Summary of Recommendations of ETC Cross-instrumental Issues Group 

Membership:  Santiago Arribas, John Biretta, Ralph Bohlin, Danielle Calzetti, 
Guido De Marchi, Harry Ferguson, Mauro Giavalisco, Perry Greenfield, Tony Keyes, 
Anton Koekemoer, Chris O'Dea, Kailash Sahu, Ken Sembach, 

Priorities for Development 

High
- Update background values in the appropriate tables in both the CGI and APT ETC
- Include new menus for specifying backgrounds in APT ETC
- Include new menus for specifying backgrounds in CGI ETC. (this should either
be done soon to assist with testing of the APT ETCs or not at all)
- Implement STIS with circular apertures

Medium
- Include both optimum and aperture-based S/N calculations in STIS (as currently
in ACS).
- Add ability to specify labels for all plots in preference menu
- Add new plots and tables of inputs
- For spectroscopy an additional plot should be given - S/N at the input
wavelength as a function of exposure time.

Low
- Change ACS apertures from square to circular in APT (can wait until Jun 02).
- Update help files to give conversion from V to J,H,K Zodi normalization
- When users input an aperture-based S/N and request an exposure time,
they should be given the exposure time for both the optimum and
Aperture-based S/N. S/N vs exposure time plots should show both
optimum and aperture-based S/N (with appropriate labeling).
- Implement library of common input spectra (can wait until Jun 02 when NICMOS
is included, since the current library includes the ones useful to STIS and ACS)



General Requirements.
All instruments will use the same background normalization in the ETC and 
the same input files. 
Users will be able to specify the Zodi background, Earthshine, and Airglow
separately. 

Format for Background menu.
Zodiacal background will be specified using the following options
(1)  High, Medium, and Low, (2) RA, DEC, and Sun angle, 
(For implementation of the detailed model, a lookup table should be 
used - as in WFPC2).
(3) a V band normalization for the Zodi light, (4) Low Sky.

Airglow will be specified using:
High - Daytime
Low - Night 
In redoing the airglow, we should double-check that the resolution 
of the lines in the input spectrum is sufficient for COS needs. 

Earthshine will be specified using 
High -  20 deg Earth limb angle
Typical (Random) - 28 deg Earth limb angle (typical for CVZ obs)
Low - 40 deg Earth limb angle
And a pull down menu for Degrees from Earth Limb Angle

The actual values:
Zodi spectrum 
We should adopt a Zodi background which is well documented. Greg Aldering's Zodi
spectrum is based on the Leinert etal (1998, A&AS, 127, 1) compilation and is
in agreement with the 1.25 micron DIRBE measurement of the Zodi background from 
Kelsall etal (1998, ApJ, 508, 44). Thus, we will adopt the Zodi spectrum provided 
by Greg Aldering. We will adopt the Aldering NEP spectrum for our "Low" value. 
The Average and High values will be re-scaled versions of the NEP spectrum. 
The normalization (and a look up table for the detailed model) will be determined 
by Mauro, Ralph, Santiago and Kailash. 

Thermal background:
NICMOS and WFC3 will require use of a thermal background from HST plus
an instrument specific contribution (for the light path after the 
filters). 

EarthShine:
The Paresce spectrum seems to be a bit too bright and too red.  
Kailash has analyzed some deep STIS CCD images and determined the
typical background due to Earthshine. He confirms that the typical
background is much less than that currently used in the ETC. 
Kailash will continue to pursue this (including Larry Petro when
he becomes available in November). 

Output format:
For imaging we should report the fluxes (ergs/s/cm2/A) for each of the 
different input components of the calculation (in addition to the counts
which are given now).
For spectroscopy, we should report these values as a function of wavelength 
in both a table and a plot. 
The help files should give the scaling for the Zodi background in the 
J,H,K bands for a given V magnitude. VEGAMAGS should be used. 
For spectroscopy an additional plot should be given - S/N at the input
wavelength as a function of exposure time. 
The units of all the ETC plots should be specifiable in the exposure 
calculator preferences menu, so it is possible to show the source spectrum 
in e.g., F-lambda. 
Flux units to include:
	F-lambda	and F-lambda/sq. arcsec
	Fnu and Fnu per sq. arcsec
	VEGAMAG and VEGAMAG per sq. arcsec
	ABMAG and ABMAG per sq. arcsec

Apertures:
We should provide a menu of circular apertures for the S/N calculation
for all working instruments. 
The output should include (1) the fraction of the encircled flux which is
included in the chosen aperture, and (2) the counts and count rate 
in the brightest pixel.  

Common library of input spectra:
We will provide a uniform set of input spectra which use the same files
for all instruments.
These will include the following spectra (compiled from the existing ETCs)
User Supplied Spectrum
Flat continuum (in lambda)
Power law (in lambda) with user specified index
Black body with user specified Temperature
No continuum (for use when specifying emission lines only)
Kurucz models
HST Standard Star spectra (including NICMOS standards)
Object-based Templates
 QSO spectrum       with user specified redshift
 Seyfert Galaxy            "
 Elliptical Galaxy         "
 Orion Nebula       at zero redshift
 Planetary Nebula         "
 cool stars (GL 229B, GL 752B, GL411, GL406)  "

Optimum point source S/N:
We should provide both the "optimum S/N" and the Aperture-based
S/N in the ETCs when users input an exposure time.
When users input an aperture-based S/N and request an exposure time,
they should be given the exposure time for both the optimum and
Aperture-based S/N.
S/N vs exposure time plots should show both optimum and aperture-based
S/N (with appropriate labeling).




Wish list:
Lower priority items include
1. provide a plot of Zodi background as a function of time at a given
RA, DEC 
2. For a given V mag normalization, provide the normalizations
at J, H, and K. 

Appendix:
Thermal Model for NICMOS
The current NICMOS-ETC uses a detailed thermal model which takes into
account the contributions from all relevant components of HST (major
contributor) and NICMOS.  This model uses updated values for key input
parameters (e.g. M1-M2 emissivity), and it shows good agreement with
data. (details in Anand et al NICMOS-ISR 00-001; Robberto et al, 2000,
SPIE, 4013, 386).

This model also includes the effects of the misalignment between the
telescope entrance pupil and the cold mask according to the values
found by Krist et al. (1998, PASP, 110, 1046). This misalignment amounts
for an increase in the thermal background of 11 %. For NIC 1 and NIC3
these effects are scaled assuming the same misalignment, which is
probably ok within a 20% uncertainty.

However, in order to check this and/or to improve this accuracy, the
NIC3
and NIC1 misalignments must be modeled. In the case of NIC3 this
requires new non trivial observations (i.e. well exposed PSFs with NIC3
in focus, which implies  moving the secondary).

In summary, for ETC purposes the current thermal model is OK for NIC2,
and probably for NIC1 and NIC3 too. However, if accuracies for the
thermal background better than, say, 20 % are required, the NIC3 (NIC1)
misalignment should be modeled, and new observations should be planed
for that.


