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: Spectroscopy, Imaging, Imaging Target Acquisition, and Disperse Light Target Acquisition.
- Spectroscopic ETC and User Guide
- Imaging ETC and User Guide
- Target Acquisition and User Guide
- COS Known Issues and Features
- HST ETC User Guide
- Current ETC Known Issues
- Current ETC Release Notes
- Issues affecting previous ETCs
COS ETC Cycle 21 Warnings
last updated: 11/30/2012
Exposure Time and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings are not correct
The focus of the G130M/1055 and G130M/1096 settings were recently adjusted to increase their resolution. As a consequence, the spectral height and size of the resolution element (resel) have changed. Because of these two changes, exposure times may be underestimated by as much as a factor of 4, and S/N may be overestimated by as much as a factor of 2. The ETC assumes a resel of 30 pixels and an extraction height of 47 pixels, but the true resels are smaller and depend on wavelength; the true extraction heights are larger. The ETC will be updated for phase II of Cycle 21, in the meantime the correction factors given in the Known Issues page of the ETC should be used. For more detailed information on how to determine precise corrections, users can refer to ETC Corrections for resel size and extraction height.
COS BOA Calculations for Wavelengths <1200 Å
The transmission of the BOA aperture at the short wavelengths seen by the G140L 1105 and 1280 cenwaves, while not fully characterized, is expected to be close to zero (due to MgF2 cutoff for wavelengths shorter than 1200 A). The values reported by the Spectroscopic COS ETC, for calculations using the BOA at wavelengths shorter than 1200 A, are not accurate and should not be used for planning any COS observations.
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, HZ4, 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.
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, the ETC will set the FWHM to the resolution element of the mode being used if the user supplied FWHM is smaller than the instrumental width.
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.