Blue ring around star??

[middy]

Was out doing some more testing of the scotch mount last night. On the last image I took one of the stars seems to have blue smears around it. None of the other stars seem to be affected. Is that star overexposed? Does anyone know what causes this?

The first image shows the overall shot. The star in question is near the right edge of the picture and seems to be alpha Carina.

The second image is a crop around the star from the full size image.

[[1ponders]]

It looks to me like chromatic aberation Andrew. ya get that with achromatic lenses. The brighter the point source the more obvious it will be. I gather that is Canopus or Sirius.

[[1ponders]]

I'd guess Canopus 'cos I think I can just make out the LMC at the bottom left in the larger image

[Robby]

I'd say it is Chromatic Aberation. What lens are you using?
Also is seems to be on one side only, so could be CA combined with coma. Again probably due to lens, but I could be wrong.
:ship2: Might be a UFO :P

[middy]

Yes it is Canopus. I tried to look for the LMC but I don't know what I am looking for having never seen it before. I can never see it in the sky and I certainly can't see anything different in the bottom left of the photo. I'm glad one of us knows what we are doing. :P

[middy]

I am using the lens built into the camera. ;) Unfortunately it is not a DSLR with interchangeable lenses. :(

[Itchy]

Yep, that will be the problem. The lenses in most point & shoot digital cameras are not really up to astrophotography. Even with a DLSR, you would need to spend a lot on money on an Apochromatic lens to minimise this type of chromatic aberation. And I do mean lots (>$3K).

Most of us simply put up with the problem and try to minimise its effects through various software solutions. It is almost impossible to get rid of it completely.

Having said that, don't loose heart. you can still get some great shots with a point & shoot.

cheers

[[1ponders]]

It's very faint but it's there. I estimate you took the photo sometime between 9:30 and 10:00

[middy]

Very close ..... it was a series of shots between 10:00 and 10:15 pm.

Now lets see if you can tell my lat/long from the image (to the nearest minute please) :lol:

[RB]

27.583 South
153.033 East

;)

[middy]

:doh: D'Oh! I forgot I'd entered my lat/long in my profile ...... cheater!!! :lol:

[RB]

:hi: Couldn't resist middy.
I like your pics BTW.

[middy]

Thanks. I am going to move to wide field shots for a while now that the scotch mount is almost complete. I am hoping to get a ball and socket mount today so I can image areas of sky other than SSW and SSE. :)

[tornado33]

there is a filter called a Minus Violet filter, that is specifically designed to remove these blue and violet fringes.
This baader filter
http://www.alpineastro.com/filters/filters.htm#FringeKiller
is said to be even better