It has already been some time from first light of ACF telescope and I was looking forward a clear night. And it happened, a few days after supermoon 🙂 Main goals for this another test night were: to set distance between Astro Physics CCDT67 telecompressor and sensor and to check guiding.
CCDT67 telecompressor is simple two element optic device, that reduces instrument focal length. Reduction factor depends on distance to sensor, and for 85mm the factor is 0.67x with illuminated circle of 29mm diameter. Increasing distance lowers both reduction factor and illuminated circle, as presented at the graph below. CCDT67 also flattens curvature, but only a little. It is designed to work with instrumets of low focal ratio – from f/9 to f/18.
I started with a distance of 85mm, so 0.67x compression factor. And that combination gave effective focal length about 1900mm. It is much more than 2540 x 0.67. The reason for that is well known for SCT users. SCT focal length is not constant, it depends on focal plane position. Once you move focal plane away from the tube, the focal length increases. In my setup there are two factors that affect it. One is additional focuser that adds some distance. And second one is telecompressor itself, that “eats” some backfocus, so it is able to compress focal length.
Good thing is, that CCDT67 is just a 2″ barrel that can be insterted deep into the focuser. If it would need to be mounted at focuser tube far end, then focal plane would be moved even more far away. 1900mm was a little bit higher than I expected, so I added two more 2″ extenders between telecompressor and camera. Then the total distance was 115mm. And in this configuration I achieved effective focal length about 1700mm. I aim for 1600mm and ideally for 1500mm, but at this moment I do not have more 2″ extenders, so I leave it at 1700mm that gives pixel scale of 0.45″/px with QHY163M camera.
Then I shot some test images. Atmosphere transparency was good, but it was the first and last good thing this night. There were bright Moon shining in the sky, strong wind and the seeing was horrible. Stars diameter was in the range of 3.5-4.5 arc seconds. Such conditions happens about twice a year at my location. Usually seeing is in the range 2-2.5″, such it was when I made ACF first light. But I just started guiding and shot some open clusters around Cassiopeia to check how mount tracks such heavy load and if there is any vignetting or field curvature in that setup composition.
All components seem to work fine. Mount does not look like overloaded and tracking was ok, and also field curvature does not seem to affect image much. However it may be more visible, when seeing will be better and stars will shrink.
CCDT67 telecompressor seems to work fine with ACF f/10 telescope. Effective focal length is affected by focal plane position (it is SCT telescopes feature), so it will be longer one may think basing on telescope specifications. But image quality is good, and no curvature is visible at QHY163M sensor. However it needs to be verfied, when seeing will be good.
And few more pictures images on December, 13th: