How to reduce noise in a picture of the night sky?

Question

I have heard multiple opinions about pros and cons of using a high ISO (ex. 1600) versus a low ISO (ex 100-400) to take pictures of the night sky. I am really confused what actually works better. Some say you need a high ISO, or others say a low ISO, etc.
I don't have a high-end DSLR for budget concerns- I have a Nikon D3000. While I have gotten slightly better at night photography, I am still confused on how to reduce noise in the photograph. Some photographers take multiple pictures and then paste them together in Photoshop and that seems to help cut the graininess down. All the same, I don't really understand how to do this.
I have Photoshop Elements 11 and Photomatix.

Answer 1

It depends on what your goal is.



If you want static star fields, then you need to keep your exposure time to around 25 secs or less. Because of this, you need to have your ISO high and a wide aperture (f2.8 or wider) to get enough light for a decent image. I use Topaz Denoise (Photoshop plugin) to reduce noise.



If you want star trails, then that's a different method. You have the ISO low (100-200) aperture around f4-5.6 and use LOTS of separate exposures of about 30 secs. You then stack those exposures in post. Doing it that way, rather than one long exposure stops the sensor heating up and eliminates the need for any long exposure noise reduction to be turned on.



I have a few examples of both methods on my Photostream.



Edit: This is the free software I use to stack star trails;

http://www.startrails.de/html/software.html



Also, you do NOT want Long Exposure Noise Reduction switched on if you are doing 30 secs exposures, as the camera will immeditately take another 'dark field' exposure for the same length of time, which will mess up your stacking. You don't need it on for 30 sec exposures anyway. Only use LENR if you are doing exposures of several minutes to an hour long and make sure your have enough battery power for the camera to cope with 2 exposures of that length!!

Answer 2

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Answer 3

Each equipment has its limit. Your job as a photographer is to work with that limit and produce the best image possible.



D3000 is a fine camera. Not the latest and the greatest, but still a fine camera. Having said that, it does have limitations. If you go beyond ISO 800, you may start to see noise, especially if you take long exposures. Many of this is correctable via software.



Having said that, night sky (astro-photography or land-scape?) requires very little dynamic range. My suggestion would be to test between ISO 800 and ISO 1600, correct as much as you can via software, then if you still have issues, use extensive touching up via software. If you accept under-exposure, enhance contrast, then do some touch-up, you should be able to do a reasonable job with your equipment.

Answer 4

If you're trying to do non-star trail type shots, then stacking is an effective method to average random noise out. Noise is reduced by the square root of the number of pictures in the stack, thus 5 shots will improve the signal to noise by 2.2 times. With stacking, you can shoot at high ISO. An easy program to use is StarStaX, however, the output is only 8-bit which gives you limited post processing abilities.

http://www.markus-enzweiler.de/StarStaX/StarStaX.html



Another type of stacking can be used to create star trails. StarStaX is also useful for doing this and has useful features to fill gaps.



If you want 16-bit TIFF output (gives you more latitude for post processing), something like enfuse works well. Enfuse is part of the Hugin distribution which is primarily used to stitch panoramas.

http://hugin.sourceforge.net/

Hugin has the ability to align your images due to earth rotation or camera movement between pictures.



Do a search for stacking programs. There are a few more free programs out there which are popular in the astrophotography circles.



Doing night sky stuff in Photoshop is tedious and slow. Use Photoshop to post process your stacked images. Use the other two programs to stack.

Answer 5

Is using a low iso not a possibility? Low iso + long shutter speed= good night shots as far as I know. Do you shoot in jpeg or raw? If you shoot Raw there are more options for post-processing and you'll be able to get out any additional noise that you wouldn't with a jpeg.

Answer 6

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