UVIT Calibration Database is structured in a hierarchical order of sub-directories, starting from the type of calibration data down to each detector or filter. There will be one calibration data file at the lowest level directory. All the files are in fits format. Some of these folders may not be used by the current version of Level 2 pipeline software.
CalDB Files
A brief description of the directory tree is given below. This listing is also available in the README.txt file (plain text), within the CalDB distribution.
Bad Pixels
Bad pixels were found in the ground calibrations. Further, they can be different in PC and IM modes. These bad pixels could change in orbit. These values are being monitored periodically. The processing pipeline considers all the unexposed pixels as bad-pixels. Therefore, a separate calibration file is used for each selection of the Window.
- Number of files: 42. One for each combination of detector, window-size and mode (PC/IM).
- CALDB/BAD_PIXELS/FUV/IM/[100×100, 150×150 .. 512×512]
- CALDB/BAD_PIXELS/FUV/PC/[100×100, 150×150 .. 512×512]
- CALDB/BAD_PIXELS/NUV/IM/[100×100, 150×150 .. 512×512]
- CALDB/BAD_PIXELS/NUV/PC/[100×100, 150×150 .. 512×512]
- CALDB/BAD_PIXELS/VIS/IM/[100×100, 150×150 .. 512×512]
- CALDB/BAD_PIXELS/VIS/PC/[100×100, 150×150 .. 512×512]
NOTE: FUV and NUV channels are expected to be used only in PC mode. VIS channel is always used in IM mode and typically VIS channel uses full frame (512×512). Other modes and windows for the channels are included for the sake of consistency. Those files may not be kept up-to-date.
Centroid (Bias) Correction
These data are obtained from Ground calibration. No change is expected in orbit.
- Number of files: 6. Two for each channel (one each for 3×3 and 5×5 centroiding).
- CALDB/BIAS_CORR/FUV/[3×3, 5×5]
- CALDB/BIAS_CORR/NUV/[3×3, 5×5]
- CALDB/BIAS_CORR/VIS/[3×3, 5×5]
Dark frames and Pixel noise
Initial data were obtained during ground calibrations. As of 2016-December-14, processing pipeline will use the dark frames from the CALDB.
- Number of files: 6. Two files (1 for dark, one for noise) for each channel.
- CALDB/DARK_NOISE/FUV/[DARK, NOISE]
- CALDB/DARK_NOISE/NUV/[DARK, NOISE]
- CALDB/DARK_NOISE/VIS/[DARK, NOISE]
Distortion Correction
The distortion has two components: One is due to the aberrations of the optics, and it is a low frequency effect. The second is due to the detector and it can have low and high frequencies. The use of MCP/fibre/electric-fields makes for a complex transfer function. Distortion due to the optics is modeled by ray-tracing. Distortion of the detector is calibrated in the laboratory. None of these are expected to change in orbit. However, distortion correction will be verified periodically in orbit by observing astrometric fields. If needed, the CalDB will be updated by applying corrections to the existing distortion matrix.
- Number of files: 25. 22 files - one for each filter-optics combination,
and 3 - one for each detector (channel).
- CALDB/DISTORTION/OPTICS/FUV/[F0, F1, .. F7]
- CALDB/DISTORTION/OPTICS/NUV/[F0, F1, .. F7]
- CALDB/DISTORTION/OPTICS/VIS/[F0, F1, .. F7]
- CALDB/DISTORTION/DETECTOR/[FUV, NUV, VIS]
Flat fields for filters
This refers to the flat fields at the central wavelength of each filter, for each combination of filter, detector and mirror. It can be broadly broken into a low frequency and a high frequency component. The low frequency component is best determined by a 9-point photometric calibration. It is best that the observations of 9-point photometric calibration are complemented by 9-point photometric observations of sources with different spectra. The high frequency component is determined by the on-ground flat-calibration of the detector, and on ground flat-calibration of the filters.
- Number of files: 44. One each for the combination of filter, detector and mode.
- CALDB/FLAT_FIELDS_FILTER/FUV/IM/[F0, F1 .. F7]
- CALDB/FLAT_FIELDS_FILTER/FUV/PC/[F0, F1 .. F7]
- CALDB/FLAT_FIELDS_FILTER/NUV/IM/[F0, F1 .. F7]
- CALDB/FLAT_FIELDS_FILTER/NUV/PC/[F0, F1 .. F7]
- CALDB/FLAT_FIELDS_FILTER/VIS/IM/[F0, F1 .. F5]
- CALDB/FLAT_FIELDS_FILTER/VIS/PC/[F0, F1 .. F5]
Local Dark
This is a set of dark frames which are only used to find the dark frames during an observation by interpolation (in time) by median of the dark frames taken immediately before and immediately after the observation. These measurements are taken for all regular observations. Therefore, Level 1 data always come with these data. Processing pipeline will use the dark frames that is available within the Level 1 data-set. Information in CalDB is never used.
- Number of files: 6. Two per channel.
- CALDB/LOCAL_DARK/FUV/[DARK1, DARK2]
- CALDB/LOCAL_DARK/NUV/[DARK1, DARK2]
- CALDB/LOCAL_DARK/VIS/[DARK1, DARK2]
NOTE: These files are not used by Level 2 pipeline as of 2016-December-14. Dark frames in FUV and NUV channels (PC mode) are found to have minimal counts and are very stable. Therefore, Level 2 pipeline is not applying any dark correction for these data. However, VIS channel (IM mode) requires dark correction. Level 2 pipeline uses the Dark frames in CALDB/DARK/VIS/ for dark correction of VIS data. Dark counts in VIS channel are found to be stable over time, so the pipeline is not using the dark frames that come along with Level 1 data.
