Focal reducer selection

[Hicks]

Up until recently I've imaged with an MX716 and F3.3 reducer. I've since upgraded to the SXVR-H9 which has an 11mm diagonal as opposed to the MX716's 8mm and I get the feeling will make using F3.3 a problem.

To make matters worse (at least from a FR pov ) I've just received a filter wheel + OAG. If I'm not mistaken that means at an absolute minimum I'm looking at 17.5mm (sxvr ccd distance), 42mm (wheel+oag) and then another 7.75mm due to the thread adapter ring needed to connect to the focal reducer, giving 67.25mm total between focal reducer and ccd.

That suggests a focal ratio of (85-67.25)/85 ~= f2.0 and considering f3 was already pushing it with the smaller mx716 sensor, I'm assuming larger sensor plus even faster focal ratio is going to be a non starter.

I image with an 8" f/10 LX90 SCT (~2002 model), so having some degree of focal reduction is useful. Is there any downside to relocating the focal reducer to after the filter wheel then using extension tubes to hit a slightly higher ratio of say f/5?

I'm expecting there's downsides to that positioning and I've read that reducers are picky about exact spacing to avoid coma? So that leads onto, should I look at getting an f6.3 reducer instead? According to this site the f6.3 has a spacing of 85mm which I could get to within 3mm of with my existing adapters.

Does an f6.3 sound like a reasonable choice given the above gear and does anyone know roughly what size sensor would be the limit when operating around f6.3?

If anyone is using a similar setup I'd like to hear about your experience whether it's with or without a FR.

[Davey-T]

The Meade 6.3 f/r needs to be as close to the rear of the scope as possible so the first thing in the imaging train, on the other hand the distance is quite forgiving and I've used one from 70mm to 150mm from the sensor, don't have any experience of your camera.

Dave

[Hicks]

I've had another look at my setup and realised I can get everything connected with a total of 59.5mm which would be FR->Filterwheel->CCD no T-adapter needed.

The F3.3 has a rough focal length of 90mm which give or take inaccuracies in my measurement, tallies with the 85mm I've read online. So with that spacing I should achieve a compression ratio of 0.33 (give or take). I read elsewhere that CR * Clear Apperture is the illuminated circle on the chip. The meade F3.3 afaik is 38mm so 0.33 * 38mm = 12.54mm which is a little larger than the diagonal of the SXVR-H9 (11.22mm).

Have I made some fundemental error in my calculations? Or does it look like, at least with the current setup, I could get away with using an F3.3? Even if it's just within spec to avoid vignetting, would coma be an issue?

I'll likely end up getting an F6.3 anyway though as I'd like to add a zero image shift focuser (absolute version) at some point to my setup which is going to increase the optical train length, but I'm now curious if the F3.3 may be usable in the mean time without wasting an imaging session to find out it's not