|
I've used the celestron 0.63x on a number of larger scts, c8, m10, c14 and have generally aimed at 105mm (but +/- a mm or2 depending on the scope). Stars are decent across a 4/3" sensor (asi294), and just need to do a bit of a crop on an APS-C sensor (asi2600). Its going back a while but I think I remember trying 145ish mm as was recommended by a few people and got quite bad results. 85mm is not possible for me to fit an OAG and filter drawer so never tried it. The corrected image circle produced by the reducer is indeed about 24mm, but there is still a bit of useable image circle outside of this IMO.
Regarding using the OAG - this is when things get interesting. The illumination of an OAG is right on the outer edge of this circle. What I found was this means its almost impossible to get full illumination of the OAG prism, and for some reason I found that this gets worse with the larger aperture scts. I had to push the prism just inside the cameras imaging circle (just enough so that flats would fix it ok), and even then I still had crazy comet shaped stars to guide with, and only on approx 1/2 of the guide sensor used. Now, you can still guide on a comet-shaped star and this is pretty much how I did all my guided shots, but the real problem was that with such a small percentage of the guide sensor illuminated, there were so many targets I just gave up on due to inability to find a guide star.
I recently got the Starizona SCT corrector -L for the c14, and this has all changed. I'm not convinced that the optics are any better than Celestron's reducer, but I get full illumination of the guide sensor even when using the asi2600 and much better stars to guide with. Plus lots more backspacing to work with.
|