PhotToNat contains three tables, viz. PHOTTONAT COINCIDENCE and DEGRADATION.
The PHOTTONAT table contains coefficients used to calculate the MIC CCD
framerate for a given detector configuration (stored in the binary field
In the extension header the fraction of events above the detection threshold
is stored in the keyword PHDFRAC together with the associated
event detection threshold (keyword: THRESH).
The value of PHDFRAC can be used to correct the detected counts for the global loss of detector efficiency (ageing). The pair of values THRESH and PHDFRAC are the settings at the time the OM photometric calibration was performed (cf. CCF file ColorTrans, section 4.5.4
The parameters characterising the clocking of the OM CCD are stored in the 10 element field PARAM00 of the extension PHOTONAT. The sequence witin PARAM00 and the field values are
|description||value||unit||variable in SAS|
|total horizontal clocks for one row||407||H rows||NHP|
|total vertical clocks for a frame transfer||288||V rows||NVPS|
|time required for a shift by one pixel|
|- in vertical direction||6.E-7||sec||VTRANS|
|- in horizontal direction||1.E-7||sec||HTRANS|
|number of rows in image area||290||V rows||NVPI|
|vertical to horizontal switch time||2.E-7||sec||V2H*1.E-9|
|horizontal to vertical switch time||1.E-7||sec||H2V*1.E-9|
|start of logical CCD window in hardware CCD||15||row||ROFFSET|
|the remaining 2 parameters are not used at the moment||0.||n.a.|
The logical CCD window of the prime channel has its starting address at (52,27), while the redundant is at (64,15).
The format of the binary table in the PHOTTONAT extension follows the description of the CCF ICD . The following two additional keywords are present in the extension header:
|keyword||keyword type||keyword unit||keyword value|
|PHDFRAC||E||not applicable||may vary|
The table COINCIDENCE holds the coefficients used to parameterize the empirical coincidence loss curve. There is no filter dependence of the parameters. The binary table has the following format:
Binary table: COINCIDENCE
|PLINFUNC||xE||n.a.||set of coefficients for linearity curve parameterization|
The table DEGRADATION contains the coefficients of the time dependent sensitivity degradation correction. Since sensitivity degradation is due in part to the photocathode, it is wavelength dependent and therefore there is one set of coefficients for each one of the OM lenticular filters.
The current correction is a linear function of the MJD of observation. However the table has place for a future cuadratic function. The result of the function is a correction factor to be multiplied by the coincidence loss corrected counts. The binary table has the following format:
Binary table: DEGRADATION
|FilterID||xE||n.a.||set of coefficients for correction function|
|(U B V UVW1 UVM2 UVW2)|