Need help to find error.
My first attempt to photograph M45 with 135mm lens.
I focussed at F5.6 then stepped down to F8.
After processing in Registax 3 the stars look like rings.
The first shot is 17x2s, the second is 8x2s.
(Nikon D80, ISO 1600, Nikkor 18-135 lens)
The Nikon D100 does a nice trick in autofocus mode.
Set the samera up and focus it like normal, slightly depress the button. If it thinks it can't find focus, it goes to the infinity setting and stops. Then flick it back into manual focus.
Try Autofocus on Sirius, it might be bright enough to work. Alternatively, autofocus during the day and put the lens into manual and don't bump the focus ring.
I did some tests on Canopus. Manual focus, F8, 2sec. the size of the star is 3x5 pix, at F38 15sec. 2x16 pix.
The 'Doughnuts' in the earlier shots are 10x10 pix!
btw Autofocus on the Moon works, but if it cant find focus, the infinity is not good enough for stars.
I will try a Hartman mask, then shoot a star trail and adjust the focus. But this would be hard as the focussing ring is not very smooth and has some play as well.
