Calibration Access and Data Handbook

Description

The COLORTRANS CCF component is used by the SAS.
COLORTRANS contains one extension named COLORMAG.

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. $(b-v)_1 < (b-v)_2 < ... < (b-v)_n$
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

keyword name	keyword	keyword	description
	type	unit
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/$cm^2$/A/count	AB Flux conversion factor of U
ABF0B	E	erg/$cm^2$/A/count	AB Flux conversion factor of B
ABF0V	E	erg/$cm^2$/A/count	AB Flux conversion factor of V
ABF0UVW1	E	erg/$cm^2$/A/count	AB Flux conversion factor of UVW1
ABF0UVM2	E	erg/$cm^2$/A/count	AB Flux conversion factor of UVM2
ABF0UVW2	E	erg/$cm^2$/A/count	AB Flux conversion factor of UVW2
FCFU	E	erg/$cm^2$/A/count	Flux conversion factor of U
FCFB	E	erg/$cm^2$/A/count	Flux conversion factor of B
FCFV	E	erg/$cm^2$/A/count	Flux conversion factor of V
FCFUVW1	E	erg/$cm^2$/A/count	Flux conversion factor of UVW1
FCFUVM2	E	erg/$cm^2$/A/count	Flux conversion factor of UVM2
FCFUVW2	E	erg/$cm^2$/A/count	Flux conversion factor of UVW2
APEU	E	pixel	$r_{aperture}$ of photometric calib. U-filter
APEB	E	pixel	$r_{aperture}$ of photometric calib. B-filter
APEV	E	pixel	$r_{aperture}$ of photometric calib. V-filter
APEUVW1	E	pixel	$r_{aperture}$ of photometric calib. UVW1-filter
APEUVM2	E	pixel	$r_{aperture}$ of photometric calib. UVM2-filter
APEUVW2	E	pixel	$r_{aperture}$ of photometric calib. UVW2-filter
APEMAGNI	E	pixel	$r_{aperture}$ of photometric calib. MAGNI-filter
APEWHITE	E	pixel	$r_{aperture}$ 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
			transformation
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.