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Corrector plate placement - C8 Edge

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#1 CTYankee

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Posted 21 May 2024 - 09:43 AM

I have a C8 Edge, and would like to make sure the corrector plate is seated/placed properly. I have taken it apart to clean it before but can't remember if I replaced it properly (i.e., if I marked its position correctly before removing it, etc). So it could very well be significantly off of its correct position. And I am not sure if the cork spacers or outer plastic ring are placed correctly either. Can anyone assist?

 

I have done some reading of other posts on CN about how the corrector plate should be situated. These posts have been helpful, but some have fairly complex or time-consuming procedures on using a laser collimator, taking exact measurements with calipers to ensure correct distances, etc. A few posts mention an engraving on the top of the corrector plate that should be placed at the three o'clock position within the OTA, and one post mentioned that a mark on the secondary mirror must be placed opposite the focuser. One post mentioned rotating the corrector plate 45 or 90 degrees periodically, and repeatedly checking the optics after each rotation to find the sweet spot.

 

Bottom line- does anyone have any straight-forward, easy-to-follow guidance on how the corrector plate, cork spacers or plastic outer ring should be placed on the OTA? Is there a Celestron (or other) video tutorial or good pictorial written reference showing the proper basic orientation of the corrector plate, spacers, etc? I am sure there are some things that can easily be done (i.e., making sure the corrector plate engraving is at the 3 o'clock position, etc) without involving the use of a laser collimator or a time-intensive process; I just don't know where to find the info. 

 

Thanks in advance. 



#2 TOMDEY

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Posted 21 May 2024 - 12:39 PM

It's pretty much the same procedure as putting the engine back into your airplane, after you've decided to take it apart to clean it.    Tom

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#3 KTAZ

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Posted 21 May 2024 - 01:35 PM

Here is my advice if you really don't know what you have in hand; send it to Celestron for a cleaning and collimation.Yes, that will cost you a few Benjamin's (do people still say that?) but it will be correct when you get it back.

 

The Edge is more complex than your standard SCT. The additional lenses in the baffle tube require a more precise alignment of all the optics.

 

When I got my Edge, I had an issue when I removed the retaining ring for the secondary; the baffle inside the corrector came loose. I had to pull the corrector to snug it back down. I could now see that the corrector was offset several mm from one side to the other; so much so that the grub screw on one side was almost fully extended.

 

Had to be a mistake, right? I shoud instantly loosen up all those grubs, grab my calipers and center that corrector so my secondary is back in the precise middle ot the OTA, right?

 

Not so fast. I spoke with the customer service manager at Celestron in Torrance, CA. He said that it is very precisely centered at the factory during double-pass collimation; don't assume that your baffle tube is perfectly centered with your OTA. It is very common for it not to be. Very little impact to a standard SCT, but the edge requires a nice straight line from your secondary through those baffle tube optics.


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#4 TOMDEY

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Posted 21 May 2024 - 08:21 PM

Here is my advice if you really don't know what you have in hand; send it to Celestron for a cleaning and collimation. Yes, that will cost you a few Benjamin's (do people still say that?) but it will be correct when you get it back.

 

The Edge is more complex than your standard SCT. The additional lenses in the baffle tube require a more precise alignment of all the optics.

 

When I got my Edge, I had an issue when I removed the retaining ring for the secondary; the baffle inside the corrector came loose. I had to pull the corrector to snug it back down. I could now see that the corrector was offset several mm from one side to the other; so much so that the grub screw on one side was almost fully extended.

 

Had to be a mistake, right? I should instantly loosen up all those grubs, grab my calipers and center that corrector so my secondary is back in the precise middle of the OTA, right?

 

Not so fast. I spoke with the customer service manager at Celestron in Torrance, CA. He said that it is very precisely centered at the factory during double-pass collimation; don't assume that your baffle tube is perfectly centered with your OTA. It is very common for it not to be. Very little impact to a standard SCT, but the edge requires a nice straight line from your secondary through those baffle tube optics.

Yes! Double-pass AC (using a full-aperture precision optical flat mirror and interferometry, etc. from system focal surface) is the professional Gold Standard for alignments and certification. With those proper tools and expertise, and care --- the factory nails and locks in the best optical alignment, as well as certifying that performance specifications are met or better. That's how we build our space-based imagers. To the layman, a system may superficially appear to be non-nominal, completely unaware that the factory tech painstakingly tweaked the screws to optimize the optical centers of the elements, which are often noticeably different from the mechanical centers. In that sense, each and every unit is a one of a kind deviation, which minimizes cost while maximizing performance.

 

I'd return it for servicing, with a note describing what I discovered and what did to it.    Tom


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#5 quilty

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Posted 22 May 2024 - 06:22 AM

another rason the shun the edge versions



#6 davidc135

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Posted 22 May 2024 - 11:06 AM

If it's to be sent to Celestron then you may as well make sure it's re-assembled as best you can according to common sense and collimated at high power at or near focus on a star. If the performance is still lacking then at least you've done what you can and the scope can be returned for adjustment.

 

David


Edited by davidc135, 22 May 2024 - 11:07 AM.


#7 freestar8n

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Posted 22 May 2024 - 06:53 PM

If this is an EdgeHD8, I thought they also had small plastic screws for centering - so I'm not sure why there is mention of cork spacers.

 

Either way - with any sct there is probably an orientation of the corrector that works best - but how much that orientation matters is unknown.  There is an impression that the variability has gone down over the years - so if it is a relatively recent model it may not be sensitive to rotaton.

 

As for getting it centered - it's true that the EdgeHD systems do have an axis that multiple components need to be aligned with.  But the corrector is very smoothly varying and doesn't have much change in slope - so it won't be very sensitive to slight errors in centering.

 

If money is no object then - sure - send it to celestron.  But it may work fine just putting it together and roughly centering the corrector.

 

No matter what - you then need to collimate carefully with the secondary.  That adjustment will take out much of the "slop" in the overall system errors.  If this is mainly for visual use - it's the center that matters most.  If it's for imaging - check for aberrations in the outer stars that don't go away with tighter collimation.

 

If things seem ok - you are probably fine.

 

Frank




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