Weird inward / center pointing triangled stars

[astrobena]

For the past few weeks i've notived that on all widefield images my stars (except for the middle of the image) are not round, but a traingle shape, with the tip always pointing into the middle. Probalbly some kind of lens distortion but really weird. I've attached an image, shot at 28mm. You can see this best in the corners... if anyone has any ideas why this might be happening and how I could fix that I would be very grateful.

01_05_2021_22_23_29_0077.CR2

[Stargazer33]

It might be helpful if you could post an image rather than a link. PNG is a good lossless format. Also, what equipment are you using?

[michael8554]

CR2 file suggests a Canon DSLR, you didn't say ?

Was this a zoom lens at 28mm, you didn't say ?

Looks like Coma, the amount may depend on what focal length was used.

For daytime snaps coma doesn't really show, so to save money often the makers don't correct it.

Michael

[astrobena]

On 26/05/2021 at 09:14, michael8554 said:

CR2 file suggests a Canon DSLR, you didn't say ?

Was this a zoom lens at 28mm, you didn't say ?

Looks like Coma, the amount may depend on what focal length was used.

For daytime snaps coma doesn't really show, so to save money often the makers don't correct it.

Michael

 

On 25/05/2021 at 22:43, Stargazer33 said:

It might be helpful if you could post an image rather than a link. PNG is a good lossless format. Also, what equipment are you using?

Yes sorry, it's a canon eos 400D with kit lens @ 28mm, f/4 ...
Is the only way to correct this digitaly (f.e. with photoshop) or buy a better lens or what do people do to avoid this?
 

[Astro Noodles]

My first reaction would be to suggest 'pinched optics'. Although that doesn't seem likely in May. Perhaps it is camera wobble caused by mirror/shutter activation.

Edited May 30, 2021 by Astro Noodles

[vlaiv]

31 minutes ago, astrobena said:

Yes sorry, it's a canon eos 400D with kit lens @ 28mm, f/4 ...
Is the only way to correct this digitaly (f.e. with photoshop) or buy a better lens or what do people do to avoid this?

It is very hard and next to impossible to digitally solve such issue on whole image. Although it is easily seen on stars - this sort of blur affects whole image.

You can always stop down your lens a bit - that tends to solve some of such issues.

Another option is to get better lens - one that is sharper / has less aberrations.

Third option - one that is most involved is to use longer FL better quality lens - or even better - small scope that is properly corrected and diffraction limited - and make mosaics to get really wide field shots of the sky. This option is really involved as it requires you to take numerous shots - and then process them in particular way - like mosaic stitching that is by no means easy on wide field as there are geometric distortions in the image (Microsoft ICE does good job of this but it is no longer available for download from Microsoft download site). For example, in order to create equivalent image to 28mm lens with scope like this:

https://www.firstlightoptics.com/william-optics/william-optics-redcat-51-v1-5-apo-f49.html

Would require about 81 exposures / panels and 9x9 mosaic.

Another good lens that you can use would be this one:

https://www.samyanglens.com/en/product/product-view.php?seq=311

With that - you would need only 9 panels and 3x3 mosaic. Bonus would be shooting at F/2.8 (F/1.4 and F/2.0 are a bit soft for astronomical use).

[alacant]

On 25/05/2021 at 22:36, astrobena said:

how I could fix that

Hi

It's easily fixed in software. To perfect it,  stop down the lens to around f5 (not using the lens diaphragm) and use the app.

If you are serious about 28mm, then a cheap way is one of the old Takumars.

HTH

Edited May 30, 2021 by alacant