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Davis, L. E. 2001, in ASP Conf. Ser., Vol. 238, Astronomical Data Analysis Software and Systems X, eds. F. R. Harnden, Jr., F. A. Primini, & H. E. Payne (San Francisco: ASP), 511
Automated Photometric Calibration Software For IRAF
Lindsey E. Davis
National Optical Astronomy Observatories, Tucson, AZ 85719
Abstract:
New IRAF tools for the photometric calibration of optical/IR images are
presented. The new tools are standard star catalog and world coordinate driven,
requiring both accurate standard star catalog coordinate and accurate image
coordinate data. Network access to standard star catalog servers is supported.
The new tools are suitable for embedded use in pipeline reduction software.
The new tools described here were written to support the photometric
calibration of NOAO Deep Wide Survey data and are sufficiently general
that, with minor modifications, they can be used with any standard
star catalog server and applied to any optical/IR image data.
Sections 2 and 3 describe the current standard star catalog and image data
requirements. Sections 4 and 5 provide an overview of the software and a
summary of its principle features. The current status of and future plans for
the software are summarized in section 6.
The standard star catalogs must satisfy the following requirements.
- The standard star catalog must be a text file or a catalog server either
local or remote. The catalog server must support simple region extraction
queries and text output. The standard star text files and catalog servers must
provide ids, celestial coordinates, and one or more magnitudes for all the
extracted standard stars.
- The standard star catalog coordinates must be defined in a standard
celestial coordinate system. The coordinates must be accurate enough to permit
unambiguous location of the standard stars in the input image data.
- The standard star catalog magnitudes must be defined in a standard
photometric system. Extraction of catalog magnitude errors is desirable but
not required.
The input image data must satisfy the following requirements.
- The image coordinate system must be accurate enough to permit unambiguous
location of the standard stars in the image. The image coordinate system need
not be the same as the standard star catalog coordinate system. The software
will transform from one coordinate system to the other.
- The effective exposure time, the effective airmass, the filter name, the
effective time of observation, the effective gain, and the effective readout
noise must be stored in the input image headers before beginning photometric
calibrations.
- Basic CCD reductions including overscan, zero level, dark current, flat
field, and fringe corrections must be completed before beginning photometric
calibrations.
- Mosaic specific CCD reductions such as crosstalk and interchip gain
corrections must be completed before beginning photometric calibrations.
- The photometric zero point must be constant across the images. Variable
image scale effects must be removed before beginning photometric
calibrations.
Given a standard star catalog and an input image list which satisfy the
criteria defined in sections 2 and 3, the photometric calibration
software performs the following functions. An outline of the calibration
software is shown in Table 1.
- Selects a standard star catalog from the list of supported catalogs.
- Selects the images containing standard stars from the list of input
images using the standard star coordinates and the image coordinate system.
- Groups the standard star images by filter and time of observation
using information in the image headers.
- Creates standard star pixel position and magnitude tables for each
standard star image using the standard star coordinates and the image
coordinate system.
- Computes aperture photometry for the extracted standard stars using
parameter settings appropriate for the instrument and exposure times in the
image headers.
- Creates a standard star observations files by combining data for the
same field taken through different filters and at different effective airmasses.
- Calibrates the photometry by solving the system of transformation
equations defined by the user.
Table:
The Photometric Calibration Package
![\begin{table}
\begin{tabular*}{\textwidth} {r@{\extracolsep{\fill}}c@{}l}
\hline...
...the 4m mosaic aperture photometry parameters\\
\hline
\end{tabular*}\end{table}](img1.gif) |
The photometric calibration software supports the following features.
- The standard star catalog may be either a file or a local or remote
catalog server which supports region extraction. The software is independent
of the type and format of the standard star catalog and does not need
to be modified to support new standard star catalogs.
- Standard star list filtering can be performed by the catalog server as
part of the extraction or by the photometric calibration software after
extraction. Common filtering operations include changing coordinate systems,
imposing magnitude limits, and sorting by magnitude.
- All the standard celestial coordinate systems are supported. The
photometric calibration software will automatically convert the standard star
coordinates from the standard star coordinate system to the image coordinate
system.
- Standard star image selection, standard star list extraction, standard
star identification, and standard star measurement are world coordinate system
driven and fully automated.
- The image grouping and final calibration steps still require some user
interaction mostly in the interests of quality assessment.
The current photometric calibration software uses a set of catalog access
tasks and catalog access API developed for use with astrometric catalogs
but not yet part of the standard IRAF distribution (Davis 2000). The code
which does the actual aperture photometry, matches the observations, and
computes the photometric transformations is an adaptation of the existing
IRAF APPHOT and PHOTCAL packages (Davis and Gigoux 1993).
Although most of the software is automated and runs without intervention by
the user, some user input is still required in the areas of grouping the
standard star images and interacting with the fitting process to get an
optimal fit. More work is required in these areas in order to make the
code fully automated in a pipeline environment.
The author would like to thank Daniel Durand for help in the early stages
of developing the catalog interface, the NOAO Deep Wide Survey team for
providing motivation for the project.
References
Davis, L. E. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, ed. N. Manset, C. Veillet, &
D. Crabtree (San Francisco: ASP), 667
Davis, L. E. & Gigoux, P. 2000, in ASP Conf. Ser., Vol. 52, Astronomical Data Analysis
Software and Systems II, ed. R. J. Hanisch, R. J. V. Brissenden, & J. Barnes (San Francisco: ASP), 479
© Copyright 2001 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
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