Munipack ‒ Photometry Calibration

# Photometry Calibration

A photometry calibration.

## Synopsis

`munipack phcal [.. parameters ..] file(s)[,result(s)]`

## Description

Purpose of this routine is the photometry calibration of CCD frames. The calibration establish an exact relation between observed instrumental counts and expected amount of photons by using a (small) set of known calibration stars. On base of the calibration, all known objects and the image itself should be transformed from instrumental counts to aboslute fluxes of photons, energy or magnitudes.

## Detailed Description

Every single picture elements exposed by light for a time period is collecting electrons created due to photo-electric effect from photons. Counts of the electrons is digitised and stored in output images. One can be directly used for additional light analysis. We will use term (instrumental) count(s) for their calculated amount (widely used by astronomers).

For a non-ideal detector, amount of digitised electrons will less than for photons. These counts will depends on the instrumental equipment (quality of optical path, lenses, mirrors, weather, quantum efficiency of CCD camera, etc). As consequence of the measurement process, the counts is unique per an apparatus and observation conditions. Two telescopes equipped with the same instrumentation will not product the same counts due to atmospheric conditions. The different apparatus (filters, detectors) generally produces different counts. To compare and to process observations from different observatories, we need unify all observations to same scales. Traditionally, in various parts of physics, the calibration is done with setting of calibration scale on a priory known calibrated sources. The sources are calibration stars for astronomers. The scale is relation between observed counts and produced photons.

More detailed description of the calibration can be found in Photometry Overview.

The calibration implemented by Munipack follows these steps:

- Detected stars are merged using of coordinates against to a reference photometric catalogue.
- With known filter and photometry system, expected count of photons is determined from magnitudes in the catalogue.
- Instrumental counts are transformed from an instrumental to a standard (photometry system) filter using of provided transformation matrix. (This step is optional.)
- By using of a telescope area, gain and exposure time are observed counts normalised on count rates.
- The calibration itself is determined as the ratio of reference amount of photons and observed rates.

Image data values are converted on a physical quantity (like photons, fluxes, magnitudes). The conversion is applied on frame data as well as on photometric tables. The conversion requires known astrometry and photometry calibration.

The conversion is very useful for converting observed quantities to a derived ones. One is ideal for construction of multi-spectral pictures of objects.

## Algorithm

The calibration is computed by the way:

- Reference catalogue and frames are merged in spherical coordinates and by search in close neighbourhood.
- From known filter and photometry system, from catalogue magnitudes photon fluxes are computed.
- Observed counts are normalised to rates by knowing of telescope area and exposure time.
- The calibration ratio between expected photons and observed counts is determined by a robust method.
- All values in frames and tables are recomputed on photons.

When photometry transformation table from instrumental to standard system is available `--tratab`, the counts are transformed from instrumental to a relative standard counts and the calibration is performed on the kind of counts.

## Operational modes

Photometry calibration is a very complex task so various ways are implemented:

Reference Catalogue
  Reference sources are stars provided as a table. The table can be prepared by hand or a photometrical catalogue can be used. Precision of calibration is given by quality of the catalogue and also by true colour transformation of instrumental to the catalogue. Usually, the calibration has small systematical offsets but larger random errors.
Reference Frame
  Reference stars are pulled from previsouly calibrated frame. Relative precision is generally better than in previous case because no colour transformation is required.
Manual Calibration
  The calibration values are provided by an user.

The choice suitable for specific situation would be very difficoult.

## Results

Output calibrated fits frames contains a new extension described in Photometry Calibrated File. Its table contains coordinates of stars on frames and various photometric quantities.

See Common options for input/output filenames. If advanced parameters `-O` are not set, default `-O --mask '!\1_XXX.\2'` is used according to some selected quantity. For example, `-q MAG` and the input file `blazar_01R.fits` will produce the output file `blazar_01R_MAG.fits`. Be warned, that output files are overwritten in any case by default.

## Preparation Of Photometry Catalogue

The table is a fits table and must contains columns with Right Ascension, Declination, reference magnitudes (and optionally with standard errors of the magnitudes). The column naming is by default RA, DEC, MAG and can be changed with --col-* options.

The table may be prepared by any standard FITS utility (for example fcreate utility of FTOOLS).

To save the time, you can just edit file `mtable.lst` in Munipack distribution (carefully handle with NAXIS1 and NAXIS2, etc.) and create a table

    $ munipack fits --restore mtable.lst

The output in mtable.fits can be used to a right frame as

    $ munipack phcal -c mtable.fits frame.fits

## A Low-Precision Calibration

We are using just one filter. No transformation matrix is used.

## A High-Precision Calibration

We are using multiple filters. Transformation matrix is used, we get the maximum possible precision.

## Examples

Calibrate against to UCAC5 catalogue:

    $ munipack cone -c UCAC5 -o 0716cat.fits -r 0.1 110.47 71.34
    $ munipack phcal -c 0716cat.fits --col-ra RAJ2000 --col-dec DEJ2000 \
      --col-mag Gmag  --photosys Johnson 0716_?R.fits
    $ munipack phcal -c T_Phe.fits --tratab phfotran.fits  --col-mag B,V,R,I \
       TPhe_B.fits,b.fits TPhe_V.fits,v.fits TPhe_R.fits,r.fits TPhe_I.fits,i.fits

    munipack phcal --verbose -r grb140423-2_C_0002_cal.fits --photsys-ref Johnson  grb140423-2_C_0003.fits

## See Also

Photometic System Transformation, Common Options, Photometry Format, Photometry Overview.