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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 23.2 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 Instead old calculation results from Cycles 17 and 18 have been archived.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 23.1:


  • 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.

  • ETC Emission Line Bug

    In ETC calculations involving emission lines, if any emission line falls outside of the bandpass of the observation, emission lines declared further down the list may be dropped without warning. In order to work around this issue, ensure that all emission lines are within the observation bandpass.


  • 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.


  • 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 spectroscopic slits, in particular for slits that have numbers followed by one or more letters (i.e. 0.2x0.6). The ETC will inform the user that this is an unsupported option.


  • 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.

Read the Release Notes for ETC 23.2