STScI Logo

simbackgd stsdas.hst_calib.synphot.simulators



simbackgd -- Two-dimensional HST background simulator


simbackgd obsmode output


This task computes a simulated background level for HST instruments with two-dimensional imaging formats (FOC, WFPC, WFPC2, NICMOS, and STIS) for a given observing mode. The output is a single number representing the background flux (in detected counts) per detector pixel, and optionally an entire image with the same pixel format as the detector specified by the mode. The output image could, for instance, be added to a user-constructed astronomical background image for use as input to the `simspec' task.

The calculated background has contributions due to zodiacal light, scattered earthlight, and thermal background. Zodiacal light is a function of the pointing of the telescope relative to the sun. The telescope pointing is set by task parameters "det_ra" and "det_dec", the sun position is set by task parameter "time", which controls the date of the observation. The earthlight background is calculated from task parameter "earthtab", which specifies the maximum permitted earthlight spectrum, and task parameter "eshine", which specifies a fraction of the maximum permitted earthlight. The thermal background is calculated from "thermtab", which specifies the spectrum of the thermal background.


obsmode [string]
Telescope observation mode. The observation mode is a comma- or space-separated list of keywords that specifies a valid light path through the telescope. (Use the `obsmode' task to review valid values for the observation mode.) The observation mode is used to compute the instrument throughput, and select the point spread function and the detector dimensions.
output [string]
Output image name. If the name is left blank or set to "none", no output image will be created. The image pixels will all be set to the computed background level, in detected counts.
(exptime = 1.0) [real] [min = 0.0] [max = INDEF]
Observation exposure time in seconds.
(det_ra = 0.0) [real] [min = 0.0] [max = 24.0]
Right ascension (in hours) of the center of the detector aperture.
(det_dec = 0.0) [real] [min = -90.0] [max = 90.0]
Declination (in degrees) of the center of the detector aperture.
(det_ang = 0.0) [real]
Detector position angle (in degrees) relative to equitorial coordinates. The position angle is measure counterclockwise from north.
(verbose = yes) [bool]
If this parameter is set to yes, the task will print the number of background counts to STDOUT.
(counts) [real]
The number of background counts. This is an output parameter and should not be set by the user.
(wavetab = "none") [string]
Wavelength table An appropriate table can be generated by using the `genwave' task. If a binary table is used, the wavelength column name must be WAVELENGTH. If a text table is used the first column is taken to be the wavelength column. The subdirectory synphot$data has text wavelength tables useful for specific HST passbands.

If no wavelength table is specified, a default wavelength set is used. The default wavelength table covers the wavelength range where the telescope and magnitude passbands are non-zero. Wavelengths are spaced logarithmically over this range.

(simbackp = "") [pset]
The parameter file containing the noise and background parameters. These are:
(eshine = 0.0) [real] [min = 0.0] [max = 1.0]
Fraction of maximum earhlight to include in spectrum
(time = "Jun 1997 1:55:42 PM") [string]
The time of the observation. The time is used to compute solar position. The background light is a function of the angle between the sun and the telescope, so this date controls the background light contribution. All times are UT (Universal time). The time may be entered in a variety of formats. The month field must be first, followed by the day of month and year. All other fields are optional and will be set to zero if omitted. Fields must be separated by one non-alphanumeric character. Months may be specified by number or any unique abbreviation of the English name.
(simcatp = "") [pset]
The parameter set containing the file and catalog parameters. The parameters used by this task are:
(detcat = "simulators$data/detectors.dat") [string]
Catalog of detector dimensions. The catalog is a table containing four columns: the observation mode, the pixel scale, the number of pixels in the X dimension, and the number of pixels in the Y dimension. If the catalog is a binary table, the columns are named OBSMODE, SCALE, NX and NY. If it is a text table, they are the first through fourth columns. The row in the catalog is selected if the input observation mode is a superset of the catalog observation mode. If the table is a binary table, the units of the detector scale are read from the SCALE column units. If the table is a text table, the units are assumed to be arcseconds.
(zodtab = "simulators$data/zodiac.dat") [string]
A table of zodiacal light flux. The units of the flux are tenth magnitude solar type stars per square degree. The table is a function of ecliptic latitude and heliocentric longitude of the detector. The heliocentric longitude is the absolute value of the difference of the ecliptic longitude of the detector and the sun. The first row in the table contains the latitudes and the first column contains the longitudes at which the flux values are tabulated. The default table is taken from "A.C. Levasseur-Regourd and R. Dumont, "Absolute Photometry of Zodiacal Light," Astr. and Ap., 84, 277 (1980)". If the table name is blank or set to none, the zodiacal contribution to the background will be omitted.
(earthtab = "simulators$data/earth.dat") [string]
The earthlight spectrum at its maximum value. The flux is given in units per square arcseond. If the table name is blank or set to none, the earthlight contribution to the background will be omitted.
(thermtab = "simulators$data/thermal.dat") [string]
The thermal background spectrum. The flux is given in units of square arcsecond. If the table name is blank or set to none, the thermal contribution to the background will be omitted.
(refdata = "") [pset]
The parameter set for reference data used in calculations. This pset contains the telescope area, and the names of the graph and component tables. For more details, type "help refdata".


1. Simulate the background seen by detector 2 of the WFPC-2 camera, using the F555W filter:

    cl> simbackgd wfpc2,f555w,2 output.hhh


Written by Bernie Simon.


simimg, simnoise, obsmode, refdata

Source Code · Package Help · Search Form · STSDAS