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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 22.1.2 release.

Browser Bug

A browser related bug has been reported which causes some of the form fields to be reset to their default values when the users presses the browser back button to return to the previous form input page.

This is due to a bug in the web browser, rather than in the ETC. We recommend using one of the following browsers, which have been observed to demonstrate the correct behavior (back button appears to remember the previous form settings):

  • On the Mac:
    • Using Mavericks (10.9):
      • Safari v7.0
      • Firefox v24.0
      • Chrome v30.0.1599.101
    • Using Mountain Lion (10.8.3):
      • Safari v6.0.4, 6.0.5, 6.1
      • Firefox v20.0 — v24.0
      • Chrome v30.0.1599.101
    • Using Lion (10.7.5):
      • Safari v6.0.4, 6.0.5
      • Firefox v20.0 — v24.0
    • Using Snow Leopard (10.6.8):
      • Firefox v6.0.2, v22.0
      • Safari v5.1
    • Using Leopard (10.5.8):
      • Firefox
  • On the Linux platform (2.6.9-78.0.8.ELsmp):
    • Firefox v3.6.23, 17.0.9
  • On the Windows platform:
    • Using Windows 8.1
      • Internet Explorer
    • Using Windows 7
      • Firefox 24
    • Using Windows XP
      • Internet Explorer 7.0.5730.11
    • General
      • Firefox 23.0.1

Please be aware of incorrect behavior on the following platform/browser combinations. Please do not use these versions in order to avoid encountering problems in your work:

  • On the Mac:
    • Using Mavericks (10.9):
      • Chrome 31.0.1650.57
    • Using Mountain Lion (10.8):
      • Chrome 31.0.1650.57
    • Using Lion (10.7.5):
      • Chrome 27.0.1453.116, 30.0.1599.101
      • Safari 6.0.2
    • Using Snow Leopard (10.6.8):
      • Chrome 27.0.1453.116
    • Using Leopard (10.5.8):
      • Safari v5.0.6: Problem reported with defaults being reset in some fields
  • On the Linux platform (2.6.18-194.3.1.el5):
    • Firefox v3.6.20: Problem reported with defaults being reset in some fields
  • On the Windows platform:
    • Internet Explorer v7.0.5730.13 showed several issues (using the STIS Spec input form as an example; the same issues may show up in other instruments since many of the forms are shared). After a wavelength warning is produced, the warning does not disappear when the user updates the wavelength range. In addition, upon pressing the back button, the wavelength checking appears to reset to the default Filter/grating range, while leaving the selected button as the users choice.
    • Internet Explorer 10.0.9 on Windows 7


  • ETC Calculation Dates

    ETC Calculations are currently performed with MJD = 56747.5 (March 31, 2014)

  • 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 21.2:


  • CTE corrections and the ETC

    Phase I proposers should note that the ETC applicable to CCDs (ACS, STIS, WFC3) does not take into account the effects of CTE when predicting S/N ratios.

    The Pixel-based CTE correction code currently only works for ACS (Anderson & Bedin). It corrects the CTE trails by restoring the flux to the source, but it will not improve the S/N in the source to be equivalent to that without the effects of CTE.

    Proposers are advised to consult the relevant ISRs (e.g., ACS ISR 12-03 by Ubeda & Anderson and ACS ISR 12-05 by Chiaberge for ACS, STIS ISR 03-03R by Bohlin & Goudfrooij and subsequent ISRs for STIS) to determine the percentage of flux lost and increase their exposure times accordingly.

    Proposers should also be aware of these issues:

    • The effects of CTE can be minimized by placing the source close to a readout amplifier. For the same total exposure time, the CTE losses are reduced by taking fewer longer exposures rather than more shorter exposures.
    • CTE degrades S/N through flux loss of the source into CTE trails, increased noise from CTE trails of warm pixels/cosmic rays, and increased noise in dark calibration files.
    • In WFC3 and ACS, the post-flash mode can help mitigate CTE effects, particularly for low S/N sources, and for low background levels; for more details, please see the white paper on CTE in the WFC3 UVIS.
  • SNR/Time Calculations at emission lines

    In spectroscopic and spectroscopic acquisition modes, emission lines that are narrower than the instrumental resolution are broadened by the ETC to be at least one resel (resolution element) wide. This is a flux-conserving operation performed by pysynphot when preparing the input target spectrum.

    However, the photometric extraction calculation makes use of a simplifying assumption. Instead of integrating over the wavelength range corresponding to a resel, the target count rate per pixel at the specified wavelength is simply multiplied by the number of pixels per resolution element.


  • ACS Spectroscopic Webpage does not have Javascript validation of obswave or displayed wavelength ranges for each filter

    On the ACS spectroscopic webpage, it is possible to select a wavelength outside of the passband of the selected filter, and instead of a warning appearing that the selected wavelength is outside the passband, the form will instead produce an error when submitted.

  • 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.


  • Dark Current Column Header in ETC Tabular results is labeled as per-pixel, but values are per-resel

    As such, the dark current reported in the ETC tabular results are high by a factor of 6. Both the information at the top of the results page and the information in the plots provided are correct (this issue affects only the ETC tabular results).

  • Calculations with BOA at wavelengths shorter than 1200A

    The transmission of the BOA aperture at the short wavelengths seen by the G130M 1055 and 1096 central wavelength and G140L 1105 and 1280 central wavelengths, while not fully characterized, is expected to be close to zero (due to MgF2 cutoff for wavelengths shorter than 1200A). The values reported by the Spectroscopic COS ETC, for calculations using the BOA at wavelengths shorter than 1200A, are not accurate and should not be used for planning any COS observations.

  • Count rate calculations for extended targets

    All ETC versions including this one have been affected by a bug in the count rate calculation for spectroscopic observations of extended targets. The diameter of the target input by the user is incorrectly ignored in the cross-dispersion direction, and the aperture size (2.5") is used instead. As a result, the reported extracted target rate is overestimated by a factor 2.5/target diameter for target diameters smaller than 2.5". Therefore the SNR will be overestimated, and the time will be underestimated.

  • Wavelength dependence on the Exposure Times and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings due to changing resel size.

    The resolution, and hence the resolution element (resel) size of the G130M/1055 and G130M/1096 settings is wavelength dependent. The ETC assumes resel sizes of 8 pixels for G130M/1055 and 9 pixels for G130M/1096, which corresponds to the region of highest resolution for each setting. For G130M/1055 this is roughly around 950 A, and for G130M/1096 this corresponds to roughly around 1050A. ETC calculations away from these wavelengths will continue to use the fixed 8 or 9 resel width for the S/N calculation even though the actual resel becomes larger. If you instead wish to calculate the S/N for the appropriate resel size at each wavelength, results will differ by as much as a factor of five in exposure time and a factor of two in S/N. Exposure times scale as Resel_nominal/Resel_lambda, while S/N scales as SQRT(Resel_lambda/Resel_nominal). The table below gives detailed corrections for these two settings. The correction is multiplicative, that is t_new=t_etc*correction or S/N_new=S/N_etc*correction.

    Exposure time corrections:

    Cenwave Segment Lambda Resel size Correction Factor
    (Angstroms) (pixels)
    1055 B 900 10.6 0.75
    950 8.3 0.97
    1040 19.0 0.42
    A 1050 21.5 0.37
    1120 32.1 0.25
    1190 42.6 0.19
    1096 B 940 11.9 0.75
    1000 8.4 1.07
    1050 8.6 1.04
    1080 11.9 0.76
    A 1100 15.3 0.59
    1170 23.9 0.38
    1230 31.6 0.28

    S/N corrections:

    Cenwave Segment Lambda Resel size Correction Factor
    (Angstroms) (pixels)
    1055 B 900 10.6 1.15
    950 8.3 1.02
    1040 19.0 1.54
    A 1050 21.5 1.64
    1120 32.1 2.00
    1190 42.6 2.31
    1096 B 940 11.9 1.15
    1000 8.4 0.97
    1050 8.6 0.98
    1080 11.9 1.15
    A 1100 15.3 1.30
    1170 23.9 1.63
    1230 31.6 1.87


  • 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 0.4 arcseconds

    Please note that the default aperture radius for WFC3 is 0.4 arcsec (90% of encircled energy). For 80% of encircled energy, use an aperture with radius 0.2 arcsec, comparable to the ACS default.

Read the Release Notes for ETC 22.1.2