Exposure Time Calculators

New COS ETC Release Notes Updated 06/27/2008

The Exposure Time Calculators (ETCs) are online tools that predict the count rates and S/N in various observing modes given specified source characteristics. COS provides ETCs for four separate scenarios:

Spectroscopic Science Exposures

Use this ETC to calculate exposure times and, as needed, BUFFER-TIMEs for spectroscopic observations of external targets.

• COS Spectroscopic ETC

Imaging Science Exposures

Use this ETC to calculate exposure times and, as needed, BUFFER-TIMEs for imaging observations of external targets.

• COS Imaging ETC

Imaging Target Acquisition

Use this ETC to calculate exposure times for target acquisition exposures using ACQ/IMAGE mode.

• COS Imaging Target Acquisition ETC

"Dispersed Light" Target Acquisition

Use this ETC to calculate exposure times for target acquisition exposures using ACQ/SEARCH, ACQ/PEAKD, and ACQ/PEAKXD modes. Note that either a disperser or an imaging mirror may be used with ACQ/SEARCH, but that only dispersers may be used with ACQ/PEAKD and ACQ/PEAKXD.

• COS Dispersed Light Target Acquisition ETC

Previous Calculations

Use this ETC to recall previous calculations, using the ETC ID number. Note all COS ETC ID numbers must include the preceding "COS" to recall a previous calculation.

• Recall Previous Calculations

Known Issues and Features

• The current version of the COS ETC does not provide the correct value for
  the Global Screening Limit for the Imaging and Imaging Target Acquisition
  Modes.
  
  The correct value for the Global Screening Limit for the Imaging and
  Imaging Target Acquisition Modes is 170,000 cts/sec.
  Users might have performed calculations where this limit has been exceeded
  without triggering a warning flag in the ETC. This would affect mostly
  observations of extended sources; observations of point sources would
  likely trigger the local screening limit warning before triggering the
  Global Screening Limit.
  
  
  To verify that the Global Screening Limit has not been violated in their
  ETC calculations users are advised to check the value given in the "Count
  rate entire detector" field in the ETC output. GOs and GTOs that think
  that their observations exceed the Global Screening Limit are advised to
  contact their Contact Scientist as soon as possible.
  
  Comparison between COS and STIS in the FUV for faint targets in
  background noise-limited regimes:
  
  The COS and STIS spectroscopic ETCs do not consistent cross-dispersion
  extraction height definitions for accumulation of detected signal and,
  especially, detector backgrounds, in the FUV. As a result, for equivalent
  cross-dispersion extractions when making a direct comparison between STIS
  and COS ETC simulations, the COS backgrounds provided by the ETC are
  roughly a factor of 3 too high for FUV cases. These differences are
  especially important for faint targets in background noise-limited
  regimes. The ETCs provide a complete breakdown of the quantities used in
  the S/N determinations, so a manual re-determination of noise-limited S/N
  may be performed post facto.
  
  The net effect is that, in general, COS backgrounds are expected to be
  much lower than the STIS backgrounds and that COS will require shorter
  exposure time than STIS to yield the same S/N.
  The STIS FUV ETC uses an extraction height of 3 pixels (1.5 resolution
  elements) while the COS FUV ETC uses an extraction height of 47 pixels
  (4.7 resolution elements). For a direct comparison to the height used by
  the STIS ETC, 15 pixels should be used as the COS FUV extraction height.
  Assuming that comparable encircled energies are recorded by both STIS and
  COS within their respective cross-dispersion resolution elements a direct
  COS/STIS comparison requires that the COS FUV backgrounds be reduced by
  factor of 3.
  

• Use of HST Standard Star Spectra to model the spectral distribution of your source for COS/FUV observations:

  Please be aware that use of some of the HST Standard Star Spectra to model the spectral distribution of your source can have a significant impact on the total count rate. This can lead to bright object protection flags being raised, can affect the estimated buffer time, and could lead you to chose a different observation mode (ACCUM instead of TTAG)

  For some of the stars in the HST Standard Star Spectra, the spectra currently available are contaminated, to varying degrees, by geocoronal Ly alpha emission (some of these stars were observed with IUE).

  Note that this applies only to the FUV detector and to gratings/central wavelength combinations that cover the Ly alpha region at 1216 A: G130M for all central wavelength settings; and G140L for the 1105 A central wavelength setting.

  To completely avoid this problem we suggest that you do not use the following HST Standard Star Spectra with the settings described above for your ETC calculations: G93-48, LB227, LDS749B, HZ4, GD50, GD108, GRW +70 5824, and HZ21.

  If you have used these before please redo your calculations with one of the other HST Standard Star Spectra.

• All COS wavelengths are vacuum wavelengths.

• The "extended source" option in the COS ETCs assumes a uniformly-filled 2.5 arcsec diameter aperture.

• No spectral convolution of user-supplied input spectra is performed by the COS ETC.

  • If supplying a high-resolution spectrum from another instrument, such as STIS, in order to estimate an exposure time with COS, be sure to smooth the spectrum to a resolution listed in Table 5.1 of the COS Instrument Handbook. Failure to do so could cause the ETC to falsely indicate that a narrow emission line violates the local count rate limit.

  • If adding an emission line in part 3 of the ETC, choose a width that is the quadrature sum of the intrinsic line width and the instrumental width, or the same problem described above could occur.

  • A problem in the opposite, and more dangerous, sense could also occur. If supplying a low-resolution spectrum from another instrument, IUE for example, with intrinsically narrow emission lines, the ETC will calculate a lower count rate than COS will actually record, and could lead to a bright object protection violation. A robust estimate of the true emission line width must be provided and used in the ETC, in order to properly estimate the local count rate.

COS ETC Help Pages