It lists the coefficients defining the colour transformations, the zero points for the different filters and the aperture radii used in the photometric calibration analysis.
In the COLORMAG extension the instrument zero points of non-dispersive filter elements are stored as header keywords as follows: the zero point of the filter FilterID is stored in the keyword ZPTFilterID, e.g. the zero point of the the U-filter is stored in ZPTU.
The AB system zero point of the filter FilterID is stored in the keyword ABM0FilterID, e.g. the AB zero point of the the U-filter is stored in ABM0U.
The flux conversion factor derived from White Dwarfs of the filter FilterID is stored in the keyword FCFFilterID.
The flux conversion factor in the AB system of the filter FilterID is stored in the keyword ABF0FilterID.
The aperture radii applied to extract the source counts in the photometric calibration analysis are listed for the different filter elements in the keywords APEFilterId (e.g. APEU for the U-filter). The listed radii define the aperture widths of the different filter wheel elements for which the zero points are valid and for which the colour transformations are applicable.
The coefficients of the colour transformation are stored in the binary table
of the COLORMAG extension.
The columns FILTERID1 and FILTERID2 identify the type of colour used in the transformation. FILTERID1 holds the identifier of the shorter wavelength compared with FILTERID2, e.g. FILTERID1=B and FILTERID2=V. The sequence of filters sorted from short to long wavelength is: UVW2, UVM2, UVW1, U, B, V.
The validity range of a specific colour transformation are stored in the column TRAFOLIMIT. The lower limit is stored as the first and the upper limit as the second element in the TRAFOLIMIT column. Note that the validity ranges of the colour transformations must not overlap, i.e.
Different colour transformations can be defined in the same colour interval. The different branches of the colour transformation are identified using an integer number which is stored in the column BRANCH. The application of the different transformations is explained in the CCF release note (i.e. which value of BRANCH corresponds to which metallicity). However generally the value 0 in the BRANCH column is reserved for the colour transformation of the main sequence stars.
The coefficients to convert the colour index (mag1-mag2) into the colour index of the standard system (MAG1-MAG2) are stored in the column TRAFOP1. The coefficients to calculate the standard magnitude MAG2 from the colour index (mag1-mag2) and the brightness (mag2) are stored in the column TRAFOP2. The uncertainties of the coefficients are stored in the columns TRAFOP1E and TRAFOP2E respectively.
Currently the colour transformations are described with a quadratic functions of the colour index. Therefore only three parameters per column are used. The calibration file can keep up to 10 parameters and the not used coefficients are set to zero.
The binary table COLORMAG has the following format:
Binary table: COLORMAG
|ZPTU||E||magnitude||zero point of U-filter|
|ZPTB||E||magnitude||zero point of B-filter|
|ZPTV||E||magnitude||zero point of V-filter|
|ZPTUVW1||E||magnitude||zero point of UVW1-filter|
|ZPTUVM2||E||magnitude||zero point of UVM2-filter|
|ZPTUVW2||E||magnitude||zero point of UVW2-filter|
|ZPTMAGNI||E||magnitude||zero point of MAGNI-filter|
|ZPTWHITE||E||magnitude||zero point of WHITE-filter|
|ABM0U||E||magnitude||AB zero point of U-filter|
|ABM0B||E||magnitude||AB zero point of B-filter|
|ABM0V||E||magnitude||AB zero point of V-filter|
|ABM0UVW1||E||magnitude||AB zero point of UVW1-filter|
|ABM0UVM2||E||magnitude||AB zero point of UVM2-filter|
|ABM0UVW2||E||magnitude||AB zero point of UVW2-filter|
|ABF0U||E||erg//A/count||AB Flux conversion factor of U|
|ABF0B||E||erg//A/count||AB Flux conversion factor of B|
|ABF0V||E||erg//A/count||AB Flux conversion factor of V|
|ABF0UVW1||E||erg//A/count||AB Flux conversion factor of UVW1|
|ABF0UVM2||E||erg//A/count||AB Flux conversion factor of UVM2|
|ABF0UVW2||E||erg//A/count||AB Flux conversion factor of UVW2|
|FCFU||E||erg//A/count||Flux conversion factor of U|
|FCFB||E||erg//A/count||Flux conversion factor of B|
|FCFV||E||erg//A/count||Flux conversion factor of V|
|FCFUVW1||E||erg//A/count||Flux conversion factor of UVW1|
|FCFUVM2||E||erg//A/count||Flux conversion factor of UVM2|
|FCFUVW2||E||erg//A/count||Flux conversion factor of UVW2|
|APEU||E||pixel||of photometric calib. U-filter|
|APEB||E||pixel||of photometric calib. B-filter|
|APEV||E||pixel||of photometric calib. V-filter|
|APEUVW1||E||pixel||of photometric calib. UVW1-filter|
|APEUVM2||E||pixel||of photometric calib. UVM2-filter|
|APEUVW2||E||pixel||of photometric calib. UVW2-filter|
|APEMAGNI||E||pixel||of photometric calib. MAGNI-filter|
|APEWHITE||E||pixel||of photometric calib. WHITE-filter|
|ALGOID||I||n.a.||parameter to select type of colour transformation|
|column name||column type||column unit||comment|
|FILTERID1||9A||n.a.||short wavelength filter id|
|FILTERID2||9A||n.a.||long wavelength filter id|
|TRAFOLIMIT||2E||magnitude||upper/lower limit of validity range of colour|
|TRAFOP1||10E||n.a.||coefficients of colour transformation|
|(mag1-mag2) into (MAG1-MAG2)|
|TRAFOP1E||10E||n.a.||uncertainties of parameters in TRAFOP1|
|TRAFOP2||10E||n.a.||coefficients of magnitude transformation|
|(mag1-mag2) into MAG2|
|TRAFOP2E||10E||n.a.||uncertainties of parameters in TRAFOP2|
|BRANCH||I||n.a.||identifier of branch of colour transformation|
Each record holds the transformation coefficients related to one filter combination of a certain colour interval and one branch, e.g. the record with FILTERID1='U', FILTERID2='B', TRAFOLIMIT=[0.0, 0.5[ and BRANCH=0 holds the coefficients of the colour equations to transform main sequence stars with an instrumental u-b colour between 0.0 mag and 0.5 mag into the standard photometric (U-B) colour and B-magnitude. (The transformation into filter magnitudes generally has a higher uncertainty than the transforming of colours).
The binary table is designed to allow colour transformations for any filter combinations. However it is intended to use colour tranformations only to correct the magnitudes measured in the optical filters, i.e. U, B, V, although the colours between and UV- and optical measurement may be used for this purpose, e.g. UVW2-B colour to correct a B-magnitude.