How much does a light pollution filter help?

[cs_pixinsight]

I'm considering a light pollution filter for use when I can't get to my normal dark site.  However, I'd like to see the difference it makes before spending the money on it.  I current image with a Canon XTi from 20mm - 320mm effective focal length, so I'm thinking a clip in filter may be my best option.

Does anyone have a visual comparison of the same astro target taken from a light polluted site with the filter, the same light polluted site without the filter and at a dark site without the filter?  If you could describe the light pollution level at those two sites via the Clear Sky Clock light pollution descriptions at the bottom of http://cleardarksky.com/lp/CstlRckCOlp.html?Mn=astronomy  it would be most helpful for comparisons to my sites.  I know this is asking a lot, but I'm hopeful.

I'd like to see how much the filter helps in the light polluted site vs. going to a dark site.  If you don't have any comparisons I'd still love to hear your opinion on how much you feel it helps and which filters you have tried and recommend.  I'm leaning toward the Astronomik CLS clip filter at the moment.

Craig

[pfile]

i do all my imaging from a red zone. i have never imaged outside a red zone, so i don't have any personal images to compare to.

i use the astronomik cls with a modded canon 50d.

the cls is something like a broad-spectrum Ha filter combined with a broad-spectrum OIII filter. not exactly, but close. many people using the CLS complain of difficulty when trying to color balance their images, but it's completely trivial with pixinsight. in fact, just doing an auto-STF with the channels uncoupled gets you 80% of the way there.

i'd get the ccd version of the CLS even if you don't need it, just because it has slightly better transmission characteristics.

here's one image taken thru the CLS...

[astropixel]

The CLS-CCD filter is better with DSLR's. I have used one, but exposure times need to be increased substantially,  not helpful with unguided imaging.

Under my sky 3.5 minutes at iso800 is optimal with no filter. Increase by 70 - 100% with a CLS-CCD filter.

Here is feedback from Astronomik - asking the same question.

The reasonable extention of the exposure time depends on at least the following factors: sensor of the camera, light pollution, tracking quality and brightness of the object that is imaged.

We recomend the longest exposure time that meets the following cirteria:

*Your tracking is perfect on at least 80% of your images

*The backgroung is not brighter then 15% of the dynamic of your sensor

*The brightest portions of the object is not overexposed (This works for many objects, but Objects like M42 need seperate short exposures for the very brightest areas)

Typicaly that leads to 1 to 15 minute exposures.

I hope that this is a key fo your to get the most reasonable exposure times for your setup, object and location.

[ruediger]

I'm using an IDAS LPS-P2; the performance should be comparable to the Astronomik Clip-In Filter. The IDAS filter is basically a 37mm threaded filter that can be placed inside the camera body, but can also be used in front of the lens, together with some cheap step-down filter adapters. This has the two advantages, that I can use the filter also on a full format camera and can avoid stopping down my favorite lens (EF 2/135mm), therefore not getting diffraction spikes.
The filter itself is able to reduce skyglow by about 1 EV (you can double your exposure time), therefore improving SNR by about sqrt(2). Example shot from my light polluted balcony (IDAS front-mounted, single shot 4:25min, unstacked): http://www.flickr.com/photos/urlaubsknipser/6198250916/sizes/o/in/set-72157629786380045/

Rüdiger

[cs_pixinsight]

Guys, thanks for the input.  Looks like it may well be worth the money from my backyard anyway.  Getting 1EV+ extra signal strength will get me closer to the 3EV I get from my dark site.  Some additional questions if you don't mind.

I assume that the clip in filters would eliminate any worry about using the wide angle lenses since the light would be coming in fairly perpendicular at that point.  Would you agree or do I still have to worry about short focal lengths for wide angle shots at 20-50mm?

Will any of these filters help with shooting when the moon is present or are narrow band filters necessary for this?

Feel free to comment on my question regarding the Astronomik UHC vs CLS-CCD to astropixel if you have some insight.

pfile:  I was thinking that PI would help a lot with any color balance issues - thanks for confirming that.

astropixel:  I see you say the CLS-CCD is better with DSLRs.  I also noticed that Astronomik's site does the same in comparison with their UHC filter.  Do you happen to know why?  The smaller pass bands and higher transmission levels on the UHC would indicate better rejection of unwanted frequencies, but perhaps the CLS-CCD is optimized for DSLRs in some way that I'm unaware of.

ruediger:  Your suggestion of using the filter on the front of the lens on full frame cameras is a definite plus.  At only 37mm, do the step-down rings cause really bad vignetting to the point flat fielding doesn't fix it and you have to crop the image to remove the edges?  My lenses have 77mm front elements, so I'd be cutting almost half of my diameter with the step-down ring.  Right now, I could use this filter inside my camera, but I may go full frame in the future if they come down a bit in price.

Was your image off a full frame camera and if so, is it cropped at all?

Thanks again,
Craig

[astropixel]

Craig. I mean by comparison to the CLS filter. Personally, I found the filter useless for short exposures of a minute or so when imaging M42 for instance. The practical advantages over longer exposures is a different matter.

[pfile]

Will any of these filters help with shooting when the moon is present or are narrow band filters necessary for this?

i doubt it. i'd go for narrowband in that case.

[ruediger]

ruediger:  Your suggestion of using the filter on the front of the lens on full frame cameras is a definite plus.  At only 37mm, do the step-down rings cause really bad vignetting to the point flat fielding doesn't fix it and you have to crop the image to remove the edges?  My lenses have 77mm front elements, so I'd be cutting almost half of my diameter with the step-down ring.  Right now, I could use this filter inside my camera, but I may go full frame in the future if they come down a bit in price.
Was your image off a full frame camera and if so, is it cropped at all?

The flickr NGC7000 was taken with a EOS 500Da + EF 2.0/135mm and is shown uncropped. Don't worry about vignetting! Even with full format 5D II + EF 2,0/135mm and 37mm front aperture, I measured a light falloff from center to edge of only 40%, easily corrected with flats. For shorter focal length lenses (I use 1.4/24mm + 1.2/50mm) the front lens diameter is much smaller than the filter thread, so there is even less vignetting. But these two lenses must additionally be stopped down for acceptable star sharpness, so you will still get diffraction spikes.
37mm of course is not enough for focal lengths larger than 200mm, because you get very small effective apertures. But the IDAS filter is also available in 72mm width.
Rüdiger