[cometcatcher (Kevin)]

I've been experimenting with shooting deep sky through achro refractors, for curiosity to see how much can be squeezed out of one and how well the CA can be managed. Now days with fairly cheap semi-apo's, achros have become redundant for astrophotography.

In the raw, achros produce Chromatic aberration as a result of not being able to focus red, green and blue wavelengths of light at the same focal plane. One of the colours will be in focus, the others will not. Usually this results in blue or red halos around bright stars. See attached examples.

There's a few ways of fixing or reducing the problem without buying another scope. Shoot narrowband, use filters and / or process out the artifacts.

The last few nights I've been collecting data to experiment on the Orion nebula to see if I can outsmart the CA with filters. The Astronomik CLS filter reduces a lot of CA, reduces light pollution and also enhances nebula.

The Baader Semi-apo filter does much the same thing but to a lesser extent. The advantage is it lets through more light.

A simple yellow filter like the Hoya K2 almost eliminates the blue fringe by itself. Colour balance correction can be done in the camera itself.

GSO make a bunch of 48mm filters and I've been experimenting with those also. Their yellow is much deeper than the Hoya K2. It completely eliminates all blue fringe around bright stars but at the expense of a stronger yellow colour shift.

Anyway here's a couple of pics. More subs data would have been nice but the weather and moon had different ideas. There is a plain frame with no filters or retouching for comparison. The camera was an unmodified Pentax K-x.

The deeper frame used all the above mentioned filters and also an orange filter. I would like to have added some red or Ha but now the moon is interfering.