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Observatory Support

ETC News and Known Issues

Be sure to read the Release Notes for new features and resolved issues in the ETC 24.2 release.


  • Browser Bug

    Some web browsers have a bug in which using the Back button causes ETC form values to reset. More information can be found in the User's Guide.

  • ETC Calculation Dates

    ETC Calculations are currently performed with MJD = 57462.3 (March 15, 2016)

  • Contact the Helpdesk for access to Cycle 18 or prior calculations

    The Cycle 17 and Cycle 18 Java ETC is no longer available. Instead old calculation results from Cycles 17 and 18 have been archived. Contact the STScI Helpdesk at in order to retrieve any old calculations necessary.

Using the ETC with archival data

There are a number of issues involved in using the ETC to support archival research. In particular,

  • The ETC is optimized for projected instrument sensitivity and status during the upcoming observing cycle, and will thus supply incorrect answers for past instrument performance.
  • The default SYNPHOT and pysynphot data files — graph (TMG), component (TMC), and thermal (TMT – for NICMOS and WFC3) tables are also optimized for projected use, and will thus also supply incorrect answers for past instrument performance.
  • ETC calculations are not required to have the same standard of reliability and absolute accuracy with respect to sensitivity as instrument calibration files and pipelines. In particular, for instruments with health and safety concerns the ETC provides a conservative estimate.

The following issues remain unresolved in 24.2:


  • An invalid value in an input field, even a field for an option that is not selected, will cause the ETC to return with an Input Error.

    An invalid value in an input page field (say, a negative extraction region radius), even when that field is not selected, still triggers the checks for validity in the web server. This causes error pages to be displayed even though the invalid field is not selected.

  • ETC User's Guide Display Issues

    On the ETC User Spectra page, the units will appear garbled if the character encoding is not correctly set to Unicode. The Firefox browser defaults to ISO-8859-1 (Western) encoding on this page. Manually setting the encoding to Unicode via View —> Character Encoding will fix the issue.


  • ACS results with CLEAR do not include known effects of PSF degradation

    Filterless ACS observations (obtained by setting both filters to "CLEAR") are available but not supported by ACS calibration programs. Such observations are out of focus, resulting in a broader PSF (less encircled energy) than otherwise expected, as documented in ACS ISR 2003-03. This PSF degradation is not included in the ETC calculations, and the ETC will thus tend to overestimate SNR in those cases.

  • It is possible to enter a “Post Flash” value in SBC observations.

    The ETC will allow a value to be entered for post-flash in SBC imaging observations. The ETC will then ignore this value, as post flash does not apply to the SBC.


  • NICMOS ETC No Longer Available

    The NICMOS ETC is no longer available for new calculations. Given the fact that NICMOS is not being offered for use in the current proposal cycles, and due to the substantial maintenance and development costs in a constrained budgetary environment, the decision was taken to discontinue support for the NICMOS ETC.

    Users can still retrieve NICMOS ETC results from Cycles 17 and 18 by contacting the STScI Help Desk.

    In order to (for example) calculate the effective wavelengths of NICMOS filters, pysynphot may be used, provided that the Throughput Tables have been downloaded. For example, to calculate the effective wavelength of the NICMOS f222m filter,

    >>> import pysynphot as S
    >>> sp=S.FileSpectrum(’$PYSYN_CDBS/calspec/gd71_stisnic_003.fits’)
    >>> bp=S.ObsBandpass(‘nicmos,3,f222m’)
    >>> obs=S.Observation(sp,bp)
    >>> obs.efflam()
    For assistance with pysynphot, please contact the helpdesk.


  • Option for in-band normalization

    The option for in-band normalization currently is not supported for certain STIS modes, in particular for the 50CCD filter in imaging modes and for slits that have numbers followed by one or more letters (e.g. 0.2x0.6). Performing such calculations will cause the ETC to produce an error.

  • Error in brightest pixel count rate for observations with 52x0.05 slit

    In cases where the brightest pixel geometry is less than one pixel in size due to aperture effects, the ETC code will ignore the effects of the aperture and calculate the brightest pixel as a full pixel. This will result in the brightest pixel count rate being overestimated.


  • Incorrect background computations for the UVIS grism G280

    The background is being computed incorrectly for the UVIS grism G280. We expect to correct this problem within a few weeks of the release of ETC version 25.1.

  • Incorrect computations for UVIS quad filter FQ937N when the wrong CCD chip is selected

    As discussed below, it is possible to select the wrong CCD chip for the UVIS quad filters. FQ937N is on chip 1. ETC version 25.1 computes the source and background flux incorrectly for FQ937N only if the wrong chip (2) is selected.

  • Wrong CCD chip can be selected for WFC3/UVIS quad filters

    Each WFC3/UVIS quad filter (named F*Q) covers somewhat less than a quadrant of the UVIS FOV on a particular chip, 1 or 2. The ETC will let the user select either chip, when only one is physically possible. The results are significantly different for the bluest quad filters, FQ232N and FQ243N, because the quantum efficiency on the two chips differs by ~20% in the passbands of these filters. (See Section 5.4.1 in the WFC3 Instrument Handbook). The ETC default input value for “Detector chip” is 2, which is the correct value for these two filters. The location of all of the quad filters is given in the Position column of Table 13.5 in Section 13.4.1 of the Phase II Proposal Instructions. Amps (quads) A and B are on chip 1; amps (quads) C and D are on chip 2.

  • Option for in-band normalization

    The option for in-band normalization currently is not supported for certain WFC3 modes, in particular for the 50CCD filter and any mode which begins with a number but contains letters.

  • WFC3IR Imaging has a limited range of values for ‘Percentage of all light’

    In the WFC3 IR Imaging ETC with default settings, when ‘User a circular region containing X percent of total light’ is chosen, there is not sufficient data to handle values less than 50% or greater than 99.9%. Selecting such values will result in a ‘Value out of range’ error.

  • Default Aperture for WFC3 (UVIS and IR) is 0.2 arcseconds

    Please note that the default aperture radius for WFC3 is 0.2 arcsec. This aperture includes ~84 to 86% of encircled energy for wavelengths in common to WFC3/UVIS and ACS/WFC, and ~76 to 80% of encircled energy for wavelengths in common to WFC3/IR and ACS/WFC. (See Tables 6.8 and 7.6 in the WFC3 Instrument Handbook for WFC3/UVIS and WFC3/IR, respectively.) For direct comparison to the ACS default aperture, use a circular region with 80% of total light. Note that this option will not work in all cases.

  • Chrome browser behaves badly with back button for WFC3 stare and scan spectroscopic modes

    When disperser G141 has been selected, the simulation is run, and the option to return to the previous page is used in the Google Chrome browser, the input parameters are preserved but there is an inappropriate error message on the input line for the vacuum wavelength (Error: wavelength is not in the passband) and the message below that input line is “Filter G102 pass band 0.8 – 1.1645” instead of “Filter G141 pass band 1.0665 – 1.7205”. The next simulation will fail if the wavelength error message is present. It can be removed and the appropriate pass band statement restored by selecting G102 then G141 as the disperser.

Read the Release Notes for ETC 24.2