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SCT Collimation Process Question

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#1 Dave Lee

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Posted 08 August 2013 - 04:29 PM

I have a question about the proper process for collimating a typical (as in Meade/Celestron) SCT. For context assume that we are talking about the older, '3 screw' secondary mirrors with no center screw.

It is my understanding that the assembly here is the secondary mirror which is mounted on some kind of plate, 3 screws that move the mirror/plate in and out at points spaced at 120*, and a center pivot point that is fixed (I assume).

It would seem to me that if this is the structure, then balancing the in and out motion of the 3 screws to keep the proper pressure on the center pivot point would be an extremely important consideration. It would be easy to (I would think) 'screw in' all the screws such that you actually start to bend the secondary support plate around the pivot point. Or it would be easy to move the 3 screws out to the point where the secondary is not contact at all with the pivot point.

But when I read the collimation instructions in the online Celestron Manual, this consideration is totally absent. Following those instructions you would move the 3 screws at will to achieve collimation with no considerations about keeping the proper pressure between the pivot point and mirror support plate.

I must be missing something here - but what is it?

Thanks.

dave

ps. Related to this is another question. I am going to replace the 3 screws with 3 knobs from Bob. When you start to remove the screws (this is a 15 year old C11) the screws will either (eventually) start to back out of the plastic secondary assembly, or it will stay there just pushing the secondary mirror further and further away (the instructions are clear to do one screw at a time for obvious reasons). Is there a gotcha in here somewhere? I'm just being cautious here.

#2 David Pavlich

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Posted 08 August 2013 - 04:35 PM

My guess is before you bent the secondary plate, you'd either strip the threads on the collimation screws or you would strip the head of the screws. That plate is pretty thick.

David

#3 rmollise

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Posted 08 August 2013 - 04:46 PM

The three screws are threaded into a backing plate. You ain't gonna bend nuttin' unless your name happens to be "Magilla Gorilla." ;)

Just follow Bob's instructions and you will be fine.

Oh, and with the exception of a few Meades, it is the older scopes that have the central screw.

#4 Dave Lee

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Posted 08 August 2013 - 04:53 PM

OK, thanks. Sounds like common sense will work in this case.

dave

#5 Gil V

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Posted 09 August 2013 - 09:12 AM

Usually, the secondary mirror has a cork backing. Not much chance of warping it.

Just keep the screws snug at all times.

#6 EFT

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Posted 09 August 2013 - 12:00 PM

Really nothing to worry about here. The 1/2" thick (approx.) aluminum plate that the screws thread into isn't going anywhere. While you always want to tighten the screws, you still must keep it within reason, not because of warping the secondary, but because of the possibility of stripping the screw holes or cracking the plastic front that the screw heads rest on. I would actually recommend changing the screws to allen heads rather than knobs. The allen heads give you better leverage for fine adjustments than the knobs do. And yes, the screw will back out eventually.

#7 rmollise

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Posted 09 August 2013 - 02:13 PM

Usually, the secondary mirror has a cork backing. Not much chance of warping it.

Just keep the screws snug at all times.


The screws are not in contact with the mirror or any cork, but with the aluminum mirror backing plate.

#8 fldba

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Posted 10 August 2013 - 10:10 AM

Is there a good picture or diagram somewhere that would help beginners like me visualize what is actually happening when you tighten or loosen these screws?

#9 rmollise

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Posted 10 August 2013 - 10:34 AM

I don't know if there is a good drawing of a secondary mount on the Internet, but one is simple:

Secondary mirror is attached to the round aluminum backing plate with adhesive.

The backing plate has threaded holes for screws.

The secondary mount, which is the thing that extends through the corrector, has three screws and a central protrusion, a "bump."

The screws thread into the backing plate, and the central bump on the mount means that screwing the screws tilts the secondary and backing plate in teeter-totter fashion.

;)

#10 EFT

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Posted 10 August 2013 - 10:40 AM

If you mean what is happening to the collimation when you turn each screw, then this might be helpful: http://www.asterism....als/tut14-1.htm.

#11 Dave Lee

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Posted 10 August 2013 - 10:47 AM

If you mean what is happening to the collimation when you turn each screw, then this might be helpful: http://www.asterism....ls/tut14-1.htm.


Ed, pretty useful info - thanks

I corrected the link here.

http://www.asterism....als/tut14-1.htm

dave

#12 REC

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Posted 10 August 2013 - 12:35 PM

Great article to bookmark!

#13 T1R2

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Posted 10 August 2013 - 01:43 PM

Bob's Knobs are one of the great inventions of our time.

#14 Dave Lee

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Posted 20 August 2013 - 03:57 PM

For completeness my collimation knobs arrived a week or two back. And the trend of one clear night per month is continuing here on the east coast.

FWIW, the knobs are a BIG improvement over trying to adjust it with a phillips head screwdriver. However I will say that I can see how a standard hex end screw (as suggested by Ed in this thread) would work as well or possibly better (easier to adjust without moving the scope).

Since I am doing this on a C11 I can't reach from the focuser to the front of the tube. So I just took a roll of masking tape out and ran a piece of tape from the outside of the secondary to the edge of the scope. That made identifying the proper screw adjustment quite easy.

Still waiting for another clear night when I have more time for temp equalization (and decent seeing) for final adjustments.

dave

#15 WesC

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Posted 20 August 2013 - 07:33 PM

Since I am doing this on a C11 I can't reach from the focuser to the front of the tube. So I just took a roll of masking tape out and ran a piece of tape from the outside of the secondary to the edge of the scope. That made identifying the proper screw adjustment quite easy.



I do the exact same thing, it works great! :waytogo:

#16 dcornelis

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Posted 21 August 2013 - 02:22 AM

Actually, on my second hand C11 bending of the plastic plate is the reason it doesn't keep collimation well. An owner before me has put large diameter bob's knobs on the secondary and overstressed the cap to the point where the plastic is showing a discoloration. Still unsure what I am going to do to this problem. So yes it is possible to overstress the assembly.

#17 Live_Steam_Mad

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Posted 07 September 2013 - 02:50 PM

There are 3 very good pages with pictures which illustrate this SCT secondary collimation mechanism very well ;-

http://www.astronomy...aryremoval.html

http://astroshed.com...aga/c11saga.htm

http://www.cloudynig...at/Number/60...

I was dismayed to see that my own second hand C11 has a secondary holder that rotates with a little pressure from my hand. I need to get a mount for it though before I can test it out properly.

Regards,

Alistair G.






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