How to Take Calibration Frames: Darks, Flats, Bias, Flat Darks...
Astrophotography calibration frames are extremely important if you're aiming to get the best possible images of deep sky objects. Although they make complete sense and become second nature once you are used to them, the different types of calibration frames can be very confusing to beginners.
While "light" frames are simply the name used to refer to our regular images, darks, bias, flats and dark flats are what we call "calibration frames." These are used to enhance the quality of our data by reducing the noise, getting rid of small artifacts, and taking care of vignetting around the edges.
Below, we go over how to take each of these frames.
Light Frames
Let's get this one out of the way right at the beginning: Light frames are not calibration frames. Light Frames are simply what your regular raw pictures are called. When importing all your raw data into your stacking software, for example PixInsight, you will add your files using the "Add Lights" button.
Now that this is clear, let's jump to the actual calibration frames. If you're unsure as to how many calibration frames to take for each category, we usually take 15 which works well.
Dark Frames
Dark frames are taken in complete darkness, when no light can reach the sensor. This is done by placing the cap of the camera (or the telescope if you do not wish to remove the camera from the camera) on. Darks help reduce the noise in our data, although new sensors are such low noise that darks are becoming less and less crucial.
How to Take Dark Calibration Frames
Temperature: Must match your lights.
Exposure time: Must match your lights.
ISO/Gain: ISO or Gain must match your lights.
Special technique: Place the cap on your camera or telescope to block any light from reaching the sensor.
When to take: Dark calibration frames can be taken at home, as long as your camera can reach the same temperature it was at during your imaging night. This is easily done if you own a cooled astronomy camera, but can be tricky for DSLR/Mirrorless owners. When we use our DSLR camera, we usually play it safe and take our darks right after imaging, while packing our gear.
Number of uses: You can re-use your dark calibration frames for several months. It is recommended to create a "Darks Library" on your computer, that includes darks for each different ISO/Gain settings, temperature, and exposure time you often use. You can then keep the master calibration frames and use them for future images. It is often recommended to re-shoot your dark frames every year or so, and, of course, whenever you get a new camera.
Do Filters matter for Dark frames? Filters do not matter when taking Dark calibration frames.
Flat Frames
Flat frames are by far the most disliked type of calibration frames by amateur astrophotographers. They can be a bit tricky to take if you are a beginner, and aren't re-useable in most cases.
Flat frames slightly help with noise, but their primary goal is to help remove vignetting and dust spots.
How to Take Flat Calibration Frames
Temperature: Does not matter.
Exposure time: Needs to be exposed with the Histogram peak around the middle. Start with 1 second exposures and observe the histogram. Take shorter or longer exposures until the peak is near the center. Nowadays, most image acquisition programs have a "Flats Wizard" built-in which will automatically find the ideal exposure time for you and take all your flats easily!
This can be very fast if using a color camera, as you'll only have to take one set of pictures, but can be a bit time consuming if you're doing sets for each filter when using a monochrome camera. Still, it goes by pretty fast since the exposure times is usually very short.
ISO/Gain: ISO or Gain must match your lights.
Special technique: There are several ways to take Flats:
Using a light panel - Point your telescope up and place a light panel on top of it. Make sure it covers the tube perfectly and turn it on.
Using the sky - Called "Sky Flats", this is done when the sky is evenly lit right after sunset or, even better, right before sunrise.
Using a light panel and a white t-shirt - Use a plain white tee if you find that your light panel is too strong or the light doesn't shine evenly. Most light panels have an adjustable power setting so you should not need to use a t-shirt.
When to take: Flats are best taken right after your imaging session, ideally just before sunset if you want to do Sky Flats.
Number of uses: You can only re-use the same flats if nothing has been moved in your imaging train. This is usually fine for telescopes installed at remote observatories, assuming the camera rotation is the same.
Do Filters matter for Flat frames? Filters matter when taking Flat calibration frames, so you'll need to take different sets of flats with each filter.
Bias Frames
Bias Frames help take care of some noise, and are extremely similar to Dark Frames. They are taken the exact same way, except for the exposure time which needs to be set at the fastest shutter speed your camera can do.
We only use Bias frames when shooting with a DSLR or Mirrorless camera. There is, in our opinion, no point in taking Bias frames with a cooled astronomy camera. The best calibration frames to use in that case would be the Dark Flats, which we'll cover next.
How to Take Bias Calibration Frames
Temperature: Must match your lights.
Exposure time: The fastest shutter speed your camera allows.
ISO/Gain: ISO or Gain must match your lights.
Special technique: Place the cap on your camera or telescope to block any light from reaching the sensor.
When to take: Bias calibration frames can be taken at home or on the field. They're extremely quick to take so we suggest just taking them on the field right after your imaging session. It should only take 5 seconds or so since your shutter speed is so fast!
Number of uses: You can re-use your bias calibration frames for several months. Just like darks, you can create a "Bias Library" on your computer and keep re-using these whenever needed. Just re-shoot them once a year or so.
Do Filters matter for Bias frames? Filters do not matter when taking Bias calibration frames.
Dark Flats
The last type of calibration frames is the Dark Flats. These are what we take when using a cooled sensor camera, instead of Bias.
Dark flats are the same as flats, expect the cap is now on your telescope so that no light can reach the sensor. These are usually taken right after your regular flats.
How to Take Dark Flats Calibration Frames
Temperature: Does not matter.
Exposure time: Must match your Flat frames.
ISO/Gain: ISO or Gain must match your lights.
Special technique: Place the cap on your camera or telescope to block any light from reaching the sensor.
When to take: Dark Flats should be taken right after each set of flats, with the cap on the telescope. The Flats Wizard will usually tell you to place the cap on your optics after taking the flat frames.
Number of uses: You can only re-use the same dark flats if nothing has been moved in your imaging train and you are using the same set of flats. This is usually fine for telescopes installed at remote observatories, assuming the camera rotation is the same.
Do Filters matter for Dark Flats? Filters do not matter when taking dark flats, so just use any filter.
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Final Thoughts on Calibration Frames
Astrophotography calibration frames can be a bit daunting at first because it seems like there are so many different ones, but quickly become easy to understand.
Because Bias and Dark Flats depend on which type of camera you use, you only have 3 different types of calibration frames to worry about instead of 4!
If you use a DSLR or mirrorless camera, we recommend calibrating your data with:
Darks
Flats
Bias
If you use a cooled CMOS astrophotography camera, we recommend calibrating your data with:
Darks
Flats
Dark Flats
Consider building a library for your darks and bias frames. You should only have to take care of flats (and dark flats depending on your camera) whenever you go image on the field.
Clear Skies,
Galactic Hunter
