COS ETC Cycle 18 WarningsCOS Web Form Bug
Version 18 of the ETC, which was installed in the Fall of 2009, contained a bug in the web forms for COS imaging ETC that affected calculations performed using a point source with a size other than the default. A fix to this problem was installed today (2/17/2010). Users who think their previous ETC calculations may have been affected by this problem should repeat their calculations now.
The problem corrected by this new version of the ETC occurred when the "Point Source" option is selected in section 2 of the ETC input form and "circular region" is selected via the pull down menu option, the calculation is performed using a circular region of radius 0.4 arcsec, regardless of the value selected in the pull down menu.ETC calculations using the G140L grating (updated 02/16/2010)
Cycle 18 COS/G140L ETC users are advised that the there is a disconnect between the wavelength ranges covered by the throughput files used by the COS/ETC and the wavelength ranges given in the COS IHB for the G140L grating (i.e., wavelength ranges recorded on-orbit for FP-POS=3). G140L/1105: For this cenwave the throughput file only has non-negligible throughput for wavelengths larger than 1152 Å , even though the ETC will allow calculations at wavelengths as low as 1104.5 Å. The shortest wavelength recorded on orbit for FP-POS=3 of this setting is 1120 Å. G140L/1250: For this cenwave the throughput file only has non-negligible throughput up to 1145 Å in segment B even though the recorded range for this segment, for FP-POS=3, goes out to 1165 Å.
To get around these issues and avoid getting erroneous exposure time or S/N calculations:
- When using the G140L/1105 mode in the ETC, do the calculations only at wavelengths larger than 1152 Å. If you require the exposure time or S/N for 1120 < λ < 1152 Å you can use the G140L/1250 mode in the ETC to do this calculation (but only for 1120 < λ < 1145 Å).
- When using G140L/1250 note that the throughput increases very rapidly toward the long wavelength range seen in segment B. However, since there are ~20 Å of counts that are not reported by the ETC you should be careful that the contribution to the signal from this wavelength region does not cause a local rate violation (if there is an emission line in this region for instance). If you need to perform a S/N or exposure time calculation at a wavelength that is not covered in the ETC range for this setting, i.e., 1145 < λ < 1165 Å you can use the G140L/1105 setting of the ETC to do this calculation (but only for 1145 < λ < 1165 Å).
The NUV detector dark rate used in the COS ETCs is lower that the dark rate current seen in the NUV detector. The first on-orbit measurements of the dark rate in SMOV indicated a background of ~0.65e-4 cts/sec/pixel, which is the value used in the COS ETCs. Since these initial measurements, however, the level has steadily risen, and as of mid-January 2010 it is approximately 2.9e-4 counts/pixel/sec. The reason for this increase is not yet understood. Users should take into account these differences when planning observations of targets where the background level is important. The dark rate of both detectors is measured weekly, and we will be monitoring this closely during the coming months.
Warning regarding G140L cenwaves in ETC vs APT
Cycle 18 users are advised that the "1250" G140L cenwave in ETC 18.0.4 is identical and refers to the same mode as the cenwave called "1280" in APT 18.0 and the Cycle 18 COS Instrument Handbook. GOs preparing Cycle 18 phase I should do ETC calculations with G140L/1250, but use G140L/1280 in their APT phase I. Cycle 17 users should NOT change and re-submit their phase IIs in relation to this change in cenwave name. All Cycle 17 phase IIs should use the G140L/1230 nomenclature instead.