[ctron]

Been using the KX for astronomy for a while, but at home my background skies are just a touch above my remote site in terms of light pollution and I want to try to negate the pollution. I am considering the Astronomik CLS, but have some concerns. First of all, it looks like I need the screw on the front lens of the camera kind as the CLS clip version won't fit my camera, is that correct? Also, as I have found out with unmodded Canon cameras, the CLS causes a color shift to blue which some find difficult to correct color balance. With an unmodded Pentax, is the color shift also present as well? How long does exposure need to be increased to offset filter losses? Thanks in advance for your help....

[JimmyDranox]

In fact, colors are not shifted, but transmited in different proportion. And Astronomik CLS do this, not so much as an UHC filter, but still do. AFAIK, all pollution filter do this more or less. From what I have herd, IDAS LPF has the smallest ''shift'', but I cannot confirm.

About exposure, I can't tell exactly, but I think that about two stops. I can't tell exactly because I had a 2'' Astronomik CLS for a few days, and I have used it visually, to see the efect, but I didn't take pictures, so my estimation is not precise.

[Learjet]

I have the 48mm (2") CLS. Yes it does produce a blue colour cast that can be difficult to balance. Two stops sounds about right. In general I prefer the Baader Semi-Apo filter. Virtually no colour shift and only 1 F stop dimmer. Or you could go for the Astronomik UHC-E which also claims to have little colour shift.

The 48mm filters are designed to be used inline down the optical path of a telescope. With most telescope to camera adapters they just screw in.

[Learjet]

Here is a single frame, about 61 seconds at ISO6400 and the CLS filter. This is with no post processing but in camera balance settings shifted away from blue. The problem with this filter and an unmodified K-x, when the temperature gets warm in summer it can suffer from red clipping making the red channel very noisy. With a full spectrum modified camera it's much better.

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