Astrophotography with relatively large DSLR sensor using fast telescope has some consequences. Well known parabolic mirror issue is coma. To fight it we need to use coma corrector. Here is probably the most entry level coma corrector – photovisual device made by GSO.
Optically it is built with four lenses in two groups. According to manufacturer it works with telescopes as fast as f/3.5. Corrector seems to be well made, MC layers are of decent quality, and blackening is also good.
To work properly with f/5 instrument we need to keep 72mm distance between rear corrector lens and sensor. To keep this distance I needed to use 10mm extender that is placed between corrector and EOS bayonet adapter.
I used for comparison some starry area in Coma Berenices constellation. Each frame is 3 minutes subexposure made with Canon 20D at ISO800. There is no coma corrector image at the left, and with corrector at the right
Pictures are resized to 66%. Good news is that coma is basically eliminated, only in left bottom corner there is still some coma effect visible. It is most probably due to bad collimation or some tilt in the optical train. Another good news is, that there is not any visible quality degradation in the frame center. Unfortunately I cannot compare this corrector with other, but I think it will meet my expectations, at least for some time.