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
- Issues affecting previous ETCs
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
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.
The above warnings also apply to spectral lines being studied in the G130M/1055 and G130M/1096 modes where the resel size is different than assumed by the ETC. Please refer to the table above of resel size versus wavelength to appropriately choose a linewidth.