Hello all.
I'm pretty new to astrophotography. I noticed some distortions near the edges of images. It also seems like stars on the edges, especially on the bottom, are slightly out of focus while stars in the center are in focus. I have a f/4 200 mm Newtonian reflector and a 1x coma corrector. ASIAIR reports a focal length of 783 mm instead of the expected 800 mm. Is this an issue of back focus, collimation, guiding, something else, all of the above?
Thanks a lot.
Nikhil
📷 1000010474.png
Your issues with the asymmetrical defocus is from tilt, while the defocus itself is caused by mis-spacing the sensor to the coma corrector’s optical elements. Should be pretty easy to fix, adjust your tilt-plate and space the corrector and sensor out to the optimal distances.
F/4 is notoriously unforgiving! If the distortions are mostly in the corners, I'd double-check your back-focus distance to the coma corrector. Even a 1mm difference can cause issues. Also, check for any 'sag' in the focuser - those Apertura stock focusers can sometimes tilt a bit under the weight of a heavy camera train.
Thanks for the response! It turns out a slight mismeasurement of back focus length by a few mm was the culprit, just like you say!