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What's up with my frac?

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#1 leroybrown

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Posted 06 November 2019 - 03:49 AM

Hi - bit of a newbie here, picking up the hobby again after a lengthy break. When I set up my scope again (Celestron 102HD) I noticed the objective was not clean on the inside of the scope so took it off for cleaning - now i think I've disturbed it as i cannot get full focus and CA is pretty terrible. I've tried to draw what I can see in the attachment (this was about as good as I can focus on a brighter star)  - I've tried without the diagonal and and the view is the same so it must be the lens? Just wondering if there is a way to fix it? The scope itself is at least 15 years old (maybe more) so it doesn't exactly owe me anything. It has provided some great views in the past and I'd love to keep it - I'm even looking at upgrading the mount

 

 

 

 

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#2 Tapio

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Posted 06 November 2019 - 04:10 AM

First, I would not have bothered to clean unless it was really dirty.
But now it looks like a bad collimation issue.
Someone can maybe help you fix.
I'd say spend $100 or something like that for a newer used similar like scope.
Or little more for ED scope for better views.

#3 clearwaterdave

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Posted 06 November 2019 - 04:20 AM

My guess would be that you didn't get it screwed back in correctly.,Not sure how your scope goes together but I would take it back apart and reasemble.,slowly.,good luck.,


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#4 leroybrown

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Posted 06 November 2019 - 04:40 AM

My guess would be that you didn't get it screwed back in correctly.,Not sure how your scope goes together but I would take it back apart and reasemble.,slowly.,good luck.,

There are three screws around the outside that hold it in place but nothing to collimate it. I had to clean as there was a thick dog hair that had got in and was stuck behind it somehow - I'll take it off again and see if there is something obvious - cheers. Nothing to lose now!!


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#5 sg6

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Posted 06 November 2019 - 04:44 AM

Would say that you have replaced the lens and it has not gone in straight and so is sat at a small angle, easy enought to occur.

 

If the lens is in a cell then unscrew the retaining ring tkae the cell out and carefully put it back in and hopefully it will seat itself square, the replace the retaining ring. Then have a look. Hopefully should be OK.

 

If the lens was not in a cell and so was 2 separate lumps of glass then you could have the orientation of one or both wrong - that would tend to give poor image and CA but I would think not the "fan" shape you have drawn.

 

If 2 separate bits of glass ask again as they need to go in in the correct order.

 

If a cell then one option is hold scope vertical with the retaining ring released and gently tap the top (objctive) end of the scope. It can help the cell settle right.


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#6 clearwaterdave

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Posted 06 November 2019 - 04:45 AM

Just one small chunk of something between surfaces could be an issue.,good luck.,


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#7 Redbetter

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Posted 06 November 2019 - 04:46 AM

Cleaning is necessary at times.  And this often results in the need for disassembly/reassembly because liquid can get between the lenses. 

 

This looks like a misalignment/collimation issue and it will produce a soft image with a lot of color.   Seen it before.  Not sure how the retainer ring is on this one, but when you take it apart again, place the innermost objective in and make sure it seats properly on whatever the inner cell support ring is.  Then slip in the outer lens, turn it to vertical facing up and carefully tap the sides of the tube to get it to fall in place properly.  With some tapping around the sides of the OTA it should self-center on the spacers (unless they are askew and not uniformly positioned...had that with some cheap scopes.)  Then gently tighten the ring down and do some tapping as it starts to make contact.  Tighten down some more until slightly firm so you know it has fully seated, then back off a little until just loose, then tap again.  Then gently retighten so that it is just holding the objective with minor pressure. 

 

I don't want rattle, some swear by it, I don't, I learned to swear at it.  I used to do the "leave it loose so that it rattles" routine, but that was consistently giving me coma/collimation problems, so now I tighten slightly more at the end until there is no rattle.  If you see pinch during the star test, back off a little on the tightening.  The problem with rattle is the outer lens can move about, producing collimation/wedge/tilt errors in use.  No thanks, had enough of that.


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#8 leroybrown

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Posted 06 November 2019 - 04:54 AM

Thanks all - much appreciated. Will have a look later



#9 Grounddweller

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Posted 06 November 2019 - 05:47 AM

All sound advice given in earlier posts, I will add only one more. When you removed the screws from the assembly to detach the objective, were there any spacers present around the screwthreads? If one fell off and it was put on the wrong hole, that would throw off the collimation as well, or if there is a felt liner present and it is worn, reattaching it with a different orientation might have also misaligned the objective. Did you identify the screw holes to align them back in the same order? If not, you may want to rotate the objective 120° ( if it is a 3 hole assembly, more or less to match the amount of screw holes there are) backwards and forwards to see if that may be part of the solution.


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#10 dwmedic

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Posted 06 November 2019 - 10:00 AM

Search Youtube for Skywatcher ST80 Mods & Tweaks, it sounds like yours is removed the same way. In the video he shows how to reassemble and to make sure the lenses are aligned correctly. Good Luck


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#11 AndresEsteban

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Posted 06 November 2019 - 11:55 PM

Cleaning is necessary at times.  And this often results in the need for disassembly/reassembly because liquid can get between the lenses. 

 

This looks like a misalignment/collimation issue and it will produce a soft image with a lot of color.   Seen it before.  Not sure how the retainer ring is on this one, but when you take it apart again, place the innermost objective in and make sure it seats properly on whatever the inner cell support ring is.  Then slip in the outer lens, turn it to vertical facing up and carefully tap the sides of the tube to get it to fall in place properly.  With some tapping around the sides of the OTA it should self-center on the spacers (unless they are askew and not uniformly positioned...had that with some cheap scopes.)  Then gently tighten the ring down and do some tapping as it starts to make contact.  Tighten down some more until slightly firm so you know it has fully seated, then back off a little until just loose, then tap again.  Then gently retighten so that it is just holding the objective with minor pressure. 

 

I don't want rattle, some swear by it, I don't, I learned to swear at it.  I used to do the "leave it loose so that it rattles" routine, but that was consistently giving me coma/collimation problems, so now I tighten slightly more at the end until there is no rattle.  If you see pinch during the star test, back off a little on the tightening.  The problem with rattle is the outer lens can move about, producing collimation/wedge/tilt errors in use.  No thanks, had enough of that.

After many years with refractors, which I love and I have deep respect, I've learned that tightening utill you hear the doublet kust "rattle" a little is not a good idea. Both elements are loose and I really don't like it because with time they get in the wrong position. So, a too much tightening will pinch the optics, too loose, misalignment. So yes, just the right tightening a notch after you don't hear the rattle anymore. That's the right tightening for me! So I totally agree with you!


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#12 leroybrown

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Posted 07 November 2019 - 03:42 AM

Search Youtube for Skywatcher ST80 Mods & Tweaks, it sounds like yours is removed the same way. In the video he shows how to reassemble and to make sure the lenses are aligned correctly. Good Luck

Yep - that is exactly how it fits and I think the locking ring was on too tight - I had to get pliers to remove it but have replaced it as per the video. Unfortunately not had chance to test it on an actual star yet as it seems to have been raining here for the last 24hrs (and mostly for about 2 months  now rolleyes.gif ). Thanks again to everyone - fingers crossed i have it sorted now.

 

Or course, having watched that video I want an ST80 as well now....


Edited by leroybrown, 07 November 2019 - 03:43 AM.

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#13 dwmedic

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Posted 07 November 2019 - 09:35 AM

Watch out telescope addiction is real and there is no known cure. lol.gif

 

Let us know if that fixed it.


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#14 leroybrown

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Posted 08 November 2019 - 03:20 AM

Will do as soon as we get a break in the weather. Telescope.gif cloudy.gif



#15 Nippon

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Posted 08 November 2019 - 08:39 AM

Did you just remove the cell by removing the three screws or did you also remove the two lens elements from the cell?


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#16 Mitrovarr

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Posted 08 November 2019 - 03:00 PM

Given the image. I'd say your lens is badly tilted. I'd guess it caught on the side and stuck in a tilted position when it was being reinstalled. Try disassembling it to the same degree as before and reinstalling it, this time being sure it goes fully back into position.

#17 Nippon

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Posted 08 November 2019 - 05:10 PM

Is everyone assuming the OP took the lenses out of the cell. If he just took the cell off to clean the rear element and the cell attaches with three screws than it could be the cell was just not reinstalled squared on to the tube. There is usually enough play in screw holes to tilt a cell. Tele Vue scope's cells are squared on by that very method.


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#18 leroybrown

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Posted 10 November 2019 - 02:29 PM

I originally just took the cell off for cleaning and it was after this that I noticed the problem. I hadn't looked through the scope for at least 5 years and it was stored in my garage probably in the same position. I've now taken the lenses out and put them back as per the video recommended in a previous post - it hasn't solved the problem. I've rotated the cell to the three possible positions and the image just rotates as well so either one of the lenses is tilted even though it looks ok or I'm wondering if it's possible for the glass to degrade?



#19 SteveG

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Posted 10 November 2019 - 05:45 PM

I originally just took the cell off for cleaning and it was after this that I noticed the problem. I hadn't looked through the scope for at least 5 years and it was stored in my garage probably in the same position. I've now taken the lenses out and put them back as per the video recommended in a previous post - it hasn't solved the problem. I've rotated the cell to the three possible positions and the image just rotates as well so either one of the lenses is tilted even though it looks ok or I'm wondering if it's possible for the glass to degrade?

No, not possible unless it’s covered with fungus.

 

I think you might consider flipping lenses around at this point. It is possible that one or more are backwards.



#20 Redbetter

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Posted 14 November 2019 - 01:02 AM

If star images were uniform before (apparently years before?) and are showing a large degree of comatic collimation error since it was cleaned (and apparently before the retainer was even removed for the second round?), then something definitely moved during the interim.  Since the problem rotates with the cell it is in the objective or cell rather than any sort of focuser tilt, etc. 

 

I assume this scope has spacers and that they remain near the edges and are still evenly distributed relative to one another.  If one has moved out or in, or shifted considerably to one side or another, or if it has a fold or some other defect that has altered its thickness then it might give you some strange results.  They are typically adhered to the lower/thicker lens. 

 

You can get an idea of which lens is the source of the problem by rotating one while leaving the other in place.  For instance, you could rotate the outer lens 90 degrees or so and see if the elongation/coma rotate.  If the detect rotates then you know the problem is something related to the outer lens.  If the defect's orientation remains unchanged then the issue is with the bottom lens:  either the way it sits in the cell or the way the top lens is centered or tilted relative to it.


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#21 leroybrown

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Posted 14 November 2019 - 09:19 AM

If star images were uniform before (apparently years before?) and are showing a large degree of comatic collimation error since it was cleaned (and apparently before the retainer was even removed for the second round?), then something definitely moved during the interim.  Since the problem rotates with the cell it is in the objective or cell rather than any sort of focuser tilt, etc. 

 

I assume this scope has spacers and that they remain near the edges and are still evenly distributed relative to one another.  If one has moved out or in, or shifted considerably to one side or another, or if it has a fold or some other defect that has altered its thickness then it might give you some strange results.  They are typically adhered to the lower/thicker lens. 

 

You can get an idea of which lens is the source of the problem by rotating one while leaving the other in place.  For instance, you could rotate the outer lens 90 degrees or so and see if the elongation/coma rotate.  If the detect rotates then you know the problem is something related to the outer lens.  If the defect's orientation remains unchanged then the issue is with the bottom lens:  either the way it sits in the cell or the way the top lens is centered or tilted relative to it.

I think I'd had pretty much reached this conclusion (in fact only came back now to post and voice these ideas) - and yes, one of the spacers has indeed moved. Looks like I have something else to try now! Thanks for this - very useful as I hadn't considered rotating just one of them.

 

Thinking about it, it'll be quite a detailed process cos I have to wait for a clear night, then zero in on Polaris (cos it stays in place, and is high up which removes the possibility of a lens falling out) whilst ensuring I can reach both the objective and the eyepiece comfortably (without too much stooping for my bad back). Hopefully re positioning the spacers will sort the issue and I won't need to do much in the way of rotating the lens. Still cheaper than a new scope tho!!


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