STScI Logo

Hubble Space Telescope

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.


COS ETC Cycle 21 Warnings

last updated: 09/11/2013

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 Å). The values reported by the Spectroscopic COS ETC, for calculations using the BOA at wavelengths shorter than 1200 Å, are not accurate and should not be used for planning any COS observations.

COS/NUV Imaging Target Acquisition now supports specification of acquisition mode

This version of the ETC now supports ACQ/IMAGE and ACQ/SEARCH target acquisition modes. In the case of the ACQ/IMAGE mode, a 9x9 pixel checkbox is assumed, while the ACQ/SEARCH imaging option simulates a single dwell point of the ACQ/SEARCH algorithm, with a 220x470 pixel subarray. Previous calculations assumed the parameters of the ACQ/IMAGE acquisition mode and may find an increase in exposure time with the new ACQ/SEARCH option for faint targets.

Dark Current Column Header in ETC Tabular results is labeled as per-pixel, but values are per-resel

As such, the dark current reported in the ETC tabular results are high by a factor of 6. Both the information at the top of the results page and the information in the plots provided are correct (this issue affects only the ETC tabular results).

Count rate calculations for extended targets

All ETC versions including this one have been affected by a bug in the count rate calculation for spectroscopic observations of extended targets. The diameter of the target input by the user is incorrectly ignored in the cross-dispersion direction, and the aperture size (2.5") is used instead. As a result, the reported extracted target rate is overestimated by a factor 2.5/target diameter for target diameters smaller than 2.5". Therefore the SNR will be overestimated, and the time will be underestimated.

Wavelength dependence on the Exposure Times and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings due to changing resel size

Since ETC version 21.2, resel sizes and extraction heights appropriate for the optimized focus of G130M/1055 and G130M/1096 have been implemented. However, the resolution, and hence the resolution element (resel) size of the G130M/1055 and G130M/1096 settings is wavelength dependent. The ETC assumes resel sizes of 8 pixels for G130M/1055 and 9 pixels for G130M/1096, which corresponds to the region of highest resolution for each setting. For G130M/1055 this is roughly around 950 A, and for G130M/1096 this corresponds to roughly around 1050Å. ETC calculations away from these wavelengths will continue to use the fixed 8 or 9 resel width for the S/N calculation even though the actual resel becomes larger. If you instead wish to calculate the S/N for the appropriate resel size at each wavelength, results will differ by as much as a factor of five in exposure time and a factor of two in S/N. Exposure times scale as Resel_nominal/Resel_lambda, while S/N scales as SQRT(Resel_lambda/Resel_nominal). The table below gives detailed corrections for these two settings. The correction is multiplicative, that is t_new=t_etc*correction or S/N_new=S/N_etc*correction.

Time Correction
Cenwave Segment λ (Å) Resel size (pixels) Correction Factor
1055 B 900 10.6 0.75
950 8.3 0.97
1040 19.0 0.42
A 1050 21.5 0.37
1120 32.1 0.25
1190 42.6 0.19
1096 B 940 11.9 0.75
1000 8.4 1.07
1050 8.6 1.04
1080 11.9 0.76
A 1100 15.3 0.59
1170 23.9 0.28
1230 31.6 0.28

S/N Correction
Cenwave Segment λ (Å) Resel size (pixels) Correction Factor
1055 B 900 10.6 1.15
950 8.3 1.02
1040 19.0 1.54
A 1050 21.5 1.65
1120 32.1 2.00
1190 42.6 2.31
1096 B 940 11.9 1.15
1000 8.4 0.97
1050 8.6 0.98
1080 11.9 1.15
A 1100 15.3 1.30
1170 23.9 1.63
1230 31.6 1.87

Known Issues and Features