STScI Logo

Observatory Support
ETCs

ETC News and Known Issues

Be sure to read the Release Notes for new features and resolved issues in the ETC 21.2 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 Mountain Lion (10.8.3):
      • Safari v6.0.4
      • Firefox v20.0
    • Using Lion (10.7.5):
      • Safari v6.0.4
      • Firefox v20.0
    • Using Snow Leopard (10.6.8):
      • Firefox v6.0.2
      • Safari v5.1
    • Using Leopard (10.5.8):
      • Firefox
  • On the Linux platform (2.6.9-78.0.8.ELsmp):
    • Firefox v3.6.23
  • On the Windows platform:
    • Firefox v3.6.3

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 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.

Notes

  • 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 Instead old calculation results from Cycles 17 and 18 have been archived.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 help@stsci.edu 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:

General

  • 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.

  • All Imaging modes

    Pixel areas for dark, thermal, and readnoise are not rounded up to an integer number of pixels

  • All Spectroscopic modes

    Slitless spectroscopic modes use an extraction region that is not correctly rounded up to an integer number of pixels.

COS

  • ETC Tabular Results show total dark current per-pixel rather than 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 LambdaResel sizeCorrection Factor
    (Angstroms)(pixels)
    1055B90010.60.75
    950 8.30.97
    104019.00.42
    A105021.50.37
    112032.10.25
    119042.60.19
    1096B94011.90.75
    10008.41.07
    10508.61.04
    108011.90.76
    A110015.30.59
    117023.90.38
    123031.60.28

    S/N corrections:

    Cenwave Segment LambdaResel sizeCorrection Factor
    (Angstroms)(pixels)
    1055B90010.61.15
    9508.31.02
    104019.01.54
    A105021.51.64
    112032.12.00
    119042.62.31
    1096B94011.91.15
    10008.40.97
    10508.60.98
    108011.91.15
    A110015.31.30
    117023.91.63
    123031.61.87

NICMOS

  • 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()
22150.506378238068
    For assistance with pysynphot, please contact the helpdesk.

STIS

  • Extended targets

    When an extended target is specified with a diameter that is greater than the visible area of the detector, the entire target area is incorrectly used when calculating the counts due to the target, which are therefore overestimated.

  • Potential Issue with Imaging Calculations of Point Targets

    For imaging calculations of point targets, if the photometric extraction region is specified as a percentage of the total light that is smaller than is supported by the enclosed energy table, the code does not return an error. Instead, it computes a negative value for the radius, and an incorrect value for the angular area of the extraction region. The negative value is reported on the results page. Users who have selected percentage extraction should be sure to check that this reported value ("circle with radius N.NN arcsec") is positive to verify that a valid calculation was performed.

  • 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.

  • STIS CCD with Binning

    The following problems exist:

    • In spectroscopic mode with spatial binning:
      • For Extended Targets:
        • when biny=1, the calculation is performed over an extraction height of two native pixels
        • when biny=2, the clculation is performed over an extraction height of two native pixels (one binned pixel)
        • when biny=4, the calculation is performed over an extraction height of four native pixels (one binned pixel)
      • For Point Targets:
        • if the extraction height is not an integer number of binned pixels, the height is not rounded up to an integer number of binned pixels before the enclosed energy fraction for the target is computed. This causes the target rate to be slightly underestimated compared to the background.
    • In imaging mode, the value of the brightest binned pixel is overestimated by a factor equal to the binning value selected.
  • STIS Target Acquisition
    • The area of the slit is ignored when computing the extraction area (extended targets) or enclosed energy fraction (point targets)
    • The effect of slits with a width narrower than one pixel is ignored when computing the brightest pixel rate.
  • STIS Slitted Spectroscopy

    The convolution kernel is incorrectly computed by rounding to the nearest, rather than the next highest, integer number of pixels

WFC3

  • The global background value reported on the results page correctly includes the thermal contribution for IR calculations, even though the web page mentions only "sky + dark".

Read the Release Notes for ETC 21.2