APT Exposure Time Calculator Details
As of July 15, 2003, we provide a new improved version of the ACS ETCs. For details on the new ETCs and
the tests we performed on them, see ACS ISR 2003-08
Baseline Tests for the Advanced Camera for Surveys Astronomer's Proposal Tool Exposure Time Calculator
by Boffi et al. The list of improvements with respect to the previously available ETCs is:
***New modes:
- A coronagraphic mode is now available
and comes in two flavors: one to be used together with
imaging filters and one to be used with ramp filters.
The newly implemented tool estimates exposures times for
coronagraphic exposures, accounting for the additional
background contribution from the central source PSF, as
indicated in the
ACS Instrument Handbook chapter 6.
***Enhancements of already existing modes:
- The Signal-to-Noise and exposure time calculations can now be calculated over a variety
of regions (or apertures). The user can select square regions or circular
apertures. Several options are provided in the form of pull-down menus.
- The sky contributions are from the Earth shine, the zodiacal light and the geocoronal lines.
There are a few improvements with respect to the old ETCs:
- 1) both Earth shine and zodiacal light can be selected as low,average,high (or very
high in the case of the Earth shine), following the prescriptions of Giavalisco et al.
WFC3-ISR-2002-12.
In addition, each contribution can be scaled with a user's selected factor or normalized to a user's selected magnitude;
- 2) The geocoronal lines now have a gaussian profile, instead of being a delta function.
- 3) The geocoronal lines intensity can be high, average, low or zero.
- Emission lines are now included in all modes. They can be
added to any continuum or used by themselves.
- The source counts are now calculated more accurately,
because the fraction of encircled energy is taken at the
effective wavelength instead of the pivot wavelength of the
observing mode. The effective wavelength is the weighted average of the
system throughput AND source flux integrated over the wavelength.
This is particularly important for red targets in F850LP, see the
April 2003 STAN.
- The calculation of the brightest pixel is now more precise.
- Any contunuum can now be red-shifted, not just that of QSOs.
- For the spectroscopic mode, the source counts are calculated
with the appropriate fraction as a function of wavelength, while
the old ETCs used a constant.
- For extended sources, a source diameter is selectable
which enables the proper global limit calculation for SBC exposures.
