Odd shaped stars, what causes them?

[Kaptain Klevtsov]

The problem arose when I bought a 200mm Nikon fit telephoto lens off E-Bay and stuck it on the Atik 16HR. The stars are all oddly shaped like this.

(click to make it the same sizeish)

I managed to bag an M31

(click to biggerise)

And an M45

(click to biggerise)

Does anybody know of a cause that might make the stars go the shape they are doing? BTW guiding doesn't make a difference, on or off the stars are the same, and Capella does that when focusing.

Bemused of Huddersfield

(Kaptain Klevtsov)

[blinky]

Is it not astigmatism? TBH I dont actually know what that is! but think it is what causes it. If you get what I mean..and I am not sure I do!

[barkis]

Probably not much help this KK, but I remember reading that zoom lenses were not really suited to astro photography. Having said that, I now need to try and find the author of that statement, as I am sure me saying it carries no weight.

Those star shapes bear the hallmarks of astigmatism, but it may not be inherrent, but rather induced by the zooming action.

Try imaging at max and minimum magnifications to see if the anomaly is reproduced at both states.

Ron.

[blinky]

Try changing the aperature as well, I read that lenses work best at 1 or 2 stops short of maximum (or was that minimum )

[steppenwolf]

I read that lenses work best at 1 or 2 stops short of maximum

I believe that to be case too .....

[FLO]

The problem arose when I bought a 200mm Nikon fit telephoto lens off E-Bay and stuck it on the Atik 16HR. The stars are all oddly shaped like this.

It might be a sticky or misplaced aperture blade. Check that it actually produces a circular, or at least evenly misshaped, aperture.

HTH

[Kaptain Klevtsov]

Thanks guys, the aperture diaphragm looks OK in the daylight, so it must be astigmatism (whatever that is). The M31 was stopped down to f/5.6 and looks a lot better on the subs. I did a flaming star Neb. widefield to include the tadpoles on IC410 wide open with a Ha filter and all the subs came out pants.

Time to use this thing stopped down only then I reckon.

Kaptain Klevtsov