Introduction
The Space Telescope Science Data Analysis System (STSDAS) synthetic photometry (synphot) package is an IRAF-based suite of tasks designed to simulate photometric and spectrophotometric data resulting from Hubble Space Telescope (HST) observations of astrophysical targets. Tasks in the synphot package can be used to make plots of HST instrument sensitivity curves and stellar library spectra, to predict count rates for observations in any available mode of the HST science instruments, and to examine photometric transformation relationships amongst the various HST observing modes, as well as conventional photometric systems such as UBVRI and uvby. Synphot is available to assist guest observers in preparing HST observing proposals and has proven useful in planning and optimizing HST observing programs due to its cross-instrument simulation capabilities.
How It Works
Throughput curves for all HST instrument components---such as mirrors, apertures, filters, gratings, and detectors---are stored in STSDAS-format tables. In addition to HST instruments, throughput data for several conventional photometric systems, such as Johnson-Cousins UBVRI, Stromgren uvby, and Walraven VBLUW, are also included. The individual throughput data tables are referenced via master instrument graph and component lookup tables. The instrument graph table essentially provides a map of all of the HST instruments and describes all allowed combinations of the instrument components. The synphot passband calculator utilizes user-supplied keywords that describe a desired instrument observing mode in order to trace a path through the instrument graph table, load the necessary throughput tables, and then calculate the composite passband from the product of the throughputs of the individual optical components that make up that mode. The fact that all pertinent instrument data resides in external tables means that any telescope and instrument can be simulated simply by providing the appropriate throughput data and instrument graph table.
The Synphot Tasks
There are three main categories of tasks in the synphot package:
- Tasks that create, manipulate, and plot passbands and spectra.
- Tasks that fit model passbands and spectra to photometric and spectrophotometric data.
- General utility tasks that convert IRAF/STSDAS image data to and from table format and tasks that check or display information from the instrument graph and component lookup tables.
The following sections describe the basic functions of the tasks that create and manipulate passbands and spectra and perform synthetic photometry on these data.
Generating and Manipulating Passbands
The tasks calcband and plband calculate and plot passband (throughput) data. They can either read existing throughput curves from data tables or synthesize passbands from gaussian, box, and polynomial funtional forms where the user specifies the central wavelength, width, or polynomial coefficients. For example, the following IRAF command will produce a plot of total throughput for the Faint Object Spectrograph (FOS), using its blue-side detector, its 4.3 arcsecond entrance aperture, and its G270H grating:
The string "fos,blue,4.3,g270h" specifies the desired HST instrument observing mode and simply consists of keywords associated with the particular instrument components that make up the observing mode. Throughput data for the HST Optical Telescope Assembly (OTA) is included by default in calculations of any HST instrument mode.
Generating and Manipulating Spectra
The tasks calcspec and plspec calculate and plot photometric and spectrophotometric data. They can read or synthesize spectral data and convolve it with selected passbands or HST observing modes to simulate observed data. Spectral data can be read from tables of your own observational data or from available spectral libraries. Synthetic spectra can be generated through the use of built-in blackbody, powerlaw, and HI emission functions, where the user specifies the desired temperature, slope, or column density. Spectra can also be modified to add or remove extinction effects and can be renormalized to a chosen absolute flux level within a given passband. The spectrum calculator supports the use of data in a variety of physical units, including F_lambda, F_nu, counts, AB_nu and ST_lambda magnitudes, Jy, and mJy. Unit conversion is performed automatically when necessary to combine data of different forms or to produce results in a chosen form that differs from that of the original data.
The following IRAF command demonstrates how one might synthesize an 8000 K blackbody spectrum that is normalized to an AB magnitude of 15.5 in the Johnson V passband and includes the effects of interstellar extinction at a level of 0.2 E (B--V):
The substring "bb 8000" specifies the desired blackbody function, "rn v abmag 15.5" specifies that the spectrum is to be renormalized to a level of 15.5 AB mags in the V passband, and "ebmv 0.2" specifies the desired level of extinction. The resulting spectrum will be stored in the table "bb.tab" and the data will be in units of F_lambda ("flam").
Estimating HST Countrates
The tasks calcphot and countrate are especially useful for simulating HST observations and estimating detected count rates. Calcphot provides the ability to compute photometric quantities for spectra within selected passbands. Countrate computes count rates for spectra within selected passbands and will also compute count rate spectra for spectrographic instruments. The countrate input parameter list has been designed to mimic the specifications on an HST proposal exposure log sheet. For non-spectroscopic instrument modes, such as any of the HST Wide Field/Planetary Camera (WFPC) or Faint Object Camera (FOC) modes, countrate computes the total detected counts within the specified filter passband that one would expect to receive from a target having a given spectrum.