Noise. The Enemy of a good astro image.
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Noise.....what do we mean by noise? Not the sort of thing that you complain about when the neighbours are having a party at 3am, and you haven't been invited, but just as bad in an astro-image!!
Noise is the enemy of a good astro-image, and we need to do all we can to keep it to a mimimum. Signal is what you are imaging....the objects out there countless light years away in space and time. What we have to do is aim for a good ratio of lots of signal to very little noise. This is known as a good signal to noise ratio.
Noise is anything that isn't the actual object that you're trying to image. There is random noise, such as cosmic ray hits on the detector, meteors, satellite trails and the like, and fixed pattern noise (FPN), which is created by the equipment you are using. FPN can be removed by correct calibration, and will be dealt with separately. There is also a random element to how the photons hit your detector, which changes for each exposure, and this too adds noise. All of these are known as shot noise.
The other type of noise is known as read noise, and this is the noise generated by the electronic processes involved in reading the electrical charge created by the photons hitting your detector. This is one of the reasons why some cameras are much more expensive than others with the same chip.....the electronics are much better and hence the read noise is lower.
Signal increases in a linear fashion with exposure, but noise increases as the square root of exposure. This means that the signal to noise ratio goes up (improves) as the square root of the exposure. A 100 second exposure will have a signal to noise ratio ten times better than a 10 second exposure, 10 being the square root of 100. A 15 minute (900 second) exposure will have a signal to noise ratio ten times better than a 30 second exposure, 30 being the square root of 900...etc etc
Here are the main ways to improve your S/N ratio.
1. Take longer exposures.
This will increase the amount of photons detected by your sensor...the more photons you detect, the more the signal will increase in comparison to the noise. There are practical limits to how long you can expose for however. There is the accuracy of your mount....how long you can track for without the image showing trailing, and how bright your sky background is. In a really dark site where the sky is black, very long exposures can be done, but in a light polluted environment, the image will soon begin to fog up due to the brightness of the sky. Another factor, independant of skyglow, is the likelihood of something ruining the exposure....a cloud coming across for example. While losing a 5 or 10 minute exposure isn't too bad, losing a 30 minute exposure can be really annoying!
2. Use a bigger telescope and/or a faster focal ratio.
A bigger telescope and/or a faster focal ratio will gather more light in a given exposure time. The light gathering power of a telescope is related to the square of its diameter. A 12 inch telescope will have 4x the light gathering power of a 6 inch telescope, and 9 times the light gathering power of a 4 inch telescope. It can easily be seen why when you look at the area of the lens or mirror. The 12 inch telescope mirror has a surface area of 73062 square mm, a 6 inch telescope 18146 square mm, and a 4 inch has 8171 square mm.
Focal ratio is similar. The exposure that you will need from a given telescope at F8 will be 4x as long as with the same telescope at F4 to achieve the same brightness. So a 6 inch telescope imaging at F4 will produce the same brightness of image as a 12 inch telescope at F8. This is because at F8, the image the telescope produces covers 4x the area of the image produced at F4. An easy way to envisage this is to imagine you have a film projector, and you want to make the projected image bigger, so you move the screen away to double the distance. This makes the image twice as big, but it also covers 4 times the area, and is therefore 4 times fainter as the same amount of light is spread over an area 4 times as large.. To get the same image brightness as before, you now need a projector 4 times as powerful.
3. Use a camera with a higher quantum efficiency (more efficient sensor)
The more efficient the sensor of your imaging device, the more actual photons that hit it get converted into digital data, and the brighter your image will be. This is known as the Quantum Efficiency, and can been seen in the camera specifications. Particular sensors have differing quantum efficiencies for particular wavelengths.
4. Combine multiple shorter exposures.
This is known as stacking. The effect of stacking is to produce the effect of a single long exposure by combining multiple short exposures. The more exposures you use, the better the S/N ratio becomes. At first, as the S/N ratio is proportional to the square root of the total integration time, it improves rapidly, but after a while, the improvement becomes less noticeable.
I always try to get a minimum of 20 exposures in a stack, and really prefer about 40-50. With a very faint target, I may go above this and on occasion will get around 100 exposures in a stack. After this point any improvement is so slight, and takes so many extra exposures, that it's not really worth adding more. Remember that you will only get 6 x 10 minute exposures done in an hour, and that assumes that all of them will be useable, which is often not the case.
This image shows the effect of stacking ever increasing numbers of exposures....
The image is a single exposure, then stacks of 5,10, 20 & 35 exposures.