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Photometric System Transformation

An approximation of photon fluxes in a standard set of filters by a linear combination of an instrumental set of filters is determined on a field with known calibration sources.

Synopsis

munipack phfotran [.. parameters ..] file(s)

Description

A common astronomical apparatus composed from a telescope, filter and a detector has slightly different spectral sensitivity than the standard one which had established the primary (stellar) standards of a photometric system. Fortunately, a commonly used equipment close fits the standard spectral sensitivity due to effort of manufactures. Therefore, any differences are small and can be, with suitable precision, approximated by a linear approximation.

This action determines such transformation by application of the linear approximation between observed sum of counts and expected photons from calibration stars.

The transformation table can be used to convert observed counts c in an instrumental system (identified by PHOTSYS1) to counts c' in a standard system (identified by PHOTSYS2).

c'i = Σj Cij cj, i = { B,V ...}

The transformed counts c' will generally proportional to observed photons and can be used for calibration.

The transformation is designed to be used on a calibration field. The sparse field with many of well calibrated stars. There are sources of such fields (which can be supposed as the secondary standards):

Algorithm

The transformation is determined by the way:

A result of the transformation is a nearly tri-diagonal matrix (elements around diagonal dominates over other ones). The limitation of the shape is forced due to ill-conditioning of the problem.

Prerequisites

Needs both astrometry and instrumental photometry of frames.

Headers would contain all the exposure time, filter, telescope area and photometry system keywords.

Specify photometric system (a conventional set of filters). Default is used value from frame header, use it when value is missing or needs correction. The option is important while determining of photometry calibration.

Specify filter. Default is used value from frame header, use it when value is missing or needs correction. The filter is important while determining of photometry calibration.

When calibrated frame contains FWHM parameter, the first aperture larger then the radius is used. When the parameter missing, the first aperture or user provided aperture is used.

Important.

The exposure time, filter, gain, area and an instrumental photometry system are absolutely necessory for calibration and none of them can not be omitted. At first, all values are obtained by reading of headers of FITS files. If at least one is not found, the calibration process is stopped (a wrong calibration which looks as valid is much more worse than any fail).

The situation can be solved by the ways:

While common values of exposure times, filters etc. are included to every header, the keywords can differ from Munipack's defaults. In the case, set ones via environment variables.

Input And Output

On input, FITS frames in several filters are required. Ones must be passed in order from short- to long-wavelengths. Composited frames are recommended.

On output, a new FITS table representing the transformation is created.

Parameters

Reference Catalogue:

-C,--cal
The calibration is specified by an exterior entity. The values are saved to a PHOTOMETRY extension header and counts are converted to photons for both image and table values.
-c,--cat
Reference photometric catalogue. A fits table with coordinates and magnitudes of reference stars. One can be a selection from a "real" catalogue or a table prepared by oneself. See section Preparation of Photometric Catalogue, how create this table by hand.
-r,--ref
Reference frame is used. The reference frame usually created by -c,-C options. Useful for relative photometry.
-f,--filters
Filters used for calibration.
-F,--filter-ref
Reference filter for calibration.
--col-ra
Right Ascension column. Default is RAJ2000.
--col-dec
Declination column. Default is DEJ2000.
--col-mag
Magnitude column(s). Default is the filter (-f option) with 'mag' suffix like -f V --col-mag Vmag (?).
--col-magerr
Magnitude std. error column(s) (no default). If this parameter is omited, errors are estimated as the square of photons derived from magnitudes.
--tol
search radius for object identification in degrees (default 5*FWHM)

Calibration specific:

--photsys-ref
reference (standard) photometric system (catalogue)
--photsys-instr
instrumental photometric system (frames)
-q, --quantity
calibrated quantities by default: PHRATE, FLUX, FNU, FLAM, MAG, ABMAG, STMAG (see description), multiple quantities can be used, separated by colon

Photometric system:

--tratab
Table describing conversion from instrumental to reference photo-system. Usually product of phfotran.
--phsystab
A table with photometric system definitions (specification)
--list
Lists available photometric systems. Their identifiers are names of extensions in (photometric system definitions) file.

Helper:

--area
Area of telescope aperture in square meters [m2].
--saper
selects appropriate aperture from PHOTOMETRY extension. By default, flux in infinite aperture is used.
--advanced
Advanced format. Additional extensions (results of star find, photometry and residuals) are included. This format is not used by default because result FITS is twice or more bigger.

See Common options for input/output filenames.

When options for the area and the reference and instrumental systems are used, FITS header is updated according to provided values.

Default values for coordinates will be usually unsatisfactory.

Add -E,--extin option description.

Caveats

Just equal number of instrumental and standard filters is implemented.

Examples

Calibrate against to UCAC5 catalogue:

$ munipack cone -c UCAC5 -o 0716cat.fits  -r 0.1 110.47 71.34
$ munipack phfotran --area 1.86 --photsys-instr DK154 -c T_Phe.fits --col-ra RA --col-dec DEC --col-mag B,V,R,I T_Phe_000001.fits T_Phe_000003.fits T_Phe_000005.fits T_Phe_000007.fits

See Also

Common options, Photometry Calibration.