Exposure Time Calculators

New COS ETC Release Notes Updated 10/02/2009

New ETC User Guide

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

• 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, HZ4, GD50, and GD108.

  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