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The Jackal

Light polluted ISO800 or ISO1600

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The histogram on the back of the camera is of a camera processed jpeg.

Don't be confused by it just use it as a guide.

I always shoot for around about 25%, whatever the ISO and other settings.

If this is good enough for Jerry Lodriguss it's good enough for me.

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Generally, the ISO of the DSLR is nothing more than an in-camera gain control that does nothing more than scaling the image brightness. As long as you have correct signal being captured, you can post-process to wherever you like. You only need to worry about ISO if you're interested in using the image you get straight from the camera

However, with the Canon imaging chips, there is an ISO setting that captures the most dynamic range in the incoming signal, and for my camera (600D) its at about ISO800, for the newest Canon DSLRs it's at ISO1600. Knowing this, I'll always shoot at ISO800 for deep sky stuff.

As for shooting under light polluted skies, as long as your histogram does not at all reach the bright side of the graph, you should be able to work with those images as you've not blown out anything. You'd want to have your lowest colour curve away from the dark side of the graph to ensure that you've as good a chance as any to collect enough signal to stack above the noise. It's interesting doing the processing as I'm discovering!

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Normaly photography rules do apply for astrophotography, but there are more important things to consider.

We are taking photos of extremely dim objects.  This means that we use very long exposure times.  The sky is constantly moving, so it is a battle to make sure that the image is pin sharp whilst capturing the data.  As we are capturing photons at low light levels, we need to make sure to have an exposure long enough to get a good signal to noise ratio.  Frankly, this means that we will end up capturing hours and hours worth of photons to make that image.  The more we get the better that signal to noise ratio.

I was recently told that there is no point going above ISO800.  This has something to do with how the cameras deal with the signal amplification. from what I'm told, it's a straight multiplication in software, so you are not really capturing anymore data.  So, the longer the exposure the better - if you can lower the ISO that's better as it means that you'll be capturing more data in the camera rather than fixing it in post.  Taking lots of sub frames is great for helping to combat the sky movement issue - that's why we do it.   And stacking them, isn't to make them brighter, it's to reduce the noise in the photo.

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