COS ETC Help Page

New ETC Release (20.1.1): Changes in the FUV sensitivity and new throughput curves

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.

Contents

• Warnings
• 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

New ETC Release (20.1.1): Changes in the FUV sensitivity and new throughput curves

Throughputs for all FUV gratings have been updated to take into account that the sensitivity of the FUV channel seems to be decreasing at substantially higher rates than before. The new throughputs are projected to Jan 1 2013.

These new projections may increase the required time to reach a certain S/N by as much as 50% and so users should repeat any calculations performed in preparation for the Cy 20 call for proposals.

More details

Resolution of New COS/FUV G130M Modes

The two new COS/FUV G130M modes (cenwave=1055 and cenwave=1096) have resolution of R ~ 3000 at 950 A, decreasing to R ~ 1000 at 1150 A and dispersion of ~ 0.01 A/pixel, similar to the dispersion of the other, higher resolution, modes. This implies that at 950 A one resolution element is around 30 pixels. Since the ETC uses a resolution element that is 5 times smaller (6 pix), it will overestimate the time required to achieve a certain S/N per 30 pixel resolution element, while underestimating the S/N per 30 pixel resolution element for a given time. To calculate the correct exposure time per real 30 pix resolution element, divide the exposure time returned by the ETC by 5 (= 30/6). To calculate the correct S/N per real 30 pix resolution element, multiply the S/N returned by the ETC by 2.2 (= 5^0.5).

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, 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.

• 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.