Quote:
Originally Posted by naskies
Hi Ray,
Thanks for the nice little write-up. Any further updates on your f/4 newt + RCC1 combination?
This seems like a great combination for my STF-8300M plus SX AO, but I've found very mixed reviews on the RCC1 (difficult to collimate being one of the main criticisms).
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Quote:
Originally Posted by Lee
Ray, is the RCC thought to have advantages over the MPCC, over its increased back focus (which could be seen as a disadvantage too!)???
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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.
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. 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.
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?
Hi lee. The advantage over the MPCC is that it has 3 elements vs 2 and my understanding is that it could be designed to correct coma without introducing SA. According to the reference below, the original MPCC possibly introduces about 0.7 waves of SA (makes a bit of a mess of 1/7 wave optics!), but even that amount of aberration is generally within the seeing blur for DSO imaging. However, with small pixels in good seeing, the RCC1
may have an advantage (haven't done a comparative test though).
The extra back focus is great for fitting stuff in, but it comes at a cost of vignetting, since the RCC1 must be pushed back further towards the mirror to work properly and give the required distance to the focal plane. This design would be better if it had a larger aperture - but then it would cost a lot more. As it is, it is a very cost effective compromise for high res imaging close to the optical axis (it works fine with the 694 and is usable up to APSc). Forget it with a full frame DSLR or large chip CCD though (eg like the 11002).
I am seriously considering glueing one into the focuser of the 200f4 and of my next scope as permanent parts of each scope - they don't cost much as a fraction of the total cost and I can't think of any reason why I would use either scope without a CC.
regards Ray
ref:
http://www.telescope-optics.net/sub_..._corrector.htm - discussion around Figure 155 is relevant