ETC 19 Bugs and Special Notices
This page holds information about know issues or ETC bugs identified since ETC v19.1 was installed.
For the ETC 19.1 Release Notes and Known Issues, see the ETC 19.1 Release Notes.
All Instruments
Extinction can only be applied to a non-zero continuum, and is not applied to emission lines
A non-zero E(B-V) value may only be used when a non-zero continuum has been selected. In addition, the extinction will only be applied to the continuum, and not to any emission lines specified separately.
Default Wavelength Range used for Throughput Plots
Please note that the default wavelength range used when plotting “Throughput”, [0,35000] angstroms, is in most cases much larger than the original wavelength range of the throughput for the mode under consideration.
Flux normalization value should not have spaces:
In section 4 of the ETC form, the flux value for the Flux Normalization in ergs /(cm^2 s A) must not contain a space
Currently, if you enter a normalization flux similar to “1.5 e-15” (note the space), then the ETC will not be able to process the calculation, and will return an uninformative error. The solution for this issue is to enter the value without any spaces.
Use the “gal3” extinction curve instead of “Average Galactic”
If at all possible, we recommend using the “gal3” extinction curve instead of the “gal1” or “Average Galactic” curve.
ACS
CTE corrections and the ETC
Phase I proposers should note that the ACS ETC does not take into account the effects of CTE when predicting S/N ratios. While the pixel-based CTE correction code of Anderson & Bedin will correct the CTE trails by restoring the flux to the source, it will not improve the S/N in the source to be equivalent to that without the effects of CTE. Proposers are therefore advised to consult ACS ISR 09-01 by Chiaberge et al. 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 longer exposures rather than more shorter exposures.
COS
Incorrect Warning Message for Irregular Sources
In spectroscopic calculations, when the count rate per segment/stripe exceeds the limits for irregular sources (6,000 cts/sec/segment for FUV, 12,000 cts/sec/stripe for NUV) the warning message displayed by the ETC INCORRECTLY states that “60% of the global count rate limit” has been exceeded. The warning message should state instead that “40% of the global count rate limit” has been exceeded (the global count rate limits are 15,000 cts/sec/segment for FUV and 30,000 cts/sec/stripe for NUV).
FUV Background Rate is higher in the ETC than in the Cycle 19 Instrument Handbook
In Cycle 18, a COS FUV dark rate of 2.25e-6 counts/s/pixel was reported in the IHB and used by the ETC. For the Cycle 19 IHB we updated this value to 1.8e-6 counts/s/pixel, which is based on a slightly different analysis technique, and includes pulse-height filtering. However, ETC 19 continues to use the more conservative value of 2.25e-6 counts/s/pixel in its calculations, and COS Acquisitions do not use any pulse-height filtering, so the higher background rate is appropriate in those cases.
Spectral resolution of G130M 1055 and 1096 reported incorrectly by the ETC
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).
Using extinction curves other than “gal3” may cause errors with analytic spectra
When employing analytic spectra (blackbody, power law, flat) with applied E(B-V), using an extinction curve other than “gal3” will cause errors. The current workaround is to use the “gal3” extinction curve, or to generate the analytic spectrum separately and supply it as an uploaded spectrum.
COS ETC S/N Plots and tables overstate dark current by a factor of 6
When plotting signal to noise from the COS ETC Results page, the dark current contribution is calculated incorrectly. As a result, the signal-to-noise reported in S/N plots only is lower than the actual signal-to-noise. This effect is more apparent for targets with lower count rates (i.e. where the fraction of counts due to dark current is larger). Note that the S/N reported on the results page is accurate.
NOTE that this also applies to the tables that can be obtained through the “View results in tabular form” link.
STIS
Typo in STIS E230H 2263 Wavelength Range
The STIS E230H 2263 currently shows 2524-2801 Angstroms as its wavelength range. This is actually the wavelength range for the 2663 central wavelength. The correct wavelength range is 2124-2401 Angstroms. Note that, if you need to submit a calculation using this setting, that you may do so by disabling Javascript before entering the calculation.
Using extinction curves other than “gal3” may cause errors with analytic spectra
When employing analytic spectra (blackbody, power law, flat) with applied E(B-V), using an extinction curve other than “gal3” will cause errors. The current workaround is to use the “gal3” extinction curve, or to generate the analytic spectrum separately and supply it as an uploaded spectrum.
Adding binning to STIS observations causes the count rate/pixel to decreate
When setting the binning to values other than 1 for STIS observations, the count rate (and other reported variables) may be divided by the number of pixels per bin rather than multiplied. The only available workaround is currently to contact the ETC Helpdesk with the details of your calculations. Please also provide the binning you wish to use, as that information is not available when retrieving past calculations.
WFC3
WFC3 Grisms report per-pixel SNR not per-resel SNR
The WFC3 G280, G102, and G141 grisms report per-pixel SNR in the ETC, although the description states that they are reporting per-resel SNR. For these grisms, the resel consists of the following number of pixels:
G280: 3.3 pixels/resel
G102: 1.9 pixels/resel
G141: 2.3 pixels/reselPlease keep this in mind when planning your observations.