What are calibration frames you might ask?
First of all, the photos you take of the target that you are capturing are called light frames. Calibration frames are photos you take in particular ways, with a particular configuration or setup and or under particular conditions with the purpose of improving the quality of your light frames.
The calibration frames are used in processing your light frames in order to remove or reduce different kind of noise, vignetting and spects, dust or other imperfections of your optical setup.
There are different kind of calibration frames, each with a specific task and each to be taken using specific configuration, settings or methods. I must point out that I’m describing my way of taking calibration frames and the ones that I use in my work flow. There are other ways as well.
Bias frames
Bias frames are used to take care of the electronic noise the sensor produces when reading the value of the pixels. This is also called the readout nose of the sensor. For high-end cameras, ususally astro photography dedicated cameras this type of noise can be extremely low, but normally never zero. For DSLR cameras, this type of noise is almost always present and need to be removed.
The settings for bias frames are:
Fastest shutter speed possible for your camera (normally 1/8000).
The same ISO setting as your light frames.
Place the lens cover on your telescope and also cover your view finder. If you have a DSLR you can remove the camera house all together and place it under a blanket. You can capture bias frames for every session, but since they are not temperature sensitive, or dependent on camera rotation, zoom or any other optical setup you can capture a set of bias frames for every ISO setting that you use and store them in a library. Refresh that library from time to time and you will be set. I have 100 bias frames in my library, but you will probably be fine with 20-40.
Dark frames
Dark frames are used to record the electronic noise your camera produces during the capture of a particular exposure. The noise level depends on a number of factors such as the lenght of the exposure and the amplification of the signal, in other words the ISO value you used for the exposure. During long exposures the sensor will heat up more than during short exposures and this generate more noise and that is why it’s also called thermal noise. Dedicated astro photography cameras ususally handle this better since they often come with cooling and can be cooled 30-40 degrees celcius below ambient temperature.
Dark frames will also correct issues with stuck pixels in your sensor, for example pixels with too high sensitivity or pixels with too low sensitivity. These are normally called hot pixels and cold or dead pixels. Hot pixels can be red, green or blur and much brighter than they should and also permanent which is why you want these to be removed.
The settings for dark frames are:
The same shutter speed as your light frames.
The same ISO setting as your light frames.
Place the lens cover on your telescope and also cover your view finder. You will probably need to capture dark frames for every session since they are temperature sensitive and the temperature need to be the same as it was when you captured your light frames. Dark frames can take a long time since you need to use the same shutter speed but they are not dependent on the optical assembly so you can remove your camera house and take them separately while you are packing up.
If you have a dedicated astro photography camera with the ability to control the temperature you can build a library of dark frames using different shutter speeds and ISO settings and refresh that from time to time. If you have a DSLR this is much more difficult but can be done by placing the camera in controlled temperature environments. Take at least 20 dark frames, but if you have the time 50 won’t hurt.
Flat frames
Flat frames are primarily used to correct uneven field illumination, vignetting caused by a filter or the optics that you use but also used to remove dust or spect that is present in your optical assembly. Please note that flat frames can’t correct for light pollution or light gradients.
The settings for flat frames are:
The same ISO settings as your light frames.
At first glance this might look easy, but there are several considerations to be made. The goal is to take frames with an exposure that will evenly illuminate the frame. It is very important that you keep the camera connected to the optical assembly and also keep the camera rotation and focus at the exact same settings are the light frames. Flat frames are not sensitive to temperature so you can put your gear away for the night and take flat frames the next morning or inside, but as I said don’t tough the focus or camera.
Flat frames are normally taken with the AV mode of you have a DSLR, which mean that you let the camera decide the shutter speed. The next step is to evenly illuminate and that can be done by poiting the telescope to the sky and place a white t-shirt or other thin cloth over the lens. You can also use a light source, like a flat LCD panel and reduce the light with cloth or like I used, high grade photo printer paper. I had to use a few layers since my light source was too bright.
Adjust the light level so that your histagram will show an even exposure for all channels and the histagram is placed 30-50% from the left. Below is a picture of me taking flats. Take at least 20 flat frames, but if you have the time 50 won’t hurt.
The calibration process (if you use Deep Sky Stacker) is complicated but I will try to explain it in short. The bias frames are combined into a master bias frame. The dark frames are combined into a master dark frame. Both of these are subtracted from the light frames. The flat frames are combined into a master flat frame and then used with a division algorithm with the light frames. This will result in calibrated light frames and those are then stacked together to complete the final stacked image.
That final light imare you will want to edit in PixInsight, Photoshop or your software of choice.
Clear skies everyone!