Calibration Access and Data Handbook


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Procedure

Colour transformations are supplied only for the three visual bandpass filter (U, B, V). Due to time constraints or due to bright source restrictions the normally used colour terms may not be available and non-standard colour transformations have to be used instead. For that purpose non-standard colour transformations are supported, which make use of the UV filters. Colour transformations making use of the UVM2 filter are not considered at all because of the strong dependence of the UVM2 brightness on interstellar absorption.
The accuracy achieved by a colour transformation differs between the various transformations. Therefore a priority is assigned to the various transformations, which should be followed by the calling task: if possible, the colour transformation with the highest priorities should be used first.
The following table shows the supported colour transformations sorted according to their priority (1 is highest). Only the listed combinations are possible.

FILTERID1 FILTERID2 priority
B V 1
U V 2
UVW1 V 3
UVW2 V 4
U B 1
UVW1 B 2
UVW2 B 3
UVW1 U 1

The colour indices of the correction terms (mag$_1$-mag$_2$) are always defined as (shorter wavelength passband - longer wavelength passband), e.g. (u-b), (b-v) or (uvw1-u) etc.

A new keyword ALGOID is used in the CCF to associate parameters sets with different algorithm. An ALGOID keyword value of 1 corresponds to the transformation described in this version (2.0 or later) of the calibration handbook. An ALGOID keyword value of 0 corresponds to transformation as described in calibration handbook version 1.1 or earlier.

The CAL provides the following functionality (assuming the keyword ALGOID has a value of 1):

The colour transformation can only be applied when a transformation exists between the specified filters filter1 and filter2 with the colour (mag1-mag2) falling within the validity range of the colour transformation equation and if the specified transformation branch branch is listed in the COLORMAG extension of the CCF ColorTrans component.

The colour in the instrumental system (mag1-mag2) must fall within the validity range of the colour transformation:

\begin{displaymath}
r_0 \le {\tt (mag1-mag2)} < r1
\end{displaymath} (31)

where
$r_0, r_1$ defines the validity range of the colour transformation as defined in the column
  TRAFOLIMIT of the ColorTrans CCF component.
mag1-mag2 the colour index calculated from the two input magnitudes in the instrumental
  system.

and the parameters of the colour equation must be defined for the requested branch (specified by the optional input parameter branch) of the colour-colour diagram:
\begin{displaymath}
{\tt branch} = branch
\end{displaymath} (32)

where
$branch$ value in column BRANCH in the COLORMAG extension of the CCF
  ColorTrans component.
branch value of optional input parameter

The transformation of the colour term (mag1-mag2) into a standard system is described by the equation

\begin{displaymath}
(MAG1-MAG2) = \sum_{i=0,9} k_i \cdot ({\tt mag1-mag2})^i
\end{displaymath} (33)

where
$k_i$ are the parameters stored in the TRAFOP1 column of the ColorTrans CCF
  component.
mag1-mag2 the colour index calculated from the two input magnitudes in the instrumental
  system

The transformation of the instrumental magnitude mag2 into the magnitude MAG2 of a standard photometric system is described by the equation:

\begin{displaymath}
MAG2 = \sum_{i=0,n}(l_i \cdot ({\tt mag1-mag2})^i) + {\tt mag2}
\end{displaymath} (34)

where
$l_i$ are the parameters stored in the TRAFOP2 column of the
  ColorTrans CCF component.
mag1-mag2 the colour index calculated from the two input magnitudes
  in the instrumental system
mag2 brightness in the instrumental system of filter2
MAG2 brightness in the standard system of filter2


next up previous contents
Next: Calling Parameters Up: CAL_omGetColorTransform Previous: CAL_omGetColorTransform   Contents
Michael Smith 2011-09-20