Struggling to get sharp stars

[jackp93]

Hi all

sorry if this is a silly question but I’ve just bought a new Nikon d750 and 50mm lens and the last couple of nights I’ve been trying to do some astrophotography with it however I cannot get the brighter stars to go sharp. The fainter ones don’t appear too bad but the brighter ones tend to have a disc around them. I did think maybe it was just a bad evening but I have tried it twice now with the same outcome. What’s causing this? 
 

 

thanks

Jack 

 

[happy-kat]

When you focus are you right at the end of focus travel, or do you get better focus back from maximum focus travel?

[jackp93]

Hi happy kat

no I started by going to infinity on the markings but found it was slightly off so I zoomed in on live view and adjusted until it was as sharp as I could get.

[happy-kat]

So your best focus of back from maximum focus travel, then that's good.

The next reason might be the lens is exhibiting chromatic aberration.

You might need to stop it down a bit to improve stars across the range, but try to focus where the thirds intersect not in the centre.

[Alien 13]

I would say that the abundance of tight faint stars suggests you focus is good and is the method I use to get the focus correct. The distortion with the brighter stars is down to the lens itself and sky conditions but can be improved by stopping down as Happy Kat mentioned or fixed in post processing or even by using lens correction data in PS or whatever.

Alan

Edited June 23, 2020 by Alien 13

[jackp93]

My attempts have been with it wide open at 1.4 so as long as the weather stays I’ll try stepping it down  tonight, hopefully that will sort it. Thanks for all the info.

[jackp93]

Hi Alan 

thanks for the info. I use affinity photo so will have a look around that and find the lens correction tool.

 

thanks

 

Jack

[Alien 13]

40 minutes ago, jackp93 said:

Hi Alan 

thanks for the info. I use affinity photo so will have a look around that and find the lens correction tool.

 

thanks

 

Jack

Hi Jack, I use Affinity too and find it brilliant. If you open a RAW file it will show the RAW file editor which allows you to correct for lens defects on specific Canon lenses, there are adjustments for Geometry/De fringing/Chromatic aberration and Vignetting plus lots of other stuff.

Alan 

P.S. Just had a quick look in Affinity and it seems to list every lens on the planet in its correction tool 

Edited June 23, 2020 by Alien 13