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MartinB

Filters for DSO imaging

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On 26/11/2008 at 14:35, MartinB said:

Hutech IDAS – like the Astronomik this is an interference filter of very high quality. It’s most important characteristics is that it is probably the most colour neutral LP filter around. This means that it is an excellent choice for DSLRs and one shot colour imaging. It will still reduce transmission of light requiring longer exposures to achieve the same non filtered signal. You will still need significantly longer exposure times – up to double compared with no filtration.

I'd really like to settle the issue in my mind about whether a light pollution filter will need much by way of increase in exposure time. The Hutech blurb says:

Filters increase exposure times.
While light pollution suppression filters will slightly increase the exposure times of emission line objects in their bandpass and affect broadband sources somewhat more, they do not require increasing the length of an exposure. However, because they increase the contrast of emission line objects compared to the sky background, it is usually desirable to increase exposure times to make better use of the imaging media's dynamic range.

This is a little bit ambiguous about white light sources, but certainly if one looks at the transmission curves, the transmission is nigh on 100%, apart from the notches of course. I find it hard to reconcile that the reduced contribution of light intensity due to those notches could amount to half of the total exposure. So when you say up to double the exposure, I presume that that is from practical experience?

Ian

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Hi

That's a very old thread! Yes, lp filters enable longer exposures but don't help if the lp is a white/broadband source.

Louise

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Thanks Louise,

I can see that they will enable longer exposures as a result of a reduction in apparent sky fog, but to what extent do they reduce the light from white light sources like galaxies/stars?

Ian

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49 minutes ago, The Admiral said:

Thanks Louise,

I can see that they will enable longer exposures as a result of a reduction in apparent sky fog, but to what extent do they reduce the light from white light sources like galaxies/stars?

Ian

There is only a small reduction in white light transmission but there will be an affect on colour balance which is easily corrected in processing. You just have to look at the transmission spectrum of the lp filter you're interested in. Lp filters are useful if, for example, you are badly affected by yellow sodium light which can be filtered. However, they are less useful if your sky is blighted by white/broadband light. In that case there can be an advantage to narrowband imaging though there's no substitute for dark skies!

Hth

Louise

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Coming back to this 8 years on I would say that an IDAS filter will reduce transmission a little.  The most significant change however is the reduction in sky glow.  As a result of this you need to expose for longer before sky glow swamps the effect of read noise.  

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I use the  NEODYMIUM  filer by Baadar (to filter out light pollution) which is screwed into the nose piece of my t-ring connected to my Canon DSLR.  I am planning to move to a cooled CCD mono camera and use LRGB filters.  Is it OK to also use the NEODYMIUM filter along with the LRGB, or would I need to remove it?  

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