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Doubles to test Optics

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#1 mike174

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Posted 20 February 2013 - 11:17 PM

Hi,

What are some good doubles to try to split using my C11 to test it's optics? The shadow on one side of focus is not the same as the other side for the same given amount of focus travel. I believe this indicates SA. Wondering what a double star test would prove?

My Orange C8 is pretty much identical on either side. Rings look practically identical too, unlike the C11. The AT6M blur circles look like the C8's. Wondering if I got a so-so C11. Like to try a C8HD and put it on my EQ5P but not sure I want to roll the dice anymore with SCT's.

Tnx,

Mike

#2 WRAK

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Posted 21 February 2013 - 06:24 AM

You don't need a double star for this - just have a look at the diffraction pattern for any reasonable bright star with a magnification of about x280 or more and compare the result with the examples on http://www.telescope...berrations.htm.
Wilfried

#3 jrbarnett

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Posted 21 February 2013 - 05:53 PM

Double stars are poor test subjects for testing optical quality. Particularly for larger aperture instruments. They are excellent, however, for testing *seeing* quality. Larger scopes are more affected by poor seeing than smaller. Similarly larger scopes cool more slowly, and while cooling suffer thermal issues which are not optical quality issues but nonetheless impair double star performance.

As WRAK suggests, you're better off doing a star test on the scope. Because it is an SCT, collimation will be absolutely critical for double star performance. Slight miscollimation will stack with any thermal issues and aperture-related seeing challenges, and cripple the scope's potential for splitting doubles.

Regards,

Jim

#4 azure1961p

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Posted 22 February 2013 - 09:27 AM

I would stress here that thermal issues can mimic spherical aberration to a degree. What Im thinking here is that the bigger optics of the 11 aren't as equalized as the smaller more manageable 8". Often folks use fan cooling like a Lymax. I wouldn't rush to judgement just yet in the glass alone till its truly equalized. Even blowing a fan on the OTA for a good hour or more outside would be good.

Pete






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