How Long Should My Exposure be?

The SNR (signal to noise ratio) of your image increases with more exposures. Not directly, but with the square root of the number of exposures. So with 4, 9, 16, 25, 36, 49, etc. exposures your SNR will go from 2 to 3, 4, 5, 6, 7, etc.

So the more exposures you can get, the better your image will be, even though there is a diminishing return here. In order to go from an SNR of 7 to 10, you need to add 51 more exposures of the same length, practically doubling the time it will take you to finish the photo.

The shorter the exposure is, the more you can take in the same night. So why not go and take 3600 x 1 second exposures per hour? Yes, it is a lot to process afterwards, but that is what giant hard drives and super-fast CPUs are for – right?

The answer is your number one enemy in astrophotography – ok, perhaps number three right after being tired and clouds – Read Noise.

When the camera reads data from your sensor there is a bit of uncertainty, because none of the sensors are made to perfection. So a bit of noise is added to the signal, not just the regular BIAS noise – you could subtract that one, but really nice random noise.

The only way to make that random noise not matter is by having a signal that is much greater than the potential noise perhaps 20 times as big or perhaps    3 times as big as the square of the noise. Which one? Your call. It is whatever rule you want to pick, test out what works for your gear. Thank you to Jon Rista for publishing the formula and background here!

So how do you get that signal? Stars, Galaxies – no problem, what about the bits between the stars? That’s the sky background, and that is what will make the difference between an image where you have to crush it all to black or where you can see the nice dust between the stars (IFN). Take a test exposure once you are set up and check the median value of your image. This is very often a good approximation of the sky background.

Then take as short an exposure as you can, based on the formula below and take as many as you can in about 80% of the available darkness per night. The remaining 20% will often be used for focusing, image download etc.

You can use the calculator below to determine the minimum value for your sky background, i.e. the median value of your image. If the value is higher in the test image you take, you can shorten the exposure.