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UHC and the Moon

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#1 mic1970

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Posted 10 October 2019 - 08:25 AM

I have seen some of the big YouTubers switch to narrow band filters Ha and OIII when the moon is out and getting nice results.  Does a UHC filter have too much bandwidth frequency wise to be of use when the moon is out?

 

 



#2 Midnight Dan

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Posted 10 October 2019 - 09:21 AM

Generally speaking yes, a UHC has too much bandwidth.

 

But, there are a lot of variables.  The amount of light pollution your image will receive from the moon depends on how illuminated the moon is, how far from the moon you're imaging, and the level of transparency of the sky.  A night with poor transparency has lots of aerosols in the sky, which scatter the moon's light and make light pollution much worse.  On a night with excellent transparency, with say a 50% moon, and imaging to the north, the moon will have less effect than you might think.

 

And even though an UHC won't do nearly what a narrow band will, it will certainly help.  Any filter which blocks out portions of the wide spectrum will reduce light pollution.  Some UHC's have very wide bands, others have narrower ones.  Nebula filters like the IDAS LPS-V4 have very narrow bands, and the new Optolong L-Enhance is even narrower.  So the narrower your filter, the more it will help.

 

For myself, if I'm imaging away from the moon on a night of good transparency, with my IDAS LPS-V4 filter, I'll image with a one-shot color camera if the moon is less than about 50% illuminated.  The subs will not be as good as on a dark night, but very usable.

 

-Dan


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#3 WadeH237

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Posted 10 October 2019 - 04:07 PM

I would not think that using a UHC filter in moonlight would offer much benefit for imaging.

 

In general, light pollution filters work by blocking wavelengths of light that are commonly emitted by terrestrial sources, but not by astronomical targets.  The problem with moonlight is that it's a very broadband light source, and emits all the wavelengths that sunlight emits.

 

A UHC filter works by allowing light through that is emitted by ionized oxygen (OIII) and hydrogen (H Beta).  There are two problems with this for imaging.  The first issue is that moonlight is a very strong in OIII.  For imaging in moonlight, you want as narrow a filter as you can get.  Something like a 3nm OIII filter is about the best you can do.  Such a filter would probably be worthless for visual, but is excellent for imaging.  The second problem that you'll have is that your UHC filter blocks Ha, which is probably the main wavelength that you want to catch.  All of that wonderful, red nebulosity that you see in astro photos comes from this - and your UHC filter blocks it completely (and there's not much point for any visual filter to pass it, because your eyes are not very sensitive to it at all).

 

And finally (and you may already know this), but narrow band filters of any kind will only work with objects that emit light in the specific bands that they pass.  Most commonly, that will be emission nebulae that glow red from hydrogen, and planetary nebulae that are rich in oxygen, with some hydrogen.  Narrow band filters will not work for reflection nebulae, dark nebulae, galaxies and star clusters (well, since stars are broad band light sources, you'll see them - but their color will be seriously messed up by using narrow band filters).

 

Finally, despite all of this, if you already have the UHC filter, why not give it a try?  It certainly won't hurt anything, and you might get an interesting result.



#4 Midnight Dan

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Posted 10 October 2019 - 07:27 PM

... The second problem that you'll have is that your UHC filter blocks Ha

 

Some do, some don't.  For example, the Astronomik UHC passes the Ha and SII area. 

 

http://tinyurl.com/y2y3jckc

 

-Dan




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