Light Pollution and LRGB Filters

[badgers]

With my DSLR I always imaged with a UHC-S filter in place as it was very effective at dealing with the LP issues of my city location.

I was always under the impression that RGB filters such as the Baader ones were supposed to be have wavelength gaps to filter out a lot of light pollution.

Since I moved to CCD and RGB filters I took out the UHC-S filter from my imaging setup as I didn't think I'd need it anymore.

Actually the difference it makes is still very significant.

I took out the UHC filter and took around 3 hours of RGB data..... the results were much worse than the previous data with the UHC-S filter in place.

Top two shots are UHC-S and Red Filter, Bottom two are just the Red Filter (10 min subs, auto stretched in PI)

Could seeing explain such a stark difference in contrast ? I didn't see anything too dissimilar about the seeing, these were consecutive nights on 18th and 19th August.

Looks like my UHC-S filter will be staying in place, and my RGB filters seem very susceptible to LP regardless of the advertising.

[RikM]

Interesting post.

I have been thinking about removing the Skywatcher LPR filter which normally lives on the end of my coma corrector. I am also using the Baader LRGB filter set, so was thinking aiong the same lines as you. I.e. I don't need it any more for RGB. The only reason I haven't taken it off yet is that I know I will still need on there for the L subs. Looks like I may as well just leave it in place.

[badgers]

Yes, that's exactly why my UHC was still on, for all the luminance shots.

I'm focusing on Ha RGB right now, so figured I was loosing some signal with the filter in place.

[badgers]

On a second note Rik... Do you find the Baader RGB filters to be parfocal on your 150P ?

I'm seeing loss of focus as I move through the RGB cycle, but I'm not sure whether or not it's focus slip or just that they are not parfocal.

[RikM]

My Lum, red and green seem to be very close but the blue is out sligthly. (I'm going to reshoot the blue for my bubble because the star sizes don't align very well.) I refocussed for Ha but I didn't have to move it very far.

[badgers]

I'm glad to hear that, I was worried my moonlite was letting me down.

[rfdesigner]

Take a look at

http://www.astrosurf.com/~buil/filters/curves.htm

and you will find that there is little suprise.

If you look at the UHC filter trace in the above you'll see it's effectively two narrowband filters in one and light pollution that is dominated by high pressure sodium light is quite broadband, but is almost completely removed by the UHC.

The gap between R and G filters is very narrow and just takes out low pressure soudium and the spike from high pressure sodium.

Derek

[badgers]

That's a really useful link Derek, cheers.

The UHC-S was the first thing I bought on astro buy and sell and it's probably the best purchase I ever made. The difference on DSLR shots was staggering, and it's clearly still making a difference for LRGB.

Anton