Taking Dark Frames - What Are They, And When Should You Take Them

Taking Dark Frames
When using an eVscope, you will find references to Dark Frame or Sensor Calibration in three different situations:
 * 1) The 'Sensor Calibration' option shown on the final tab of the mobile application
 * 2) The pop-up at the end of most Citizen Science mode observation launches
 * 3) In the description of the procedures to follow for most Citizen Science observations

In all cases, the meaning and use of Dark frames and Sensor Calibration means the same thing, but the particulars of each use is different.

What Are Dark Frames?
A dark frame is an image captured with the sensor in complete darkness (i.e. with a closed shutter or the lens and viewfinder capped). Such a dark frame is essentially an image of noise produced by the sensor. . In the case of the eVscope, dark frames are produced by typically doing 2 minutes of image captures with the cap on and with the scope set to the target exposure times and gain settings that the dark frames are intended to be used with. In call these cases, the sets of dark images are combined together to produce a single 'reference' dark image, which can then be used to subtract the effect of both imager flaws (stuck pixels) and the accumulation of dark current during the exposure, allowing the resulting images to be 'calibrated' so that they only contain data from the light received by the scope when using the same exposure and gain settings.

What Is The 'Take Dark Frames' Action Used For?
Activating this function is done to create a reference dark frame for the Enhanced Vision mode. This process should be done periodically (every couple of months) to handle possible flaws and aging of the imager (dead pixels, change in dark current, dust), and when the operating temperature of the scope is non-trivially different than what it was when 'Sensor Calibration' was last done. Consequently, the process should be done after allowing the scope of acclimate (warm or cool to the outside temperature it will be used to make observations). When the scope processes EV mode images, all the frames captures are done with a consistent set of parameters - exposure time of 3970ms, and gain of 25db. Consequently, the reference dark frame is produced using these same settings, allowing proper calibration of the images taken during EV mode. This dark frame is NOT used with Citizen Science observations, nor with Live Mode display.

What Are Dark Frames During Citizen Science Observations Used For?
During most types of Citizen Science operations (the current exception being Asteroid Occlusions), the observations are quite sensitive to accurate calibration of image brightness, and the captured images used a variety of exposure times and/or gains. Consequently, these observations will call for capturing dark frames either after the observation or, for long observations like exoplanet transits, both before and after the observation. In all cases, the dark frames are captured by putting the cap on, and capturing 2 minutes of frames while using the same settings as the science observation will use.

For short term observations, like Planetary Defense or Comets, the darks can be done by waiting for the pop-up at the end of the Launch duration, and selecting OK to capture the darks.

For long term observations, like Exoplanet Transits, it is necessary to collect darks at the start and end of the observation. This is generally done by temporarily setting the Duration to 2 minutes and 'Launching' an observation (and noting the time of this action in your logs, as reporting when you captured your darks is a necessary part of most Citizen Science reporting). This is done before the first observation Launch, and right after the last Launch has completed.

In all cases, these data will be used by the Citizen Science astronomers to calibrate the science images ('lights') provided during the observations. For long duration observations, the presence of dark frames at the start AND end can help account for likely temperature changes during the evening, as exoplanet observations often run 4 hours or longer.