GSO 0.5x 2 inch reducer
My 6 inch Cassegrain GS-150CC has f=1830mm. It's too long for full Moon with APS-C, and too slow (F12) for DSOs. (also too long for guiding with old Vixen GPD mount)
Then I began to think about a focal reducer at a reasonable price.
AstroStreet, a shop in Japan, has a 0.5x reducer (produced by GSO, Taiwan) at ~5000JPY. It's originally for eyepieces and when I tried on my camera, I couldn't get focus. The focal position was too into the tube. I needed to shorten the back focus of the reducer so that the focal position moves back.
I made a short mount adapter from Kenko's product. It has a very short camera side ring and the back focus is adjusted by T2 rings in the front for various camera mouts. I picked up ones for Fuji-X and Nikon-F (my camera is Fuji) and combined Fuji-X body side ring and Nikon-F T2 ring, then it made this:
With this, I got the focus! and the image scale was 0.47x of the straight prime focus. Maybe this is too much reduction and the peripheral zone is much degraded.
I gave another try by putting the reducer's lens into the T2 ring for Fuji-X. It is much longer than that for Nikon, and the inner dia. just matches the reducer's lens dia.!
From the front:
Now the reducer lens is just in front of the camera's mount. The back focus to the sensor is only ~20mm.
It made this↓ The scale is 0.74x.
0.74x ... Humm. Maybe I will put a few millimeters of spacer at the back of the reducer, then it gives a bit more reduction, like ~0.6x.
Anyway, I tried this reducer on stars. An example is like this:
It seems working pretty well :-)
GS-150CC connection arrangements
Here's what the back end arrangements with GS-150CC are... Note that I usually use 50mm+25mm extension tubes that came with the scope. (It gives the focus with 1.25" diagonal mirror and eyepieces).
Straight prime focus
With GSO 2" 2x ED Barlow (2" ext. tube added)(2.2x)
The reducer (0.74x)