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Troubleshooting a C11

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#1 cshine

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Posted 03 June 2019 - 06:06 PM

I currently have on loan an older C11 XLT. I was pretty excited about trying it out for my EAA and planetary imaging endeavors as it's a big upgrade on the C8 I've been using for the last year. Unfortunately it is not performing as well as I'd hoped and I'm trying to figure out if I'm doing something wrong or if there is an issue with the optics.. I'm a relative newbie but I'm comfortable with collimation basics and have used metaguide to get my C8 dialed in quite nicely. 

 

Two things jump out as a little weird to me, hopefully someone can steer me straight here or give some general troubleshooting pointers. 

 

Firstly - I can get the collimation close to good but found that one of the adjustment screws is fully snug while the other 2 are quite loose. I don't remember getting into that situation with the C8. It doesn't feel right and I can't fine tune the collimation. Should I "hit reset" and loosen all 3 and start over?

 

Second - even with the collimation close to good I can't get any distinct diffraction rings either side of focus. The are portions of rings in places, example attached. Tube had been outside for hours, no sign of thermal currents. Seeing was quite good and I tested on a few stars close to zenith. The Bahtinov pattern was also somewhat indistinct, not nearly as well defined as I see on the C8.

 

Thanks in advance.

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#2 David_Ritter

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Posted 03 June 2019 - 06:26 PM

Having two collimation screws that stay loose is a pretty strong indicator of a real problem. Can you take the secondary assembly out and carefully inspect the screws and secondary mounting plate? The screws should all be snug and they should firmly hold down the mirror on a central pivot point. If this is not possible, something is wrong.

 

I know of one problem that was similar to this. It was Meade that utterly refused to be collimated. Turned out that the secondary had come loose from its tape backing and was flopping around. Eventually, the mirror fell off. Apparently, a prior owner had installed collimation screws that were too long and they had pushed the mirror right off it's backing. So no matter what, it was not possible to collimate that particular scope and repeated attempts caused the tape to finally fail altogether and at that point the mirror then fell off.

 

Happily, the inside of that scope was felt lined and it was sitting horizontally when the mirror finally let go. The mirror fell on the felt and amazingly, it was completely undamaged. Replacing the tape and using screws of the proper length fixed it and the scope worked very well afterwards.


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#3 Chris MN

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Posted 03 June 2019 - 07:06 PM

I "second" Mr. Ritter. I have owned a C11 (several in fact) since 1992 (and I have dissected all of them).  Those collimation screws must be snug.  1/8 of a turn of those fine thread screws can throw collimation off.  Get that fixed first.

 

One night recently, stars didn't appear "right".  Something was "off" (but not by much. I'm starting to get picky in my old(er) age because I know the C11 can deliver great views).  Did a quick collimation check and yup, slightly off.  I turned one screw no more than 1/8 turn and BINGO!  Back to tack sharp.

 

You said the C11 didn't perform as well as you had hoped.  Can you be more specific? What were you hoping for and what did it deliver?  A C11 is a big step up from a C8 (I took that step in 1992!).

 

 

Chris N

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#4 Asbytec

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Posted 03 June 2019 - 07:11 PM

The image above is way too far from focus for accurate collimation. You are seeing an image of the secondary shadow centered on the primary. Collimation is not done looking at the image of the secondary on the primary, that only shows the secondary is centered on the primary and is not a collimation signature. Collimation is about making the optical axes (sic) aligned by centering the diffraction effects. Not the shadow of the secondary. Make sure the diffraction rings are concentric to the Poisson spot in the center of the diffraction pattern. To do that, you need to be much closer to focus and only a few diffraction rings visible. As close as seeing will permit a good look at the slightly defocused pattern. Then collimate it in focus if seeing permits. 


Edited by Asbytec, 03 June 2019 - 07:13 PM.

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#5 CHASLX200

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Posted 03 June 2019 - 07:40 PM

If it gets collimated and the image is still bad then it could be the optics. These scopes are all over the place from super sharp to total duds.



#6 cshine

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Posted 03 June 2019 - 10:02 PM

Having two collimation screws that stay loose is a pretty strong indicator of a real problem. Can you take the secondary assembly out and carefully inspect the screws and secondary mounting plate? The screws should all be snug and they should firmly hold down the mirror on a central pivot point. If this is not possible, something is wrong.

 

 

Thanks, I was afraid of that.. I'll need to check with the owner first but that sounds like a sensible approach. Glad that Meade turned out ok.

 

I "second" Mr. Ritter. I have owned a C11 (several in fact) since 1992 (and I have dissected all of them).  Those collimation screws must be snug.  1/8 of a turn of those fine thread screws can throw collimation off.  Get that fixed first.

 

One night recently, stars didn't appear "right".  Something was "off" (but not by much. I'm starting to get picky in my old(er) age because I know the C11 can deliver great views).  Did a quick collimation check and yup, slightly off.  I turned one screw no more than 1/8 turn and BINGO!  Back to tack sharp.

 

You said the C11 didn't perform as well as you had hoped.  Can you be more specific? What were you hoping for and what did it deliver?  A C11 is a big step up from a C8 (I took that step in 1992!).

 

 

Chris N

Cedar, MN

It's hard to describe it but I was expecting more sharpness and detail than I get in the C8, especially with the bigger planets and PNs. Hopefully addressing the apparent secondary issue will solve the softness I'm seeing right now. Thank you.



#7 Eddgie

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Posted 03 June 2019 - 10:09 PM

A possibility, and hopefully not the issue with your scope, but this is the main reason that this kind of condition exists (running out of collimation screw travel).

 

The SCTs use a pivot behind the back of the steel plate that the secondary mirror is mounted on.  This little pivot is how the scope is collimated. As you loosen or tighten the three screws, the steel disk see-saws on this pivot to tilt the mirror in the appropriate direction.

 

And this is what happens if the collimation is ever made too tight.  A lot of people think you can warp the secondary if you put to much pressure on the collimation screws but that steel plate is really thick.  What happens if you use too much pressure is that the secondary housing where the pivot contacts it cracks or deforms or the pivot breaks and you run out of travel in one direction before you can reach collimation. (newer SCTs have plastic secondary housings and these crack when the collimation screws are excessively tightened).

 

I am not saying that this is your problem, but what I am saying is that the symptoms are consistent with a deformed secondary housing or worn/deformed pivot.  

 

I would not be surprised if this was your problem, but my fingers are crossed for you that it is not.  Not that it is all that big a deal (it can be replaced) but it does require pulling the corrector and rebuilding the secondary housing. 


Edited by Eddgie, 03 June 2019 - 10:32 PM.


#8 cshine

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Posted 03 June 2019 - 10:17 PM

The image above is way too far from focus for accurate collimation. You are seeing an image of the secondary shadow centered on the primary. Collimation is not done looking at the image of the secondary on the primary, that only shows the secondary is centered on the primary and is not a collimation signature. Collimation is about making the optical axes (sic) aligned by centering the diffraction effects. Not the shadow of the secondary. Make sure the diffraction rings are concentric to the Poisson spot in the center of the diffraction pattern. To do that, you need to be much closer to focus and only a few diffraction rings visible. As close as seeing will permit a good look at the slightly defocused pattern. Then collimate it in focus if seeing permits. 

Thanks for this detail. I should probably clarify that I wasn't using that defocused-star for collimation, I used a progressively tighter focus for that and then tried to complete the accurate collimation with metaguide. That's when I ran into the "one tight, two loose" problem. The point I was trying to make with the image above is that the diffraction pattern looks really "muddy" to me, no clearly defined concentric rings at any point during de-focusing. I don't know what that means but I know I don't like it!   The fact that nobody here is saying "wow that looks wrong" means I'm probably over-analyzing this. 

 

 

If it gets collimated and the image is still bad then it could be the optics. These scopes are all over the place from super sharp to total duds.

I didn't know that actually, I always thought that the bigger stuff was built to a much higher standard. I guess like anything they are subject to misuse and abuse over the years too.



#9 Eddgie

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Posted 03 June 2019 - 10:21 PM

Here is a link that explains the problem that I described.  Again, I do not know your problem, but the symptom you give is consistent with a cracked or deformed secondary housing or a broken pivot (older SCTs used aluminum cans, and they would dimple. Newer ones use plastic, and they crack).

 

Anyway, here is a link that explains the condition:

 

https://translate.go...gspiegeleinheit



#10 Eddgie

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Posted 03 June 2019 - 10:27 PM

And the above image looks like 30 or 40 waves of defocus.  This is far to much to use for collimation.  This much defocus makes it insensitive. This is also why it is very dim.  Way to much defocus. 

 

Defocus about 10 waves (bright outer ring, two or three dim rings, bright inner ring, and then secondary shadow).

 

As dim as it is though, I am pretty sure I see a zonal error.  Not bad one and common on the C11. This is not causing your problem.  I just can't look at a star test and not see errors.  It is a curse. (30 Waves of defocus is in the ballpark of what you would use for a zonal error and that is why it is easy to see). 


Edited by Eddgie, 03 June 2019 - 10:45 PM.


#11 Asbytec

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Posted 04 June 2019 - 01:00 AM

Thanks for this detail. I should probably clarify that I wasn't using that defocused-star for collimation, I used a progressively tighter focus for that and then tried to complete the accurate collimation with metaguide.


Ok, I'm sorry if I missed that.

#12 cshine

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Posted 04 June 2019 - 01:54 PM

Here is a link that explains the problem that I described. Again, I do not know your problem, but the symptom you give is consistent with a cracked or deformed secondary housing or a broken pivot (older SCTs used aluminum cans, and they would dimple. Newer ones use plastic, and they crack).

Anyway, here is a link that explains the condition:

https://translate.go...gspiegeleinheit

Eddgie, thanks for this and the other guidance you posted, much appreciated. I do see some discoloration at that same spot in the center of the secondary housing which may indicate some previous stress. It doesn't look deformed as far as I can tell.

Is it a question of now removing it and inspecting visually or is there something to be gained from additional star testing here? I'm inclined to first try backing off all 3 screws and see if I end up in the same situation when recollimating from scratch.. if I do then next step would be removing the CP and secondary components.

I know Starizona sells replacement hyperstar-ready secondary kits, Celestron apparently does not.

#13 CHASLX200

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Posted 04 June 2019 - 06:32 PM

Thanks for this detail. I should probably clarify that I wasn't using that defocused-star for collimation, I used a progressively tighter focus for that and then tried to complete the accurate collimation with metaguide. That's when I ran into the "one tight, two loose" problem. The point I was trying to make with the image above is that the diffraction pattern looks really "muddy" to me, no clearly defined concentric rings at any point during de-focusing. I don't know what that means but I know I don't like it!   The fact that nobody here is saying "wow that looks wrong" means I'm probably over-analyzing this. 

 

 

I didn't know that actually, I always thought that the bigger stuff was built to a much higher standard. I guess like anything they are subject to misuse and abuse over the years too.

They all vary, don't matter on size or brand.  I have had so many SCT's.  The best ever was a 1984 C8 , and the worst i ever saw was a friends 1982 or 83 C11.



#14 David_Ritter

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Posted 04 June 2019 - 07:38 PM

If the owner agrees, I think the best thing would be to remove the secondary assembly and inspect it. Further testing might help, but it might also make things worse.

 

If two screws are freely moving right now, it's possible that some threads are stripped or damaged and the only thing left holding the mirror is the remaining third screw. Even if that's a remote possibility, if for any reason the secondary falls into the scope, the worst-case outcome means the scope is ruined.

 

And loosening all three screws too much will for sure allow the mirror to fall in. So, even if it were all good, it's still best to only loosen and tighten one screw at a time. Bob's knobs talks about this and they recommend that you replace the original screws one at a time if you install their thumbscrews.

 

It's also worth considering that the secondary assembly is actually designed to be user removable. That's to support customer installed Hyperstar imaging systems. So it's easy to do and fairly safe as long as care is taken to prevent damage to the exposed optical surfaces.

 

If I recall correctly the assembly itself is made with an outer flange so it cannot fall in the tube by accident. And the base is keyed so it can only be installed one way. Otherwise, its just held on by the big ring on the outside. Unscrew the ring and lift out the assembly. But that's just offhand from memory, so don't take my word for it. Best to read the manual first and maybe even watch a YouTube video first if it initially appears confusing.

 

Also a good idea to do it with the scope sitting horizontally with the front over a soft surface like a folded bath towel or large flat pillow or something like that. Just in case.

 

 

 

 



#15 Eddgie

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Posted 04 June 2019 - 08:55 PM

Well, the symptom is that there is someting wrong in there, and the easiest way to find out is pull the assembly out, open it up, and take a look see.

 

If you have not done it, I can tell you that it is not difficult, but of course you have to make sure the secondary goes back in the same orientation that it came out.  Celestron applies the correction of all errors in the system to the secondary mirror, so if it goes back in 120 degrees off, then if the secondary has had correction applied, it will affect the performance.


Edited by Eddgie, 04 June 2019 - 08:56 PM.


#16 Migwan

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Posted 06 June 2019 - 07:30 AM

Bought an 1100 used for a price too good to be true.   Initially had the very same issue with collimation screws.    What it ended up being was the corrector was not properly placed within the adjustable plastic centering pins.   Seems the previous owner had it out and not replaced it correctly.   One of the bottom pins was actually flattened.   The corrector was also in the wrong orientation relative the tube by about 30 degrees.  

 

After re-centering the corrector inside all the pins, all was well.   Can't say this is your problem, but as it was a simple fix, I kind of hope so.  Good luck. 

 

jd


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#17 cshine

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Posted 06 June 2019 - 10:47 PM

Bought an 1100 used for a price too good to be true.   Initially had the very same issue with collimation screws.    What it ended up being was the corrector was not properly placed within the adjustable plastic centering pins.   Seems the previous owner had it out and not replaced it correctly.   One of the bottom pins was actually flattened.   The corrector was also in the wrong orientation relative the tube by about 30 degrees.  

 

After re-centering the corrector inside all the pins, all was well.   Can't say this is your problem, but as it was a simple fix, I kind of hope so.  Good luck. 

 

jd

That would be a much easier fix alright, I'll check it out. This corrector looks to have been removed at some point alright, judging by the tape marks. Thanks

 

Well, the symptom is that there is someting wrong in there, and the easiest way to find out is pull the assembly out, open it up, and take a look see.

 

If you have not done it, I can tell you that it is not difficult, but of course you have to make sure the secondary goes back in the same orientation that it came out.  Celestron applies the correction of all errors in the system to the secondary mirror, so if it goes back in 120 degrees off, then if the secondary has had correction applied, it will affect the performance.

Understood, thank you.

 

If the owner agrees, I think the best thing would be to remove the secondary assembly and inspect it. Further testing might help, but it might also make things worse.

 

If two screws are freely moving right now, it's possible that some threads are stripped or damaged and the only thing left holding the mirror is the remaining third screw. Even if that's a remote possibility, if for any reason the secondary falls into the scope, the worst-case outcome means the scope is ruined.

 

And loosening all three screws too much will for sure allow the mirror to fall in. So, even if it were all good, it's still best to only loosen and tighten one screw at a time. Bob's knobs talks about this and they recommend that you replace the original screws one at a time if you install their thumbscrews.

 

It's also worth considering that the secondary assembly is actually designed to be user removable. That's to support customer installed Hyperstar imaging systems. So it's easy to do and fairly safe as long as care is taken to prevent damage to the exposed optical surfaces.

 

If I recall correctly the assembly itself is made with an outer flange so it cannot fall in the tube by accident. And the base is keyed so it can only be installed one way. Otherwise, its just held on by the big ring on the outside. Unscrew the ring and lift out the assembly. But that's just offhand from memory, so don't take my word for it. Best to read the manual first and maybe even watch a YouTube video first if it initially appears confusing.

 

Also a good idea to do it with the scope sitting horizontally with the front over a soft surface like a folded bath towel or large flat pillow or something like that. Just in case.

I'll have that conversation over the weekend. This C11 is older and not hyperstar ready unfortunately, that would have been easier. Thanks for the tips.


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#18 cshine

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Posted 12 June 2019 - 12:12 PM

...

And this is what happens if the collimation is ever made too tight.  A lot of people think you can warp the secondary if you put to much pressure on the collimation screws but that steel plate is really thick.  What happens if you use too much pressure is that the secondary housing where the pivot contacts it cracks or deforms or the pivot breaks and you run out of travel in one direction before you can reach collimation. (newer SCTs have plastic secondary housings and these crack when the collimation screws are excessively tightened).

...

This indeed appears to be the root cause. The exterior of the housing has visible stress lines around the center point along with a small hairline crack. It's also noticeably warped outward by ~5mm at center. 

 

C11.JPG

 

Googling around I see quite a few threads on this and there are various DIY solutions. I like the one you linked above, thanks again for highlighting that. The auto-translation from that German article is a little hard to follow so I'm wondering - what exactly is recommended as this replacement pivot point? Would I just use the end of the bolt that holds the new backing washers either side of the plastic surface or do I need to cap it with something rounded/pointed for a more linear pivot response? 

 

newPivot.JPG

 

Plan B would be to re-enforce the surface with Gorilla Glue epoxy as suggested in other threads, but I do like the "German Solution" a little better :)

 

Incidentally I have also reached out to Celestron to see if they unofficially offer a replacement housing and I might also run a Want-ad in the hopes that someone has one lying around after a Starizona upgrade.

 

Cheers



#19 Eddgie

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Posted 13 June 2019 - 11:04 PM

Yes, trying to get the part form Celestron is the best path forward.

If I recall, the above picture shows a big washer and a pan head screw. A hole is drilled in the can and the washer is placed inside. The screw is then inserted into the washer and hole, and out the front, and a washer and nut go on the outside.  Screw is 25mm long.

 

This may or may not work.  The problem here is that it limits the amount of tilt you can get. Note the the outside edge of the inner washer can contact the back of the secondary mounting plate much sooner than if the washer were not there.  

 

Since the components are cheap though, and the can is already junk, there is zero risk and the hardware is less than $1.50.


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#20 EuropaWill

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Posted 09 July 2019 - 02:55 PM

This indeed appears to be the root cause. The exterior of the housing has visible stress lines around the center point along with a small hairline crack. It's also noticeably warped outward by ~5mm at center. 

 

attachicon.gif C11.JPG

 

Googling around I see quite a few threads on this and there are various DIY solutions. I like the one you linked above, thanks again for highlighting that. The auto-translation from that German article is a little hard to follow so I'm wondering - what exactly is recommended as this replacement pivot point? Would I just use the end of the bolt that holds the new backing washers either side of the plastic surface or do I need to cap it with something rounded/pointed for a more linear pivot response? 

 

attachicon.gif newPivot.JPG

 

Plan B would be to re-enforce the surface with Gorilla Glue epoxy as suggested in other threads, but I do like the "German Solution" a little better smile.gif

 

Incidentally I have also reached out to Celestron to see if they unofficially offer a replacement housing and I might also run a Want-ad in the hopes that someone has one lying around after a Starizona upgrade.

 

Cheers

Did Celestron get back to you about the replacement housing?  If so how much do they charge for that? I may have a similar need as I've had some collimation issues recently and also noticed some hairline cracks and a slight pushing out on my plastic housing. 



#21 cshine

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Posted 09 July 2019 - 05:06 PM

They replied after about a week to say that they did not carry a replacement part. I was able to source a used one in good condition and it's working great.

#22 EuropaWill

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Posted 14 July 2019 - 11:47 PM

Wondering if anyone else has a C11 secondary housing just taking up space somewhere after they upgraded to the hyperstar kit.....Anyone? flowerred.gif  



#23 Orion68

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Posted 15 July 2019 - 09:03 PM

Having two collimation screws that stay loose is a pretty strong indicator of a real problem. Can you take the secondary assembly out and carefully inspect the screws and secondary mounting plate? The screws should all be snug and they should firmly hold down the mirror on a central pivot point. If this is not possible, something is wrong.

 

I know of one problem that was similar to this. It was Meade that utterly refused to be collimated. Turned out that the secondary had come loose from its tape backing and was flopping around. Eventually, the mirror fell off. Apparently, a prior owner had installed collimation screws that were too long and they had pushed the mirror right off it's backing. So no matter what, it was not possible to collimate that particular scope and repeated attempts caused the tape to finally fail altogether and at that point the mirror then fell off.

 

Happily, the inside of that scope was felt lined and it was sitting horizontally when the mirror finally let go. The mirror fell on the felt and amazingly, it was completely undamaged. Replacing the tape and using screws of the proper length fixed it and the scope worked very well afterwards.

I have a similar issue with my C11. In my case there is one really tight screw that does not want to tighten any more and two screws that are able to be adjusted. Wondering if the tight screw is all the way through the backing plate and pressing up against the back of the secondary mirror. My screws are allen head #6-32 by 1/2" long with sae threads. I replaced the original phillips head screws with these so that I could use an allen wrench for adjustments.

 

Does anyone know the thickness of the backing plate? The replacement screws are a tad longer than the original screws so now wondering if they are too long at 1/2".



#24 EuropaWill

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Posted 15 July 2019 - 09:19 PM

I have a similar issue with my C11. In my case there is one really tight screw that does not want to tighten any more and two screws that are able to be adjusted. Wondering if the tight screw is all the way through the backing plate and pressing up against the back of the secondary mirror. My screws are allen head #6-32 by 1/2" long with sae threads. I replaced the original phillips head screws with these so that I could use an allen wrench for adjustments.

 

Does anyone know the thickness of the backing plate? The replacement screws are a tad longer than the original screws so now wondering if they are too long at 1/2".

I'm being very careful here so please use your discretion if you follow my lead....but last night my scope became essentially un-collimatable with two of the screws super tight (one especially so) and one very loose.  The star test indicated I needed to loosen the lose screw and tighten the super tight one which was just not possible physically and it just wasn't making any sense so I did the intuitive thing which was to slowly loosen the other two screws while slightly tightening the very loose one knowing it might completely drop the mirror, but I was backing it out slowly and checking the star-test every minor adjustment. My goal was to re-establish some kind of balanced tension between the three screws to start again from scratch. I had the scope pointing up at Vega which was close to zenith. Which of course was the most risky position for it to be in since if the secondary mirror would drop it would drop the hardest in that orientation.  I was eventually able to get back to a good pattern though its not perfect and with the crack I can't expect it to really hold.  I'm reluctant to say do the same thing because you really  need to make small adjustments and check each iteration but I do think if any one screw is super tight, that should be reversed because its probably locking the mirror from moving. 


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#25 Orion68

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Posted 18 July 2019 - 08:34 PM

I'm being very careful here so please use your discretion if you follow my lead....but last night my scope became essentially un-collimatable with two of the screws super tight (one especially so) and one very loose.  The star test indicated I needed to loosen the lose screw and tighten the super tight one which was just not possible physically and it just wasn't making any sense so I did the intuitive thing which was to slowly loosen the other two screws while slightly tightening the very loose one knowing it might completely drop the mirror, but I was backing it out slowly and checking the star-test every minor adjustment. My goal was to re-establish some kind of balanced tension between the three screws to start again from scratch. I had the scope pointing up at Vega which was close to zenith. Which of course was the most risky position for it to be in since if the secondary mirror would drop it would drop the hardest in that orientation.  I was eventually able to get back to a good pattern though its not perfect and with the crack I can't expect it to really hold.  I'm reluctant to say do the same thing because you really  need to make small adjustments and check each iteration but I do think if any one screw is super tight, that should be reversed because its probably locking the mirror from moving. 

Thanks so much for your feedback. I'm going to try tonight to see if that one screw is truly locked up.




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