RCC1 coma corrector impressions

[Shiraz]

The Baader RCC1 is designed to correct coma on fast Newts. I fitted one to an imaging system based on an f4 Newtonian (currently 200mm, but will later upgrade to a 250mm). The chip is an icx694 with 6M small pixels and a field of view of 0.89 x 0.72 degrees in the current system.

The RCC1 has a required back focus of 91.5mm, which allows a lot of ancillary gear such as filter wheels, OAG etc. to be inserted. It does not do this by any optics magic though - it relies on you to set the focal plane to be a long way out, so that the RCC1 can be set back closer to the mirror than other CCs. Since the RCC1 is 75mm long, this means that the focal plane needs to be more than about 166.5mm from the OTA surface or the RCC1 will intrude into the input light column. On my GSO Newtonian, this required cutting 55mm from the OTA tube to move the mirror up and the focal plane out. I also cut down the focuser to minimise its height and fitted a 70mm secondary to allow for the larger light cone intercept. With these mods, the centre of the field of view is fully illuminated (just). Vignetting sets in at small angles off axis (from light missing the secondary and from the smallish input aperture of the RCC1). However, the centre of the fofv is of primary interest for galaxy imaging and the vignetting is not excessive over the fairly narrow field of view of the icx694. The system is usable with APSc (tried it with a QHY8), but anything larger and vignetting would be a big problem.

The system is laser collimated through the RCC1 – it needs to be right, but it is not too difficult to get it aligned. The attached composite is from a typical short sub with stretched small crops at native resolution taken from the field centre and the four corners. There is perceptible distortion in one corner at the 1.17 arc sec sampling, but I expect that to be fixed when the camera image plane alignment is tweaked up a little bit better. The additional segment shows the lower right corner without the RCC1 – it made a huge difference.

There is no noticeable decrease in central resolution with the RCC1 in place, so the corrector is not introducing excess aberration of any sort – in good seeing, the FWHM has occasionally been well under 2 arcsec. Coatings look to have very low reflection and I have not noticed any artefacts from bright stars.

All-in-all, this appears to be an excellent CC for a fast Newtonian, particularly if used with a relatively small chip for high resolution imaging.

thanks for reading. Regards Ray

[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).

[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!)???

[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.

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_aperture_corrector.htm - discussion around Figure 155 is relevant

[naskies]

What's involved in shortening the tube / moving the primary up the tube? It sounds like some major scope surgery?



Sounds great!



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.



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).