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Do longer exposures = more noise with NB DSLR?

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#1 Footbag

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Posted 30 November 2012 - 11:49 AM

My goal is to get my rig ready for some NB imaging from my red zone site. I'll probably be using an NB Ha EOS clip in filter. But I've been wondering. Since I'm going to be taking 20-30m exposures, will I need more lights to get rid of the additional noise? Or will the noise from the longer exposures average out easily?

In other words, would I be able to get away with taking 8 30m exposures and applying darks, or would I need considerably more lights to average out the noise?

#2 Jerry Lodriguss

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Posted 30 November 2012 - 01:50 PM

Hi Adam,

It's not so much the noise that you have to worry about, it's the signal-to-noise ratio that is more important.

It's a little bit of a misconception that longer exposures produce "more noise" and that that is a bad thing.

What most people see as "noise" in their image is actually made up of thermal signal and true noise. You can remove the thermal signal with darks.

You can't remove the noise. But you can reduce its effect by increasing the signal by stacking more exposures.

The photons collected in an exposure actually have noise associated with them also. This is called photon or shot noise. It increases with longer exposures, BUT ... the signal goes up faster than the noise, so you are always ahead of the game. You don't really have to worry too much about this noise.

So to actually answer your question... if you would shoot in the summer and go 20 - 30 minutes, your thermal signal would probably saturate some of the pixels. At that point you don't have any data left, and subtracting a dark would only make that pixel black.

If you shoot during the winter, or with a cooler box, your darks should be effective. But this will depend on the thermal signal characteristics of your particular camera.

Basically, the total exposure you are going to need is going to depend on how bright the object is, how narrow the filter is, how slow the focal ratio is, and how bright the sky is.

You will probably find that you need more lights and more total exposure for Ha shots.

Notes:

Only shoot the Ha filter with a modified camera.

Judge the correct exposure by the histogram in the red channel only.

Jerry

#3 Footbag

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Posted 30 November 2012 - 05:48 PM

Thanks Jerry for the thorough explanation.

#4 fco_star

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Posted 30 November 2012 - 06:53 PM

Jerry, you always explain things in a very easy way... finally I understood some of this theories






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