Subject: SWG phonecon minutes, 4/25/03
From: strauss@astro.princeton.edu
Submitted: Fri, 25 Apr 2003 18:30:32 -0400 (EDT)
Message number: 126
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LSST Science Working Group
Minutes of Phonecon 4/25/03
Attending:
Andy Connolly
Kem Cook
Daniel Eisenstein
Al Harris
Dave Monet
Robert Nemiroff
Mike Shara
Michael Strauss
Sidney Wolff
This was a relatively brief meeting. Only 1/3 of the Science
Working Group showed up (most of you did not respond when I asked who
was going to attend), and so we were stymied to make much progress on
the science drivers for the project.
We left the March meeting in Tucson having put together a series of
teams to produce science cases and requirements for the various
drivers for LSST. The idea was to write a report describing these
things, which would help us put together the Design Reference Mission
which is our goal. What these reports should include is outlined in
the minutes from the March meeting, lsst-general #102. We had hoped
to get first drafts of these reports in place by roughly this time.
At the current writing, only one of these (namely that on Kuiper Belt
objects, lsst-general #114) has been posted; Dave Monet has of course
posted a number of notes on astrometric requirements/calibrations as
well. See also Mike Shara's outline (lsst-general #107) of the science
case for dwarf novae.
Daniel Eisenstein pointed out that it is becoming increasingly clear
for the weak lensing group that quantifying the requirements is a
larger job than perhaps had been realized earlier. In particular, one
would like to relate basic observables such as image quality (seeing),
depth, choice of filters, sky coverage, and so on, to constraints on
cosmological parameters; to do a full-up job of this will require
extensive simulations. We agreed that the full job is beyond the
scope of the reports in many (all?) cases; what we need now is a
description of the problem, and where appropriate, a description of
the job that would be needed to do the full calculation. Our eventual
goal is, as it were, the derivative of science gained as a function of
survey specifications: how do we lose if we go 0.5 mag less deep? If
the seeing is 0.1" worse? If A Omega is a lower value? And so on.
Sidney Wolff suggested that funding could be found to support the more
detailed calculations, where needed, on a few month timescale.
We then went briefly through the various science cases. As
mentioned above, weak lensing realizes that the job of fully
understanding the relationship between requirements and science
output is quite complicated. Daniel said that there did not seem to
be any obvious threshold in science output as a function of seeing,
depth, and so on. (Tony Tyson was not here, but he may have something
to add here; I remember a year ago a vociferous discussion of what
would happen if the DMT design were scaled down to a smaller
aperture...)
Andy Connolly said that he has been working on several approaches to
optimal filter design with photo-zs for weak lensing in mind. As the
weak lensing kernel is quite broad in redshift, he is not trying
necessarily to minimize the rms scatter, but he is trying to minimize
the number of catastrophic errors, i.e., points with redshifts off by
many sigma.
Supernovae: Kem Cook described a meeting a few weeks ago, led by
Andy Connolly, to explore astrometric and photometric calibration for
LSST; see lsst-general #125. One idea that has come up there is to
have a series of photometric calibrator fields to which one would
return several times a night; the 2MASS survey had a similar scheme.
This would give detailed variability information on the several hour
timescale over ~100 square degrees, which could be used for
detailed supernova light curves. It is not yet clear how
scientifically useful that will be in ten years time; this needs
exploration. Mike Shara said that this would be a fine way to find
dwarf novae as well, although the sky coverage is small enough that
the numbers would be small, and it would not allow one to map out the
distribution of extragalactic stars in the sky.
Stellar populations: Knut Olsen chaired a meeting in Tucson (see the
minutes at lsst-general #103), which came up with an outline of a
report, to be completed on April 30, describing the science drivers.
They are particularly keen on observations over the sky in a series of
metallicity/age sensitive bands; it wasn't clear how deep these data
need to be.
Near-Earth Asteroids: Al Harris has been working, in the context of
a NASA panel on the NEA hazard, on the "residual risk" from NEA's,
once the Spaceguard survey is complete. It comes from three sources:
-The 10% of >1 km NEA's that the Spaceguard survey will miss, and
which will cause a global catastrophe;
-Several hundred meter objects which hit the ocean and cause a tsunami;
-Several hundred meter objects which hit the land.
Very roughly speaking, the risk is 50-50 between the remaining 1 km
objects, and the tsunami objects. He plans to post something soon to
lsst-general on this.
He is feeling moderately confident that LSST could do the NEA search
in a relatively small fraction of the observing time (e.g., observing
towards quadrature after dusk and before dawn). The trade-off of
filter choice, exposure time, etc., still needs more work. Sidney
said that people at NOAO plan to put together an exposure time
calculator, which may be useful for this purpose.
Al described the experience with Pan-Starrs, where each of the
science groups was asked what observing resources they would need to
achieve a modest realization of their science goals (i.e., they were
not told, "you have the telescope all to yourself; what are you going
to do?"). He said that this was a useful exercise; when all was said
and done, the telescope ended up not being terribly oversubscribed.
As some programs in fact need the same sort of data, one can put
together an observing program that makes everyone happy. The LSST SWG
is only starting to grapple with the fact that a single cadence is not
going to work; we don't yet have a clear idea of what is needed. This
is part of what the Design Reference Mission is all about.
Dave Monet pointed out that with Pan-Starrs, LSST, and other surveys
producing asteroids in huge numbers, the Minor Planet Center, which
computes orbits mostly by hand, is probably not going to keep up; the
community definitely needs to plan a replacement/enhancement to the
MPC to handle all this.
As I said above, this was not a terribly productive meeting, both
because of the small number of people attending, and the small amount
of work that has been posted. I would like to call for another
phonecon in about 3 weeks time, when I hope that there will both be
better attendance, and more work in hand to discuss.
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