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A Galaxy Filter for Light Polluted Skies.

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#1 Otto Piechowski

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Posted 25 January 2013 - 12:21 PM

The post on the 82 Wratten filter for galaxy observing is the cause of this post.

For visual astronomy only; regarding the use of filters and other observing techniques for enhancing the views of galaxies in light polluted skies:

1. What have you tried and how successful has it been?

2. Of what (filters/observing techniques) have you heard?

3. What might be tried?

Otto

#2 blb

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Posted 25 January 2013 - 02:06 PM

I have used the Orion Sky Glow with some success on gakaxies. It does not help much but there is some improvement from the filter darkening of the sky. I live in a white zone where the naked eye limiting magnitude varies from about 3.7 to 4.3

Although I have not used one, DGM makes a filter for galaxies that, I think, works like the Sky Glow filter.

#3 David Knisely

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Posted 25 January 2013 - 02:09 PM

A broad-band "Light Pollution Reduction" filter like the Lumicon Deep-sky, Orion Skyglow, or Astronomik CLS may help slightly on some of the larger more diffuse galaxies, but mainly only under mild light pollution or skies that are already fairly dark. Under heavier skyglow, no filter will help much at all, although with some of the smaller galaxies, increasing the power may help make them more visible. DGM Optics makes a broad-band filter that also has some effect on galaxies, but again, it is most effective when the sky is already fairly dark:

CN REPORTS: The DGM Optics GCE Filter

As has been mentioned before, the best filter for galaxies is a "gasoline" filter (i.e., drive as far away from city lights as you can). Clear skies to you.

#4 Ed D

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Posted 25 January 2013 - 07:40 PM

I have tried M&SG, UHC, etc., and none have helped me. I live in suburban Miami with bad LP. I hate to sound like a broken record, but the only filter that has worked for me is the 'gas' filter David and many others refer to.

What has worked is to observe late on a Sunday night, or any other night when LP is at its lowest. The night after New Year is another good one, when most are tired (hung over) and the stores are closed. A smaller scope that can be left set up by the door and will acclimate relatively quickly is another one I do. If I wake up in the wee hours of the morning I can bring the scope outside and observe for a while, although my dogs think I'm nuts and I feel like a zombie later on at work.

Ed D

#5 starrancher

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Posted 25 January 2013 - 07:49 PM

I have tried M&SG, UHC, etc., and none have helped me. I live in suburban Miami with bad LP. I hate to sound like a broken record, but the only filter that has worked for me is the 'gas' filter David and many others refer to.

What has worked is to observe late on a Sunday night, or any other night when LP is at its lowest. The night after New Year is another good one, when most are tired (hung over) and the stores are closed.

Ed D


The dark sky real estate filter works great too . If you have one of those , you don't really need the gas filter .
:grin:

#6 Feidb

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Posted 26 January 2013 - 01:32 AM

I've found that if you need a filter, you should stick with planets or clusters.

On the other hand, if you have no choice, broad band is about the only way to go.

Under dark skies, I've experimented around with filters on the showpieces. I've used an O-III on the bright galaxies like M-33 and M-32 and have had mixed results finding some of the IC and NGC galaxy knots. The same with the UHC. The H-beta didn't help at all, so far.

I tried an LPR which I guess is the same as broad band (I guess that's what they were called in the 90's?) and it helped a bit on the galaxy knots also.

However, those were very bright galaxies and for faint fuzzies, no filter really does anything because when it comes to taking something away, there just isn't enough light to go around, at least not in my experience.

#7 nytecam

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Posted 01 February 2013 - 12:39 PM

...The dark sky real estate filter works great too.... :grin:

I've got one of those filters for my cam called 'Remove sky background' and it works even with severe LP or moonlight - how else could I survive :grin:

#8 tnakazon

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Posted 01 February 2013 - 10:00 PM

I also find the broadband filter useful in sketching large emission & planetary nebulae when I just want a moderate increase in nebulae contrast without darkening the background sky too much so as to keep enough of the background field stars visible in my FOV.

Me and a friend once noticed that the Andromeda galaxy showed somewhat improved contrast when using our broadband filters on our 100mm F/4 Orion SkyScanner scopes in a green/blue-zone dark sky.

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#9 jgraham

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Posted 02 February 2013 - 10:42 AM

I have an on-again, off-again relationship with filters for visual deepsky observing. I've had mixed luck with Orion's broad band filters. The best I've used was the Baader semi-Apo filter, which combines their Fringe-killer and Moon and Skylow filters. The one that intrigues me the most are imaging skyglow filters. These are different in that they remove narrow bands of light to preserve as much color as possible. The color doesn't mean much for visual, but they do pass more light than broad band filters. I recently bought a 2" Orion imaging skyglow filter for use with my Lightbridge 16 since it has a lot of light to work with. I also happen to have a 2" semi-Apo filter. I'll give these a try as soon as we get any clear weather, but that may not be until spring!

#10 jgraham

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Posted 07 February 2013 - 01:26 PM

Well, I finally had a chance to try out my new 2" Orion Imaging Skyglow Filter for visual observing. I've been using interference filters designed for imaging for visual observing for quite some time as they tend to have higher transmission and sharper spectral cut-offs than the simpler (and less expensive) colored glass filters that are common for visual use. The imaging sky glow filters are interesting in that they selectively remove narrow bands of the spectrum in an effort to retain as much of the color information as possible. For visual observers this translates into more light passing through the filter while at the same time reducing skyglow.

As filters go I was reasonably inpressed with this filter. I used it on my Lightbridge 16 with a Meade 24mm UWA eyepiece with and without a 2x Barlow. The sky conditions were on the poor side of fair with moderate seeing and poor transparency (the best we've had in a month). The effect of these filters is always subtle and this was no exception. Overall I noticed a significant drop in sky brightness without a notable drop in the brightness of the object resulting in increased contrast. The improvement was very subtle for bight targets like M42 (a bit more of the fan was visible with the filter and the structure in the core stood out better), but it was immediately apparent for faint targets like M1 and M79. For the first time ever I had absolutely no problems finding M1 from my back yard. It wasn't bright and contrasty by any means, but easy to see as a ghostly puff of smoke rather than a slightly brighter smudge of sky glow. M79 was particularly nice as it is so low above the horizon and buried in schmuck.

Soooo, I give this filter a modest thumbs-up. It's not like turning on the lights, but it is a nice addition to my toolbox.






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