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The all important star test analysis, SCT.

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#1 Julio

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Posted 11 January 2013 - 01:03 PM

Recently had an opportunity to acquire a SCT,I'm quite a rookie in star test interpretation. I frequently read the articles and posts on this great forum and read in amazement how some of us can interpret the quality of optics from a simple de-focused image. I sat out last night with a well collimated SCT in an attempt to understand what i see after some light reading on star testing. Conditions were 2-3/5 seeing with some light clouds rolling in every so often. I'm aware that star testing should be performed under ideal seeing, but i figure id give it a shot. I saw a nice diffraction pattern outside of focus and slightly darker less distinct pattern within. I have uploaded some a-focal videos,and would love to hear the opinions of the more experienced. These were shot a-focal on a non tracking mount. :jump: :question:

Extra Focal

http://s99.beta.phot...zps17093e96....


Intra Focal

http://s99.beta.phot..._zpsedbe9d92...

http://s99.beta.phot...zps6600e63c....

#2 David Knisely

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Posted 11 January 2013 - 01:43 PM

You are testing things way too far from focus. When doing the star test, you should be using pretty high power and just *slightly* de-focus the star image. If you can easily see the dark shadow of the secondary mirror, you are way too far inside or outside of focus. Clear skies to you.

#3 Julio

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Posted 11 January 2013 - 07:18 PM

Ill try that next time thank you, do you know if a Roddier test will work with my current start test images?

#4 REC

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Posted 13 January 2013 - 11:43 AM

Julio, that was a great idea! Mine is a little like that and that was good advise David gave as I was doing it too far out of focus too!

Thanks to both of you!

Bob

#5 David Knisely

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Posted 13 January 2013 - 01:39 PM

Ill try that next time thank you, do you know if a Roddier test will work with my current start test images?


Probably not. Again, the diffraction effects will be most notable in the slightly defocused images. I prefer Dick Suiter's "breakout" star test where you defocus only just barely enough to observe the very first hints of the central shadow from the secondary becoming visible (the 'breakout' point). Remember that in SCT's, there is often a little residual spherical aberration present even in some halfway decent instruments, so don't panic if you see just a little in the star test. Clear skies to you.






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