Strange shaped stars...why?

[Jezphil]

I've been taking wide field views of The Milky Way with a Canon EOS Ra camera and two lenses (Canon 15-35 and 24-105) set at anything from 15mm to around 30mm. I'm using a SkyWatcher Star Adventurer, carefully polar aligned, for tracking at anything from 45" to 2' exposure. The aperture is usually quite wide, up to f/2.8. ISO between 400 and 800. Whatever variations I have been using, I am getting strange artefacts on the outer, larger stars. I've blown some up in the image here. What's causing this? It's driving me crazy. Whatever the settings, I can't seem to get rid of them. 

 

 

Artefacts.tif

[vlaiv]

23 minutes ago, Jezphil said:

The aperture is usually quite wide, up to f/2.8.

Lens don't perform good at the edges of the field when using wide aperture settings.

If you want good correction to the edge of sensor - try stopping down lens to F/4 or even less.

[Craney]

Looks like a form of Coma.

Found this test picture on-line...

http://2.bp.blogspot.com/-VaJDcEcHYAI/USeMN1rSCgI/AAAAAAAADKM/4cRbKXIWpMM/s1600/24mm-coma-test_800px.jpg

Do a google image search of  "star aberrations"    and you will get some similar shapes.

 

[Jezphil]

28 minutes ago, Craney said:

Looks like a form of Coma.

Found this test picture on-line...

http://2.bp.blogspot.com/-VaJDcEcHYAI/USeMN1rSCgI/AAAAAAAADKM/4cRbKXIWpMM/s1600/24mm-coma-test_800px.jpg

Do a google image search of  "star aberrations"    and you will get some similar shapes.

 

Ah! That's it! The exact same shape at the wider apertures. Thank you for this.  Thanks also to Vlaiv for cracking this one - I will stop down. Much appreciated.