thermback -- Evaluate the thermal background count rate for an observing mode.
This task computes the predicted thermal background flux for a specified telescope observing mode. At the user's option, it can also create an output spectrum containing the flux as a function of wavelength. The observing mode of the HST is specified in several task parameters. These are instrument, detector, spec_elem, and aperture. The instrument must always be specified, but the other three parameters may be blank, depending on which instrument and instrument configuration is used. More than one observing mode keyword may be specified in a task parameter by separating the keywords with a comma. The obsmode task has more information on the legal observing mode keywords.
The feature described in this paragraph Thermback extends the usual obsmode to permit the user to optionally specify a temperature associated with any component in the optical chain. For example, if the usual obsmode is "nicmos, 2, f222m", one can override the default temperature of the primary mirror to 300K with "nicmos, 2, f222m, primary#300". (But see Bugs, below.) Legal names for specifying the components are contained in the THLMODE keyword of the component emissivity file.
At present, CDBS only contains thermal related data files for the NICMOS instrument. Thermback will generate the error "Component names not found in lookup table" for unsupported instruments.
The output table is specified by the parameter output. If this parameter is set to "none" (the default) or left blank, no output table will be created. Otherwise, the task will create a table of wavelength versus flux.
- obsmode [string]
- The full specification of the observing mode, with optional per-component temperature overrides.
- (output = "none") [string]
- The name of the output spectrum produced by this task. If this parameter is set to "none" (the default) or a blank string, no output table will be created. Otherwise, it will create an STSDAS table with two columns, WAVELENGTH and FLUX. The WAVELENGTH column will contain wavelengths in Angstroms. The FLUX column will contain the flux at the corresponding wavelengths. The table will also contain the header parameters GRFTABLE, OCMPTBL, TCMPTBL, and OBSMODE. These will contain the name of the graph table, the optical component lookup table, the thermal component lookup table, and the observation mode.
- (tcmptb = "mtab$*_tmt.fits") [string]
- The name of the thermal lookup table. This table is analogous to refdata.cmptbl: it translates the thermal component names in the graph table to the filenames containing the emissivity tables and other relevant data for the component.
- (detcat = "synphot$simulators/data/detectors.dat") [string]
- The name of a file containing detector properties, including the pixel scale.
- (wavetab = "crmodewave$wavecat.dat") [string]
- The name of the wavelength table over which to perform the calculation. If blank, synphot will compute a wavelength table that matches the specified obsmode. The catalog is a table with two columns. The first column contains an observation mode string and the second contains a wavelength table name. This task reads the wavelength catalog and searches for the observation mode that best matches the input observation mode (constructed from task parameters) it then uses the wavelength table to resample the spectrum on the wavelength grid of the detector. If the wavelength catalog is an STSDAS table, the observation mode column must be named OBSMODE and the wavelength table column must be named FILE. If the wavelength catalog is a text table, the observation mode column and the wavelength table columns are the first and second columns, respectively.
- (refwave = INDEF) [real] [min = 1.0, max = 200000.]
- The reference wavelength in the spectrum. If this parameter is not INDEF, the output will contain the flux at this wavelength.
- (verbose = no) [boolean]
- If yes, Thermback will write detailed information including a calculation of the thermal background rate at each component in the optical train.
- (form = "counts") [string]
- Desired output form for the calculation. The standard set of synphot forms
are recognized :
FNU ergs / s / cm^2 / Hz FLAM ergs / s / cm^2 / A PHOTNU photons / s / cm^2 / Hz PHOTLAM photons / s / cm^2 / A COUNTS photons / s ABMAG -2.5 log_10 (FNU) - 48.60 STMAG -2.5 log_10 (FLAM) - 21.10 OBMAG -2.5 log_10 (COUNTS) VEGAMAG -2.5 log_10 (F/F_vega) JY 10^-23 ergs / s / cm^2 / Hz MJY 10^-26 ergs / s / cm^2 / HzNote that if form = counts or obmag, the result will be integrated over the passband and normalized to the collecting area of the HST.
A standard magnitude system is VEGAMAG, for which Vega by definition has magnitude 0 at all wavelengths. The AB and ST magnitude systems are based on constant flux per unit frequency and per unit wavelength, respectively. The zero points for these two systems are set for convenience so that Vega has magnitude 0 in both systems for the Johnson V passband.
- (thermflux = INDEF) [real]
- An output parameter containing the calculated thermal flux per pixel in the units specified by "form".
- (refdata = " ") [pset name]
- A parameter set containing the HST telescope area, the name of the HST graph table, and the name of the component lookup table. These parameters have been placed in a pset because they are common to most of the tasks in this package.
1. Compute the thermal background rate for the NICMOS 2 camera using the f222m filter. Store the result in table nic2def.fits:
sy> thermback.obsmode = "nicmos, 2, f222m" sy> thermback.output = "nic2def.fits" sy> thermback
There is a sporadic bug that may manifest when the temperature override option is used, that causes the program to crash. This appears to be a memory problem. Since the cause of the problem is not well understood, use caution in relying on results generated with the temperature override parameter.
Written by V. Laidler based on work by A. Sivaramakrishnan SEE ALSO synphot opt=sys and refdata for more details of the graph and component tables.