
           DRAFT REQUIREMENTS FOR THE COS EXPOSURE TIME CALCULATOR

Prepared by: Ken Sembach
Revision Date: 30-Nov-2001
===============================================================================

This document contains a brief description of the requirements for the APT
exposure time calculator (ETC) for the spectroscopic observing modes of the 
Cosmic Origins Spectrograph (COS).  The APT/ETC requirements for COS target 
acquisition modes will be given separately at a later date.  It is expected 
that whenever possible the COS spectroscopic ETC inputs will conform to those
selected to be the standards for all of the Hubble instruments by the 
ETC Cross-Instrumental Issues Group chaired by Chris O'Dea.

This document describes the generic requirements for inputs in the following
categories : 1) Instrument parameters, 2)  Exposure parameters, and 
3) Output information.  Data, parameterizations, and equations needed to 
perform the exposure time calculations will be provided to the APT/ETC 
Development Group, as needed, by the COS Instrument Scientists (Tony
Keyes and Ken Sembach).  Additional requirements may be added at a later 
time, but those listed below are expected to represent the majority of those 
needed to set up a basic COS ETC.


1) INSTRUMENT PARAMETERS

Grating Selection:
-----------------
The following gratings should be selectable for the far-ultraviolet (FUV)
detector: G130M, G160M, G140L.

The following gratings should be selectable for the near-ultraviolet (NUV)
detector: G185M, G225M, G285M, G230L.  

The approximate wavelength coverages of these gratings are as follows,
and multiple grating settings may need to be used to cover the full wavelength
range specified.  The exact wavelength coverages may vary slightly for the 
NUV gratings from those presented here depending upon the final selection
of flight optics.

G130M: 1150-1450 A   (~300 A per setting, small gap between detector segments)
G160M: 1405-1775 A   (+370 A per setting, small gap between detector segments)
G140L: 1230-2050 A   (~820 A per setting, small gap between detector segments)*

*Note: The G140L wavelength coverage may extend to slightly shorter wavelengths
depending upon final throughput measurements.  We expect that there may be 
useful data that could be obtained down to ~1150 A.  

G185M: 1750-2000 A   (3x28 A portions of spectrum per setting)
G225M: 2000-2500 A   (3x35 A portions of spectrum per setting)
G285M: 2500-3200 A   (3x41 A portions of spectrum per setting)
G230L: 1700-3200 A   ([1 or 2] x 398 A portions of spectrum per setting)

Only one grating may be selected at a time.


Grating Central Wavelength:
--------------------------
The following central wavelengths should be selectable for the seven gratings.
The central wavelengths and default values for the NUV gratings may change 
depending upon the final selection of flight optics.  The nominal central 
wavelengths (in Angstroms) are as follows:

G130M: 1298, 1309 (default), 1320 
G160M: 1586, 1600 (default), 1614 
G140L: 1150, 1230 (default)

G185M: 1794, 1807, 1820, 1833, 1847, 1857, 1872, 1884, 1897 (default), 1910, 
       1923, 1929

G225M: 2120, 2127, 2138, 2149, 2154, 2171, 2189, 2206, 2322 (default), 2339,
       2355, 2372, 2388, 2400

G285M: 2642, 2662, 2683, 2694, 2708, 2722 (default), 2742, 2971, 2993, 3012, 
       3032, 3050, 3070

G230L: 2635, 3000 (default), 3360

Only one central wavelength per grating may be selected at a time.


Aperture Selection:
------------------
Two circular (2.5") entrance apertures should be selectable (PSA or BOA).  The 
Primary Science Aperture (PSA) is a full-transmission aperture expected to 
be used for most normal science observations. The Bright Object Aperture (BOA)
is used for observations requiring flux attenuation.  The BOA contains a  
neutral density (ND2) filter that attenuates the flux by a factor of 100.   
The PSA aperture should be the default aperture.  Both apertures will be 
calibrated.

Two additional apertures, the Flat-field Calibration Aperture (FCA) and the 
Wavelength Calibration Aperture (WCA) are also available but are not expected
to be used by the science community and therefore need not be incorporated 
into the ETC.  Light from external sources cannot pass through these 
apertures.


Operational Mode:
----------------
The user should select one of the two operational modes (Normal, RVMM) 
available.  The Normal operational mode should be the default.  The Rear View 
Mirror Mode (RVMM), which is only available for the NUV settings, can be used 
to attenuate the flux of objects in either aperture to ~4% of that obtained in 
the Normal operational mode by rotating the grating mechanism.  Both modes can 
be used with any of the grating or aperture selections.


2) EXPOSURE PARAMETERS

Exposure Time / Signal-to-Noise, and Reference Wavelength:
---------------------------------------------------------
The user should be able to specify either the total exposure time (in seconds) 
or the S/N per spectral bin size (see below) required at a reference 
wavelength within the bandpass covered by the grating setting selected 
above.  If the exposure time is specified, this should be the summed (total)
exposure time for all FP-split (sub)exposures.  [FP-split exposures are 
designed to reduce the fixed-pattern noise in the spectrum created by 
summing the exposures.] The ETC does not have to worry about whether an 
exposure is FP-split or not since it will have no knowledge of the 
fixed-pattern noise contribution to the total noise budget.  

If the reference wavelength falls outside the bandpass covered by the 
grating/central wavelength selection made by the user, an error message 
should be given to the user and a new (in-band) wavelength requested.

If the S/N specified or calculated exceeds a minimum value then the following 
alert should be communicated to the user:

"Signal-to-noise calculation does not include the effects of fixed-pattern
noise that may be present in COS data at some wavelengths.  The user should
refer to the COS Instrument Handbook for more information about fixed-pattern
noise."

(This minimum value is should be set initially to S/N=30:1, but this may be 
revised upward depending upon the results derived from ground test data.)


Spectral Bin Size:
-----------------
The user should be able to select the spectral bin size (in Angstroms) for 
the S/N or exposure time calculation.  The default value should be the size 
of a single resolution element appropriate for the grating and central
wavelength selected.  The approximate point source resolution element sizes 
for the various gratings are as follows, but will change slightly depending 
upon final selection of flight optics.  A table of refined values will be 
provided to the APT/ETC Development Group.

G130M: ~0.059 A   G185M: ~0.085 A
G160M: ~0.072 A   G225M: ~0.102 A
G140L: ~0.54  A   G285M: ~0.129 A
                  G230L: ~1.10  A

The user should also be informed of the default resolution element size in 
pixels as an informational point.  These sizes will be specified once the 
final grating selections are made and the resolutions are measured.


Point / Extended Source:
-----------------------
The user should be able to specify whether the object to be observed is a 
point source (default) or an extended source that will fill the aperture.  
For extended sources, the ETC should assume that the source fills the aperture.
The spectral resolution for a filled slit is still to be determined.
  

Spectral Energy Distribution Shape:
----------------------------------
The user should be able to specify the shape of the spectral energy 
distribution of the object being observed.  The following options should be 
available:

a) User supplied spectrum in either an ASCII, SDAS, or FITS table.  This 
   file would have to be placed in an anonymous ftp area by the user, so the 
   filename must be specified.  Format of the user supplied spectrum
   should be the standard two column format used for the STIS ETC 
   [wavelength (A) and flux (erg cm^-2 s^-1 A^-1) for point sources or 
   flux (erg cm^-2 s^-1 A^-1 arcsec^-2) for extended sources].  

b) Kurucz model spectra for stars of varying spectral types (like those 
   listed in the existing STIS ETC).

c) HST standard star spectra (like those listed in the existing STIS ETC).

d) Red-shifted QSO energy distribution, with redshift specified by the user.
   The rest-frame energy distribution should be that of the FOS composite
   QSO spectrum given by Zheng et al. (1997, ApJ, 475, 469) or some 
   updated version thereof.  

e) A power law spectrum [F_lambda proportional to lambda**(alpha)] with a 
slope (alpha) specified by the user.

f) A black-body spectrum with a temperature specified by the user.

g) A flat continuum.

The user should also be able to add emission lines to the input spectrum,
as is currently available for the STIS ETC.  The inputs to specify a 
particular emission line would be the line center (in A), the FWHM of the 
line (in A), and the integrated flux of the line (in erg/cm**2/sec for a 
point source or in erg/cm**2/sec/arcsec**2 for an extended source).


Spectral Energy Distribution Normalization:
------------------------------------------
The user should be able to specify a spectral energy distribution
normalization based on one of the following choices.  Magnitudes or fluxes 
should be specified on a per square arc second basis for extended sources.

a) A photometric magnitude, for which the photometric band (UBV) and magnitude
   are specified.

b) A flux at a given wavelength, for which both the flux (in erg/cm**2/sec/A
   for a point source) and wavelength (in A) must be specified.  The 
   wavelength should not need to be within the spectral bandpass of the 
   selected grating/central wavelength.

c) No normalization, which should be used for user-supplied spectra only.


Spectral Attenuation:
--------------------
The user should be able to specify the spectral energy distribution 
attenuation [in magnitudes, E(B-V)] resulting from interstellar extinction.  
Several choices for the shape of the extinction curve (Galactic, LMC, SMC, 
extragalactic - see STIS ETC) should be available.  The default should be 
standard Galactic extinction of E(B-V) = 0.00 magnitudes.  The extinction 
laws currently in the STIS ETC are as follows:

The average Galactic extinction law from Seaton (MNRAS, 187, 73p, 1979). 
The LMC extinction law from Koorneef & Code (ApJ, 247, 860, 1981). 
The SMC extinction law taken from Prevot, et al. (A&A, 132, 389, 1984). 
The general extra-Galactic extinction Law taken from Calzetti, Kinney, &
  Storchi-Bergmann (ApJ, 429, 582, 1994). 


Backgrounds:
-----------
The user should be able to specify the magnitudes of the backgrounds affecting
the observation.  These backgrounds include:

a) Detector dark background, a combination of both intrinsic and cosmic 
   detector contributions.  This differs between the FUV MCP and the NUV
   MAMA detectors.  The count rate per square centimeter should be supplied 
   by the user, with the displayed default choice appropriate for the grating 
   chosen.   Default detector background rates for the FUV and NUV detectors 
   are still to be determined.

b) Zodiacal light, which is expected to be low in the FUV.  The standard 
   APT/ETC model should be used along with several options for the strength of 
   the radiation.  The "Low", "Medium", and "High" choices adopted by the 
   APT/ETC framework should be used.  "Low" should be the default.

   The APT/ETC Group intends to use Greg Aldering's zodiacal light spectrum 
   based on the work of Leinert et al. (1998, A&AS, 127, 1).  The Aldering 
   North Ecliptic Pole value will determine the "Low" value.  The Medium and 
   High values will be rescaled versions of the Low spectrum, with 
   normalizations to be determined by the APT/ETC Cross-Instrumental Issues 
   Group.

   The APT/ETC may also provide a means for specifying the zodiacal background
   as a function of RA, Dec, and Sun angle.  This is not necessary for the 
   COS ETC and could be omitted for simplicity if desired.

c) Airglow, which can be represented by two values (Day, Night).  Day should 
   be the default.  At Ly-alpha, values of 20 kR (Day) and 2 kR (Night) are
   probably sufficient.  For the O I 1302, O I 1356, and O II 2470 lines,
   the emission will be considerably weaker and should be scaled to the 
   Ly-alpha emission by factors that are to be determined (see STIS ETC for 
   an initial guess at this).  The O I airglow will be negligible during 
   Night observations and can be set to zero unless Day is specified.


4) OUTPUTS

The standard outputs being developed for the APT/ETC should be available for 
the COS ETC.  Standard output should include a summary of the ETC results.  
This list should include:

a) S/N and exposure time assumed or calculated by ETC.

b) Maximum local count rate and wavelength of maximum.

c) Global count rate.

d) Any warnings associated with bright object protection (BOP) violations.
   These must stand out boldly.  These warnings will need to be specified for 
   both the global count rates on each detector as well as the maximum local 
   count rates on each detector.  Local count rate maxima may need to be 
   specified per pixel; thus, a conversion from count rate calculated per
   resolution element to one calculated per pixel may be necessary.  This 
   conversion is trivial once the resolution element size in pixels is 
   specified.  Final details of the BOP violation rules still need to be 
   defined by the COS group.  

e) Buffer time.  This value, which should be calculated by the ETC, is the 
   length of time (in integer seconds) that the user will need to enter into 
   the Phase 2 form to specify the size of the buffer needed to hold the 
   accumulated data.  The value of the buffer time specifies the estimated 
   minimum time in which 2.35x10**6 photon events (the half-buffer capacity)
   will be accumulated during a time-tag exposure.  The value of buffer time
   should be set equal to the half-buffer capacity divided by the estimated 
   maximum average count rate in photons per second.  The value of this 
   quantity must be greater than 80 seconds.

   If this quantity is less than 110 seconds, then a warning should be issued
   indicating that data loss may result if the observation is performed in
   TTAG mode.

f) Effective area at the wavelength of interest (useful for cross-wavelength
   comparisons).

g) Observational parameter summary (key inputs used), which should include the 
   following information in a compact form

        -grating and setting
        -aperture
        -reference wavelength
        -spectral bin size (in A and in pixels)
        -point/extended source
        -SED type
        -SED normalization
        -SED attenuation
        -zodiacal light choice
        -airglow choice
        -specified dark rate
        -number of source events per spectral bin at specified wavelength
        -number of background events per spectral bin at specified wavelength
        -number of airglow events per spectral bin at specified wavelength
        -maximum local count rate
        -wavelength of maximum local count rate
        -global count rate
        -recommended buffer time
        
The following outputs should also be available to the user by selecting from a 
set of choices.  Output should be available in both tabular (ascii) and 
graphical forms, and it should be available in a variety of units to be 
determined by the APT/ETC Cross-Instrumental Issues Group (e.g., F_lambda,
F_nu, VEGAMAG, ABMAG, etc).

a) Input spectral energy distribution, with a choice of wavelength range. The  
   default wavelength range should be the wavelength range covered by the 
   selected grating/central wavelength combination.  User should also be able 
   to view the input backgrounds (detector, zodi, airglow) over the same 
   wavelength range.

b) Output S/N per spectral resolution element, number of counts, or count rate
   as a function of wavelength for the exposure time specified by the user.  
   The default wavelength range should be the wavelength range covered by the  
   selected grating/central wavelength combination.  

c) Output S/N per spectral resolution element, number of counts, or count rate
   as a function of time for the reference wavelength specified by the user.  
   The times shown should be equal to 0.10, 0.25, 0.5, 0.75, 1.0, 2.0, 3.0, 
   4.0, 5.0, and 10.0 times the exposure time entered by the user.



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