What Are Calibration Frames and Why Do We Use Them?
Essentially, calibration frames are a way of improving our signal-to-noise ratio, or SNR. We take images of some very dark things, and as such the amount of signal that the camera sensor receives is inherently low. The noise however, because it’s dark, is a lot higher than say taking an image in bright daylight. Therefore the SNR in a daytime image is a lot higher. What we need to do when doing astrophotography is improve our SNR, and calibration frames allow us to do that.
The other method of improving our SNR is to take long exposures and then to do something known as “stacking” which, simply put, is the taking of lots of images and then using software to digitally put them together to produce a final single image that contains more detail and signal than a single capture. Calibration frames help us to improve that signal further.
Different Types of Calibration Frames
So what are the different types of calibration frames, what do they do, and how do we shoot them? There are four types that we use; dark frames (or “darks”), flat frames (“flats”), dark flats and bias (sometimes called “offsets.”)
Darks – these are taken in order to reduce the thermal noise that a sensor chip (be it DSLR, CCD etc) introduces into an image. When you take the long exposures that astrophotography requires, the chip heats up and the level of noise it introduces is down to three things; ISO (or gain with a CCD), temperature, and exposure time. We take dark frames in order to mitigate this noise. There’s a little more to it but that’s it in a nutshell.
We shoot darks at the same ISO or gain setting, using the same exposure time as our “light”, or sub frames, and as close to the same temperature as possible. The only difference is that we put the lens cap on. Before the advent of cooled cameras this was usually at the end of an imaging session. But with the ability to now pretty much pick a temperature to image at, we can do them almost anytime if you have such a camera.
Flats – a lot of astrophotographers tend to get stressy when it comes to shooting flats, especially in the early days. There’s a bit of a misconception that they’re difficult to do. They’re not. As with all things related to producing good images, it takes patience. Plan how you’re going to shoot them first.
I’ve seen some people say that we shoot flat frames in order to remove light pollution. From my own experience this is simply not true. Flats are there simply to remove any dust or vignetting picked up in the optical train.
There are a number of ways you can shoot these, such as using a dedicated flats light panel, tying a white t-shirt over the objective end of the OTA and then pointing it an evenly lit patch of sky, or even using your phone or tablet and an app that produces a pure white screen and placing that over the objective end. The latter is the method I use, although I must warn that if you’re going to do this then make sure that you’re pointing as vertical as possible and that the device doesn’t slide on the smooth screen. I’ve already written off one tablet doing that.
The exact settings you use will be dependent on your specific camera, but as a rule of thumb, try and aim for the histogram to be between a third and halfway along.
The great thing about flat frames is that you can use them time and time again for your setup, so long as you make absolutely no changes to it, such as rotating the camera, or adjusting focus, or anything else that might alter your optical train. If you make any adjustments to it then you’ll need to shoot a new set. The good thing about this is that they’re not temperature-dependent like darks are so you can do them at any point.
Dark Flats – are simply the equivalent of the dark frames for your light frames…but for your flats. It’s best to shoot these at the same time as your flats. The only change is that you put the lens cover on. Make no other changes to any settings or adjustments to the optical train.
Bias, or Offsets – are an inherent base level of readout noise, or an inherent pattern to the sensor. Because we are primarily interested in the fixed noise of the electronics, we shoot these at zero length, or as close to zero length, as we can get to minimise any other noise.
These may or may not be needed depending on your camera. I’ve recently read that with some cameras the bias is taken into account when you shoot the darks, and that including them in your stacking process can actually make matters worse because the software will essentially remove the bias twice and produce some odd effects. With my own setup this certainly seems to be true so I don’t shoot them.
But if you need to, then they’re perhaps the easiest to do. Place the lens cover on, set your exposure for the shortest possible time and then take them. Don’t forget that these need to be shot using the same ISO or gain as your light frames.
There are no real hard and fast rules for this from my own experience. The more the better perhaps, but you do come across the law of diminishing returns, so I personally aim for around 50 of each type. With darks that can be a pain, especially if you’re using quite long exposure light frames. I’ve personally used less than 50 quite frequently with no major issues, so I guess it’s down to you how many you ultimately want to use.
Calibration frames can be a bit of a pain, especially if you’re shooting them at the end of a long night, but to my mind they’re essential if you want to try and produce something absolutely beautiful and make your images “pop.” So try not to skip them because you’ll only regret it later if you do.
Thanks for reading, and if you have any useful tips yourself, or think I’ve missed anything, please do leave a comment either on here or my Facebook Page. For now, keep your heads up, and clear skies!