coma?

[alacant]

Hi

At the edges of a photo on a refractor, I get stars like this enlared top left corner crop. Apart from chromatic aberration, what is the effect called where the false colour extends away from the star to the centre of the field?

Thanks

[Leveye]

Are you using a field flattener?

[Ruud]

There are two types of chromatic aberration: lateral and longitudinal.

The image possibly shows a combination of both, but as it happens way off-axis in the image, my guess would be that this is mostly longitudinal chromatic aberration, probably made more evident by a curved focal plane.

[MarsG76]

This looks like lens distortion, happens in a lot of lenses and ED refractors... if you use a field flattener and/or a triplet, the problem is a lot less.

[alacant]

Hi

It's a petzval so I suppose already has a sort of field flattener. I wonder whether this would do better:

http://www.teleskop-express.de/shop/product_info.php/info/p1010_TS-Universal-2--Field-Flattener-for-Refractors-f-5-to-f-8---Flat-Field-for-Astroph.html

They also so a 2.5" version which also fits my telescope but it's really expensive.

[alacant]

Hi

Update.

Thanks for your input and ideas. They've all sent me in the right direction on Google. I also got onto Bresser, the manufacturer. They reckon it is field curvature which is why you only see it in photos. Maybe it's worth getting a flattener? Oh and you can cheat the false colour with the really nice lens corrections in Darktable, without having to pay a monthly subscription;)

[alacant]

Hi

OK, finally got it. You have to adjust the lens set closest to the focusser. Moving it closer to the main lens rounds the stars correctly. Bresser-ES call it 'Petzval'.

Round stars, right into the corner!

HTH others diagnose.