Software Requirements Specification for the ETCs
			(Based on the CGI ETCs)
			 Draft of 17-May-2000

Intended Users 
--------------

The ETCs provide support to Instrument Scientists, DAs, IDTs, and
Guest Observers proposing for HST observation time for calibration 
or science targets.


Functional Requirements 
-----------------------

The fundamental requirement of the ETCs is to estimate the total counts 
from an astronomical object, the counts that can be expected from other 
sources (sky, detector, optics), and the signal to noise ratio that will 
be realized from the source for a well formed exposure specification. 

The ETCs shall provide as output either the SNR realized within a given 
exposure duration, or the exposure duration required to achieve a 
specified SNR. 

The ETCs shall compute the desired quantities for the most important 
instrument configurations, while permitting the use of "engineering only" 
modes only by authorized users.  The "most important" modes are those 
that support the vast majority of the science (~90%+), and is generally 
taken to mean all imaging and spectroscopic modes, with the exception 
of coronography. 

The ETCs shall support a wide variety of specifications for the target 
spectrum, including functional forms that can be computed within synphot, 
star atlases that reside within synphot, specialized spectra defined by 
STScI staff, and spectra in allowed formats that are provide by the user. 

The Spectroscopic and Ramp Filter ETCs shall support the addition of up 
to three emission lines in the input spectrum, specified by line center, 
width, and total flux.  The user will be allowed to specify these lines 
independently of any selection (or no selection) of a continuous spectrum 
for the source. 

The ETCs shall properly account for background sources, such as Earthshine 
and Zodiacal light and, for UV modes, the Geo-coronal emission.  The ETCs 
shall also account for thermal backgrounds, including radiation from the 
telescope/instrument optics. 

The ETs output shall include the S/N ratio and exposure time (one of which 
will be the result of the computation), and the effective wavelength for 
which the computation was performed, if relevant.  The output shall also 
include a summary of the instrument configuration, the specification for 
the source and background spectra, and a breakdown of the counts from 
the source, background, and the detector. 

The ETCs shall issue warnings if the exposure will exceed the allowable 
brightness limits for the specified detctor; this includes both local and 
global limits. 

The ETCs should trap and report error conditions (including those reported 
by the synphot throughput engine), and provide a mechanism for users to 
uniquely identify their invocation of the ETC, so that the logs and other 
records (which are staged on STScI computers for a few days) can be used by Institute Staff to diagnose problems.

The ETCs permit execution from the host (command line), and allow for the 
processing of multiple test cases.  The format for the input file(s) and 
the output file must be simple enough that non-software developers can 
construct test cases, and can evaluate the results of regression testing 
in a straightforward way. 


Non-functional Requirements 
---------------------------

The ETCs shall be accessible to all authorized users via the Web, and 
the software and supporting data will be hosted on Institute machines. 

Historical data on ETC use indicates that peak use occurs during the two 
weeks immediately prior to the Phase 1 deadline for each cycle.  The ETCs 
for STIS are invoked a few thousands of times per day in that period. 
The CGI program and supporting machine+Web server must support this load 
and produce the computations within 30 seconds of program invocation. 

The ETCs shall be well documented, with the meaning and scientific impact 
of each input parameter being described in an on-line document.