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Old 18-07-2018, 07:14 PM
ericwbenson (Eric)
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Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 209
Quote:
Originally Posted by Russj View Post
Hello Paul, I've been reading this thread with interest, sounds like ur working thru the problems on the new scope and its starting to come together for you.

Could you answer a newbie reflector owner question? what are the indicators on miscollimation in your photos, I cant see a problem with my newbie unpracticed eye. Nice image by the way.
Regards russ
Hi Russ,

Miscollimation causes aberrations (other than spherical) to appear. Usually coma is the first and worst, but astigmatism can also be generated.

A simple way to think about it: the 'corrected' field is only a finite size, collimation shifts it around, when it is perfectly centered you see no aberrations, or the same aberration in each corner, depending on the size of the sensor relative to the 'corrected' field. Symmetry is the goal.

Image tilt will only induce defocus (inwards in one corner and outwards in the opposite). Defocus means the stars stay round but get bigger. NOT the same as coma or astig which makes the stars non-round. So when you hear someone blaming their eggy stars on image tilt it usually is not the case (but can be, read on...)

In reality separating the two factors (aberrations and tilt defocus) is really hard unless you have very good seeing to be able to examine the shape of the non-round stars. What can really be confounding is a little bit of inherent astigmatism in the optics (from mounting or built-in), which is not detectable in focus, but is revealed by slightly going out of focus. So now is that astig in the corners from image tilt or field curvature or collimation?!?

It takes practice and patience to iterate the setup, which is what Paul is doing now...

and then throw in a camera rotator and it becomes a really tough nut to crack!



HTH,

EB
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