This is an old revision of the document!
Generating A Science Frame
The raw light frame reported by a CMOS or a CCD and displayed in your favorite image software contains unwanted signals (i.e., counts) from the optical system and detector as well as from the atmosphere. The process of removing or reducing these signals such that only the source signal is present is called “Data Reduction”. Currently, our data reduction pipeline is broken into a few distinct steps. (1) Removing Local Effects, (2) Removing Atmospheric Effects, and (3) Calibration to Physical Units.
The optical system and detector introduce spatial non-uniformities in how radiant energy is collected over different pixels as well as extra signal from the electronic components in the detector owing to voltages necessary for the operation of the detector as well as voltages generated by thermally agitated electrons. These three effects are handled by specially collected calibration frames (usually multiple FLAT and DARK frames). Properly addressing these results in a light frame that represents (to the best of our abilities) the collected radiant energy that was incident on the primary optical device of our optical system (i.e., the 0.8 m primary mirror).
The atmosphere introduces some extra radiant energy owing to light pollution herein referred to as 'sky brightness'. Additionally, the atmosphere removes some radiant energy that would otherwise reach the detector via a process called 'atmospheric extinction'. Properly addressing these results in a light frame that represents (to the best of our abilities) the radiant energy that would have been collected were the detector at the “top of the atmosphere” (i.e., outside the bulk of the atmosphere).
Data Reduction to Energy Incident on Telescope
Data Reduction to Energy Incident at Top-Of-Atmosphere
Calibration to Physical Units
TL;DR
Vocabulary
- Data Reduction
- Astrometric Calibration
- Photometric Calibration