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:
and back:
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
(2"-M42 AD/M42 ext. 15mm/mount AD)
With GSO 2" 2x ED Barlow (2" ext. tube added)(2.2x)
(2" ext./Barlow/2"-M42 AD/M42 ext. 15mm/mount AD)
The reducer (0.74x)
(2"-M42 AD/M42 ext. 15mm/reducer installed in Kenko mount AD)
END.