Subject: Minutes of August 8 LSST phonecon

From: Michael Strauss

Submitted: Fri, 8 Aug 2003 18:09:03 -0400

Message number: 164 (previous: 163, next: 165 up: Index)

			LSST Science Working Group phonecon August 8, 2003
				     Minutes

Attending:
Gary Bernstein
Chuck Claver
Andy Connolly
Kem Cook
Daniel Eisenstein
Richard Green
Fiona Harrison
Lee Holloway
Zeljko Ivezic
Inga Karlinger
Dave Monet
David Morrison
Beatrice Muller
Knut Olsen
Abi Saha
Mike Shara
Michael Strauss
Chris Stubbs
Nick Suntzeff
Jon Thaler
Tony Tyson
Sidney Wolff
Dennis Zaritsky
Apologies if I missed your name!

  The main subject of the discussion today was the process of writing
the Design Reference Mission.  We are starting with a concept of a
telescope capability as endorsed by various decadal surveys, namely
one with an etendue in the ballpark of 250 square degrees meter^2
operating for of order ten years, which is capable of carrying out
surveys for near-earth asteroids, KBO's, supernovae and other variable
objects, and deep lensing.  Our job in the design reference mission is
to flesh out and quantify the science goals, describe what
requirements each has on such things as seeing, astrometric and
photometric accuracy, cadence, choice of filters, and so on, and
describe the derivative of the science with respect to these
requirements: how fast, e.g., does the weak lensing science degrade as
the seeing worsens?  We also have to ask the question of the extent to
which this facility is likely to remain compelling by the time in
actually gets on line.  What we do *not* have to do is to come up with
a complete, coherent observing program for the full ten-year lifetime
of the mission; while we should and will point out how the different
programs fit together and can use the same data, the scientific
landscape is likely to change dramatically over the next 5-10 years,
to make the details of what we work out now interestingly obselete.
It is also not our job to make basic design decisions such as choosing
between a single large monolithic telescope a la the DMT design, or a
distributed aperture like a super-PanSTARRS.  As I think Sidney Wolff
phrased it, "Just write down what we want to do, not what we want to
build".  It would be appropriate to have appendices to the DRM
document describing these various options.  After the SWG has finished
its task, it will be the job of advocates of these different
approaches to write proposals to the funding agencies showing that
their telescope concept can meet the science goals laid out in the
DRM.

  It was edifying for me at least to have the discussion that resulted
in the above paragraph, and makes our job of writing the DRM more
straightforward and less politically charged.  

  We would like to get at least a first draft of this DRM out over the
next 2-3 months.  The news is good, as lsst-general has seen quite a
bit of traffic of people posting drafts and outlines of sections they
would like to write.  However, it is clear that we cannot answer all
relevant questions on that timescale; what the document will have to
do in such cases is to describe the issues involved, give what
qualitative answers we can give, and outline the research project
needed to do a full and proper job. 

We then went through each of the major science areas we've been
discussing for LSST, and talked about where things stand. 

********************************Near-Earth Asteroids***************

Al Harris, who has been leading the SWG discussions on this topic, was
unfortunately unable to join us.  David Morrison was here, and told us
that the NASA Science Definition Team on the hazard from NEA's would
soon (i.e., in the next few weeks) be releasing their report, and that
they would touch upon the LSST question; we look forward to what they
have to say.  
  Al, who has been involved in this NASA panel, is (among other
things) quantifying the risk associated with these NEA's.  He has
suggested modifying that section slightly and incorporating it into
the LSST DRM. 

*******************************Kuiper Belt Objects*****************

Gary Bernstein has posted a write-up (lsst-general 114) to be included
in the DRM.  Recent comments from Bea Muller (lsst-general 162), as
well as some rethinking on his part about the KBO population, based on
some results he's getting from HST, are prompting him to plan a
rewrite of his text; he hopes to do this over the next short while. 

*******************************Variable Objects******************

Fiona Harrison had posted a note (lsst-general 160) suggesting that
the emphasis here should be on the region of as-yet unexplored
parameter space (apparent magnitude and variability timescale) opened
up by the LSST.  Kem Cook pointed out that in a mode in which we
return to a certain set of fiducial fields in order to carry out
photometric calibration, we would be able to carry out quite exquisite
studies of variability on all sorts of timescales in those regions of
sky.  Daniel Eisenstein suggested, however, that the concept of
fiducial standard fields for photometric calibration was unnecessary
with a survey repeating any given area of sky multiple times: the
survey calibrates itself.  In a given lunation, a given area of sky is
covered of order six times, three pairs of two in Zeljko Ivezic's
suggested cadence mode (lsst-general 62), for example.  If one wants
observations in multiple bands, one needs to divide this number by the
relevant number of bands.  We didn't resolve the photometric
calibration question, but it is definitely important.

  Chris Stubbs discussed the synergy between LSST and other missions
to look for transient phenomenae, mentioning the EXIST mission
(all-sky imaging in hard X-rays) and LISA (gravitational wave
detection using interferometers in space) as cases in point.  

  Tony Tyson mentioned the transient sources that he has been finding
in his deep lens survey; sources that are undetected in one ten-minute
exposure (i.e., fainter than ~25 mag), which will pop up to 22nd mag,
and then disappear again.  Nick Suntzeff seems to find similar things;
we should definitely mention the existence of these mysterious sources
in the DRM.  Tony said he had relevant figures on his web site. 

  Mike Shara described his posting, lsst-general 61, about dwarf and
classical nova science to be done with LSST.  He emphasizes that of
known types of variable stars, four types would dominate in LSST data:
contact binaries (W Ursa Majoris stars), classical novae, dwarf novae,
and RR Lyrae stars.  We agreed that the DRM should emphasize both the
new discovery space for variable objects, and the populations of known
types of variables that LSST could fruitfully explore. 

************************Supernovae********************************

  This naturally led to a discussion of supernovae.  In our meeting in
March, there was not a great deal of enthusiasm about pushing SN
science as a science driver for LSST, partly because of a sense that
most of the relevant science questions would be answered on the LSST
timescale.  The topic came up here again, and several people,
including Nick Suntzeff and Abi Saha, argued that supernovae are
definitely worth another look.  Peter Garnavich was unable to join us,
but is working on a document describing the sensitivity of LSST to
measuring the rate of change of the redshift-distance relationship. 

  We all agreed that with supernovae, follow-up is the limiting
factor.  At 23th mag (the relevant depth for SN in the right redshift
range for w measurements), we estimated there would be of order of 100
SN in *every* LSST image.   LSST of course will not obtain spectra of
these objects, and it is unclear whether LSST will get sufficiently
sampled light curves, in enough filters, to get good light curves.
This is a calculation that is crying out to be done.  Remember that
23rd mag is only a bit above the plate limit, so the S/N of the
photometry of such an object will not be terribly high. 

  Abi stressed the importance of nearby supernovae (m=20).  These are
rare enough and bright enough that follow-up can be done
straightforwardly on other telescopes.  One can imagine doing peculiar
velocity studies for such objects, for example. 

******************Galactic Structure/Stellar Populations**************

Dave Monet and Dennis Zaritsky have put together a first draft of a
document on that subject (lsst-general 154); Knut Olsen is working
further on this.  Dynamical studies of the halo from proper motions,
among other things, are stressed here; the constraints this puts on
halo structure needs modelling, given the inevitable errors of stellar
distance determination, etc. 

*******************Weak Lensing******************************

  While a technical discussion of this in the context of LSST exists
(cf., lsst-general 24), what is needed is more on the science
drivers.  Tony Tyson, Daniel Eisenstein, and Gary Bernstein are
currently working on that. 

******************Putting it all together**********************

Michael Strauss has the job of taking these disparate pieces and
making them coherent in a single document.  He has a web site,
http://astro.princeton.edu/~strauss/LSST, on which documents can be
placed.  Thus he will put versions of this DRM there, and any pieces
that don't easily display on the lsst-general archive (i.e., anything
that is not ascii: figures, highly formatted text, etc.); if you have
such things, send them to Michael and he will put them there.  He will
look into creating a CVS respository, which is a mechanism for
multiple people to work on a document simultaneously and keeping it
all straight.  And Sidney Wolff volunteered the help of folks at NOAO
for the mechanics of making a proposal pretty.  


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