Picture Noise

[jasperuk]

Stupid question i bet but how do i keep my noise levels down when taking pictures is there a trick or tips ?

[Philip.]

use the highest ISO you can get away with

[ollypenrice]

There are lots of things to do.

Take the longest sub exposures you can within the limits of your tracking and LP.

Take a huge number of them. Diminishing returns only set in as you approach fifty.

Take a huge number of darks at exactly the right temperature.

Use a Sigma clipping algorthm for stacking your subs.

Take a very large number of flats and calibrate them with a master bias made of a very large number of sub exposures.

Ideally dither guide between sub exposures.

It is a long hard slog getting and calibrating good data!

Olly

[rfdesigner]

Short questions tend to yield long answers

Things that damage Signal to Noise Ratio (SNR) are numerous but the main ones are:

1. Sky glow

2. Camera readout noise

3. Camera dark current

4. Guiding

There are other forms as well, but we'll ignore these right now.

Skyglow is best removed by taking longer images. The 'noise' in skyglow equals the square root of the glow level. So if you double the imaging time you'll get twice as much signal and twice as much skyglow, but as the skyglow noise is the square root you'll only get 40% more noise, so your signal to noise ratio will improve by 40%.

Readout noise: every time you read out an image from your camera you will add some noise due to the readout mechanism in the CCD or CMOS sensor and the electronics of the camera. As this noise is fixed in magnitude no matter how long a shot you take longer shots get more signal but the same readout noise, so SNR improves quickly vs readout noise as your shots get longer.

From the above one would assume that one very long shot is the way to go, but the effect of readout noise is often quickly overcome, after that it's best to close the shutter, save the data and take another shot, that way you end up with many short subs which you can stack allowing you to do clever things like sigma clipping which can allow you to remove things like aircraft or cosmic ray hits. DSLRs often have good readout noise.

Dark current: every piece of silicon semiconductor leaks, be it computer memory, your Hi-Fi amp, or your camera where it slowly fills the pixels with charge. The leakage will look just like sky glow so again longer shots make for better SNR, and doubling the shot length will add 40% to your SNR. The best way to overcome this dark current is to cool the camera because the leakage halves for every 6~8C you cool the silicon. This also means if you use dark frames (highly advised) to remove pixel to pixel variations in dark current then the dark frames need to be taken at exactly the same sensor temperature as the image shots. This is a major drawback to DSLRs as even if you cool them you usually can't fix the temperature.

Guiding: If you have perfect guiding then all of the signal from a star will fall into a very small number of pixels. let's say it's 4 (2x2)

If however your guiding allows the image to shift across the camera by +/-1 pixel in both directions then you suddenly find your star has bloated to a 4x4 star covering around 16 pixels, so your signal is now attenuated by a factor 4, as you can see poor guiding will really hurt your final SNR.

Derek

[rfdesigner]

use the highest ISO you can get away with

Not necessarily true.

Take a look at:

EOS40D / EOS50D comparison

you will see the readout noise gets better at faster ISO, but the level where your camera saturates halves for each ISO setting, and for ISO above 400/800 the readout noise doesn't really change much

So.. if using DSLR then 400/800ISO is probably about optimum for deep sky work as a balance between saturating and noise.