Quote:
Originally Posted by Astrofriend
What do you think, can the tilt also be the problem of the triangular stars ? Or what cause this kind of distortion of the star profile ?
/Lars
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Hi Lars,
That sort of distortion looks like it is or might partly be due to astigmatism, perhaps with a touch of coma. Both aberrations / Astigmatism should get better at smaller apertures and away from the corners, so perhaps run some tests imaging at f/4, 5.6, 8, 11 and compare the various corners/centres to help with some conclusions. It can be painful to have to wait until nightime to run your tests so you can try a test target. I will add a link to one I think might be useful.
Best
JA
EDIT: Aberration Test (Down towards bottom of Article) .....
https://www.lonelyspeck.com/a-practi...erration-test/
The only short coming of the test is that ideally you'd want to do it at infinity for your lens since that's where we use them. Still it may give you a guide to the performance of the lens. Alternatively rather than covering the whole sensor with the whole image at a close distance, you could more further away and just image the whole target using the corners of the sensor, and one at the centre for comparison, thereby allowing testing at infinity focus (probably ~>10-20m, depending on lens).