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CPC 800 secondary and corrector alignment

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#1 GoFish

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Posted 17 December 2016 - 04:20 PM

I new-to-me CPC 800 arrived here with an incredibly dirty (on the inside) corrector.  It had a thick-ish film and several spots/rings formed around some kind of particulates.  Shipping it to Celestron was considered, as was shipping it back to the seller.  But in the end, I decided to clean the corrector myself.  Shipping these scopes around the country is neither fun nor inexpensive.

 

Since I was going to be removing the corrector, and the secondary holder was already loose and spinning, I read everything I could find about alignment of correctors and secondaries.  

 

Wow!  There are a lot of different and conflicting reports about alignment marks and shimming.  

 

So I thought I would document for posterity what I actually found on this 2015-era CPC 800:

 

  • There was no etching or serial number anywhere to be found on the corrector plate.  I looked carefully and closely more than one time.  Nothing to indicate proper rotational alignment.
  • There were no lines, marks, etc. on the secondary or the secondary holder to indicate proper rotational alignment.  There is, however, a set screw at one location.
  • There were fibrous (thick paper) shims (dark gray colored material) in 2 locations:
    • Between the glass and the retainer ring.  On my scope, this (single) shim went from 3 o'clock to 8 o'clock.  It was not glued on -- just held in place by clamping action.  The width of this shim was such that it was in contact with the aluminum front cell and also overlapped the edge of the glass.  It's purpose is a mystery to me, since it does not alter the distance between the glass and the mating surface of the cell.
    • Glued to the inside of the plastic retainer ring were 2 sections of shim.  Together these shims covered almost the entire circumference of the retainer ring.  Their purpose seems to be to provide a uniform cushion where the ring engages the glass.
  • The shims did not seem to me to be providing any kind of optical alignment purpose.
  • There are 4 nylon-tipped set screws around the circumference of the cell.  The purpose of these set screws is to center the corrector and secondary with respect to the primary.  If you look at a picture of a CPC front cell from the outside, there are 4 locations around the perimeter where it seems a screw is missing.  These set screws are in those "empty" holes and are hex socket screws (2 mm hex key) with nylon tips.
  • I was surprised how far off-center the corrector was positioned by these 4 set screws.  The set screw at the 7 o'clock position was flush with the inside rim of the cell, so the edge of the glass is in contact with the aluminum for much of the lower quarter of the "clock".  At the 1 o'clock position there was a gap > 1/8" as established by the set screw at that location.

I marked the corrector at the 3 o'clock position before disassembly, and noted and marked the location of the partial shim.

 

The secondary was loose and spinning, so the horse had already left the barn as far as marking the secondary goes.

 

After successfully cleaning the corrector, I reassembled.  I put the corrector back where it was before disassembly.  I rotated the secondary so that its set screw was at 3 o'clock, which is opposite the focuser on the CPC 800.  Information on the Celestron web pages suggest that the 3 o'clock position (opposite the focuser) is the master reference for the optics on this scope.  Hopefully positioning the secondary so that the set screw is aligned this way was the right thing to do!

 

I was able to collimate using my Hubble artificial star, and got nice looking diffraction patterns inside, outside, and at focus.  So I am cautiously optimistic that the scope is going to perform well outdoors, despite the initial problems of a filmed-over corrector and spinning secondary.

 

(Photo shows the film on the inside of the corrector when I received it.  The particulates that are circled are on the inside, too, and appeared to be small pieces of aluminum, probably left over from tapping the holes in the cell.  Disappointed to see how much swarf there was inside the OTA.)

 

 

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#2 leveye

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Posted 17 December 2016 - 05:46 PM

No pictures of it nice and clean just for bragging rights? 



#3 sebring96hbg

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Posted 17 December 2016 - 09:49 PM

I have a C9.25 which I bought new within the last year.  I contacted Celestron this past spring similarly about particulates (more like machining shavings) and an off-gas film on the inside surface of the corrector plate.  I was told not to worry about it and be more concerned about what could happen in shipping the OTA back and forth.

 

I removed the plate and cleaned it.  Like yours, there are no factory marks on the edge of the corrector.  So, I first marked it using two short pieces of electrical tape offset from each other. I should have left the tape in place which would have been otherwise concealed by the retaining ring for the corrector plate.  However, I removed them.

 

Later in the week, bored by the clouds, I wanted to remove the secondary mirror after some Hyperstar research to inspect the mirror and to see how easy it would be.  So, I tried to remove the retaining ring for the secondary holder.  In so doing, I rotated the corrector plate as well.  Ugh.  I was kicking myself for removing the tape.

 

In my correspondence with Celestron, I sent pics which I used to reorient the corrector plate.  The key thing for my OTA is that the bottom screw of the upside down triangle of collimation screws is in the 6 o'clock position.  I so rotated the corrector plate and later performed a collimation.  Subsequent viewing of Saturn was better than ever-like 1080p vs. 480i.

 

Fast forward to an hour ago, I tried the secondary retaining ring again.  This time, I placed my fingers in the slots for the collimation screws (mine has the rotating cover) to provide counter-resistance as I successfully removed the retaining ring.  Woot!

 

The big finish to my situation is that the slot for the set screw on the secondary cell is at the 2 o'clock position, not 3 o'clock.  Per Celestron, the positioning may vary from OTA to OTA made after 2009.  

 

http://www.celestron...econdary-mirror



#4 GoFish

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Posted 17 December 2016 - 11:00 PM

No pictures of it nice and clean just for bragging rights? 

I guess it would be only right  :grin:

 

I'll try and get a good image and post it here. Truthfully, though, there is not much to see on a clean corrector. It becomes sorta transparent. 



#5 GoFish

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Posted 17 December 2016 - 11:08 PM

I have a C9.25 which I bought new within the last year.  I contacted Celestron this past spring similarly about particulates (more like machining shavings) and an off-gas film on the inside surface of the corrector plate.  I was told not to worry about it and be more concerned about what could happen in shipping the OTA back and forth.

 

I removed the plate and cleaned it.  Like yours, there are no factory marks on the edge of the corrector.  So, I first marked it using two short pieces of electrical tape offset from each other. I should have left the tape in place which would have been otherwise concealed by the retaining ring for the corrector plate.  However, I removed them.

 

Later in the week, bored by the clouds, I wanted to remove the secondary mirror after some Hyperstar research to inspect the mirror and to see how easy it would be.  So, I tried to remove the retaining ring for the secondary holder.  In so doing, I rotated the corrector plate as well.  Ugh.  I was kicking myself for removing the tape.

 

In my correspondence with Celestron, I sent pics which I used to reorient the corrector plate.  The key thing for my OTA is that the bottom screw of the upside down triangle of collimation screws is in the 6 o'clock position.  I so rotated the corrector plate and later performed a collimation.  Subsequent viewing of Saturn was better than ever-like 1080p vs. 480i.

 

Fast forward to an hour ago, I tried the secondary retaining ring again.  This time, I placed my fingers in the slots for the collimation screws (mine has the rotating cover) to provide counter-resistance as I successfully removed the retaining ring.  Woot!

 

The big finish to my situation is that the slot for the set screw on the secondary cell is at the 2 o'clock position, not 3 o'clock.  Per Celestron, the positioning may vary from OTA to OTA made after 2009.  

 

http://www.celestron...econdary-mirror

 

Where is your focus knob in relation to your triangle of screws?  My focus knob is at 9 o'clock, viewed from the front. That puts 3 o'clock opposite the knob.

 

It it seems to me that the screw pattern is a funny index marker. There are 3 possible positions of the secondary that results in a screw at 12 o'clock, aren't there?



#6 sebring96hbg

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Posted 18 December 2016 - 12:35 AM

 

I have a C9.25 which I bought new within the last year.  I contacted Celestron this past spring similarly about particulates (more like machining shavings) and an off-gas film on the inside surface of the corrector plate.  I was told not to worry about it and be more concerned about what could happen in shipping the OTA back and forth.

 

I removed the plate and cleaned it.  Like yours, there are no factory marks on the edge of the corrector.  So, I first marked it using two short pieces of electrical tape offset from each other. I should have left the tape in place which would have been otherwise concealed by the retaining ring for the corrector plate.  However, I removed them.

 

Later in the week, bored by the clouds, I wanted to remove the secondary mirror after some Hyperstar research to inspect the mirror and to see how easy it would be.  So, I tried to remove the retaining ring for the secondary holder.  In so doing, I rotated the corrector plate as well.  Ugh.  I was kicking myself for removing the tape.

 

In my correspondence with Celestron, I sent pics which I used to reorient the corrector plate.  The key thing for my OTA is that the bottom screw of the upside down triangle of collimation screws is in the 6 o'clock position.  I so rotated the corrector plate and later performed a collimation.  Subsequent viewing of Saturn was better than ever-like 1080p vs. 480i.

 

Fast forward to an hour ago, I tried the secondary retaining ring again.  This time, I placed my fingers in the slots for the collimation screws (mine has the rotating cover) to provide counter-resistance as I successfully removed the retaining ring.  Woot!

 

The big finish to my situation is that the slot for the set screw on the secondary cell is at the 2 o'clock position, not 3 o'clock.  Per Celestron, the positioning may vary from OTA to OTA made after 2009.  

 

http://www.celestron...econdary-mirror

 

Where is your focus knob in relation to your triangle of screws?  My focus knob is at 9 o'clock, viewed from the front. That puts 3 o'clock opposite the knob.

 

It it seems to me that the screw pattern is a funny index marker. There are 3 possible positions of the secondary that results in a screw at 12 o'clock, aren't there?

 

My focus knob is at 9 o'clock in relation to the front. I also had a scuff mark on the secondary cell which helped figure which screw was relevant.

 

OTA%202%20annotated.jpg



#7 GoFish

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Posted 18 December 2016 - 01:20 AM

One clean collector corrector!

 

 

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Edited by GoFish, 18 December 2016 - 07:31 PM.


#8 sebring96hbg

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Posted 18 December 2016 - 08:30 AM

Very nice!  

 

When I did mine, I was sweating bullets.  However, once I saw the results, I felt so much better.




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