Does anyone know what the spacing should be between the sensor and this reducer?
ES 102mm f/7 triplet
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Posted 06 December 2019 - 04:26 PM
These don't work very well for refractors. They are really designed for catadioptric scopes that have F-Ratios at and above F10. If you look at refractor focal reducers you will see that none of them go below about 0.61x and most are at 0.7x and above. Catadioptric scopes have FRs that can go as low as 0.33x because they have a long focal to begin with. You won't find a refractor focal reducer below about $100 and most can be up in the $200-$500 and even higher. Refractor FRs come in 3 types, generic, scope line specific and single scope specific. Depending on the size will determine the flat field part which will determine the size of sensor you can use. The smaller they are the smaller the sensor you can use without vignetting, distortion and coma. I have one of these 0.5x reducers made many years ago and sold by Orion (same as the old Celestron Wide Field adapter) and it has the T-threads to screw onto the photo adapter visual back of an SCT. Some of these are available with filter threads to go on the end of a diagonal although I would not use them that way myself. As long as you use it with a camera with a tiny chip in it you might get some use in it although it may actually reverse the curved field of the refractor and make the curve negative.
There are plenty of type 2 refractor focal reducers out there designed for F7 scopes. I have a couple from Stellarvue myself. They can cost in the $200 range or less used but they will work generally up to at least APS-C sensor cameras.
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