IDAS filters and light loss..

[Coco]

I have a 2" IDAS from OSC days, now I'm using LRGB & HA should I use it still, when combining RGB the image is very Orange.

Maybe for my town skies is it worth keeping? I was wondering whether to put it back on and leave it on permanently, but to me its another glass surface, is it a 1/4 stop of light loss?

[rfdesigner]

I'd put it nearer a half stop of light loss.

Baader RGB filters form a notch to take out the low pressure sodium light, so the benefit of the IDAS filter will be muted on the colour frames, the IDAS filter might help the l frame a little.

I'm some 10 miles from Southampton, and here the light pollution is still noticable, but managable (milky way visible farily low down, due south, much better overhead) I get marked skyglow in my red frames but it's no real problem. I'm sure it's eating into my l-frames, but I've still measured my skyglow as being almost 20th mag/sq arc second so I can't really complain.

Derek

[AndyUK]

Hi Guy - I only have a 1.25" version, but I use mine instead of a luminance filter in my filter wheel... (and accept the risk that there might be a slight colour cast as a result).

I personally have no scientific proof (ie with vs without images) that it does actually improve the luminance subs, but I've seen somewhere on the internet where someone HAS done a with/without mono comparison and it does visibly improve the contrast on luminance frames. For RGB, as Derek says, the notch between R and G in the posh Baader's (and other quality filters) nicely coincides with the majority of the main Sodium light bandwidth...

[johnrt]

The notches in the RGB filters do help with cutting out some light pollution, but in my experience stacking the IDAS with the RGB & replacing the lum makes a significant improvement.

Why would using an IDAS result in light loss? It still allows through the relevant bandwidths which we are interested in, filtering out those which we are not - this would not reduce in a need to expose for longer.

Here is an example of a recent image I made using Baader LRGB filters, with the IDAS stacked for the RGB frames (Lum had no IDAS) taken at f/6 all exposures 300 seconds.

[Coco]

Well now my last target is finished I will install it between the filter wheel and reducer then maybe take out the Lum? or why not leave the lum in also as they are all par focal.?

[AndyUK]

Hi Guy - I think for use with the reducer you'll want keep the L in the wheel otherwise it'll mess around with your spacing...

[MartinB]

My location is pretty light polluted and I have done LRGB imaging with and without an IDAS filter in front of the filter wheel. In my experience using the IDAS gives a better result. You still need to use a lum filter matching your RGBs to keep everything par focal

[Coco]

My location is pretty light polluted and I have done LRGB imaging with and without an IDAS filter in front of the filter wheel. In my experience using the IDAS gives a better result. You still need to use a lum filter matching your RGBs to keep everything par focal

Super job ' I'll squeeze it in somewhere..