STIS Exposure Time Calculators
STIS ETC Release Notes Updated 05/15/2009
New ETC User Guide
The Exposure Time Calculators (ETCs) are online tools that predict the count rates and S/N in various observing modes
given specified source characteristics. STIS provides ETCs for three separate scenarios:
Spectroscopic Science Exposures
Use this ETC to calculate the count rates and S/N ratio for a simulated spectrum of ONE source in a STIS spectroscopic observation.
Imaging Science Exposures
Use this ETC to calculate the count rates and S/N ratio for imaging observations of external targets.
Imaging Target Acquisition
Use this ETC to calculate exposure times for target acquisition exposure.
Previous Calculations
Use this ETC to recall previous calculations, using the ETC ID number. Note all STIS ETC ID numbers must include the preceding "STIS" to recall a previous calculation.
*** Last updated 10/28/2009 ***
Known Issues and Features
ETC 18.0 (formerly 18.1) was released on 10/24/2009.
The following changes were made:
- The CALSPEC files that ETC uses were updated to the preferred spectra.
The current list of HST Calibration Spectra used by the ETC was updated
with the preferred set as listed in the CALSPEC website
- Dark Current and Read Noise values were updated
The Dark current and read noise changed after the repair of STIS.
The new values are as follows:
- STIS CCD:
- Read noise:
Gain = 1 5.6 e-
Gain = 4 8.3 e- (~2.05DN)
- Dark current: 0.015 e-/pixel/s
- STIS NUV MAMA:
- Dark current: 0.005 counts/pixel/s
- STIS FUV MAMA:
- Dark current: 0.00015 counts/pixel/s
- Added GALEX passband normalization option
GALEX passbands were added to the normalization menu for STIS. These filter choices are:
- GALEX FUV (1350 - 1750 Å)
- GALEX NUV (1750 - 2800 Å)
which are normalized to AB magnitudes.
- A separate report is now given for global count rates and total count rates
For all STIS ETCs, a separate sub-section was added to the detailed
information output section with the following format:
--------------------------------------------------
Global Count Rates
Source only xxxx xxxxx
Background xxxx xxxxx
--------------------------------------------------
where the x's mark the place where the numbers will be.
The left set of numbers gives the "count rates",
integrated over the entire detector,
due to just the source and just the background,
and the right set gives the total counts in
the exposure from the source or background
(i.e., the first column times the exposure time).
For extended sources in the STIS Imaging ETC and
STIS ACQ ETC the following subsection was also added:
--------------------------------------------------
Background per unit area
Sky Background (arcsec-2) xxx xxxxx
Dark Current (arcsec-2) xxx xxxxx
--------------------------------------------------
where the left column of numbers gives the rate per second per square arcsec
and the right column of numbers gives the total over the exposure.
In the case of the STIS imaging and ACQ ETCs, the new output subsections
described above replaced the following sub-section:
--------------------------------------------------
Count rate
Source (infinitely large region) 36,175.21
Sky Background (arcsec-2) 34.51
--------------------------------------------------
- ETC now uses PySYNPHOT