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C8 corrector plate orientation for collimation.

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8 replies to this topic

#1 MikeECha

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Posted 05 July 2021 - 01:18 PM

Hello there,

 

I purchased a second hand C8 Ultima (blue with bright star coating) that I had to collimate. The corrector plate was oriented such that the Celestron logo and a little rectangle for what could have been the serial number label were not horizontal as I would have expected to be when new. They were oriented about 30-40 degrees CCW.

 

Since as a read, corrector plate orientation seem to be important I wonder if those of you who still have the plate in place from factory or have put it back in the same orientation as from factory can confirm if the logo is horizontal when the scope is in home position. 

 

I am curious to know if that can be used for reference to orient the corrector plate. I looked around the edge of the plate and saw no marks when I removed mine for cleaning. 


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#2 ngc7319_20

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Posted 05 July 2021 - 02:08 PM

There was some general discussion recently here.

 

You can star test the scope for astigmatism using a star high in the sky.  If there is no astigmatism, then the rotation is ok.  If you see astigmatism, you can try rotating the corrector in 1/4 or 1/8 turn steps and see if that makes some improvement. I have seen some Celestrons where the "factory" orientation was not the best.  I am not sure they do much work testing various rotations at the factory -- probably if it passes some minimum standard they declare it "ok", mark it, and move on.  (Be wary of any astigmatism in your star diagonal.)


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#3 MikeECha

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Posted 05 July 2021 - 04:51 PM

There was some general discussion recently here.

 

You can star test the scope for astigmatism using a star high in the sky.  If there is no astigmatism, then the rotation is ok.  If you see astigmatism, you can try rotating the corrector in 1/4 or 1/8 turn steps and see if that makes some improvement. I have seen some Celestrons where the "factory" orientation was not the best.  I am not sure they do much work testing various rotations at the factory -- probably if it passes some minimum standard they declare it "ok", mark it, and move on.  (Be wary of any astigmatism in your star diagonal.)

Ok will take a look at the link

 

Thanks



#4 charlieb123

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Posted 06 July 2021 - 08:24 AM

When I removed my collector plate there was an orientation marking from what I believe was the factory.
I'm at least the 2nd owner so I may be wrong but certainly seemed that way.

 

I made my own 2 markings, cleaned and put back together.


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#5 Benschop

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Posted 22 July 2021 - 12:26 AM

If the Celestron logo / rectangle on the secondary holder is not horizontal, that does not necessarily mean that the corrector itself has been rotated.

As the scope ages, the secondary holder sometimes is not as tight as originally within the corrector plate hole, due to gasket shrink etc.

It is possible that someone rotated the secondary without turning the corrector plate itself.

 

The thread linked below contains alignment details.

https://www.cloudyni...orrector-plate/


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#6 MikeECha

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Posted 22 July 2021 - 04:24 PM

If the Celestron logo / rectangle on the secondary holder is not horizontal, that does not necessarily mean that the corrector itself has been rotated.

As the scope ages, the secondary holder sometimes is not as tight as originally within the corrector plate hole, due to gasket shrink etc.

It is possible that someone rotated the secondary without turning the corrector plate itself.

 

The thread linked below contains alignment details.

https://www.cloudyni...orrector-plate/

Benschop,

 

Thank you I did not know that the secondary can be rotated independently. That is a good data point to know.


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#7 insertnamehere

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Posted 22 July 2021 - 07:58 PM

I think the secondary holder rotation may be to correctly orient the secondary mirror.

 

In my case I also had a rotated secondary holder. I had the scope for 30 years, from new.

About a year ago I stripped it down to re-grease the mirror baffle and to do other things.

When I removed the secondary mirror I saw the alignment mark on the back  of the mirror.

 

The small angle of the alignment mark on the secondary was the same as the angle of the secondary holder.

When this C8 was made the secondary was matched to the primary.

In my case, it looks to me like the secondary holder is rotated to align the primary and secondary mirror correctly.

 

post-337912-0-94152700-1597805628.jpg

 

Chris


Edited by insertnamehere, 22 July 2021 - 08:01 PM.

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#8 MikeECha

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Posted 23 July 2021 - 09:07 AM

Chris

Good info thank you

#9 rmollise

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Posted 23 July 2021 - 03:52 PM

Hello there,

 

I purchased a second hand C8 Ultima (blue with bright star coating) that I had to collimate. The corrector plate was oriented such that the Celestron logo and a little rectangle for what could have been the serial number label were not horizontal as I would have expected to be when new. They were oriented about 30-40 degrees CCW.

 

Since as a read, corrector plate orientation seem to be important I wonder if those of you who still have the plate in place from factory or have put it back in the same orientation as from factory can confirm if the logo is horizontal when the scope is in home position. 

 

I am curious to know if that can be used for reference to orient the corrector plate. I looked around the edge of the plate and saw no marks when I removed mine for cleaning. 

 

 

There is a small engraved serial number near the edge of the corrector, which should be at 3 o'clock. If it's there, it's possible the secondary assembly has rotated.


Edited by rmollise, 23 July 2021 - 03:52 PM.



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