Is Coma Fixable in Post for Astrophotography?

[supaspiffy]

Hey fellow Photogs/Canonistas/Canon shooters or whatever you happen to call yourselves. I've been lurking for a few months in your fine forum, absorbing as much knowledge as I can of this exciting world I find myself in. Now I guess I'll make my first post.

I'm interested in getting into astrophotography and with some research, I know what equipment I need. I already have a 6D and a Tamron 24-70 f2.8. I've heard about the coma with this lens, but is it correctable in post? Can coma at all, be fixed in Lightroom or Photoshop and if so how? Is it as simple as selecting the right camera profile? With the Rokinon 14 f2.8 at only $300, I can always get it with not much trouble... but if I can get very similar results with the Tamron + post corrections, I'd rather save the $300 and the space in my camera bag.

 

[noisejammer]

Yes, but with some difficulty. I suggest you look at the Richardson-Lucy transform. You will find implementations in MaxIm and AstroArt - perhaps in others too.

 

[m8547]

Does the point spread function vary across the image? Or does the function completely describe the aberrations across the lens?

 

[noisejammer]

Does the point spread function vary across the image? Or does the function completely describe the aberrations across the lens?

Coma is a radial aberration so the psf varies radially. Richardson-Lucy deconvolution was developed to allow correction for this. Since stars are essentially point sources, it's quite easy to determine the psf over the entire field.
If memory serves, it was originally a software kludge that allowed the space telescope to be used while COSTAR was being developed.

 

[supaspiffy]

Thanks for that but when I did some research, I found a lot of dense information on the topic and thought that I had stumbled upon scientific research papers more than a few times. Unfortunately, I'm not very scientifically inclined and most of it flew past my head. I thought/hoped there would just be a plugin or filter I can use in Photoshop/Lightroom to fix the coma even if I had to purchase it.

 

[NancyP]

Coma may or may not be objectionable. It appears with very bright stars in the extreme corners of the field. Bad coma looks like a radial flattened V (seagull wings). It may or may not matter in your composition, depending on the positioning of bright stars and on the size of the print. Personally, I am fine with "square" stars (a little bit of coma), or even slight parallelogram stars (a little more coma) most of the time. I object tp seagull wings - which is why I don't use any of the stable of good old film-era 50mm lenses I have, all of which have terrible coma even down to f/4. I am imaging at 14mm, 21mm, 35mm on 6D. I usually place the most interesting area of the Milky Way or large constellation of interest away from the far corners, anyway.

My advice is to take your Tamron to a perhaps not so scenic dark location nearby and try it at different focal lengths and apertures. Remember the (600 / focal length) seconds rule for avoiding star trailing. That's 25 second exposure for 24mm lens, 12 seconds for 50mm lens, etc.

If you enjoy astrophotography, sooner or later you will break down and get that Samyang 14mm f/2.8 - great lens, huge bargain.

 

[supaspiffy]

Guess what Nancy, I already did lol. Tonight was going to be the first time I would've given her a try but just as luck would have it, it ended up being a cloudy rainy night here at Chandler, AZ. Anyway I figured it was more work researching this fangled equation (I'll leave it for the brainiacs) rather than just pony up the 300 for this jewel.

 

[lintoni]

Guess what Nancy, I already did lol. Tonight was going to be the first time I would've given her a try but just as luck would have it, it ended up being a cloudy rainy night here at Chandler, AZ. Anyway I figured it was more work researching this fangled equation (I'll leave it for the brainiacs) rather than just pony up the 300 for this jewel.

Congratulations on your new lens!

If you use Lightroom, mrsfotografie provided a correction profile for use with this lens here:

http://www.canonrumors.com/forum/index.php?topic=17252.msg366306#msg366306