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countspec -- Calculate an FOS countrate spectrum from a calibrated spectrum.
countspec spectrum obsmode output
The countspec task convolves an absolutely calibrated flux spectrum with the throughput functions of the FOS optical components to produce a corresponding spectrum that is in units of counts per second per pixel. The output is in the form of an STSDAS table containing columns of wavelength, flux, and throughput values. This task is intended to prepare spectra to be used by the bspec task which requires an input spectrum of this form.
This task accesses the Synphot library routines to generate the necessary throughput data for the FOS and to generate the fluxed spectral data. Hence, the input spectrum parameter to this task can be any legal Synphot spectrum expression, such as the name of an existing table containing spectral data, or one of the functional forms (blackbody, powerlaw, etc.) that Synphot is capable of generating. Other Synphot functions, such as the renormalize, "rn", and extinction, "ebmv", functions can also be used (see the help page for the Synphot calcspec task for full details of all the available functions).
Similarly, the parameter "obsmode" may be any Synphot-style expression that specifies the desired FOS observation mode. This will normally be a string of the form "fos,4.3,g130h,blue", for example, which includes specifications for the desired aperture, grating, and detector. To include the effects of COSTAR in the aperture throughput, simply include the additional keyword "costar" anywhere within the obsmode string, e.g. "fos,4.3,costar,g130h,blue".
The bspec task requires that its input spectral data be multiplied by the throughputs of all FOS optical elements EXCEPT for that of the grating blaze function. The combined throughput due to grating reflectivity and blaze function is contained in the standard CDBS tables accessed by the Synphot library. In order to include only the grating reflectivity in the instrumental throughput calculated by this task, it is therefore necessary to substitute the data for another optical element with similar reflectivity. The FOS collimator seems to be a good match, therefore the reflectivity of the collimator is automatically substituted in place of the grating.
- spectrum [string]
- Input spectrum specification. May be any legal Synphot spectrum expression.
- obsmode [string]
- Instrument observation mode. May be any legal Synphot-style obsmode expression for the FOS instrument.
- output [file name]
- Output table name. The table will contain three columns labeled "wavelength" (units of Angstroms), "flux" (units of counts per second per pixel), and "thr" (dimensionless). The last column contains the instrumental throughput values used to convert the input flux-calibrated spectrum into detected counts.
- (wavetab = "") [file name]
- Name of an optional wavelength table containing the desired wavelength grid on which the output spectrum will be computed. If no table is specified, a default wavelength set is used. The default wavelength set covers the wavelength range 1100 to 8000 Angstroms with a step size of 2 Angstroms per pixel.
- (grtbl = "mtab$*.tmg") [file name]
- Name of the HST graph table containing the optical layout of the FOS instrument. By default, this uses the most recent version installed in CDBS.
- (cmptbl = "mtab$*.tmc") [file name]
- Name of the HST instrument component table listing the names of the tables containing the throughput data for each of the FOS optical components. By default, this uses the most recent version installed in CDBS.
- (area = 45238.93416) [real]
- HST telescope area in cm**2. Used to convert absolute fluxes to observed counts.
1. Compute a countrate spectrum from the data for a G2V star contained in table number 46 of the Bruzuall, Persson, Gunn, Stryker (BPGS) spectral atlas. Renormalize the input spectrum to a V magnitude of 13.5 and store the output in table "fos_g2v_spec". Compute countrates for the FOS blue detector, using the 4.3" aperture and the g130h grating.
te> countspec "rn(crgridbpgs$bpgs_46,band(v),13.5,abmag)" \ >>> "fos,blue,4.3,g130h" fos_g2v_spec
2. Compute a countrate spectrum for a 3500 degree blackbody using the FOS blue detector, the 1.0" aperture, and the g190h grating. Let the blackbody spectrum have a V magnitude of 14 and store the results in table "fos_bb.tab".
te> countspec "rn(bb(3500),band(v),14,abmag)" "fos,blue,1.0,g190h" \ >>> fos_bb.tab
This task uses Synphot datasets which are not normally distributed with STSDAS. These datasets can be retrieved from the STScI ftp site.
Author: Howard Bushouse, STSDAS
For assistance using this or any other tasks, please contact firstname.lastname@example.org or call the help desk at 410-338-1082.