Exposure Time Calculators
The Exposure Time Calculators (ETCs) are online tools that
predict the count rates and S/N in various observing modes
given specified source characteristics.
IR Channel ETCs
UVIS Channel ETCs
TINY TIM PSF Simulator
Spectroscopic Image Simulator
*** Last Updated 15 February 2011 ***
Known Issues
(for all ETCs other than the ST-ECF Spectroscopic Image Simulator)
The spectroscopic ETCs for WFC3 were built upon the ACS spectroscopic ETC,
which does not convolve the input spectrum with a point spread function. In
contrast, the NICMOS spectroscopic ETC does employ this convolution. There are
two pitfalls here. First, a user can obtain an overly optimistic estimate of
the signal-to-noise ratio in an emission line if an extremely narrow line width
is specified in the input spectrum. Second, comparison of the results from the
ETCs for these instruments will not necessarily be realistic. Emission lines
can be simulated via a template, a user-supplied spectrum, or the emission
lines section of the ETC input page. It is possible via these methods for a
user to specify a line that is more narrow than a pixel. For the ACS and WFC3
ETCs, the result will be a simulation with most or all of the flux confined to
a single pixel, whereas in reality the emission line would span multiple pixels.
The WFC3 spectroscopic ETCs will incorporate a PSF convolution in a future
release, but for now, this problem can be mitigated by specifying an
appropriately broad emission line. The approximate instrumental resolution in
each of the grisms is 1.9 pixels (48 Angstroms) for the IR/G102, 2.3 pixels
(108 Angstroms) for the IR/G141, and 3.3 pixels (43 Angstroms) for the UVIS/G280.
These are the widths that should be specified for an infinitesimally narrow
line; for lines with a significant intrinsic width, the width specified in the
ETC should be the quadrature sum of the intrinsic and instrumental widths. For
example, if a line has an intrinsic width of 20 Angstroms and the observer
is using IR/G102, the line width should be specified as sqrt(400+2304)=52
Angstroms. (Feb 2011)
The SMOV calibration to the throughput of the ETC was based on observations in
a number of medium and wide band filters. For WFC3/UVIS, post SMOV data has
indicated a 10-15% higher throughput at the extreme red end of the wavelength
coverage, which was not tested during SMOV. The quad filters with
lambda=880-960 nm are the only affected filters and users should expect actual
counts to be 10-15% higher than current ETC predictions. (2 Mar 2010)
WFC3 ETC Extended Source Calculation Error Fixed:
Version 18 of the ETC, which was installed in the Fall of 2009,
contained a bug in the web forms for WFC3 UVIS and IR imaging
modes that affected all calculations for extended sources.
A fix to this problem was installed on 17 Feb. 2010. Users
who think their previous ETC calculations may have been affected
by this problem should repeat their calculations now.
WFC3 point source imaging simulations and spectroscopic
simulations using point and extended sources were NOT affected
by this problem and do not need to be redone.
The problem corrected by this new version of the ETC occurred
when the "Extended Source" option was selected in section 2 of the
imaging input forms, such that the calculation was performed using
a source diameter of 10 arcsec regardless of the value entered in
the form for source diameter and for all extraction region
selections. This resulted in the source completely filling the
extraction region when using any of the predefined square and
circular region options, resulting in much larger source count
rates and correspondingly shorter exposure times than expected.
(17 Feb 2010)
Observers are reminded that the IR detector has significant image
persistence (Handbook Sections 7.9.4 and D.2). While no restrictions are
being placed on the observation of bright sources with the IR channel,
observers should (1) be aware that subsequent images may be impacted by
residual images from saturated sources, (2) indicate via comments to STScI
those exposures which will badly (>10^7 electrons/pixel) saturate,
and (3) understand that some risk is being accepted in proceeding without
restrictions. (Oct 2009)
The UVIS shutter induces a small amount of vibration into the WFC3 instrument
optics (Handbook section 6.9.5). This occurs typically for about 200 msec
following the opening of the shutter. As a consequence, the PSF of exposures
shorter than about 5 seconds is slightly degraded. Total flux is well conserved
but the energy in the peak pixel is decreased by 5 to 15 percent in 1 second
exposures. Observers obtaining PSF reference stars are strongly encouraged to
select sources permitting exposure times greater than 10 seconds. Programs
dependent upon the exact structure and repeatibility of the PSF in short
(<10 second) exposures should so indicate via comments. Ground test and
SMOV analysis of this effect have been published in
WFC3 ISR 2008-44 and
WFC3 ISR 2009-20. (Oct 2009)
The "extended source" option in the ETCs is intended for objects at least
2 pixels across. (Dec 2008)
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