Important things for me were:
– price 🙂
– corrector should shorten my scope focal length if possible, so my camera field of view would increase
– acceptable image quality
I have made a small research and found out Sky Watcher coma corrector as a potential candidate. It is beeing sold under different brands and additionaly it was on sale at my local retailer.
So for not much more than 100$ it came to me 🙂 Built quality is decent, corrector itself is relatively lightweight, and according to specifications he gives corrected field of diameter 44mm and works for telescopes with focal ratio f/4 – f/6. And also provides focal length reduction factor 0.9 – 0.95x (yeehaa!).
So, lets go with some pictures.
Optimal corrector to image plane distance for this model is 55mm. On the image taken with this distance set you can see no coma at the corners, overall quality is good (stars FWHM was kept). Effective focal length was shortened to 679mm so it means reduction factor of 0.9x.
Next picture was shot with distance set to 50mm. We can still see no coma at the corners, good stars quality and focal length was calculated to 683mm.
This flexible CCD – corrector distance with decent correction result is probably valid for my small chip camera that covers about 1/4 of corrected field diameter for this device. When using with some larger chips one probably need to be more strict with the optimum corrector – CCD distance.
Last image is kind of comparison – it presents bottom right corner of the pictures imaged for different chip – corrector distances and without coma corrector in optical path. It is also enlarged to 150% of the original scale. You can see that within this 10mm range of corrector distance the field is well corrected and stars are properly shaped.
Overall I am pretty happy for this single piece of astro gear. It meets my requirements and comes at a very reasonable price. I also hope it will cover some larger field of camera view in the future, when I maybe some day will be able to exchange my camera with bigger sensor model (like KAF8300). I would reccomend this coma corrector for amateur use, when you need decent image quality with a very little price and you agree to have a little focal length reduction. I am eager to test it with some 8″ f/4 newtonian that will give effective f/3.6 focal ratio and that means pretty powerful astrograph.