uneven distortion of stars

Helo (Peter) · #1 23-02-2012, 01:58 AM

I have roughly polar aligned my scope and wanted to try and capture some pics of M42 using a 200mm Canon EF lens on a 500D piggybacked on the scope. There is some flaring of some of the larger stars but in different directions (outward from centre). However when I use my scope for prime-focus pics of M42, I don't see these. It seems to be some flaw in the lens optics which I didn't expect with this lens - any suggestions? Thx for any help.

http://i1255.photobucket.com/albums/...7/ORIONALL.jpg

Poita (Peter) · #2 23-02-2012, 09:19 AM

I'd need to see a higher resolution image, but it looks a wee bit overexposed and slightly out of focus which would be adding to your woes a bit.
We use the 200 EF at work, and it is prone to flaring, we can't use it for shots with a bright window in scene for example, so I think it probably is not a great lens to use for astro work.
Try a better polar alignment and doing more subs of shorter exposure times to bring down the flare, or stop the lens down a bit, were you shooting wide open?

multiweb (Marc) · #3 23-02-2012, 09:21 AM

That looks like coma. Stopping down the lens a little might help.

jenchris (Jennifer) · #4 23-02-2012, 09:30 AM

stop it down two F stops - the edge of the lenses are not that clever.

rcheshire (Rowland) · #5 23-02-2012, 09:36 AM

I use a 200mm f/2.8 EF lens and see the same. The stars however are round - image - the bright stars in Orion. Seems to be characteristic of the lens.

Not sure how to fix it, because stopping down has no effect. Perhaps an aperture ring. Not sure.

multiweb (Marc) · #6 23-02-2012, 09:52 AM

Quote:

Originally Posted by rcheshire

I use a 200mm f/2.8 EF lens and see the same. The stars however are round - image - the bright stars in Orion. Seems to be characteristic of the lens.

Not sure how to fix it, because stopping down has no effect. Perhaps an aperture ring. Not sure.

It's a lot more obvious on very bright stars even on axis sometimes. Stopping can create spikes and make the bright ones more pleasing. An aperture ring wouldn't create diffraction spikes.

Poita (Peter) · #7 23-02-2012, 11:50 AM

As I mentioned, we rarely use that lens even for normal photography as it has serious flaring issues. I think it is the lens coating, so not sure if you can do much about it other than try some filters.

rcheshire (Rowland) · #8 23-02-2012, 04:42 PM

Modifying my previous post on closer inspection of your image, are we comparing the same lenses or the same f/ratio? Coma is certainly evident at f/2.8, but not to the extent in your image. Perhaps as Plots mentioned focus is accentuating the problem.

Try f/4 or 5.6 and see what gives. Not sure I agree about lens flare with the L series lens though.

Helo (Peter) · #9 23-02-2012, 06:53 PM

Quote:

Originally Posted by rcheshire

I use a 200mm f/2.8 EF lens and see the same. The stars however are round - image - the bright stars in Orion. Seems to be characteristic of the lens.

Not sure how to fix it, because stopping down has no effect. Perhaps an aperture ring. Not sure.

Thank you all for your feedback, it is wonderful to have such a wealth of experience at IIS. It was indeed wide open at F2.8 so apart from lens quality (which I will have to accept at the moment) and better focus (I may have to accept my failing vision?), I will try stopping it down. Rowland, I love your image, do you happen to remember what settings you used and how many were stacked?

Poita (Peter) · #10 23-02-2012, 07:04 PM

Quote:

Originally Posted by rcheshire

Modifying my previous post on closer inspection of your image, are we comparing the same lenses or the same f/ratio? Coma is certainly evident at f/2.8, but not to the extent in your image. Perhaps as Plots mentioned focus is accentuating the problem.

Try f/4 or 5.6 and see what gives. Not sure I agree about lens flare with the L series lens though.

The L series isn't particularly flarey, but the 200mm 2.8 lens seems to be, we have two and they both exhibit it on different bodies.
I'd ensure it is clean and try it at 5.6 and see how you go. But also try shorter subs to see if it is flare or coma or both.

rcheshire (Rowland) · #11 23-02-2012, 08:13 PM

Hi Peter. 24 x 210 second light frames, but 40 would be much better. f/5.6 - you will get iris diffraction spikes. There is a lot of detail missing, it could do with reprocessing. The camera has been modded with a spectrum enhanced filter - that is the IR filter has been removed and replaced with an Astronomic filter.

There are several ways of obtaining good focus, a basic method is using live view at 10x magnification, focusing on a bright star until the diameter of the star is as small as can be. Focus at f/2.8 and then stop down to f4 or f5.6.