Astrophotography and dark frames
Discussion
I've just bought my first dslr, a nikon D40. Im going on holiday soon, and since i'll be away from londons light pollution, Im hoping to do a little astrophotography.
The D40 itself, i've found to be a lot less noisy than other cameras i've used. But i'm wondering what the best way to eliminate noise will be, especially as i'll be doing very long exposures.
Should I take a dark frame after my shots, and use some dark frame subtraction software. or do dark frame subtraction in photoshop?
Or should i use the in camera noise reduction? (which im not that impressed with, although i haved tried used it on long exposures yet)
Does anyone have any tips regarding astrophotograhy?. One tip i picked up from someone, is not to open the iris fully- stay a few stops under. Apparently this can get rid of a little atmospheric 'glow'. (not sure why i would though)
I think the first thing to do is not wait until you are on holiday, but go outside tonight and take a few practise shots so you can compare for yourself. Take lots of exposures of different durations so you can see what the nature of the noise is. Will you be mounting the camera on a tracking mount?
Sadly wont be using a tracking mount.- just a sturdy tripod.
i keep meaning to go take some test footage, but i cant see any stars where i live, and its hard finding a night when its not raining!
Hows does the ISO system work in a digital camera? does it simply vary the voltage inside, thus making it more sensative? (and also incrasing noise.)
i've found the D40 can go up to iso 800 before noticeable noice starts appearing- good for sports ona cloudy, dull day, but as you said, i'll keep it on 200 for night time stuff, since i'll be on a tripod.
i keep meaning to go take some test footage, but i cant see any stars where i live, and its hard finding a night when its not raining!
Hows does the ISO system work in a digital camera? does it simply vary the voltage inside, thus making it more sensative? (and also incrasing noise.)
i've found the D40 can go up to iso 800 before noticeable noice starts appearing- good for sports ona cloudy, dull day, but as you said, i'll keep it on 200 for night time stuff, since i'll be on a tripod.
Fordo said:
Sadly wont be using a tracking mount.- just a sturdy tripod.
i keep meaning to go take some test footage, but i cant see any stars where i live, and its hard finding a night when its not raining!
Hows does the ISO system work in a digital camera? does it simply vary the voltage inside, thus making it more sensative? (and also incrasing noise.)
i've found the D40 can go up to iso 800 before noticeable noice starts appearing- good for sports ona cloudy, dull day, but as you said, i'll keep it on 200 for night time stuff, since i'll be on a tripod.
Make the ISO 100.i keep meaning to go take some test footage, but i cant see any stars where i live, and its hard finding a night when its not raining!
Hows does the ISO system work in a digital camera? does it simply vary the voltage inside, thus making it more sensative? (and also incrasing noise.)
i've found the D40 can go up to iso 800 before noticeable noice starts appearing- good for sports ona cloudy, dull day, but as you said, i'll keep it on 200 for night time stuff, since i'll be on a tripod.
For astro photography you want to be using the highest sensitivity possible. You will get errors in tracking the movement of the stars (even using a guided mount) and want to capture as much light as possible, in as short amount of time as possible. Use 1600 ISO if you can. With the Nikon you want to cancel all noise reduction in the camera and do it manually (for the best results). Basically astrophotography is all about very, very dim objects and tiny signals. Therefore you have to do a lot so you can maximise your signal to noise ratio and then stretch the hell out of the levels!
Your in camera noise is split into two main areas, random (usually colour) noise, and fixed (dead pixels and amp glow etc) noise. To maixmise the signal to noise ratio in the image you need to take lots of "light" frames (ie with the subject on them), align them in PS and "stack" them - ie average them so the random noise is effectively averaged out, yet the signal (ie stars, nebulosity etc) is constant so stays put. You also need to take lots of "dark" frames and average them to give a master dark frame that you subtract from the light image (cancelling dead pixels, glow etc). You need to take lots of darks as they will have random noise that needs averaging too. Darks should be taken at the same time as the lights and at the same temp (in the winter I take them when packing up and with the camera in the fridge (to keep it at outside temp) as well!
ALignement and tracking of the mount is obviously critical. Have never used a D40 so let us know how you get on. Used my Canon 300D a lot with some success, only tried the Nikon d80 twice with no success - the shutter refused to fire (not enough light and not in focus). Think this is sorted as it was down to a dodgy ebay remote and not having the camera set to MF mode.
Can give some pointers on the Photoshop work if needed, but need to go to bed now! Get yourself over to www.ukastroimaging.co.uk
Your in camera noise is split into two main areas, random (usually colour) noise, and fixed (dead pixels and amp glow etc) noise. To maixmise the signal to noise ratio in the image you need to take lots of "light" frames (ie with the subject on them), align them in PS and "stack" them - ie average them so the random noise is effectively averaged out, yet the signal (ie stars, nebulosity etc) is constant so stays put. You also need to take lots of "dark" frames and average them to give a master dark frame that you subtract from the light image (cancelling dead pixels, glow etc). You need to take lots of darks as they will have random noise that needs averaging too. Darks should be taken at the same time as the lights and at the same temp (in the winter I take them when packing up and with the camera in the fridge (to keep it at outside temp) as well!
ALignement and tracking of the mount is obviously critical. Have never used a D40 so let us know how you get on. Used my Canon 300D a lot with some success, only tried the Nikon d80 twice with no success - the shutter refused to fire (not enough light and not in focus). Think this is sorted as it was down to a dodgy ebay remote and not having the camera set to MF mode.
Can give some pointers on the Photoshop work if needed, but need to go to bed now! Get yourself over to www.ukastroimaging.co.uk
Edited by VxDuncan on Thursday 23 August 23:44
Just read the posts above (oops!) If your not using a tracing mount you'll obviously get star trailing, just like the hot French bit did in "The Beach". So I suppose if you want maximum trails then 100ISO, minimum trails 1600ISO!
You might want to build something called a "Barn door tracker" or "Haig Mount". Simple device that enables you to track the camera at the same rate as the stars are moving. I took one to Africa, but it needed some more practice! Photo in here:
[url=http://www.pbase.com/duncan_c/astro]http://www.pbase.com/duncan_c/astro[/url]
You might want to build something called a "Barn door tracker" or "Haig Mount". Simple device that enables you to track the camera at the same rate as the stars are moving. I took one to Africa, but it needed some more practice! Photo in here:
[url=http://www.pbase.com/duncan_c/astro]http://www.pbase.com/duncan_c/astro[/url]
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