Munipack ‒ Photometric System Transformation

# 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):

- Stetson's fields

## Algorithm

The transformation is determined by the way:

- Reference catalogue and frames are merged in spherical coordinates with nearly positions.
- From known filter in a photometric system and catalogue magnitude, photon fluxes are derived.
- Observed counts are normalized to rates using of both telescope area and exposure time.
- The transformation matrix is determined.

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.

## 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.

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.