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Celestron Omni 102 AZ - Pinched Optics? Something Worse?

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#1 lxeth

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Posted 25 November 2021 - 08:59 PM

I picked up a second-hand Celestron Omni 102 AZ  (102mm f6.5)  a few days ago. Most of the posts here on CN noted the good quality of the optics, particularly for a lower-end scope.

 

102_sm3.JPG

 

First light for the scope was a couple of nights ago. The view of the Double Cluster at low power was nice, but I was sadly disappointed when I tried to split Almaak. The primary looked like a seagull with two wings spread to the side and a tail down below the star. The view remained the same with several different EPs and magnifications. I went out with the scope again last night, with the same results - The low power views (up to about x30) were fine, but anything over that and the bright stars became seagulls. Just for kicks, I tried to split the double-double, and between the three-point triangular aberrations, was actually able to see a thin black line on the east-west pair.

 

I tried quick star tests on Deneb and Lyra - the in-focus had a strong bright arc along the top and a roughly triangular-shape lower half; the out-focus picture looked like three lobes, with the shape clearly repeated in each of the diffraction rings. I wasn't able to take a picture, but made a (poor) sketch of what I saw.

 

102_startest_vsm.JPG

 

Looking through old posts here and a quick glance through Suiter's book on star testing, I'm thinking (hoping, really!) that this is a case of pinched optics, and so It might be easy to remedy. Can anyone here confirm that for me? Or if not, give me some idea of what the issue might be?

 

From what I have read here and other places online, if the issue is pinched optics, I should be bale to try to fix it by loosening the retaining ring on the objective.

But, I've never had do this before, so I'm a little apprehensive. Could someone tell me how difficult this is? And any suggestions for how to do this without making the problem worse? Or creating new issues? Here is a picture of the objective, unscrewed from the tube:

 

102_objective_sm.JPG

 

Thanks in advance for taking the to read this, and for any advice or suggestions!

 

Larry


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#2 Augustus

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Posted 25 November 2021 - 09:22 PM

That’s definitely pinched optics. Get a spanner wrench or a narrow screwdriver and turn that retaining ring till it’s loose and tighten it just enough that it secures the objective without letting it rattle. Should do the trick.
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#3 BFaucett

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Posted 25 November 2021 - 09:49 PM

Larry,

 

In case you're not aware of what a spanner wrench is (and I wasn't until I started hanging out on Cloudy Nights), here's an example:

 

"Adjustable opening from 10 to 120mm meets the need of most lens size"

https://www.amazon.com/dp/B01NBW5HV5/

 

 

A photo from the Amazon link:

 

41XRQTZNyYL._AC_.jpg

 

 

Cheers! Bob F. smile.gif


Edited by BFaucett, 25 November 2021 - 09:51 PM.

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#4 Jon Isaacs

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Posted 25 November 2021 - 09:52 PM

It looks like pinched optics.

 

For a spanner wrench, I use a pair of failed inexpensive digital calipers. 

 

Jon


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#5 lxeth

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Posted 25 November 2021 - 10:38 PM

That’s definitely pinched optics. Get a spanner wrench or a narrow screwdriver and turn that retaining ring till it’s loose and tighten it just enough that it secures the objective without letting it rattle. Should do the trick.

That's good news! Thanks!


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#6 lxeth

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Posted 25 November 2021 - 10:41 PM

Larry,

 

In case you're not aware of what a spanner wrench is (and I wasn't until I started hanging out on Cloudy Nights), here's an example:

 

Cheers! Bob F. smile.gif

 

Bob, Thanks for that! Just put an order in for one on Amazon. Should be here in a few days, and we'll see how it works out!

 

It looks like pinched optics.

 

For a spanner wrench, I use a pair of failed inexpensive digital calipers. 

 

Jon

Jon, Thanks for that!


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#7 RAK on Tour

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Posted 26 November 2021 - 06:36 PM

Bob, Thanks for that! Just put an order in for one on Amazon. Should be here in a few days, and we'll see how it works out!

Please be sure to post the results after you make the adjustments! We'd all like to know if you met with success!


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#8 lxeth

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Posted 27 November 2021 - 03:57 AM

Quick update:

 

The spanner wrench came today, and tried to loosen the ring. I pushed as hard as felt comfortable pushing (hard enough that the objective began to turn on the tube), but nothing moved. When I looked closer there appeared to be some kind of "gunk" holding the ring in place:

 

objective_gunk_sm.jpg

 

I can see this at several spots around the ring between the ring and the outside of the cell. 

 

Anyone have any suggestions as to how to loosen / remove this gunk??



#9 Jon Isaacs

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Posted 27 November 2021 - 04:09 AM

Anyone have any suggestions as to how to loosen / remove this gunk??

 

 

That is some sort of glue meant to hold the retaining ring in place.  That is a real problem.. The objective cell is probably plastic so heat is of limited usefulness.  

 

You might try using an Xacto type knife blade to cut through the glue and see it that helps.  

 

Jon


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#10 lxeth

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Posted 27 November 2021 - 05:24 AM

That is some sort of glue meant to hold the retaining ring in place.  That is a real problem.. The objective cell is probably plastic so heat is of limited usefulness.  

 

You might try using an Xacto type knife blade to cut through the glue and see it that helps.  

 

Jon

Jon,

 

Thank you as always! I was looking around the site, and found this post on removing a retaining ring from a refractor:

https://www.cloudyni...g-ring-removal/

 

One of the posters (#13) suggested using acetone:

 

Some times there is a bit of varnish or locker on part of the threads. This is there because you don't want to have the ring really tight on the lens. The stuff keeps the ring from moving since it's not very tight. look around the edge of the ring and you might see some spot that might look like varnish or paint. Acetone on a swab will usually loosen the stuff. Acetone shouldn't harm the glass or coatings, but try not to get any on the lens, Cover the lens as others said.

 

Would you recommend this as well?



#11 Jon Isaacs

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Posted 27 November 2021 - 05:37 AM

Would you recommend this as well?

 

 

If the objective cell is plastic, I would be very careful with acetone.  I would try it on some part of the cell that didn't matter.

 

 https://www.millerpl...rtain-plastics/

 

Jon


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#12 lxeth

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Posted 27 November 2021 - 06:33 AM

If the objective cell is plastic, I would be very careful with acetone.  I would try it on some part of the cell that didn't matter.

 

 https://www.millerpl...rtain-plastics/

 

Jon

Thanks Jon!

 

The retaining ring itself is metal. The lens cell (that is, the whole part that holds the lens and that I can unscrew from the tube) seems to be made of metal as well. IF it is metal would acetone be ok to use? Would it damage the objective?

 

I'd like to fix the scope to see what it really can do (and just 'cause I enjoy doing this kind of thing). But, it seems to be working ok as a low-power wide-field scope (which is why I bought it) and I would hate to make it totally un-usable.



#13 CHASLX200

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Posted 27 November 2021 - 06:47 AM

Maybe try a razor and see if that can cut into the glue holding it.


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#14 astrokeith

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Posted 27 November 2021 - 06:48 AM

Sounds like acetone will be OK.

 

The main thing is to ensure it only goes on the gloop. You dont want any running off on to lens surfaces.

 

I would use an acetone dampened cotton bud to 'paint' the gloop, perhaps two or three times with a pause to let the gloop soften and absorb the acetone. A small test should quickly tell you if the gloop is going to budge.


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#15 lxeth

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Posted 27 November 2021 - 06:53 AM

Sounds like acetone will be OK.

 

The main thing is to ensure it only goes on the gloop. You dont want any running off on to lens surfaces.

 

I would use an acetone dampened cotton bud to 'paint' the gloop, perhaps two or three times with a pause to let the gloop soften and absorb the acetone. A small test should quickly tell you if the gloop is going to budge.

Thank you! It will definitely be a SLOW and Careful process...


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#16 lxeth

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Posted 27 November 2021 - 07:15 AM

Maybe try a razor and see if that can cut into the glue holding it.

Yep, I think I'll try this first to see if it can loosen it up and then think about something stronger...



#17 lxeth

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Posted 28 November 2021 - 01:44 AM

Update:

 

The exacto knife did the trick!

 

(Wish I would have tried that before I opened up the acetone and tried it on a part of the lens cell, would have saved 5 bucks - well, at least now I'm sure that the lens cell is metal grin.gif )

 

First, I cut a piece of cardboard to cover the lens itself. I used the dust cap from my ST-80 as a guide - it was a prefect fit. Then, with the cover in place I very gently, and slowly worked my way around the outside of the retaining ring with the knife. Not as easy as it sounds - there are now a few scratches on the ring and surrounding area (maybe I can fix these later with a little flat black paint?). After going around the ring twice, I tried the spanner wrench and the ring turned easily! In fact, I think I loosened it too much - when I took the lens cell off the tube, I could hear the lenses rattle a little. So, I tightened the ring back down snuggly, then backed it off by about a quarter inch. Any advice on just how much I just loosen the ring?

 

If the weather holds, I'm going to go out tonight and do some trial and error testing - star test, loosen up a bit, star test, tighten, repeat till I get the best test I can.

 

Thank you all your suggestions and advice!


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#18 lxeth

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Posted 28 November 2021 - 08:00 AM

Well, that didn't work...frown.gif What's next?

 

Was able to try out the scope with the loosened retaining ring tonight. Unfortunately, there was no change on the star test. I loosen the ring up and tightened down again several times, but there didn't seem to be any effect. I even tried loosening the lens cell on the tube to see if this would have an effect, but no change. 

 

So, I'm wondering what I should try next? Or what the problem might be...

 

On a slight tangent, but with the ring loosened up, I was able to clearly see that the edges of the lens are not blackened. I was wondering if it would be easy enough to carefully remove the lenses from the cell and blacken the edges?  Even if I can't fix the optics, the scope still works nicely as a low-power RFT, and I thought blackening the edges of the lenses might help a little bit.

 

Thank you all for the advice and suggestions up to now! I could never've even gotten this far without it!


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#19 CHASLX200

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Posted 28 November 2021 - 08:12 AM

Update:

 

The exacto knife did the trick!

 

(Wish I would have tried that before I opened up the acetone and tried it on a part of the lens cell, would have saved 5 bucks - well, at least now I'm sure that the lens cell is metal grin.gif )

 

First, I cut a piece of cardboard to cover the lens itself. I used the dust cap from my ST-80 as a guide - it was a prefect fit. Then, with the cover in place I very gently, and slowly worked my way around the outside of the retaining ring with the knife. Not as easy as it sounds - there are now a few scratches on the ring and surrounding area (maybe I can fix these later with a little flat black paint?). After going around the ring twice, I tried the spanner wrench and the ring turned easily! In fact, I think I loosened it too much - when I took the lens cell off the tube, I could hear the lenses rattle a little. So, I tightened the ring back down snuggly, then backed it off by about a quarter inch. Any advice on just how much I just loosen the ring?

 

If the weather holds, I'm going to go out tonight and do some trial and error testing - star test, loosen up a bit, star test, tighten, repeat till I get the best test I can.

 

Thank you all your suggestions and advice!

The razor trick does work sometimes. I wonder if the lens is in right or if someone took it apart and got them reversed.


Edited by CHASLX200, 28 November 2021 - 08:15 AM.

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#20 lxeth

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Posted 28 November 2021 - 08:22 AM

The razor trick does work sometimes. I wonder if the lens is in right or if someone took it apart and got them reversed.

Would getting the lenses reversed only effect higher magnifications? At low power the views are ok...



#21 CHASLX200

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Posted 28 November 2021 - 08:32 AM

Would getting the lenses reversed only effect higher magnifications? At low power the views are ok...

Not sure really.  But i would check and see if the fouser is not tilted. Or focus a star at high power and rack in and out of focus and see if it is collimation or pinched optics.


Edited by CHASLX200, 28 November 2021 - 08:32 AM.

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#22 lxeth

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Posted 28 November 2021 - 08:35 AM

Not sure really.  But i would check and see if the fouser is not tilted. Or focus a star at high power and rack in and out of focus and see if it is collimation or pinched optics.

Thanks! More things to check out! This is actually becoming a good learning experience - the scope wasnt that expensive, so I don't mind working through the issues.



#23 Jon Isaacs

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Posted 28 November 2021 - 10:53 AM

Would getting the lenses reversed only effect higher magnifications? At low power the views are ok...

 

Astigmatism in the optics is most visible at higher magnifications. At low magnifications, you're not seeing the Airy disk.

 

I think it's unlikely that the astigmatism would be caused by a flipped lens.. and the fact that the retaining ring was secured with glue indicates it hadn't been tampered with.

 

Jon



#24 lxeth

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Posted 02 December 2021 - 10:56 PM

Well, since loosening the retaining ring didn't improve the pinched optics, I decided to try removing the lenses from the cell and rotate them to change the alignment between the flint and crown by 45 degrees, 120 degrees and 180 degrees, but to no avail.

 

I can't think of what else to do to try and fix the issue.

 

Any thoughts or advice of where to go from here?

 

Thanks and Clear Skies!


Edited by lxeth, 02 December 2021 - 10:56 PM.

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#25 EJN

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Posted 02 December 2021 - 11:07 PM

Well, since loosening the retaining ring didn't improve the pinched optics, I decided to try removing the lenses from the cell and rotate them to change the alignment between the flint and crown by 45 degrees, 120 degrees and 180 degrees, but to no avail.

 

I can't think of what else to do to try and fix the issue.

 

Any thoughts or advice of where to go from here?

 

Thanks and Clear Skies!

 

Sometimes the spacers between the elements are not the same thickness. I once had a 4" f/6 achromat which showed astigmatism even after trying everything that you did. So I took apart the lens and put in new spacers. I forget what I used, probably cut them from brass shim stock or aluminum tape. That worked, afterwards I was getting textbook star images, a clean Airy disk with a diffraction ring.

 

So that's what I would try next, if that doesn't work then the lens itself is probably bad, maybe one of the elements has wedge.

 

And a flipped element has horrible spherical aberration, obvious even at low power. That doesn't sound like your problem from your description.


Edited by EJN, 02 December 2021 - 11:10 PM.

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