With the installation of WFPC2 in December 1993, Hubble Space Telescope (HST)
gained an imager capable of high-resolution stellar photometry. This advance provided a
number of opportunities, most notably the ability to obtain deep photometry in crowded
fields such as nearby galaxies and globular clusters. However, the severely undersampled
WFPC2 point spread function (the FWHM is comparable to a PC pixel and about half a
WFC pixel) produces a challenge when attempting to obtain accurate stellar photometry.
As a result, issues of the number of PSF points calculated per pixel by the photometry
software, and of approximations of quantum efficiency variations and charge diffusion,
which cause insignificant errors of well under a percent in well-sampled data, contribute
significant errors of order a percent in PC data and greater in WFC data.
Because of the vast amount of WFPC2 data available to the astronomical community,
HSTphot was developed specifically for its reduction, allowing for the creation of a highly
specialized (and efficient) photometry program. The package (hstphot and accompanying
utilities) runs from the Unix command line, and has been successfully compiled and run on
machines running Solaris and Linux. As a manual is available with the package, this paper
is intended to describe and test HSTphot rather than to give a detailed explanation of
installation and use of HSTphot. Information on obtaining HSTphot can be obtained from
With the installation of WFPC2 in December 1993, Hubble Space Telescope (HST)gained an imager capable of high-resolution stellar photometry. This advance provided anumber of opportunities, most notably the ability to obtain deep photometry in crowdedfields such as nearby galaxies and globular clusters. However, the severely undersampledWFPC2 point spread function (the FWHM is comparable to a PC pixel and about half aWFC pixel) produces a challenge when attempting to obtain accurate stellar photometry.As a result, issues of the number of PSF points calculated per pixel by the photometrysoftware, and of approximations of quantum efficiency variations and charge diffusion,which cause insignificant errors of well under a percent in well-sampled data, contributesignificant errors of order a percent in PC data and greater in WFC data.Because of the vast amount of WFPC2 data available to the astronomical community,HSTphot was developed specifically for its reduction, allowing for the creation of a highlyspecialized (and efficient) photometry program. The package (hstphot and accompanyingutilities) runs from the Unix command line, and has been successfully compiled and run onmachines running Solaris and Linux. As a manual is available with the package, this paperis intended to describe and test HSTphot rather than to give a detailed explanation ofinstallation and use of HSTphot. Information on obtaining HSTphot can be obtained from
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