The calibration images bias, dark, flat and darkflat-frames
In astrophotography, influences of the camera and the equipment often cause several overlapping disturbances in single exposures (sub-frames or light-frames). Some can be eliminated by additional correction images. How this is mathematically solved for a sub-frame in the software is shown in the following illustration.
Schematic illustration of the use of the calibration images in the creation of a corrected light frame, using the Cirrus Nebula as an example
Bias-Frame
A bias-frame is the shortest possible exposure time of the camera, which is usually in the µs range. Such a bias frame contains an offset value set by the manufacturer and the previously defined offset value (see section 'Basics' - 'Offset') and the readout noise. Dark current almost does not occur at this short exposure time. Since the offset value is fixed and does not belong to the object signal, this value is subtracted from the actual signal. The readout noise is not eliminated in this process.
The offset-value is also included in all other calibration frames. Dark- and flat-frames, for example, also contain the offset-value. It should be noted that this value is only subtracted once during calibration. This step is realized via the dark frames when they are subtracted from the sub-frame. The flat frames are not suitable for this because they are divided by the sub-frame.
But what is the bias image needed for after all? Since each flat-frame also contains the offset-value, they must be cleared from the offset-value before they are used for the calibration of the single frame. Otherwise, the master flat frame and the light frame would not have the same offset base and the calibration would fail.
Procedure for capturing bias-frames:
- Set the shortest exposure time in the software program
- Put the cover on the telescope or camera - no light of any kind must reach the chip
- The images must have the same ISO/Gain setting as the light- and flat-frames.
- The chip temperature does not matter for these capturing, so it does not hurt if the chip is already cooled.
- Take 20 to 50 captures, which are then averaged.
After stretching (stretching the tone value curve), the individual pixels show the typical different gray levels, which only contain the offset and the readout noise. The averaged value is then the offset.
Dark-Frame
Dark frames are created to remove dark current, sensor glow, and hot- and dead- pixels from the light- or sub-frame. Dark-frames are subtracted from the sub-frame. At the same time, the sub-frames are cleared of the offset value, since this is contained in the dark-frames.
Hot pixels are created by single electrons jumping from one pixel to the neighboring pixel and react disproportionately to the additional energy. As a result, they glow red, yellow or sometimes green or blue in the final image. (https://www.fotografiewissen.de/technik/allgemein/deadpixel-und-hotpixel/)
In the case of dead-pixels, there is a technical problem so that there is no or a permanent current flow. As a result, the pixels are either black or white (burned out). (https://www.fotografiewissen.de/technik/allgemein/deadpixel-und-hotpixel/)
The sensor glow occurs because nearby electronics emit heat radiation in the infrared range and the pixels next to them react to it. (https://de.wikipedia.org/wiki/Sensorgl%C3%BChen)
180 s dark frame (stretched (stretching the tone value curve) with the program PixInsight)
Procedure for capturing dark-frames:
- Put the cover on the telescope or camera - no light of any kind must reach the chip
- Dark frames must be taken with the same exposure time, chip temperature and ISO/Gain as the sub-frames.
- With uncooled cameras it is difficult to achieve the same chip temperature. Processing such dark frames should not be used with uncooled cameras with weak sensor glow, as they lead to poorer image results.
- Take 15 to 30 captures, which are then averaged.
If a cooled camera is used, a dark frame library with different exposure times and ISO/Gain can be created, which only needs to be updated from time to time. This eliminates the need for time-consuming imaging at each night of shooting.
Flat-Frame
Flat-frames are intended to correct vignetting and uneven image illumination caused by dust or dirt in the optical system. In addition, with even illumination during flat-frames, the processing software can detect how differently the pixels react to light.
Vignetting usually occurs when several components are present in a beam path and one of them crops the edge of the image. As a result, the side edges of the image are darker in contrast to the center.
Procedure for capturing Flat-Frames:
- The camera must be in the same position as when capturing the light- or sub-frames - avoid rotating the camera.
- The focus point should be almost identical to the focus of the single frame exposures.
- The same ISO/Gain value must be set as for the sub-frames or dark-frames.
- The chip temperature plays a subordinate role for short-exposure flat frames. However, with longer exposed flat-frames, i.e. when slow optics are present or narrow band filters are used, the dark current comes into play again. - Cooling is then an advantage. For such flat-frames it is therefore important that not the bias-frame, which does not contain any dark current, is subtracted, but a darkflat-frame.
- A flatfield mask is recommended for uniform illumination. This is positioned tilt-free in front of the telescope aperture.
- The exposure time should be selected in such a way that the mountain of the histogram curve is centered at 50 % in the imaging program. - This means that the pixels all have a medium saturation.
- Some astro cameras have difficulties to expose only 1 s at a bright homogeneous illumination. Here it is helpful to increase the exposure time (e.g. to 3 s) and to reduce the illumination by the flatfield mask, so that the histogram curve is again at 50 %.
- Take 15 to 30 captures, which are then averaged.
Darkflat-Frame (auch Flatdark-Frame)
Darkflat-frames (also called Flatdark-frames) are used to remove the offset and the dark current from a longer exposed flat-frame. In general, it is always a good idea to use this type of calibration instead of bias-frames, because even short-exposure flat-frames of approx. 1 s can have a certain dark current under certain circumstances. It often happens that CMOS cameras output unsuitable signal values with very short exposed flat-frames, so that it is always appropriate to create longer exposed flat frames (> 1 s) and thus also to use darkflat-frames.
Procedure for capturing Flat-Frames:
- Exactly the same exposure time is used as for the flat-frames.
- The ISO/Gain value must also match the sub- or flat-frame.
- The chip temperature plays a subordinate role as with the flat-frames, but cooling is again very advantageous here.
- Instead of the flat field mask, the telescope is closed again as with the bias- and dark-frames.
- Take 15 to 30 captures, which are then averaged.