Calculations

Exposure Time and SNR Calculation

To specify the exposure parameters, you can give either the exposure time (in seconds), in which case the program will calculate the count rates due to the source, background, and the resulting SNR. Or, you can give the required SNR, in which case the program will calculate the required exposure time to achieve this SNR and the corresponding integrated counts. Further details follow:

If you select the exposure time:

The ETC uses the filter transmission, the throughputs of the optics, and the efficiency of the detector to calculate the expected count rate for the specified instrument configuration. It also finds the SNR, the integrated counts from the source, the sky background, and the detector background for the specified exposure time.

If you select the SNR:

The ETC uses the filter transmission, the throughputs of the optics, and the efficiency of the detector to calculate the expected count rate for the specified instrument configuration. It also finds the required exposure time so that the observation will have the specified SNR. This exposure time is then used to calculate the integrated counts from the source and different backgrounds.

Specifying the Wavelength (spectroscopic exposures):

The SNR and associated counts values listed on the results page are calculated at a specified wavelength, called the observation wavelength. The observation wavelength must be in the inclusive throughput range for the instrument, detector, spectral element(s) and disperser central wavelength (if applicable) chosen for this observation.

CTE corrections and the ETC:

Phase I proposers should note that the ETCs applicable to CCDs (ACS, STIS, WFC3) do not take into account the effects of CTE when predicting SNRs.

A pixel-based CTE correction code, first described by Anderson & Bedin (2010), is currently implemented in the calibration pipelines for ACS and WFC3. A standalone tool is available for users to run the pixel-based CTE correction on their STIS data. The algorithm corrects the CTE trails by restoring the flux to the source, but the SNR of the CTE-corrected source remains the same as that of the uncorrected source.

Proposers are advised to consult the relevant Instrument Science Reports (ISRs) on the instrument webpages 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 long exposures rather than more short exposures, as charge traps are filled by the higher background levels.
• CTE degrades SNR 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.
• For WFC3, the post-flash mode can help mitigate CTE effects, particularly for low SNR sources and low background levels. For more details, please see the white paper on CTE in the WFC3 CCD.

WFC3 scan mode:

In scan mode, the exposure parameters refer to the SNR calculation only. The ETC in scan mode does not support Time calculations. You can either give the scan rate in arcsec/sec and the scan length in arcsec, or the exposure time in seconds and the scan length in arcsec. The ETC will report the complementary quantity accordingly.

• SNR
• Calculating Time