Testing for Chromatic Aberration

[Ratlet]

I'm slowly building my kit up and I'm playing around with image processing to get a feel for things  whilst I build up the courage to pull the trigger on a mount.

I'm planning on keeping things simple and sticking to standard DSLR lenses as I have some kicking around already and there are some that are spoken highly of.  I've gotten a Talumar 200mm lens for my primary optics as the 200mm F4 lens is highly regarded for astro and it seems a good newbie focal length for learning on (also, I have one to hand).

In the mean time I was thinking about testing the lens for chromatic aberration.  Basically using an artificial star to photograph and then check for aberration.  I can change the framing of the star to test the extremes of the FOV.  The thinking being that by identifying the capabilities of the lens I will be better placed to identify issues with my technique..

Is this something that is doable/sensible?

How important is distance?  I would imagine putting the star as far as possible is best, but would there be a significant impact carrying out the test when the 'star' isn't in focus at infinity?

I've had a google and everything seems focused more on colliation than chromatic aberration.  I did find one reference which say the method is not ideal for spherical aberration due to the distance required (66m in my case).

I've had a play with some tin foil, a pin and a head torch and I can certainly get purple fringing, and eliminate it with stepping down and see the diffraction spikes from the iris when stepping down.

Many thanks and clear skys!

[Nik271]

You don't need to bother with artificial star if you want to test chromatic aberration: focus on a distant tree branch or telegraph pole or TV antenna against a bright sky. Take a picture and look for purple fringing. 

You can repeat this with real stars, Polaris for example is white and ideal for testing purposes as it doesn't move, any chromatic aberration will show as a halo around it.

 

Even if you spot chromatic aberration it's most likely to show only on the brightest stars. It's a bit unsightly but should not be a deal-breaker for the lens.

Edited March 22, 2022 by Nik271