Which filter to use?

General discussion about StarTools.

Which filter to use?

Postby ecuador » Sat Nov 26, 2016 11:55 pm

Ok, this is not directly to do with using StarTools, but I am trying to figure out which filter (if any) to use for galaxies under my skies for the intention of giving StarTools the best data I can. Obviously there are no "galaxy filters" but I thought perhaps if a filter cuts more LP than galaxy, perhaps it is better. But since this is the UK and imaging sessions are few and far between, I can't compare by doing full sessions with each filter and processing to see which stack StarTools can take advantage of the most. So I thought perhaps I'd take a shot with each filter and see the levels they get on dark sky and on the object I want and draw a conclusion.
So, for example we have an image with no filter that after linear conversion with DCRAW gives me 10% brightness on dark sky, 13.9% brightness on a particular galaxy on a set exposure. The same image with a broadband filter (Orion Imaging) gives me 5% on the dark sky, 7.4% on the galaxy. Now, the brightness difference fell from 3.9% with no filter to just 2.4%, but the ratio of object/sky is now 1.48 up from 1.39. Does this mean it is better or worse? Another example is a UHC-style (not the very narrow Lumicon) filter which further drops the sky to just 1.5%, but also drops the galaxy to 2.5%, hence only a 1% difference between object and sky, but the ratio is now at 1.67. How about that, better or worse? Any ideas of what I should be looking at? BTW the darks have a background average of about 0.1% for these exposures.
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Re: Which filter to use?

Postby ecuador » Sun Nov 27, 2016 10:53 am

I guess one way to think about it, is that the Orion filter cuts 50% of LP and 38.5% of galaxy light, a ratio of 1.3:1, while the UHC cuts 85% of LP and 74.4% of galaxy, i.e. 1.14:1 ratio, which tells me that it would be a good idea to go with the Orion for the same exposure. But still it would be hard to make other comparisons, like UHC vs no filter - the UHC cuts a bit more LP than galaxy, however it really cuts a lot of useful light, so it will be more affected by random noise. Thoughts? :)
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Re: Which filter to use?

Postby ChrisLX200 » Sun Nov 27, 2016 10:58 pm

Either way you're going to be introducing a colour bias - due to the LP (probably Red) or due to the filter itself (probably Blue or Green). You would have to discover for yourself which you find most objectionable and/or easy to correct during processing. I have an IDAS LP filter which I used to use with a DSLR and that cut the amount of red mottling, but it did result in a blue bias which I found easier to correct than removing the red.

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Re: Which filter to use?

Postby ecuador » Mon Nov 28, 2016 12:01 am

I have no problem getting the colors I want from almost any filter thanks to StarTools, so color balance is not a concern for me. Light pollution is though.
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Re: Which filter to use?

Postby ChrisLX200 » Mon Nov 28, 2016 12:26 am

What kind of light pollution do you have? LP filters are becoming less effective because of increasing use of LED lighting. The old orange Sodium and white Mercury lights are being quickly phased out - driven by power savings offered by LED lights. Pollution from broadband LED lights cannot be blocked.

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Re: Which filter to use?

Postby ecuador » Mon Nov 28, 2016 10:49 am

ChrisLX200 wrote:What kind of light pollution do you have? LP filters are becoming less effective because of increasing use of LED lighting. The old orange Sodium and white Mercury lights are being quickly phased out - driven by power savings offered by LED lights. Pollution from broadband LED lights cannot be blocked.

ChrisH


We still have way more sodium lights than LED. As I said, the LP filters cut more LP than they cut stellar light, but I wanted ideas about how to judge at which point cutting more light overall is worse, even though most of it cut is LP. I have precious few clear nights (and they "end" arbitrarily) so I can't really set up entire similar sessions on the same object to see which filter comes out ahead, so I have to get a best guess, should I use Orion, CLS, something less or more broadband or nothing at all on a galaxy... From shooting different objects, I had the feeling the CLS filter gave me the best results for galaxies (and with ST I had no problem color balancing despite the strong original color hue), but It was not the same objects/exact set of exposures as with other filters I tried, so I can't be sure...
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