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Is their any way to remove significant coma from a Sigma 18-250 lens?

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#1 Mordred182

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Posted 06 December 2021 - 07:00 PM

I have found some sever coma on all of my subs with my sigma 18-250 lens. Stars in the center seem to be ok but everything on the outside of a small area is completely useless.

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  • L_4183_ISO800_300s__10C.png

Edited by Mordred182, 06 December 2021 - 07:00 PM.


#2 firemachine69

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Posted 06 December 2021 - 07:45 PM

You'll have to stick to cropping. That lens never did live up to it's Nikon/Canon rivals.


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#3 xonefs

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Posted 06 December 2021 - 08:03 PM

an 18-250? what do you expect from a super zoom for astrophotography?



#4 whwang

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Posted 06 December 2021 - 09:46 PM

There are ways to reduce coma in post processing.  If you have a picture that's once per life time and ruined by coma, it will be worthwhile to go through the troubles.  On the other hand, if you just plan to use this lens to take many more pictures, the suggestion is don't.


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#5 900SL

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Posted 07 December 2021 - 01:16 AM

You can't turn a donkey into a racehorse



#6 sharkmelley

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Posted 07 December 2021 - 01:20 AM

The stars all look pin sharp in the 276x182 image you posted.


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#7 Tim J Fowler

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Posted 07 December 2021 - 07:50 AM

CN restrictions are demanding, but clicking on your thumbnail is about as useful as an elevator in an outhouse. My old eyes aren't nearly good enough to see a problem.


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#8 KLWalsh

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Posted 08 December 2021 - 11:18 AM

If it truly is coma, you might try stopping down the lens and increasing the exposure time. Since coma goes as the square of the inverse of the f/ratio, a lens at f/5.6 *might have* 1/4th the coma of the same lens at f/2.8. (The larger the f/number, the smaller the coma - in a very general sense.) It’s worth a try to see if it helps.

#9 vidrazor

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Posted 08 December 2021 - 01:14 PM

A lens with that much of a zoom ratio will never look good, no matter what brand it is. You're better off getting an old prime telephoto from the 70s if you can't afford a decent modern telescope, although probably even an inexpensive low-end scope may give you better results.


Edited by vidrazor, 08 December 2021 - 01:16 PM.


#10 OldManSky

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Posted 08 December 2021 - 02:17 PM

"Is their any way to remove significant coma from a Sigma 18-250 lens?"

 

Yes.  Replace it with a Sigma 135mm f/1.8 "Art" lens.  Problem solved :)


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#11 Ranger Tim

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Posted 08 December 2021 - 06:34 PM

To reduce lens artifacts one can stop them down to f/8 or so. Use a step-down filter adapter ring to do this and avoid diffraction spikes. All zooms are not created equal and most do not equal the performance of primes. That said, there are few camera lenses that equal short focal length telescopes, especially apo's and well corrected ones. Those camera lenses that do a good job for AP are well known here. Some have to be "found" by culling through successive examples. Older used lenses can be a good way to do this cheaply without a lot of heartache. It was easier before shipping became astronomical. I have had some legendary Nikkors that were real dogs for AP, and had some low budget ones that were outstanding! Still on the hunt for more -- maybe someday there will be a compendium of lenses for AP, at least for the major brands.

 

Are you trying to shoot at 250mm? Tell us your camera brand/model and maybe someone will have a suggestion for you.




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