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Old 28-08-2014, 10:23 PM
ericwbenson (Eric)
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ericwbenson is offline
 
Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 199
Hi Paul,
It's been a while...
+1 to what Joshua said. The stars in the corners when focused (so that the center is out) will show you the best that the flattener could hope to do. In order to fix anything else (mostly off-axis astig and coma from what I can see), the system needs a corrector. Remember an RC is not entirely coma free, it has a coma free zone, and only if the hyperboloids hit the prescription, and it is definitively prone to off-axis astigmatism, this may be the wall you are hitting. (Doug G. once mentioned to me that he could start to see the off-axis astig on his uncorrected 12.5 RCOS with his KAF6303, so the KAI11000 would definitively show it).

I believe changing the mirror spacing primarily affects the amount of spherical aberration in the system (SC's are like that, as are CDKs). There is a point where it is minimized. On my system it has to be better than +/-1 mm (so says PWI). PWI supplies a Ronchi eyepiece and spacer tube to adjust the mirror spacing until the Ronchi shows a null. I gather GSO doesn't supply this? If they can tell you the correct distance the Ronchi should be from the back plate, you could then borrow my Ronchi and cut a 1.25" ID tube to the right length.

The flattener can add it's own aberrations too unfortunately, to observe those rotate the flattener wrt the OTA and the camera (you sorta did that already), but keeping the camera fixed wrt the OTA might help in the diagnosis.

Best,
EB

p.s. Congrats on the Malin!

Last edited by ericwbenson; 28-08-2014 at 10:37 PM.
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