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Space Telescope Imaging Spectrograph
Correcting ETC Calculations of CCD Spectral Observations for CTE Effects

CL Script for Applying CTE Loss to Output of Spectroscopic ETC

Download the script

As all CCD's in space, the Charge Transfer Efficiency (CTE) of the STIS CCD is degrading due to radiation damage. The loss of sensitivity due to this effect can be substantial, especially for faint targets. For instance, a source spectrum which would have a signal of 100 electrons per column at the center of the STIS CCD at STIS launch, will have lost about 30% of its flux by January 2011 due to the degrading CTE. However, the STIS ETC calculations do not incorporate these CTE effects as yet.

With this in mind, we provide the IRAF CL script "cteloss" as an aid in preparing the CCD spectroscopic exposures in your phase II proposal. The script calculates the loss due to charge transfer inefficiency of the STIS CCD for a given signal level, background level, and observation epoch. This loss can then be taken into account in your exposure time calculations by applying the appropriate dimming factor to your input flux level for a second run of the ETC.

More specifically, first one has to run the ETC assuming there is no loss in the flux level and use it as input for the "cteloss" CL script (see below). The output from this script should then be used to calculate the appropriate input flux level for a second run of the ETC, which will then yield output which is corrected for the CTE effect.

The script requires three input parameters:

(a) The counts from the dark current for a given exposure
(b) The sky background counts for a given exposure
(c) The "net" object counts for a given exposure

These three parameters can be easily obtained from the output of a given run of the ETC. When you are at the "STIS Spectroscopy ETC Results" page, click on the "View results in tabular form" link. In that table, go to the row showing the main wavelength of interest and grab the values in the third, fourth, and fifth columns of that row with your mouse. Those are exactly the three input parameters the scripts needs. Here is an example of the three values to grab with your mouse in case the wavelength of interest is 4820 A.

The script has six other input parameters, which are defaulted to typical values which can and should be changed if appropriate. These input parameters are:

(d) The year of observation (currently defaulted to 2011.0)
(e) The number of CR-SPLIT readouts (currently defaulted to 2)
(f) The binning factor in the dispersion axis (defaulted to 1)
(g) The binning factor in the spatial axis (defaulted to 1)
(h) The CCD Gain setting (defaulted to 1)
(i) point source? (defaulted to yes)

If the latter switch is set to "no", the script will assume that the ETC calculation was done for an extended source.

To use the script in IRAF, download the script to your current directory and do the following:

cl> task cteloss = cteloss.cl

The parameter file should look like this:

         dark =                 Dark Current level?
          sky =                 Background level?
          net =                 Source signal counts?
     (obsyear = 2011.0)         Year of observation (e.g. 2011.0)
       (nread = 2)              Number of CR-SPLIT readouts?
        (xbin = 1)              Binning Factor in X?
        (ybin = 1)              Binning Factor in Y?
     (ccdgain = 1)              CCD Gain setting?
       (point = yes)            Point Source?
     (Version = "26Aug09")      Date of Installation
        (mode = "al")           

The output of the script is four values, namely:

(1) factor to be applied to the input flux if observing at the CCD center
(2) magnitude offset to be added to the input if observing at the CCD center
(3) factor to be applied to the input flux if observing at E1 position (near CCD row 900, see Section 7.3.7 of the STIS Instrument Handbook)
(4) magnitude offset to be applied to the input if observing at E1 position

Here is an example run of "cteloss" plus its output:

cl> cteloss 126. 0.17 285.

CCD Center E1 Apertures
fraction d(mag) fraction d(mag)
0.7 0.38 0.92 0.1

Download the script