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Attaching secondary mirror

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#1 DreamWeaver

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Posted 21 November 2015 - 09:51 PM

What's the best way to reattach the secondary mirror to the mounting plate?  I've been playing with my LX3 to try and improve the view.  Right now the secondary is offset to one side by 1.53mm.  Last year I posted the question if the secondary had to be perfectly centered, and some of the responses were that it might have been done at Meade to correct for mechanical alignment issues.  i took some measurements today and the baffle and hole in the corrector seem to be pretty well centered.  The baffle is only off center by 0.025mm and the hole in the corrector is off center by 0.43mm.  I'm thinking that that's pretty good for a scope that was rushed out the door in the mid-80's.

 

When I collimate with a star, I can almost get the rings concentric, but the poisson spot isn't quite centered.  I can't adjust any farther because the screw at the 8 o'clock position is bottomed out.  So I would like to remove the mirror from the mounting plate, and reattach it centered on the plate.  I have some black sided foam tape that's used to attach automobile trim.  Will that work, or is there a better way?

 

Here's a couple drawings to give you an idea of what I'm looking at.  (Not to scale.  ;) )

 

LX3_1_zpsj38fqdcw.jpg

 

LX3_2_zpshsfek4gk.jpg



#2 jhayes_tucson

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Posted 21 November 2015 - 10:08 PM

I'm working on a similar issue and I'm going to machine a jig to get it centered.  Let me know if you come up with a better solution.  For adhesive, I'd use something like this:

 

http://solutions.3m....93241542&rt=rud

 

John



#3 DreamWeaver

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Posted 21 November 2015 - 10:17 PM

Thanks for the link John!  My "machining" tools are made for wood so that's what I'll make my centering block out of.  I hadn't even thought about using anything to center it, so thanks for that too!

 

I tried using your procedure of looking down the front of the scope, but I haven't been able to get the secondary concentric with the other rings.  Hoping that remounting the secondary mirror will improve the views.



#4 jgraham

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Posted 21 November 2015 - 10:23 PM

After having the secondary on my LX3 recoated I remounted it with 3 spots of clear silicone adhesive. I used 3 toothpics as spacers to hold the secondary off of the backing plate until the silicone cured.



#5 PowellAstro

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Posted 21 November 2015 - 10:29 PM

The center of the whole glass may not be the center of the optical surface. Look to see if the bevel is the same all the way around. If you have a turn table, you can spin the mirror and reflect a laser off of it. When the reflected laser is stationary, the mirror is optically centered. You can then mark the center of your secondary.

#6 DreamWeaver

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Posted 21 November 2015 - 10:52 PM

Afraid I haven't had a turn table in years so I'm going to play with the secondary first.  And mark it so I can return it to where it was on the mounting plate if I need too.  I've tried rotating the secondary housing 360 degrees, stopping every few degrees, and the same adjusting screw has to be bottomed out to get the collimation close. Hoping that means the primary is centered.



#7 jhayes_tucson

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Posted 21 November 2015 - 11:22 PM

The center of the whole glass may not be the center of the optical surface. Look to see if the bevel is the same all the way around. If you have a turn table, you can spin the mirror and reflect a laser off of it. When the reflected laser is stationary, the mirror is optically centered. You can then mark the center of your secondary.

 

Be careful about spinning.  Wedge or tilt in the way the component is mounted relative to the spin axis can affect the result considerably.  For that reason, spinning can be useful for adjusting tilt relative to a spin axis, but you may get no useful information about centration.  Most (though not all) components are symmetric about the mechanical center of the part as defined by the edge.  If this is a spherical secondary (as I assume it is,) then the centering tolerance won't be very tight at all.  On the other hand if this were a RC secondary, the centering tolerance might be pretty tight and I'm not so sure that spinning will get you there (depending on the sensitivity of your test.)  That's why I'd carefully center it mechanically first.

 

John



#8 DreamWeaver

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Posted 21 November 2015 - 11:38 PM

John, everything I've read, the secondary on the LX3 is a spherical non-corrected mirror, so I should just need to get it close to center.

 

After having the secondary on my LX3 recoated I remounted it with 3 spots of clear silicone adhesive. I used 3 toothpics as spacers to hold the secondary off of the backing plate until the silicone cured.

 

Do you remember the type of silicone adhesive you used?


Edited by DreamWeaver, 22 November 2015 - 12:19 PM.


#9 jhayes_tucson

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Posted 23 November 2015 - 12:02 PM

John, everything I've read, the secondary on the LX3 is a spherical non-corrected mirror, so I should just need to get it close to center.

 

After having the secondary on my LX3 recoated I remounted it with 3 spots of clear silicone adhesive. I used 3 toothpics as spacers to hold the secondary off of the backing plate until the silicone cured.

 

Do you remember the type of silicone adhesive you used?

 

Keith,

You are correct.  With a spherical mirror, the centering tolerance goes way down; however, you still have to properly align the system.  That means that the radius of curvature of the secondary still has to sit on the optical axis of the system.  Typically that will mean that you will have to tilt the secondary mirror (typically about some point near the surface) to align it.  You can make that arrangement work but just keep in mind that you may generate an asymmetric vignetting pattern or even clip the marginal rays from the primary (which may generate some nasty diffraction issues).  Getting the mechanical centration at least close to start with is a good idea.

 

As for adhesives, a lot of glues will work but some are better than others.  If you use hardware store quality RTV, it will hold but you'll have big outgassing issues.  The best stuff is two part RTV.  It cures rapidly and has much less outgassing.  3M 2216 works well.  If you really want to use adhesive, there is a bit more to it than just squirting the glue on and squishing the parts together.  The best thing is to use three equally spaced nylon standoffs (recessed nylon balls work really well) to define the glue thickness and to apply a well defined glue volume over each point to create three uniform support points.  A good rule of thumb is to place the glue points at roughly 70% of the radius to minimize mechanical distortion.

John


Edited by jhayes_tucson, 23 November 2015 - 12:05 PM.


#10 jgraham

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Posted 23 November 2015 - 12:16 PM

I use clear silicone RTV adhesive from my hardware store. I've also used clear silicone aquarium sealer. The colored adhesives probably work as well as the color is just a filler and fillers rarely reduce the bond or bulk strength. Manufacturers like them as the filler tends to be cheaper than the silicone. Providing a small stand-off distance between the backing plate with the toothpicks and the mirror allows both the plate and mirror to expand and contract with temperature without putting a significant amount of strain on the mirror. RTV does outgas.  If I recall right it uses acetic acid as an inhibitor and as it evaporates the resin cures. I've used silicone adhesive on dozens (plural) of primary and secondary mirrors, including my biggo 16.5" without any problems. The only problem has happened when I need to take it back off! (Hint; waxed dental floss and razor blades work well.) Silicone forms a very strong bond to glass and clean aluminum (the surface of which is really a mix of aluminum oxides and hydroxides).



#11 DreamWeaver

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Posted 23 November 2015 - 12:47 PM

Thanks for the responses guys!

 

jhayes_tucson,  the drawing below is what my diffraction rings start to look like as I'm defocusing the star.  Would that be caused by the "asymmetric vignetting pattern or even clip the marginal rays from the primary" that you mentioned.  As I said in an earlier post, I can spin the secondary housing 360 degrees, and the one adjusting screw has to be bottomed out to get it close to collimated.  That makes me believe that the primary is aligned with the hole in the corrector, and the secondary mirror is offset to much on the mounting plate.  But then again, I'm new to the world of optics.  ;)

 

Defraction%20rings_zpsbvcgdi7y.jpg



#12 jgraham

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Posted 23 November 2015 - 02:01 PM

You might take a very close look around the edge of the secondary. I've seen something similar to this on an LX3 with a tiny crack than extended from the edge under the face. The optical effect extended far beyond the visible crack. To salvage the scope I blacked out the effected area. Much to my surprise it worked great and this particular LX3 remains one of my favorite visual scopes.



#13 DreamWeaver

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Posted 23 November 2015 - 02:25 PM

I'll have a look for any defects in the mirror when I separate it from the mounting plate.  I really enjoy using this scope.  I'm really surprised at how accurate it tracks with the simple timing circuit it has.  I spent a little time doing a drift alignment on it one night, and was able to do some 3 minute exposures with almost no errors visible.



#14 jhayes_tucson

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Posted 23 November 2015 - 07:19 PM

Thanks for the responses guys!

 

jhayes_tucson,  the drawing below is what my diffraction rings start to look like as I'm defocusing the star.  Would that be caused by the "asymmetric vignetting pattern or even clip the marginal rays from the primary" that you mentioned.  As I said in an earlier post, I can spin the secondary housing 360 degrees, and the one adjusting screw has to be bottomed out to get it close to collimated.  That makes me believe that the primary is aligned with the hole in the corrector, and the secondary mirror is offset to much on the mounting plate.  But then again, I'm new to the world of optics.  ;)

 

Defraction%20rings_zpsbvcgdi7y.jpg

 

Keith,

This looks more like it may be caused by a thermal problem.  Was the scope athermalized when you did looked at this?  If you have to tilt the mirror a lot to get it aligned, there is definitely a problem with centering.  Clipping the marginal rays will probably appear more like one circle clipping another so that the defocused spot looks more  like a football (greatly exaggerated) that the "bite out of the apple" like you have shown. 

John



#15 DreamWeaver

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Posted 23 November 2015 - 08:07 PM

I always see this John. No mater how many hours the scope has been outside.  I just finished centering the secondary on the mounting plate.  If this doesn't fix the problem, I guess I'll have to find a job and save my allowance for another SCT.   :lol:



#16 DreamWeaver

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Posted 23 November 2015 - 10:13 PM

I ended up just using the double sided tape I already had for holding automobile trim on.  Before I took the mirror off and recentered it, the stars were so bad that I had a hard time splitting Y1 and Y2 in Delphinus.  I just took the scope out and spent about 10 minutes collimating it.  Then I pointed it at the double-double......and split both pairs!   :jump:  And that was with only 10 minutes of temperature stableization!  I think I might just like this little scope even more now!!!



#17 astrogalio

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Posted 07 April 2016 - 05:57 AM

I know this post is from a long long time ago Keith @DreamWeaver, But I am in the EXACT same issue as you had. I am working towards fixing this issue, except on a larger C11! Same exact issue. The Corrector plate is aligned well, the primary is as well. Its just the Secondary mirror in the Corrector plate that is loose and not quite "Centered" I am working towards removing the Corrector carefully, And removing the secondary mirror baffle/holder because its now too tight that the Fastar ring on the C11 makes the whole secondary rotate which isn't supposed to rotate but stay tug and fit. I am curios how you were able to repair this issue as I am sure your issue and solution in theory should and will work on mine. I have not tested it on double stars because I haven't had any good weather conditions. But I didn't know that testing it with double stars would help clarify if the secondary is centered. But now thats a test I will use on my scope as well. Thanks and hopefully you can reply so I can resolve this exact issue as well. I would like to hear of all you did to fix this problem with the secondary mirror and holder off centered. Also I am glad to hear your telescope seems like its in back working order! I have actually been a bit on the down side lately because of this issue. How has the fix you did held up? 

 

-Edgar


Edited by astrogalio, 07 April 2016 - 06:03 AM.


#18 DreamWeaver

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Posted 07 April 2016 - 06:47 PM

Hi Edgar, I think you might have better luck if you start a new thread explaing your problem.  The problem I had was the small aluminum plate that the secondary mirror attaches to was almost all the way over to one side of the mirror.  I removed the mounting plate from the mirror, and then remounted it centered on the mirror.  Does your secondary/baffle have excess play in the center hole of the corrector?  I have no experience with Celestron scopes, but I've read in some posts that they made corrections to the secondary mirror.  I'm not sure if that would be true with your C11.  That's why I think a new post would get you the answer you need.

 

I was able to make my LX3 even better by slowly rotating the corrector while using a ball bearing as an artificial star.  All the astigmatism disappeared after rotating the corrector about 60 degrees from the factory alignment mark.  You gotta love the QC that went into these Halley's comet era scopes!




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