ETC News and Known Issues
The following issues remain unresolved in 20.1.1:
General
- Contact the Helpdesk for access to Cycle 18 or prior calculations (20.1)
The Cycle 17 and Cycle 18 Java ETC is no longer available. Instead old calculation results from Cycles 17 and 18 have been archived. Contact the STScI Helpdesk at help@stsci.edu in order to retrieve any old calculations necessary.
- Browser bug (20.1)
A browser related bug has been reported which causes some of the form fields to be reset to their default values when the users presses the browser back button to return to the previous form input page. This appears to be related to the browser itself.
The correct behavior (back button appears to remember the previous form settings) has been verified on the following platform/browser combinations:
- On the Mac:
- Using Snow Leopard (10.6.8):
- Firefox v6.0.2
- Safari v5.1
- Using Leopard (10.5.8):
- Firefox
- Using Snow Leopard (10.6.8):
- On the Linux platform (2.6.9-78.0.8.ELsmp):
- Firefox v3.6.23
- On the Windows platform:
- Firefox v3.6.3
Incorrect behavior has been observed on the following platform/browser combinations:
- On the Mac:
- Using Leopard (10.5.8):
- Safari v5.0.6: Problem reported with defaults being reset in some fields
- Using Leopard (10.5.8):
- On the Linux platform (2.6.18-194.3.1.el5):
- Firefox v3.6.20: Problem reported with defaults being reset in some fields
- On the Windows platform:
- Internet Explorer v7.0.5730.13 showed several issues (using the STIS Spec input form as an example; the same issues may show up in other instruments since many of the forms are shared). After a wavelength warning is produced, the warning does not disappear when the user updates the wavelength range. In addition, upon pressing the back button, the wavelength checking appears to reset to the default Filter/grating range, while leaving the selected button as the users choice.
- On the Mac:
ACS
- CTE corrections and the ETC (20.1)
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
- Changes to COS FUV Throughput (20.1.1, January 26, 2012)
Throughputs for all FUV gratings have been updated to take into account that the sensitivity of the FUV channel seems to be decreasing at a substantially higher rate than before. The new throughputs are projected to Jan 1, 2013.
These new projections may increase the required time to reach a certain S/N by as much as 50% and so users should repeat any calculations performed in preparation for the Cycle 20 call for proposals.
- Changes to COS NUV G285M Throughput (20.1.1, January 26, 2012)
The throughput of the G285M grating was also updated at this time. While there has been no change in the slope of the sensitivity decline, which is ~11% per year, this throughput had not been updated for the past two years, leading to exposure times being underestimated by ~25%. Users proposing for Cycle 20 with the G285M grating should also update their calculations.
- Software Bug for Dispersed Light Target Acquisition (20.1.1, January 26, 2012)
A software bug in the ETC algorithm that simulates dispersed light target acquisition (for both FUV and NUV) resulted in overestimated exposure times for a certain S/N by as much as a factor of 2. Users proposing for Cycle 20 should re-do their calculations.
- Calculations using the BOA
at wavelengths shorter than 1200 Å, are not accurate and should not be used for planning
any COS observations. (20.1)
The transmission of the BOA aperture at the short wavelengths seen by the G130M 1055 and 1096 central wavelength and G140L 1105 and 1280 central wavelengths, 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.
- Spectral resolution of G130M 1055 and 1096 reported incorrectly by the ETC (20.1)
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).
- Tables for spectroscopic modes incorrectly report dark rate per resel-width, rather than per pixel-width as for the other columns (20.1)
STIS
Typo in STIS E230H 2263 Wavelength Range (20.1)
The STIS E230H 2263 currently shows 2124-2301 Angstroms as its wavelength range. The correct wavelength range is 2124.5-2401.49 Angstroms. Note that, if you need to submit a calculation using this setting and wavelengths between 2302 and 2401 that you may do so by disabling Javascript before entering the calculation.
WFC3
- 20.1: The maximum exposure time exceeded warning message is applied based on the total exposure time, and does not take into account whether or not an exposure is divided into frames. If your total exposure time generates the warning, but the exposure time for each frame would not, then the warning may be safely ignored.
- 20.1: The newly reported global background value correctly includes the thermal contribution, but the web page mentions only “sky + dark”.