
First, there are 4 elements, not 3: corrector, primary, secondary, eyepiece. (Diag merely folds path to eyepiece.) All 4 need to be collimated for ideal performance, but that requires rotational freedom at 3 points and translational freedom at two points; we don't have that, and would be hard pressed to use it if we did. (It's hard enough to explain what is seen and what to do about it for the one adjustment point we have; explaining 5 interacting adjustments, each with 2 degrees of freedom, would be impossible.)

So, with the one adjustment you have, adjust for all the imperfections you can adjust out. If you use AltAz, the diagonal probably doesn't need to be rotated, and collimating with the diagonal is best for visual work. Even equatorial, for most visual work the diagonal is probably on one side, and probably collimation is best with the diagonal where you use it most; but this is a statistical argument about how one individual uses the scope, and the optimum answer varies. For CCD work, using the CCD to make the observations would be the most accurate, but probably painfully slow.

But, because there's not enough freedom to adjust out all the imperfections, a perfect collimation is just not possible.

It is 5 elements. You are forgetting the detector - the human eye or a CCD device. If you are wearing glasses, that lens is another element. When I first tried to collimate my SCT, I used my right eye without glasses to collimate, and I ended up correcting for the astigmatism in my eye, but my photos couldn't get a sharp image... oops.

Now, when I want to collimate my LX200, I use a SAC-IV or an electronic eyepiece, to take the eye out of the equation. Then again, I do a lot of photographing/imaging, so I do not usually use an eyepiece to collimate, except to get close after a bumpy trip.

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Subject: Collimate With or Without the Diagonal? --part 5  

From: Ed Stewart <stargazerskymtn.com>

John Mahony wrote:
> The whole point of the thread (at least the recent parts) has been what to
> do if the diagonal is misaligned (usually) and (not as often) if you're going to
> be keeping the diagonal in the same orientation. Then is it better to collimate
> through the diagonal, for best image in the center of your actual field of view...

I'm still not convinced-- if you collimate straight thru to get the best possible images with the 3 elements within the OTA (corrector, primary, & secondary) then the these elements are in their optimal positions to produce the best possible images that they can. If you then introduce a misaligned diagonal and one that has no adjustments, then the only adjustable element is now the secondary. If the secondary is adjusted to compensate for the diagonal's error(s), then it follows that the optimal collimation of the OTA elements will be lost. The diagonal is just folding the light cone which if it has become miscollimated. then it seems to me that the images will be degraded. My logic says collimate straight thru, let the diagonal fold the light cone as it will, and the image may be off center to the eyepiece, but that is better than mis-collimating.

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Subject: Collimate With or Without the Diagonal? --part 6  

From: Roger Hamlett <ttelmahntlworld.com>

This brings to mind, one aspect of collimation that hasn't been mentioned in this thread. Start with the fundamental concept already outlined, that collimation of the SCT, involves aligning the optical axis of the secondary, with that formed by the primary, at the point where the test is being made. Unfortunately, in an SCT, there is no guarantee that the baffle tube (along which the primary moves to focus), is actually aligned perfectly with the primaries optical axis. Because of this, collimation will only be exact, when the primary is at the position on the baffle tube, where the secondary adjustments are made. As you focus in/out, and the primary is moved along the baffle tube, if it's optical axis does not align to this tube, there will effectively be a small amount of lateral shift relative to the secondary, bringing back collimation errors.

Hence the 'best' collimation, setup, has to be to have the focal distance, where it is actually going to be used. This then is the 'downside' of trying to collimate without a diagonal. It is also why when setting a scope up for CCD imaging, using a focal compressor, where the effective focal plane is very different from the default 'visual' position, it may be necessary to re-collimate at this focal point for the best results.

Now collimating with a diagonal present, simply implies that the centre of the eyepieces FOV, will almost certainly not align with the centre of the baffle tube. As has been pointed out above, there is no guarantee that this is actually the centre of the optical axis of the scope anyway!. However as Doc G has pointed out, this does not prevent collimation for the best diffraction pattern (the pattern still shows the same directional offsets, as it moves away from the centre of the eyepieces FOV, which still allows good collimation to be achieved).

Hence, I would say that the 'best' configuration, would be to collimate as you intend to use the scope, and not to worry about any error in the diagonal. Separately, it is (of course), nice to have a diagonal that produces an exact 90 degree reflection, perpendicular to it's mounting face (which then should imply the FOV will remain on the same part of the sky as it is rotated), but given the small angles concerned, achieving this is not easy.

The problem I am talking about comes from internal mechanical errors. This is where the baffle tube (the actual tube on which the primary slides for focus), is not aligned to the optical axis of the primary. The problem here is that if you place the primary at (say) 300mm spacing from the secondary, and tilt the secondary to get good collimation, then refocus the scope by moving the mirror forward to give an increase in the back focus distance (a 200mm increase in back focus, will involve moving the mirror forward about 40mm), then if the internal tube is angled by 1 degree (say) to the axis between the primary and the secondary, the main mirror will effectively move sideways by 0.69mm. This will completely destroy collimation. Hence if you collimate at the rear port, then add a diagonal, and have to refocus, you will once again have lost collimation.

In the context involved, there is really no such thing as 'collimation' of a diagonal. Unfortunately, some posters are talking about the alignment of diagonals, so that they produce a centered refection, as 'collimation.' The word collimation, implies aligning the optical axes of elements, so that they are coincident with the axis of the incoming light path. A plane mirror, doesn't actually have an optical axis. If an SCT, is collimated (primary, secondary, and corrector are all aligned correctly), you can stick a diagonal in and bend the light to any angle you want, with no effect at all on this collimation. However if in doing so, you change the focus required, miscollimation may well result.

The sole possible effect of the misalignment of the diagonal (from the optimal 45 degrees, with it's surface intersecting the junction of two lines, one through the centre of the eyepiece hole, and the other through the centre of the incoming hole), is to move the viewpoint away from the optical axis. As has already been pointed out, the same unavoidable mechanical errors imply that this axis is already not in the centre of the baffle tube anyway!...

If there is an optical 'doubt' about the diagonal (producing astigmatism or some other similar distortion), the _best_ way to collimate, would be to add an extension tube to the rear port of the scope, so that the light path is the same length as it would be with the diagonal present, and collimate like this.

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Subject: Collimate With or Without the Diagonal? --part 7 of 7  

From: John Mahony <jmmahonyhotmail.com>

Let's assume a best-case scenario as far as OTA is concerned- the axes of the corrector, primary, secondary, baffle tube, and rear opening are all coincident. If you put an EP in the center of the rear opening you'll get perfect diffraction rings around a star at the center of the field. If you move the EP off center, the collimation pattern deteriorates. But it has been my experience that you can then adjust the tilt of the secondary and improve the pattern considerably. Not quite perfect as before, but to just about anyone except perhaps a professional optician, it appears to be a good collimation pattern. Stars in the center of the EP field of view have even, concentric diffraction rings, and the view at focus improves considerably.

A poorly made diagonal can produce three errors-

1) The image is offset relative to what's seen through the rear opening.
2) The light cone hits the EP field lens at an angle.
3) Wavefront distortion due to a poorly figured flat.

The last two we'll have to live with if we want to use that diagonal that evening. So we're left with the off-axis image, which, just as before, we can improve by adjusting the secondary. And you can't argue with an improved view, if that's what you want. So I collimate through the diagonal if I will be using that diagonal (in a situation where I won't be changing its orientation, which would give a offset relative to the center of the rear opening).

Doc said there's only one correct position for the secondary, but I suppose he was referring to just the three optical elements of the OTA. Another poster said that the thought of anyone collimating through a cheap diagonal makes him cringe, but there are situations where it makes sense. When I go back to imaging or astrometry, I re-collimate straight through. I see no reason to cringe.

Subject: Decreasing OTA Cool Down Times --part 1 of 6   

From: Anthony Kroes <akroesvenomtech.com>

Not sure about checking for optical quality, I just look through mine!! I've never 'star tested' a scope so it could be that or a combination of things. That said, I think an hour is NOT be sufficient cool-down time for a 10". Folks have posted here previously about times of up to 2-3 hours to ensure full cool-down depending on the size of the scope (larger-longer, all other things being equal).

1-1/2 hours seems about right for my 8", and the 12" can take 2 hours or more. Of course that all depends on how much warmer the scope was from outdoor ambient when you brought it outside or opened your observatory. Your observatory (if you have one) also makes a difference, especially on a dome, because there is less sky visible to help draw down the temp of the scope - thus longer stabilization times, roll-off roof=faster cool-down. How fast the temp outside is dropping (or rising) as the night progresses can also affect the cool-down times, particularly if it is a fast drop.

I not hip to the nuances of thermodynamics, so please excuse my numbers - I know there are other factors here...but simply, for example, if your scope cools off 25F degrees/hour and it was 60F degrees warmer than outside to begin with, like bringing a scope from a 70F degree house to a 10F degree Wisconsin winter night, you are looking at a 2+ hour cool-down. If the temp outside is also dropping by 3 degrees/hour it would stretch out to over 3 hours. That's why I leave the scope in the obs - it is usually close enough to ambient to do serious observing in an hour or less. Editor's note: to decrease these times, read on...

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Subject: Decreasing OTA Cool Down Times --part 2   

From: Scott Oates <SOates4616aol.com> Date: Jan 2003

Doc G suggested using a large fan blowing air across the optical tube to assist cooling. I have been using his method ever since. I find the cool down time for an 8" OTA to be 45 min and a 9.25 right at 1 hour. All it cost was $10 at Wal-Mart. It does not require pumping dust into the OTA and it works! I use a 14" oscillating fan blowing over the OTA. No internal fan. I have a small observatory and I leave all my equipment (camera, etc.) attached. Just the fan moving the air around works wonders.

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Subject: Decreasing OTA Cool Down Times --part 3

From: Kevin Wigell <kwemailtwcny.rr.com>

Regarding cooldown times, for some time I've been toying with the idea of putting an electronic temperature sensor inside the OTA to allow for reading the actual interior temperature. This could be done with an inexpensive, wired, indoor/outdoor type thermometer (such as sold by Radio Shack and others), or (preferably) by putting a wireless temperature sensor inside that sends a signal to a remote unit. With the wireless unit, I'm picturing sticking the transmitter inside the OTA with velcro or sticky tape so it wouldn't be loose inside the OTA.

Does anyone have any idea on whether there would be room for such a transmitter inside the OTA, say behind the mirror? The transmitters are usually a little bit smaller than a deck of playing cards. And if there is room, is there an easy way to get it in there? Also, would the weak signal from the transmitter would be able to penetrate the OTA?

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Subject: Decreasing OTA Cool Down Times --part 4   

From: Bill Arnett <billnineplanets.org>

Kevin Wigell wrote:
> Regarding cooldown times, for some time I've been toying with the idea of
> putting an electronic temperature sensor inside the OTA

I did that using one of the Oregon Scientific units that has the sensor on the end of a short wire which is intended for immersion in water. It was just the right size to fit thru the shipping bolt hole.

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Subject: Decreasing OTA Cool Down Times --part 5

From: Tom Mote <pytomtexas.net>

If I'm not mistaken, Robert Haler, at Lymax, <http://www.lymax.com>, sells an "SCT Cooler" device that blows filtered air through the "eyepiece hole" of your catadioptric telescope and brings its interior temperature to essentially the ambient outside air temperature in a relatively short period of time.

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Subject: Decreasing OTA Cool Down Times --part 6 of 6   

From: Anthony Kroes <akroesvenomtech.com>

If you are the handy type, there are plans on the internet <http://www.starcrwzr.com/cooler.htm> for making a similar "SCT Cooler" device yourself out of PVC pipe fittings and a computer fan.

Subject: Ronchi Grating Used to Reposition Secondary Rotational Position    

From: Dennis Persyk, Date: Jan 2003

> From: Mark C
> I noticed the "Easy Tester" for sale on the ScopeStuff site. It is a Ronchi
> grating mounted in a standard 1"1/4 eyepiece mount.
> <http://www.scopestuff.com/ss_jspe.htm> Would this be useful
> in trying to determine the optimal rotational position of the secondary
> and or corrector. Some LX200 users have had secondary come loose, or
> the corrector and have found no indexing marks to help in re-orientation.

My mentor and neighbor, Jack Schmidling <http://schmidling.netfirms.com/ez-testr.htm> is the maker of the EasyTester. He said that one could optimize rotational orientation of the corrector plate with his device.

Subject: LX200 Secondary Mirror Rotating?  

From: John Rostoni, Date: Aug 2001

I had a similar problem of the secondary mirror rotating out of position several years ago with my 10". Here's what I learned...

If you call Meade about it, they want you to return the scope for realignment (which is not the same as collimation). They really didn't want to talk about either what they were going to do to get the secondary back in position, or what I could do to do the same thing. I figured I had turned the secondary about 15 degrees at the most, and could most likely get it back close to where it started.

A talked to a few more people (John Piper might have been one of them), and finally got a single detail about the process - the rotational position of the secondary is adjusted within a 5 degree band about an "optimal" point. I have no idea what test is used, or what the tech is looking for while rotating the secondary - that is, I have no concrete information on what the effect of rotating the secondary is on the image quality.

With this information in hand, I decided to do some "qualitative tests". I removed the corrector/secondary (yes, this is really the only good way to retighten the loose secondary housing), and removed the secondary baffle, releasing the secondary from the corrector. A quick inspection showed several things - 1)the hole in the corrector is significantly larger than the secondary housing, making it difficult to recenter the assembly, and 2) there were no alignment marks anywhere on the corrector or secondary housing the marked the "correct" position. I did find an abrasion on the edge of the corrector that seemed to be similar to an indentation on the rubber gasket of the secondary. I marked both of these points on the outside of the corrector and secondary housing (so I could use them when the scope was reassembled. I added a few shims to keep the secondary centered in the corrector, and put everything back together.

I spent several nights going through a collimate/check-the-image/turn-the-secondary process. Each time I recollimated, I checked for focus patterns on either side of focus, in focus star images throughout the field, and for some reason, color artifacts. That was about all I could think of that would be apparent under viewing conditions.

The final result was that unless the secondary was off by at least 90 degrees, no significant image changes were apparent. Minor changes, yes...but none that would say, "wow...that's perfect!" or "Geeze...I broke it!" I also consider the proper centering of the secondary to be more critical that minor changes in the rotation position (just a guess, really). I finally got the alignment back where I was getting the best images.

Well, that's my experience. I'm not assuming that EVERY scope would have the same sensitivity to these adjustments. I'm sure there are quite a few on both sides of my single sample. After all, a sample of one is not enough to make generalizations.

Subject: Secondary Mirror Description/Optical Alignment   

From: Jim McMillan, Date: Feb 2002

I'd like to respond to your query regarding the secondary mirror by sharing a bit of my experience with it. Like you, I was curious. So, after using my scope for a couple of years (and out of warranty), I fiddled with collimation. I think I got it fairly close, but the focus just didn't want to "snap" into place. I bought the Kendrick SCT collimation laser. What I determined by it was that the optical and mechanical alignment of the OTA wasn't exactly the same.

I also had heard that the secondary hole in the corrector was a bit bigger than the secondary holder. I reasoned that I could move it to try to get better optical/mechanical alignment. So, off came the corrector. To my surprise, I discovered that the secondary mirror was not centered in the holder. Apart it came. The secondary mirror is actually glued to an aluminum disk into which the collimation screws screw. At the center of the aluminum disk is a hole which the secondary pivots on. Still thinking I could improve my optics by getting everything centered, I pried the mirror from the aluminum disk, centered, and reglued it. I used a couple of layers of black electrician's tape around the secondary holder so that it fit snugly - and centered - into the corrector.

I put everything back together and....the astigmatism was so bad, my stars were now diamond-shaped. I could barely make out medium-sized craters on the moon. The next day, I called Meade and (sheepishly) told them what I did. Ollie explained to me that part of the process of setting the optics for each scope when it's manufactured is to align them such that they compensate for the inevitable mechanical misalignment of the OTA. In other words, the fact that my secondary was not centered - both in the corrector and in the holder - was on purpose. Ollie said I'd have to send my scope back and they'd "repair" it for the standard $500 fee - or install new optics for $550.

After many, many hours of trial and error, I was able to restore my optics back to where they were - and maybe even a bit better because I had so much practice at collimation. But, I think it was more a matter of luck than skill.

So, my suggestion regarding fiddling with the secondary is to be very careful what you do - and be very sure to mark it such that you can return it to its original position.

Subject: Centering the SCT Secondary  

From: Richard Jordan, Date: Feb 2002

Like Jim's scope, my old Meade 10" SCT (2120) just didn't seem to be up to par. After a careful collimation, higher power (180X) terrestrial views were "muddy", 4-6 mag stars showed a bit of coma and planetary views were unsatisfying, even on nights of good seeing. Collimation changed depending whether the scope was inside or outside focus.

Suspecting a tilted primary mirror and/or a decentred secondary mirror, I used a machinist's caliper to check whether the plastic secondary holder was centred in the casting holding the corrector. I found that it was only slightly off-centre, so I shimmed it to the centre of the casting, recollimated and tested. There was no improvement in the image.

I made a sight tube out of a 35 mm film canister and inserted it in a 1-1/4" visual back and checked the centreing of the secondary baffle, secondary, and the slider tube. The assembly was significantly off-centre. I re-shimmed the corrector to centralize the circles as seen through the sight tube and recollimated. Still no joy.

Next, I rechecked the position of the secondary holder in the corrector holder casting and found that on one axis the secondary holder was off-centre by .5 mm. I re-shimmed the corrector so that the caliper made the same sliding fit between the secondary holder and the casting. After collimation, this time success! On the other axis, the secondary holder is 2.5 mm off-centre relative to the casting!

For the first time and after 7 years of frequent use, I saw what is passing for the GRS on Jupiter these days and the scope split Zeta Orionis. Contrast and sharpness of focus was significantly improved. Zeta Orionis had always been just a burning blob. My terrestrial test object (the print on a distant hydro transformer label) shows a noticeable improvement. Collimation is the same on both sides of focus.

In this instance a .5 mm centreing error caused a very noticeable image deterioration. There was a most definite element of luck involved in this procedure, but it was a most worthwhile effort.

Subject: Tri-Lobed Stars Fixed?  

From: Gene Chimahusky <lynol1000yahoo.com> Date: Mar 2003

Civil twilight brought clear and extremely steady skies. The last 18 months my scope, under very high power, has exhibited tri-lobed stars. I had queried a possible cause and the best that could be offered was pinched optics, as in the secondary adjustment screws too tight. Over that time I had tried multiple times adjustments with the screws and was never able to resolve the issue. The optics over this time have never seemed extremely crisp, not snapping into focus, so I also went looking for possible solutions to that.

I decided to play with the rotation of the corrector plate. I removed the bezel and clearly marked and matched were the Meade alignment marks. I started with 5 degree changes both CW and CCW. To make a long story short I ended up with 10 degrees CCW and it seems now my lobes are gone, I have a circular airy disk with a well defined circular first diffraction ring.

I replaced the bezel and tightened everything down, gave final tweaks to the collimation screws and everything stayed put, much cleaner, crisper, and now things do seem to snap much easier into focus.

These adjustments were all made at 600x and 800x, 10" f/10 with 2X barlow , 6.4 and 9mm eyepieces done straight through.

Note I could bring the lobes back and/or loose crispness with certain rotations of the corrector (obviously for the crispness) , including at the original Meade matched marks. The roughly 5 degree rotation changes brought noticeable differences at 800X and steady seeing.

Maybe the corrector versus the mating surfaces are not 'perfect' and the corrector warps slightly?

Subject: LX200 Corrector Plate Glass?  

From: Bill Keicher <wekeichercomcast.net> Date: Apr 2004

----- Original Message -----
From: Michael Blaber
> I believe the 12" LX200 classic corrector plate was made from BK7
> glass (also the 16", but not the 10" and smaller), while all the new LX
> GPS scopes appear to have "water white glass" corrector plates. Does
> Meade still use BK7 for the 16" GPS? How does the BK7 differ from the
> "water white" glass?

The 2002 and 2003-2004 Meade Catalogs specify BK7 glass (borosilicate glass) used in the corrector plate of the 7", (Maksutov Cassegrain) LX200 GPS telescope. Meade specifies "clear float glass" for the corrector plate used in all of the Schmidt Cassegrain LX200 GPS telescopes. BK7 glass is an optical grade glass available from Schott Glass and others. Achromatic doublet lenses are often fabricated with BK7 as well as a flint glass. BK7 has an optical index of refraction of 1.509 at 582 nm, has a high optical transmission, can be selected nearly free of bubbles and inclusions and has an index of refraction uniformity of about 1 part per million. BK7 glass is composed of 66% silicon dioxide, 12.4% boron oxide, 8% sodium oxide, 12% potassium oxide, and ~1.6% other chemical components.

Unfortunately, "clear float glass" may imply a commercial grade of glass used in windows! Pilkington describes typical clear float glass as having an index of refraction of 1.523 at 582 nm and being composed of 72.6% silicon dioxide, 13.9% sodium oxide, 8.4% calcium oxide, 3.9% magnesium oxide, 1.1% aluminum oxide, 0.6% potassium oxide, 0.2% sulfur trioxide, 0.11% iron oxide (Fe2O3) ( as known as soda lime silica float glass). Pilkington "Optiwhite" float glass is a low iron oxide float glass with a visible light transmission of 92%, reflection of 8% (uncoated) in a 1/8" thick sheet. "Optiglass virtually eliminates the green cast inherent in standard clear float glass" (implying "water white" glass). The polished edges of "Optiglass" float are noticeably colorless compared to standard float glass. The glass can be selected to be nearly free of inclusions. This glass is probably cheaper than optical grade BK7.

The mirrors are made of Pyrex glass (optical transmission is obviously irrelevant here).

My guess is that Meade is using the equivalent of Pilkington Optiglass in the Schmidt corrector plate. The corrector plate is thin and this is probably a good choice in this application. The Maksutov Cassegrain telescope has a thicker corrector with significant optical power and requires an optical grade glass.

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