I have a Meade LX200GPS – 12″ SCT telescope, which has a focal length of 3048mm (305mm aperture) and a focal ration of f/10.  This telescope is great for planetary imaging (what I’m mostly limited to, living in a Bortle 7-8 light pollution area, but there exist optical accessories called Focal Reducers which allow you to get this telescope down to a quick f/6.3 or an incredibly fast f/3.3.

I’ve purchased both an Anteries f/6.3 and a Meade series 4000 CCD f/3.3 focal reducer for my telescope, and have been playing around with them and my Sony A6300 crop sensor camera.

f/3.3 Meade Series 4000 Focal Reducer, 10 second exposure, ISO 400

Antares f/6.3 focal reducer, 20 second exposure (unknown ISO, possibly 400)

Native LX200GPS – 12 inch f/10, 20 second exposure

The f/3.3 focal reducer is designed to be used with a small CCD imaging sensor (sold many years ago) and has a lot of vignetting and drop off near the edge of my crop sensor. Some of this could be resolved by calibrating the optical path with light frames, but I feel that the f/3.3 reducer is better for a smaller image sensor region in the center, so you don’t really get the full advantage of the 0.839 degree FOV.

The f/6.6 focal reducer worked well with the crop sensor size and almost the entire 0.436 degree FOV was usable.

The native f/10 native focal ratio gives a relatively small 0.245 degree FOV, and a lot less light so you need higher ISO/Gain settings or longer exposure times, and is really better for planetary imaging than deep space objects astrophotography.

A video about the f/6.3 focal reducer:

A video about the f/3.3 focal reducer:

I took 11x 10 second images of the Great Orion Nebula with the f/3.3 focal reducer, and stacked them with Siril. The result isn’t bad for 110 seconds of exposure time.

Click for full size image