Subject: SST IFMOS
Date: Thu, 18 Feb 1999 13:06:25 GMT
From: Santiago Arribas <sam@ll.iac.es>
To: rfosbury@eso.org
CC: pjakobse@estsa2.estec.esa.nl, lefevre@astrsp-mrs.fr, ghasinger@aip.de,
sam@chantada.ll.iac.es
Dear Bob,
I have done a quick comparison between the Yan et al. and Thompson et
al. data and they seem to agree rather well (in the plane half-light
radius versus magnitude). Of course the data by Thompson et al, are
prefereable as they contain fainter galaxies, and are less affected by
surface brightness selection effects.
In any case, before obtaining improved curves/tables for achiving
density of objects and exposures times perhaps we should discuss about
the general procedures to obtain these tables/curves. So let me
summarize how I see this issue.
The determination of density of sources is conceptually rather
straightforward. The main problem until now is the disagreement (by a
factor 2-3) among different observational data at the faintest end
(K~23, see, for instance, Bershady et al, 1998, ApJ, 505, 50). However,
the HDF NICMOS data (although with a limited spatial coverage) should
allow to solve this well (e.g. Thompson et al.).
Regarding the exposure time determination the problem is much more
complex. One can use the NGST-ETC (Exposure Time Calculator) to have
croase estimations. However now ETC has no much flexibility and
requires some modelling. In fact for galaxies an Lstar absolute
luminosity is always assumed and it is mandatory to specify the
redshift. In addition is rather obscured for me the value of some
parameters used in the calculations (pixels sizes, 'slit width', etc).
For instance, it is not clear for me why exposures times for a point
souce and an 'extended' source of 0.03'' are different by a factor 2
(for K with R=100, S/N=10, ST=G5V, M=22.74). Therefore, it may be
dangerous to used it in a blind way especially for the faintest
galaxies.
The approach that I followed was to reduce the problem of the
determination of exposure times for 'extended objects' to 'point
sources', and then to use ETC (without any kind of hypothesis about the
source, but still with uncertainties related to the parameters
mentioned above).
For that we need to characterize the extended source in terms of its
surface brightness and to consider the size of the sampling element.
For instance, if we have an object with 22mag/"^2 which is observed
with a sampling element of 0.03"x0.03", it is assumed that this will
require approximately the same exposure time as a point source of 29.6
mag (22-2.5log(0.03x0.03)). Here it is assumed that the energy
collected by a sampling element is distributed over the same number of
detector pixels as in the case of a point source observation. This is
probably no true, but the needed correction can be approximately
determined if we learn more about ETC. In any case, I do not expect a
diffence larger than an order of magnitude in the computed exposures
times due to this effect.
>From the observational point of view we need to determine the surface
brightness. I think a good reference value can be the 'mean surface
brightness interior to a half-light radius'. This mean SB can be
directly derived from measured magnitudes and half-light radii.
However, the half-light radius determination may be bias for several
effects. The most important one is due to the seeing (for the Keck
observations by Bershady) and to the PSF in the case of the HST. This
tend to increase the h-l radius making fainter the SB. This will be
relevant for the smallest sources (at the faintest edge), but for well
resolved sources I think this should not be a problem.
Summaryzing, these are the specific things which I think should be
fixed:
1) HDF: Determination of the counts density, magnitudes and h-l radii.
The data by Thompson et al, seem good for that, but I have not analyzed
them in detail.
2) ETC: We need to know more details about how this program works. I
have tried to contact Massimo Stiaveli a few days ago but without
success. Does he belong to ESA? Should be possible to ask him to
participate in this more formally ?
3) SB determination: The most worrying issue is the effect of the PSF
on the intrinsic SB for the faintest galaxies. Perhaps I can do some
simulations convolving the HST-PSF with galaxies profiles (r-1/4), but
I do not know how realistic this may be.
Please, let me know your doubts/comments/suggestions on this before I
go further.
All the best ,
Santiago
Subject: Re: ETC
Date: Thu, 18 Feb 1999 14:36:57 GMT
From: Santiago Arribas <sam@ll.iac.es>
To: rfosbury@eso.org
CC: pjakobse@estsa2.estec.esa.nl, lefevre@astrsp-mrs.fr, ghasinger@aip.de
Dear Bob,
I have just received this mail from Massimo Stiavelli, which seems
relevant to understand the limitations of ETC, as well as his availability
for participating with us in this part of the study.
Regards,
Santiago
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>From mstiavel@stsci.edu Thu Feb 18 14:18:36 1999
Date: Thu, 18 Feb 1999 09:18:01 -0500 (EST)
From: Massimo Stiavelli <mstiavel@stsci.edu>
To: Santiago Arribas <sam@ll.iac.es>
Subject: Re: ETC
MIME-Version: 1.0
Dear Santiago,
the tool you are referring about was written years ago as a zeroth order
estimator and is now ENTIRELY INADEQUATE for any serious work on NGST.
I do not know why it produces the results you are describing since I do
not remember what I did. For point sources the assumption is that 69 per
cent of the flux ends up in a box of either 4 pixels in size or one
PSF FWHM in size, whichever is larger. The PSF was Nyquist sampled at
2 microns. There has been talk about replacing it but so far a replacement
is not available.
I have new tools that I am working on but these are not public since I
am developing them as part of a NASA funded instrument study concept.
Cheers,
Massimo
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