Re: [APML] strange halos around stars

[Chris1011@aol.com]

In a message dated 7/19/2002 2:46:02 AM Central Daylight Time, nmontec@libero.it writes:


> What can I do to mitigate this problem? would it closing the f/ratio of 
> 2 stop help me with the lateral chromatic dispersion? I was using this 
> tele 200mm at full aperture (f/4!) I was concerned of closing it more as 
> I would get too long exposure times.


Lateral color means that the lens has slightly different focval length for each color, so the red image might be slightly larger than the green, for example. Therefore the red star image does not line up perfectly with the green. You might be able to compensate for this in Photoshop by separating the 3 color channels into layers, then adjusting each image size so that they all line up, then recombining them back into the color channels. Off-axis coma and astigmatism cannot be readily fixed.

Roland Christen