Calibration frames and why do we need them

As I have already mentioned in another article, astrophotography is the art of capturing extremely low amounts of light over an extended period so that, once these tiny fragments are combined, they form complete, colourful images of the subjects. This light is extremely faint. Firstly, because it travels to us from vast distances—often millions of light-years away—and secondly, because during the final leg of its journey, it passes through various elements that affect the final image. Naturally, these include the components of our optical system, our light train. These imperfections influence the ultimate result. The lenses in our refractor inevitably have flaws, and every mirror in the reflector does too. While mostly invisible to the naked eye, their impact on the resulting image becomes more pronounced the more frames we capture for the final stack. Even the most advanced camera possesses a sensor that, being an electronic component, generates noise.

Furthermore, every filter has its own specific characteristics and irregularities that must be neutralised. Calibration frames correct these imperfections. The picture presented beside comes from my telescope. Each red arrow shows the imperfections to eliminate during the final stacking stage.

Types of calibration frames

There are several types, and each addresses a different problem.

Dark frames (or just Darks) remove thermal noise, electronic signature, hot/cold pixels, and a sensor amp glow (if your sensor belongs to that category).
To take darks, secure the lens or telescope cap tightly to block all incoming light. Remember, you must set the exposure time, ISO/Gain, and Sensor Temperature to exactly match your light frames. If your light frames (for instance, for different filters) have different exposures, you must take a different set of darks for each exposure time. You will need approximately 20 to 50 frames. I usually make 35 frames of each exposure time.
Remember that, among other parameters, the sensor temperature must also remain constant. So, if you are using the camera equipped with the sensor cooler, you are safe. You can cool down your camera to the required temperature (I am using -5° C) and create frames as above. However, some cameras (like planetary cameras or DSLRs) don’t have a cooler. In that case, take the darks right after your shooting session to maintain the current ambient temperature.

Flat Frames (or Flats) correct so-called vignetting (darkened frame edges) and shadows cast by dust motes on your lenses, filters, or sensor.
You can take flats in two ways, depending on the equipment you use. If you don’t have the motorised cover with the light panel, use an external LED light panel or even a white T-shirt held over the uniform white light source. Point the telescope vertically and place the light source at the top. Now comes the critical moment because you must calibrate the exposure time to the amount of light you send into the light train (telescope+filter+camera). The exposure should be long enough to push the camera’s histogram peak to roughly 1/2 to 2/3 of its maximum capacity. You can do this measurement in NINA, Flat Wizard. The software will automatically calibrate the time to match the best result.

The other option (a bit more expensive, but worth it!) is the motorised flat panel. Several manufacturers offer such devices. The most recognisable are DeepSky Dad, PrimaLuceLab, or Pegasus Astro. I use the DeepSky Dad because it has a well-designed mount system, and you can order it with the optional heater. First, check the required aperture, because not every manufacturer provides panels for every aperture. The procedure looks similar to the one described above. It’s even easier because you can adjust not only the exposure time but also the panel brightness.
What is critical after you take your flats, and you must always have it in mind: when you have your set of flats, you cannot alter your optical train! It must remain completely untouched; otherwise, you’ll have to repeat the procedure.

Bias Frames (Bias) address the base readout noise generated by the camera’s internal electronics (including the CMOS sensor) whenever a frame is processed. Taking the BIAS is fairly easy. Just keep the telescope capped (total darkness). Set the exposure to the shortest possible shutter speed your camera supports. Take 30 – 50 frames. Remember that the ISO/Gain and temperature must match the lights.

And this is it! To be honest, there is one more type of calibration frame: Dark Flat Frames. They are used primarily for specialised CMOS sensors that experience short-exposure artefacts or erratic behaviour when handling ultra-fast bias frames (e.g., the ASI294 series). They calibrate out the thermal noise generated during your flat frames. I have never used them (even when shooting with the ASI294 Pro).

How the process works

Your processing software handles this sequence automatically during stacking. First, it creates a Masters from the individual darks, flats, and bias frames. It averages them to create clean Master Dark, Master Flat, and Master Bias templates. Then, it subtracts the Master Bias and Master Dark noise signatures directly from your raw Light frames. You can compare this to removing faults from your lights. Next, divide the light frames by the Master Flat. Because dust and vignetting reduce illumination values linearly, this division equalises pixel brightness perfectly across the image field.

What is amp glow? Short for amplifier glow, it is a sensor-generated artefact that appears as bright, fan-like patches or starburst patterns along the edges and corners of an astrophotography image. It is caused by heat and near-infrared (NIR) light emitted by the camera sensor’s active electronic components—such as analogue-to-digital converters (ADCs), clock generators, and readout circuitry.
Because astrophotography requires long exposures to capture faint deep-sky objects, these localised electronic “hot spots” radiate energy into the adjacent pixels. The silicon in the sensor detects this infrared radiation via the photovoltaic effect, converting it into an unwanted electrical charge that mimics real starlight. These unwanted spots or areas are subject to removal by the calibration frames.