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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 27.1 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 = 58940 (Apr 1, 2020)

  • 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 Instead old calculation results from Cycles 17 and 18 have been archived.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 27.1:


  • 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 and related pages, 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 Results Plot Failure when flux or throughput is uniformly zero

    When an ETC result is returned with an observed flux of zero or throughput of zero, the related ETC plot will return an error rather than being displayed.

  • Small Extraction Heights

    Specifying extraction regions smaller than 0.1 arcsec in radius will produce incorrect results, as the enclosed energy tables do not sample below this size.


  • 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.

  • No Default Extraction Region for Extended Source Calculations

    When performing a calculation for an extended source, none of the radio buttons for extraction region size/shape under the heading "2. Specify the exposure parameters" are selected by default. One of these buttons must be selected for the calculation to succeed.


  • S/N Overestimated for Long Wavelengths with G140L/c800

    COS spectroscopy calculations for the G140L/c800 mode use an extraction height of 30 pixels, and assume that 100% of the energy is enclosed within this height. This is valid for wavelengths below approximately 1150 . At longer wavelengths, the height of the spectrum may be as high as 57 pixels, and therefore the dark-current count-rate may be underestimated by about 50%. This leads to an overestimate of the signal-to-noise. For bright sources, with sufficiently high signal, the impact on the reported exposure time is minimal. If the source is faint at these longer wavelengths, the G140L/c1105 mode may be used to calculate a better estimate of the signal-to-noise. For further questions regarding this issue, please contact the COS help desk.

  • Underestimated S/N with G140L/c800 in v26.1

    The ETC v26.1 underestimated the S/N obtained with the new G140L/800 mode, and conversely, overestimated the time required to achieve a certain S/N. Using the quantities calculated by the ETC it is straightforward to determine the correct values. The details are available in the June 2018 STScI Analysis Newsletter, Cosmic Origins Spectrograph. This has been corrected in ETC v26.2 and later, and we strongly recommend that users recalculate their results using the most recent version of the ETC.


  • 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.

  • Buffer Time in Previous ETC Results

    Older releases of the ETC did not calculate the STIS MAMA buffer times, so recalling previous results will produce the text "ERROR, number expected." Users wishing to determine the buffer time should re-run their calculations in the current release of the ETC.


  • Wrong CCD chip selection for WFC3/UVIS quad filters is permitted

    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. Even though the ETC will let the user select either chip, when only one is physically possible, the ETC will now compute the correct results, using the chip associated with the quad filter regardless of user input.

  • 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 27.1