Film vs digital for long exposures

[Pere Casals]

There are a number of points regarding digital noise which are being conflated here.

If you add multiple images together, the light-generated signal, which is (nominally) constant increases linearly; the random electronic noise (thermal and shot noise) are added as the sum of their square root values - so when the final image is scaled, the noise is reduced with respect to the desired light. This increases the signal to noise ratio (higher is better) by 3dB for each doubling of the number of images being stacked.

Sensor noise - pixels with a fixed non-zero dark value - require a black frame which can be subtracted from the finished image since they will act as point souces. Since they are also subject to thermal noise, you should also use multiple dark frames to average that noise down.

Neither of these are issues with an emulsion (though it has other issues which I discussed earlier).

Neil

There are ways to solve all that. For planets a nice approach is using video cameras at 20 to 50 frames per second and stacking some 1000 images.

It is explained here:

http://www.skyandtelescope.com/astro...etary-cameras/

I'm a user of Point Grey cameras for machine vision applications, I've been using the Flea 2 model a lot, in this example he is using the later Flea 3 model.

This way it also allows to get a very wide dynamic range, as we can even generate a 24bit per channel image is we want, from addition of component frames, of course with much lower effective bits per channel, but with no clipped bright point.


Film has other problems... today film is not very common in astronomy, still great photographs can be taken !!!

[Serge S]

Every tool is different. We as photographers need to know what to use and when.

Yes! Plus we have so many options to choose from.
Different tools offer different approaches and shape how we work as well.