Basic Use

Using the ETC is quite simple. Once you have traversed to the desired ETC web page based on the instrument and science mode, simply enter the specified parameters for each of the sections listed below, then click the “Submit” button to retrieve your results.

• Instrument settings

• Exposure parameters

  • Time or Signal-to-Noise ratio (SNR) for stare modes

  • Scan length and either scan rate or exposure time for scan modes

  • Observation wavelength (for spectroscopy)

  • Point or extended target

  • Photometric extraction region

• Target (source) specification

  • spectral distribution

  • extinction

  • redshift

  • emission lines

  • flux normalization

• Expected background levels

  • earthshine

  • zodiacal light

  • airglow (geo-coronal) lines

All the sections contain preloaded defaults, so at any point you can submit the current settings for an ETC calculation.

Examining the Results

Detailed Results

The results page includes the basic calculation results (time or SNR requested), detailed results with rates, counts, and noise associated with each component, and any relevant warnings about the calculation. For convenience, the input values are then summarized. A few items on the results page warrant additional explanation.

Spectroscopy: Regions for Rate and Counts

For spectroscopic calculations, the rate (column 1) and counts (column 2) are reported over different regions:

• The count rate (column 1) is reported over an area 1 pixel wide by pixels high.

• The number of counts (column 2) is reported over the specified extraction region used for the SNR calculation, which is one resel (resolution element) wide by pixels high. The size of the resel in pixels is reported in the heading of column 2.

Here, is the height of the extraction region specified by the user or determined by the instrument mode.

CCD Mode Calculations

CCD gain is used only for readnoise and saturation values.

Brightest Pixel Rate and Counts

For calculations in which the detector parameters specify that the exposure should be divided into independent exposures (by setting the number of frames or CR-Split), the total time will be used for most results, but the individual exposure time (total time divided by the number of frames) will be used to compute the number of counts in the brightest pixel calculation. This allows the brightest pixel to be checked against the saturation limit. This can lead to the counter-intuitive result in which the number of counts in the brightest pixel is smaller than the count rate in the brightest pixel.

Plots and Tables

Buttons appear near the bottom of the page for viewing available plots.

All the ETCs present plots of the input spectrum prior to observation (with flux units in flam or flam for extended sources), of the throughput of the specified instrument configuration, and of the observed target spectrum (extraction region not taken into account). After initial display, the X and Y ranges can be specified and the data re-plotted with the new values. This allows the user to zoom in on a region of interest, or shift a region of interest out from under the legend of a plot with multiple curves.

Spectroscopic Plots and Tables

The spectroscopic modes produce two additional plots called Total Counts and Signal-to-Noise. A link to a table of source and noise counts per pixel is also provided.

The table presents results in counts over an area that is 1 pixel wide in the dispersion direction by pixels high in the spatial direction. The Total Counts and Signal-to-Noise plots present results in counts over an area that is 1 spectral resolution element wide by pixels high. Thus, the SNR plot is consistent with the reported (or input) scalar SNR. The Observed Target Spectrum plot presents the count rate over 1 pixel wide in the dispersion direction integrated along the the full spatial dimension of the detector (rather than pixels), and thus is in units of counts per sec per dispersion-direction pixel per spatial-dimension column. At a particular wavelength, the value in the Input Target Spectrum plot divided by the energy per photon and multiplied by the area of the HST mirror (45,239 cm²), the system throughput, and the dispersion corresponds to the value in the Observed Target Spectrum at that wavelength.

Approximating ETC Results in pysynphot

The ETC constructs its observed spectrum and sky using pysynphot. For instructions on how these observations are calculated, see the pysynphot manual.