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Old 16-03-2017, 09:08 AM
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sil (Steve)
Not even a speck of dust

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Join Date: Jun 2012
Location: Canberra
Posts: 1,474
What are your expectations and how do you process? Are you expecting to press the shuuter button and have a jpeg ready to show the world saved to your camera memory card? Do you take dark, bias and flat frames too? do you use them all when processing (if at all?).

As far as I'm concerned there is no limit for ISO I deal with it in other ways (eg lots of subs). Ditto exposure time, starting to think its over rated when I can keep pin point stars and bring out faint structures. If you want "pretty pictures" from your astrophotography it all depends and my opinion is camera settings are not that critical but if you are doing actual science then yes there are probably measurable technical limits. But since no two cameras are identical in performance the various type of noise will vary widely to make it meaningless to say "never go above XXX ISO" or whatever. the answer is "it depends". I appreciate wanting to maximise the time you get to use the gear.

So I suggest, as Ken said, to practice. You dont need a telescope to sort out camera settings, or a dark site. just go outside and take photos of Orion (easy find, doesnt need long exposures), try setting say 5sec and put camera on tripod, now set iso to 400 take a shot, change iso up and take another, keep repeating. Ok the stars are probably blurred but this is PRACTICE, youre not looking for a great photo. Take those photos to your computer (with a good screen, not a laptop outside) and look at them, look to see where finer structures around orion are visible in the photos and where they are not (you didnt change shutter speed did you?) this gives you an idea where the Iso limit for YOUR camera is to gather signal you want. Now look again zoomed out and in on the same shots this time looking at the noise ISO introduces and where it overwhelms signal, you especially want little noise on top of nebulosity as it gets hard to remove without damaging structure detail. So again find an iso limit where the noise you feel is too much and where its workable.

Hopefully those two ISO limits will be the same or close so you can decide for yourself a good starting value. Now set the camera iso to that and go outside and grab say 20 subs, same shutter speed still (i havent said to touch anything other than iso). Now stack 5, 10 and 20 subs with your prefered package, this will show you how the iso noise is suppressed and you might decide there is no visible different between a stack of 10 and 20. Now you have answered yourself how many subs to be taking. compare these stacked ones to your original iso test and it may indicate to you a slight increase in iso will give you more signal for little noise increase that will disappear when stacked.

its an iterative process of testing and refining until you arrive at a set of camera settings that you will stick to for everything except solar/lunar/planetary (do testng process again for those settings). The sky is basically uniform in every direction so your settings will suit you well anywhere you point your camera in the sky. There are very few targets which easily overexpose and camera noise changes with temperature so take a few test shots at the start of your session and make sure you are satisfied your settings are giving you subs of quality you expect, orions core is easy to overexpose which you should avoid, so you need to decide on exposure times for yourself and in some cases take two sets at different exposures to combine in processing so you avoid overexposed parts.

If your subs stack up looking brownish thats light pollution, it may still be there at your dark site so you may want to look into a clipin light polution filter for your camera to block that out.
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