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Need help with totally off collimation on my SCT

Cassegrain Collimation DIY Equipment SCT Celestron
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#1 dudleyjohn

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Posted 13 September 2021 - 06:58 PM

I haven't had my CPC1100 out for quite a while and had to remove the corrector plate to get some toys my grandson dropped through the eyepiece opening. After putting it back together, I couldn't get it to focus on anything. Just got a wedge of light when looking at Jupiter, for instance. I tried to collimate it using Robert Piekiel's book on the subject, but can't get it anywhere near the ballpark. I have no reason to think the primary is out of alignment, and a visual check from the front seems to verify this. I set up an artificial star over 100 feet away and tried collimating on it, but after backing out 2 of the adjustment knobs all the way (so they were completely out of the thread), I still couldn't even get an out of focus doughnut. The images when I try to focus on anything show a LOT of astigmatism. When I defocus the artificial star, I just get a pair of parabolas that I can overlap by adjusting the focus. Both parabolas are pointing so they are opening to the lower left when viewed through my diagonal. I've tried doing this with several eyepieces of varying diameter. I can't seem to make these parabolas change in any way by adjusting the collimation knobs. The only way I can move the parabolas is by turning the focus knob.  BTW: When I put the corrector plate back in, I was careful to place the cork shims in 4 positions at the 12, 3, 6, and 9 o'clock locations so it would be centered. I've never separated the secondary mirror assembly from the corrector plate. The etched number on the corrector plate is at the 3 o'clock position.

 

Does anyone have a clue as to what is going wrong here? Is there some way for me to get the secondary collimation knobs set so they are at a known position so I know which direction I need to go to get to correct collimation?

 

I'm at my wits end trying to figure this out. Any help would be greatly appreciated.


Edited by dudleyjohn, 13 September 2021 - 06:59 PM.

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#2 stevep

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Posted 13 September 2021 - 07:10 PM

Have you checked to see if maybe something is lodged in the baffle tube, small toy maybe,

 

Steve


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#3 dudleyjohn

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Posted 13 September 2021 - 07:22 PM

I can't see anything inside that isn't supposed to be there. I had the corrector plate out and was able to see all over inside. When I take the visual back off the scope and look through it, I don't see any foreign objects. I didn't see anywhere else that looked like it could harbor a toy or anything.



#4 dudleyjohn

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Posted 13 September 2021 - 07:24 PM

I could take the corrector plate off again and inspect the secondary assembly, but I'm not sure what to look for.



#5 cuzimthedad

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Posted 13 September 2021 - 07:55 PM

Moving to Cats & Casses...


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#6 Creedence

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Posted 13 September 2021 - 08:12 PM

You mentioned visually inspecting from the front, but it's a thread worth pulling- have you taken a couple steps back and gotten rough alignment through the front of the OTA?  There are guides available online if you're not familiar with the technique, but it's a good way to at least get the black dot to fall somewhere on the unfocused star image.  Long story short (and forgive me if you're already familiar), take a few steps back, look down the OTA and adjust the collimation screws until things appear to be centered up from that vantage point.  Then you can align things as normal through the eyepiece.  If things are grossly off, starting at the eyepiece can be difficult because you don't have a central obstruction visible to start from.


Edited by Creedence, 13 September 2021 - 08:17 PM.

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#7 Scope2

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Posted 13 September 2021 - 08:13 PM

Hi John,

 

   Sorry to hear about the toys in the Telescope. before you removed the corrector plate

   did you happen to mark its original position, SCT's are very sensitive and are put into

   proper orientation at the factory. Maybe there is some kind of mark somewhere along the 

   corrector edge that will mate with corresponding mark on the tube. 

 

                                              Best Regards.

 

                                                     Pat.


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#8 cookjaiii

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Posted 13 September 2021 - 08:16 PM

I'm concerned that you removed two of three collimating screws.  Everything I have read says to never remove more than one at a time, and that is only if you are replacing the screws with Bob's knobs or some other kind of screw.  The last thing you want is to have the secondary rotate relative to the corrector plate. 

 

Here's a link describing an alternate method looking at the front of the scope, described by Creedence above.  This should get you close so you can fine tune with a star..

 

Good luck.


Edited by cookjaiii, 13 September 2021 - 08:18 PM.

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#9 PrestonE

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Posted 13 September 2021 - 08:19 PM

Following


Edited by PrestonE, 13 September 2021 - 08:20 PM.

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#10 VNA

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Posted 13 September 2021 - 08:28 PM

hello, make sure the mark on the side of the corrector plate matches with the mark on the actual tube. 

Good luck

I should add that the marks are in "flaky" white paint, and if it gone and you nave not cleaned the plate you may be able to look at the faint marks that were left.

 

I envy you with your grand kids, but kids or grand kids, no matter how cute they are should not be left around scopes--I am sure you knew that already!  ;- )


Edited by VNA, 13 September 2021 - 08:38 PM.

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#11 dudleyjohn

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Posted 13 September 2021 - 09:30 PM

Thanks for all the helpful comments. I didn't mark the shims when I took out the corrector plate. At the time, I didn't even know there were shims. I also didn't note where the etched number on the edge of the corrector plate was. I followed Robert Piekiel's directions on visually inspecting the OTA from the front to determine the primary's alignment. Tomorrow, I guess I'm going to have to remove the primary and look for the position marker on the back of it. Don't know why Celestron made it so difficult. At least I'll learn more about my equipment. I've always been leery of doing anything with the primary, but I guess I don't have a choice now.



#12 dudleyjohn

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Posted 14 September 2021 - 03:09 PM

I tried to get the primary off my CPC1100 today, but ran into a snag. After getting the corrector plate off, I went to the first step in getting the primary off: removing the focus knob. I can't find any way to get it off! According to the directions I had, there was supposed to be a set screw for me to loosen. There was no set screw I could find. I know there were many versions of this scope. I don't know which one I have. I bought it second hand and can't even remember what year it was. Any ideas?


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#13 davidc135

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Posted 14 September 2021 - 05:21 PM

Before you try to remove the primary and if the corrector/secondary assembly is free I'd inspect the secondary mirror and check to see that it is nearly concentric with it's baffle. ie The gap surrounding the 2ndry mirror and baffle should be as uniform as possible. Adjust the collimating screws to accomplish this.

 

I wonder that the collimation was so hopelessly out that it was difficult to get your bearings and just doing the above by eye will get you sufficiently close so that ordinary collimation can take over with the corrector reinstalled.

 

It may turn out that the primary position doesn't matter. I could be wrong but I'd collimate it as it is and if the imagery is poor the worst will be that you have to retrace your steps. I'd really leave the primary alone for now.

 

David


Edited by davidc135, 14 September 2021 - 05:30 PM.

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#14 Old Man

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Posted 14 September 2021 - 06:01 PM

Before you try to remove the primary and if the corrector/secondary assembly is free I'd inspect the secondary mirror and check to see that it is nearly concentric with it's baffle. ie The gap surrounding the 2ndry mirror and baffle should be as uniform as possible. Adjust the collimating screws to accomplish this.

 

I wonder that the collimation was so hopelessly out that it was difficult to get your bearings and just doing the above by eye will get you sufficiently close so that ordinary collimation can take over with the corrector reinstalled.

 

It may turn out that the primary position doesn't matter. I could be wrong but I'd collimate it as it is and if the imagery is poor the worst will be that you have to retrace your steps. I'd really leave the primary alone for now.

 

David

Most definitely THIS!!!!!!!!!!!


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#15 Bean614

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Posted 14 September 2021 - 07:15 PM

Thanks for all the helpful comments. I didn't mark the shims when I took out the corrector plate. At the time, I didn't even know there were shims. I also didn't note where the etched number on the edge of the corrector plate was. I followed Robert Piekiel's directions on visually inspecting the OTA from the front to determine the primary's alignment. Tomorrow, I guess I'm going to have to remove the primary and look for the position marker on the back of it. Don't know why Celestron made it so difficult. At least I'll learn more about my equipment. I've always been leery of doing anything with the primary, but I guess I don't have a choice now.

"Don't know why Celestron made it so difficult"...???

They didn't,  but.... there are steps to follow, and guides to be read, Before taking things apart.  


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#16 KTAZ

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Posted 14 September 2021 - 07:32 PM

You shouldn’t be playing with the primary. That is a desperation move.

 

What is the result of the inspection of the secondary? Was it damaged? Did you follow the Casady method of doing a rough alignment before attempting to collimate?

 

Celestron will handle it for about $250 plus shipping.


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#17 dudleyjohn

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Posted 14 September 2021 - 09:09 PM

I did as advised and looked at the secondary to make sure it was as level as possible. I discovered that it was tilted a lot and was off center. I centered it as much as I could and leveled it as well as I could judge it. At my next opportunity, I'll try collimating again. I've got an artificial star setup, so I can try it during the day. This CPC1100 is F10. I'm setting up the star at about 120 feet. Is that far enough? I think I read somewhere that it has to be at least 100 feet away.


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#18 dudleyjohn

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Posted 14 September 2021 - 09:11 PM

By the way: Is it possible for the secondary to come off its center post? This scope doesn't have a center screw to hold it in place. I think it's just a plastic post that the secondary sits on or something.



#19 dudleyjohn

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Posted 14 September 2021 - 09:21 PM

You shouldn’t be playing with the primary. That is a desperation move.

 

What is the result of the inspection of the secondary? Was it damaged? Did you follow the Casady method of doing a rough alignment before attempting to collimate?

 

Celestron will handle it for about $250 plus shipping.

Thanks for the heads up about the Casady method. I looked it up. I did something similar, but not quite as exacting. I'll try this tomorrow. The secondary doesn't appear to be damaged, but it was off-center and tilted way too much.


Edited by dudleyjohn, 14 September 2021 - 09:25 PM.


#20 dudleyjohn

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Posted 14 September 2021 - 09:24 PM

hello, make sure the mark on the side of the corrector plate matches with the mark on the actual tube. 

Good luck

I should add that the marks are in "flaky" white paint, and if it gone and you nave not cleaned the plate you may be able to look at the faint marks that were left.

 

I envy you with your grand kids, but kids or grand kids, no matter how cute they are should not be left around scopes--I am sure you knew that already!  ;- )

Lol. My grandson was 3 years old. I don't know how he figured out a way to unscrew the eyepiece. I guess I should have hidden my robot-like telescope in a closet. The little guy is as smart as a whip. I just hope he shows some interest in astronomy before I'm too old to help him out.



#21 lambermo

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Posted 15 September 2021 - 04:33 AM

Here's a great tool that you can build/cut yourself, or have made or even buy these days to collimate an SCT : a tri-bahtinov mask. See https://www.cloudyni...n-and-focusing/


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#22 dudleyjohn

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Posted 15 September 2021 - 08:13 AM

Here's a great tool that you can build/cut yourself, or have made or even buy these days to collimate an SCT : a tri-bahtinov mask. See https://www.cloudyni...n-and-focusing/

Great information. Could I achieve the same effect by rotating my bahtinov mask 120 degrees between collimations?



#23 lambermo

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Posted 15 September 2021 - 03:13 PM

Great information. Could I achieve the same effect by rotating my bahtinov mask 120 degrees between collimations?

No, but you can use your regular 'single' bahtinov mask for the initial focussing, star in the center, then swap the mask for the tri-bahtinov mask and cover 2/3 of it, adjust the opposite collimation screw, rotate the cover 120 degrees, next screw, and again for the last. Then refocus and recheck.


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#24 dudleyjohn

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Posted 16 September 2021 - 11:34 PM

I haven't been able to get out to try this stuff lately. It's been raining and there's been construction going on back there. I'll update when I can get back out there.



#25 dudleyjohn

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Posted 17 September 2021 - 02:31 PM

I still haven't been able to do any outdoor work, so I've been trolling the internet looking for resources. I found a great site on astrophotography that incudes a page on collimating SCTs with a laser collimator. It's not a straightforward process.

 

http://nightskyimage...collimation.htm


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