ETC News and Known Issues

Read the Release Notes for ETC 21.2

Notes

Contact the Helpdesk for access to Cycle 18 or prior calculations
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 Instead old calculation results from Cycles 17 and 18 have been archived.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.

Using the ETC with archival data

There are a number of issues involved in using the ETC to support archival research. In particular,

The ETC is optimized for projected instrument sensitivity and status during the upcoming observing cycle, and will thus supply incorrect answers for past instrument performance.
The default SYNPHOT and pysynphot data files — graph (TMG), component (TMC), and thermal (TMT – for NICMOS and WFC3) tables are also optimized for projected use, and will thus also supply incorrect answers for past instrument performance.
ETC calculations are not required to have the same standard of reliability and absolute accuracy with respect to sensitivity as instrument calibration files and pipelines. In particular, for instruments with health and safety concerns the ETC provides a conservative estimate.

The following issues remain unresolved in 21.2:

General

ETC Calculation Dates

ETC Calculations are currently performed with MJD = 56747.5 (March 31, 2014)
CTE corrections and the ETC

Phase I proposers should note that the ETC applicable to CCDs (ACS, STIS, WFC3) does not take into account the effects of CTE when predicting S/N ratios.

The Pixel-based CTE correction code currently only works for ACS (Anderson & Bedin). It corrects the CTE trails by restoring the flux to the source, but it will not improve the S/N in the source to be equivalent to that without the effects of CTE.

Proposers are advised to consult the relevant ISRs (e.g., ACS ISR 09-01 by Chiaberge et al. for ACS, STIS ISR 03-03R by Bohlin & Goudfrooij and subsequent ISRs for STIS) 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.
- CTE degrades S/N through flux loss of the source into CTE trails, increased noise from CTE trails of warm pixels/cosmic rays, and increased noise in dark calibration files.
- In WFC3 and ACS, the post-flash mode can help mitigate CTE effects, particularly for low S/N sources; for more details, please see the white paper on CTE in the WFC3 UVIS.
SNR/Time Calculations at emission lines

In spectroscopic and spectroscopic acquisition modes, emission lines that are narrower than the instrumental resolution are broadened by the ETC to be at least one resel (resolution element) wide. This is a flux-conserving operation performed by pysynphot when preparing the input target spectrum.

However, the photometric extraction calculation makes use of a simplifying assumption. Instead of integrating over the wavelength range corresponding to a resel, the target count rate per pixel at the specified wavelength is simply multiplied by the number of pixels per resolution element.
Potential Issue with Imaging Calculations of Point Targets

For imaging calculations of point targets, if the photometric extraction region is specified as a percentage of the total light that is smaller than is supported by the enclosed energy table, the code does not return an error. Instead, it computes a negative value for the radius, and an incorrect value for the angular area of the extraction region. The negative value is reported on the results page. Users who have selected percentage extraction should be sure to check that this reported value ("circle with radius N.NN arcsec") is positive to verify that a valid calculation was performed.
Browser bug
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
- 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
- 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.

ACS

The minimum exposure time threshold 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 exposure time per frame is less than the minimum time (0.1 seconds for HRC, 0.5 seconds for WFC) but your total time is not, the ETC will fail to warn you that this result is less than the minimum time for the detector.

COS

COS FUV Dark Rate has dropped

The dark rate on both FUV segments has been evolving with time, particularly in the last few months, and it is now a factor of two lower than the value currently used in the ETC for both segments. For more information, please go to the COS newsleter article “Update to the dark rates of the COS FUV Segments”.
ETC Tabular Results show total dark current per-pixel rather than per-resel

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

Exposure Time and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings are not correct. Exposure times may be underestimated by as much as a factor of 4, and S/N may be overestimated by as much as a factor of 2.

The focus of the G130M/1055 and G130M/1096 settings were recently adjusted to increase their resolution. As a consequence, the spectral height and size of the resolution element (resel) have changed. The ETC assumes a resel of 30 pixels and an extraction height of 47 pixels. The true resels are smaller and depend on wavelength; the true extraction heights are larger.

The ETC will be updated for phase II of Cycle 21, in the meantime the correction factors given in the tables below should be used. For more details on calculating a correction factor go here (COS ETC page).

Correction Factors: The ETC underestimates the dark rate due to the incorrect extraction height, and overestimates the source counts due to the incorrect resel size. The corrections also depend on the ratio of the source count rate to the dark count rate (under the assumption of negligible sky contributions):

α = C/B_dark where C is the calculated source count rate per pixel (returned in the first column, second row of the ETC output) and B_dark is the calculated dark rate per pixel (returned in the first column, fourth row of the ETC output). Faint sources will have α → 0; for very bright objects α → ∞.

For a given cenwave setting and α one can determine the correction factor as a function of wavelength from the tables below.

Caveats: The resel sizes are predicted by ray trace models of COS and may change as the new settings are better characterized.

Table 1. Exposure Time correction factors

Setting	Extraction Height (pixels)	Segment	λ (Å)	Resel (pixels)	α=0	α=1	α=10	α=100	α=∞
1055	63	B	900	10.6	3.79	3.77	3.62	3.13	2.82
			950	8.3	4.85	4.83	4.64	4.01	3.62
			1040	19.0	2.12	2.11	2.03	1.75	1.58
		A	1050	21.5	1.87	1.86	1.79	1.55	1.40
			1120	32.1	1.25	1.25	1.20	1.04	0.93
			1190	42.6	0.94	0.94	0.90	0.78	0.70
1096	59	B	940	11.9	3.16	3.14	3.04	2.72	2.51
			1000	8.4	4.47	4.45	4.31	3.85	3.56
			1050	8.6	4.36	4.34	4.21	3.76	3.48
			1080	11.9	3.16	3.15	3.05	2.73	2.52
		A	1100	15.3	2.46	2.45	2.37	2.12	1.96
			1170	23.9	1.57	1.57	1.52	1.35	1.25
			1230	31.6	1.19	1.19	1.15	1.03	0.95

Table 2. S/N correction factors.

Setting	Extraction Height (pixels)	Segment	λ (Å)	Resel (pixels)	α=0	α=1	α=10	α=100	α=∞
1055	63	B	900	10.6	0.51	0.52	0.53	0.57	0.59
			950	8.3	0.45	0.46	0.46	0.50	0.53
			1040	19.0	0.69	0.69	0.70	0.76	0.80
		A	1050	21.5	0.73	0.73	0.75	0.80	0.85
			1120	32.1	0.89	0.90	0.91	0.98	1.03
			1190	42.6	1.03	1.03	1.05	1.13	1.19
1096	59	B	940	11.9	0.56	0.56	0.57	0.61	0.63
			1000	8.4	0.47	0.47	0.48	0.51	0.53
			1050	8.6	0.48	0.48	0.49	0.52	0.54
			1080	11.9	0.56	0.56	0.57	0.61	0.63
		A	1100	15.3	0.64	0.64	0.65	0.69	0.71
			1170	23.9	0.80	0.80	0.81	0.86	0.89
			1230	31.6	0.92	0.92	0.93	0.99	1.03

All ETC versions including this one have been affected by a bug in the count rate calculation for extended target. 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.

NICMOS

NICMOS ETC No Longer Available

The NICMOS ETC is no longer available for new calculations. Given the fact that NICMOS is not being offered for use in the current proposal cycles, and due to the substantial maintenance and development costs in a constrained budgetary environment, the decision was taken to discontinue support for the NICMOS ETC.

Users can still retrieve NICMOS ETC results from Cycles 17 and 18 by contacting the STScI Help Desk.

In order to (for example) calculate the effective wavelengths of NICMOS filters, pysynphot may be used, provided that the Throughput Tables have been downloaded. For example, to calculate the effective wavelength of the NICMOS f222m filter,
```
>>> import pysynphot as S
>>> sp=S.FileSpectrum(’$PYSYN_CDBS/calspec/gd71_stisnic_003.fits’)
>>> bp=S.ObsBandpass(‘nicmos,3,f222m’)
>>> obs=S.Observation(sp,bp)
>>> obs.efflam()
22150.506378238068
    
```
For assistance with pysynphot, please contact the helpdesk.

STIS

When an extended target is specified with a diameter that is greater than the visible area of the detector, the entire target area is incorrectly used when calculating the counts due to the target, which are therefore overestimated.
The option for in-band normalization currently is not supported for certain STIS spectroscopic slits, in particular for slits that have numbers followed by one or more letters (i.e. 0.2x0.6). The ETC will inform the user that this is an unsupported option.

WFC3

Both the maximum and minimum exposure time thresholds are applied based on the total exposure time, and do 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. Likewise, if your exposure time per frame is less than the minimum time (0.06 seconds for IR, 0.5 seconds for UVIS) but your total time is not, the ETC will fail to warn you that this result is less than the minimum time for the detector.
The newly reported global background value correctly includes the thermal contribution, but the web page mentions only "sky + dark".

Observatory Support ETCs