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Question about Casady collimation method

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#1 Beeham

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Posted 13 August 2020 - 08:37 PM

Hi everyone -

 

I recently was introduced to the Casady collimation method.

 

(http://www.robincasa... and dark rings.)

 

I thought "how nifty - this seems very smart, I must try it immediately!"

 

To my horror, my trusty 1977 orange tube, when evaluated in this fashion, appeared to be significantly out of collimation.  The rings in the reflection were not concentric.  I felt as if I had failed as an astronomer.

 

Quickly regaining my composure, I thought back to my most recent collimation, done using Ed's Guide to SCT Collimation.

 

(https://astromart.co...sct-collimation)

 

I recalled how satisfying it was to have all those little diffraction rings so nice and symmetric.  How could my scope have become mis-collimated so quickly?  How could I not have noticed this last night when I was actually *looking* at stuff?

 

Like an aging fluorescent bulb, an idea flickered to life in my tired brain: if the reflections in the mirror in the Casady method are the internals of the OTA, then maybe the OTA's physical axis is not parallel to the optical axis!  I quickly retrieved a ruler from my kid's box of craft supplies, and verified - with both satisfaction and irritation - that, behold, the secondary is not precisely centered in the corrector cell!  It's offset to one side by about 1/16", or 1.6mm.  This, I believe, is why the Casady method was showing me as being out-of-collimation while actual star-viewing suggested I was collimated.

 

I can, should I so choose, readily disassemble the scope and cut a new set of cork shims to center the secondary.  This will, logically, require a re-collimation.  The question is, will doing this provide any optical improvement in the scope?  Is having the secondary centered critical to optical performance?  Or, alternatively, does proper collimation render the 1.6mm offset of the secondary irrelevant?

 

Thanks in advance for guidance on this!

 

-Beeham


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#2 Migwan

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Posted 13 August 2020 - 09:12 PM

My C11 has adjusting nylon tipped screws and the corrector is not exactly centered.   Read someplace (here I think) that the secondary and corrector are centered on the baffle tube, which may not be perfectly parallel with the tube. ???  After reading that, I left it alone. 

 

That said, the reflection rings seen from the front of my C11 are concentric.  Haven't a clue why yours aren't when your getting a good star test.  Personally, I'd go with the latter till I was told better.  Hope somebody with more experience catches this and explains what is going on.

 

jd


Edited by Migwan, 13 August 2020 - 09:15 PM.

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#3 KTAZ

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Posted 13 August 2020 - 09:21 PM

So, I’ll begin by saying, “the only stupid question is the one that is not asked”.

 

That said, I presume you were covering one eye and using the other eye to check the circles, right?

 

I find the process works really well with my 9.25SCT. After pulling the corrector for a cleaning, it is always my first collimation effort to get close before I whip out MetaGuide once it gets dark.


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#4 Beeham

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Posted 13 August 2020 - 09:35 PM

So, I’ll begin by saying, “the only stupid question is the one that is not asked”.

 

That said, I presume you were covering one eye and using the other eye to check the circles, right?

 

I find the process works really well with my 9.25SCT. After pulling the corrector for a cleaning, it is always my first collimation effort to get close before I whip out MetaGuide once it gets dark.

Not a stupid question, but "yes" - closing one eye and using a 3x5 index card with a pinhole in it, mounted on a camera tripod, to ensure repeatable eye position.



#5 gnowellsct

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Posted 13 August 2020 - 10:37 PM

I've never been very good at this technique.  I figure that's why the good lord gave us the moon.  So we could collimate our scts properly on stars and not be missing out on any deep sky.

 

The more general point is that the main downfall of the SCT lies in the mechanicals even though everyone focuses on the optics.   A road is only as good as its bridges, and a scope is only as good as its mechanical systems.


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#6 Cpk133

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Posted 13 August 2020 - 11:16 PM

 

 

Like an aging fluorescent bulb, an idea flickered to life

Nice mental imagery.  I've tried this daytime method and I detect some asymmetry in the rings.  Under the stars, no problems.



#7 TG

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Posted 13 August 2020 - 11:29 PM

I can, should I so choose, readily disassemble the scope and cut a new set of cork shims to center the secondary. This will, logically, require a re-collimation. The question is, will doing this provide any optical improvement in the scope? Is having the secondary centered critical to optical performance? Or, alternatively, does proper collimation render the 1.6mm offset of the secondary irrelevant?

Thanks in advance for guidance on this!

-Beeham

Beeham,

The secondary is spherical although it can be “touched up” a tad. Since a sphere has no preferred axis, it can be displaced laterally and needs just to be tilted to collimate it. This is what you have done using a real star. I would forget the Cassady method or whatever it’s called unless this is an itch you just have to scratch.

TG

#8 luxo II

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Posted 14 August 2020 - 12:10 AM

The Casady method is only a rough alignment - it is not accurate. What matters much more is a star test alignment.

 

Don't futz with your scope - SCT's are not particularly sensitive to lateral misalignment.

 

Lateral misalignment is a problem for the RC and classical cass folk, where the secondary is strongly aspheric.


Edited by luxo II, 14 August 2020 - 12:13 AM.

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