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Originally Posted by Shiraz
hi dave
all working very well. Only outstanding issue is a tiny bit of astigmatism, which I have not yet chased down. I think it is due to the heavy handed way I glued the primary in place, but it might just possibly be due to distortion from clamping of the RCC1 in the focuser. In any event, the system is producing nice stars and well under 2 arcsec FWHM in good seeing, so it's not a major problem - will post when resolved.
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What's involved in shortening the tube / moving the primary up the tube? It sounds like some major scope surgery?
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I have not found it to be difficult to collimate at all - noting only that, like everything at f4, it must be set up within a few 10s of microns. The biggest problem was getting the focal plane aligned in the camera, but once that was tidied up, it all collimates quite easily using a laser through the RCC1. I have been using my scope for the last 3 weeks without touching the collimation and that includes many episodes of setting up and removal from the mount. Stars are good across the 694 field.
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Sounds great!
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A previous bad report stated that a GSO f4 scope could focus without modification when the RCC1 was installed - my GSO 200f4 needed 55mm cut off the OTA to just get to focus, so maybe the previous negative report dealt with a dud RCC1.
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According to the Baader specs, the RCC1 requires the equivalent of 91.5 + 5 mm back focus with the camera at prime focus. Given the back focus of typical newts plus your 55 mm extension, that sounds about right.
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I thought about adding an AO unit to my system, but was unsure of how much SA and CA the refractive element would introduce at f4. Have you had any problems in that regard?
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I've only used my AO with my RC8 at 1625 mm focal length, f/8. I haven't noticed any obvious optical problems (I haven't specifically looked for them either), but they would be vastly outweighed by the improved tracking - especially on my EQ6.
The biggest problem I've had with AO is trying to find decent scopes/correctors that provide enough back focus, and which don't cost a fortune. My STF-8300M camera + FW + OAG requires 56 mm, but the AO takes up another 38 mm, for a total of 94 mm (which can just squeeze into the RCC1's specs via an M48 thread).