Not sure if I should direct this question to the Telescopes forum, but as it specifically relates to the visual observation of variables, I thought this might be the best place.
As I am not going to able to use my larger scopes for observing for a while, I would like to look at using my Celestron C5 SCT for visual variable star observations. Given its relatively long focal length 1250mm, my lowest power 30mm eyepiece is going to give me a magnification of around x 40 which I think maybe too high as it will give me quite a narrow field of view. I have the Celestron x 0.63 focal reducer/flat field corrector (FR/FFC) which would bring the focal length down to approx. 787mm which, if I’m correct would give me a power of around x 26 with the same eyepiece and therefore a somewhat wider field of view more suitable for VS work as it would be easier to locate the comparison stars. From what I’ve read the FR/FFC would introduce some vignetting around the edge of the field of view, but I would ensure estimates are made using only the centre of the field of view.
My question is, are there any good reasons why I shouldn’t use the FR/FFC for visual variable star observations?
I reckon I can now answer my own question – it’s not worth risking using the corrector as the instructions (which I’ve just found again) state that for “low power visual use”, the sharpest possible images across the “majority” of the field is obtained by first focusing on the central stars and then turning the focus knob approx. 1/12 of a turn CCW. So in effect nothing would be 100% in focus across the field. This might be fine and not really noticeable for general low power viewing but I wouldn’t be happy making variable star estimates using it in this way.