Subject: LSST SWG meeting minutes, 2/20/03

From: strauss@astro.Princeton.EDU

Submitted: Tue, 25 Feb 2003 14:14:08 -0500 (EST)

Message number: 76 (previous: 75, next: 77 up: Index)

    LSST Science Working Group, Telecon Minutes
		 February 20, 2003


Attending:
Al Harris
Dave Monet
David Morrison
Dennis Zaritsky
Sidney Wolff
Daniel Eisenstein
Inga Karliner 
Andy Connolly
Gary Bernstein
Chris Stubbs
Alan Stern
Mike Shara
Kem Cook
Zeljko Ivezic
Tony Tyson
Chuck Claver
Jeremy Mould
Apologies if I left you off the list!

  The main thrust of this meeting was to set the stage for the
upcoming LSST meeting in Tucson (March 18-19).  That will be a working
meeting, in which we hope to define and refine the science case and
drivers for the LSST concept.  At our October meeting in Princeton, we
defined a series of science teams, consisting mostly (although not
exclusively) of members of the LSST SWG, whose job it was to fully
develop the science cases, project, and requirements of each of the
major science driver areas of LSST.  It is now time for us to get
cracking on that!   I hope that we can make substantial progress on
these *before* the March meeting, so that the two-day intensive work
we spend at that meeting is not starting from scratch.

  The following are the teams that we defined in October (quoted from
the <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.17.html>minutes
of that meeting</a>.  Needless to say, the memberships of these teams
is not set in stone!  In each case, we identified an individual to
head the group, and coordinate the efforts of the team:

  --Near-Earth Asteroids and Main-Belt Asteroids:
	-Alan Harris, head                             
        -David Morrison
        -Dave Jewitt
        -Steve Larson 

   --KBOs + distant planets
	 -Gary Bernstein, head
	 -Dave Jewitt
	 -Alan Stern 

    --Variable Universe 
        -Chris Stubbs, head
	-Fiona Harrison
        -Mike Shara
        -Dennis Zaritsky
        -Tony Tyson
        -Abi Saha
        -Peter Garnavich
        -Kem Cook

    --Supernovae 
      - Peter Garnavich, head (volunteered for this job in abstentia!)
      - Gary Bernstein
      - Chris Stubbs
      - Fiona Harrison
      - Chris Smith
      - Dan Eisenstein
      - Nick Suntzeff

     --Weak Lensing 
      - Tony Tyson, head                                      
      - Gary Bernstein
      - Dan Eisenstein
      - Nick Kaiser
      - Chris Stubbs
      - Andy Connolly

     --Astrometry 
      - Dave Monet, head
      - Dennis Zaritsky
      - Nick Kaiser
      - Alan Harris
      - Jeremy Mould 
      - Chuck Claver

Quoting from (and expanding upon) the October meeting minutes, these
teams have the job to: 

  1- Describe the science goal for LSST in this area in some
quantitative detail.  This should also include a discussion of where
the field might be in 8-10 years.

  2- List the requirements on the instrument and the data management
system to accomplish the goal (more on this below). 

  3- Come up with a specific observing plan assuming that your program
could use all the telescope time it needed, over a several-year
baseline.  For this, assume that the telescope has the ability to
reach R = 24.0 for a 3 sigma detection of point sources in 30 sec,
with a seven square degree field of view (Tony: is this consistent
with the DMT design?). 

  4- Consider what fraction of the science goals would be reached with
the following strawman observing plan (which we can and should explore
further), using two different modes:
	     
   <a
href=http://www.astro.Princeton.EDU/~ivezic/talks/AAS201lsst.ps>Ivezic's
cadence</a>, which gives an interlocking set of fields observed twice
in two bands (r and another band) in 15-minute chunks, going to
roughly 24th in each chunk.  Repeating this gives a very deep exposure
in r, and somewhat less deep in 3 other bands (for the sake of
argument, let's suppose they are g, i, and z).  50% of the LSST time would
be taken up in that mode.

  Deep pointings of ~1 hour (made up of many 20-second exposures,
without offsetting the telescope), going to r=26, repeated ~4 times per
year in each of 2 bands (say r and i), in a region centered on the
ecliptic.  This covers a total of 5000 square degrees. 

  Dan Eisenstein has <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.73.html>suggested</a>
a third cadence mode, which involves repeated imaging on short
timescales of a given area of sky.  Let me not suggest that for the
strawman right at the moment, but it is something that you might
consider answering item (3) above. 

  On item (2) above, what are the desiderata, let me repeat the list I
gave <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.16.html>after
the October meeting </a>: 

      The area of sky imaged at any given time.
       The depth and dynamic range needed in a single exposure.
       The depth and dynamic range needed in stacked exposure. 
       The requirements on seeing, PSF, and pixel size.  
       The filters needed. 
       The need, if any, to stack the data. 
       The photometric accuracy needed (both relative and absolute).
       The astrometric accuracy needed (both relative and absolute). 
       The filters needed. 
       The cadence of observations needed (very different for moving
          objects and, e.g., distant galaxies). 
       The area of sky to be covered. 
       Requirements on the speed of data reduction needed, and the
          nature of the measured quantities. 
       Auxiliary data needed (e.g., follow-up spectroscopy,
                observations at non-optical wavebands, and/or 
                a priori calibrating data)
       Specialized data analysis tools needed to carry out the science

Let me also remind you of the <a
href=http://www.noao.edu/lsst/reqoct9.pdf>requirements document that
Wolff, Connolly, Tyson, and Zaritsky put together; this is a more
comprehensive list of desiderata than included above. 

OK, with that assignment written down, here are some brief notes from
the phone conference itself.  Much of the discussion was focussed on
the issue of cadence, partially spurred by Daniel Eisenstein's <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.73.html>posting</a>,
and also the realization that we will naturally want different
observing plans in different conditions.  Al Harris pointed out that
Ted Bowell (who will come to the meeting in March) has been thinking
about the cadence problem as well (and comes to somewhat different
conclusions than Zeljko); we all look forward to hearing the details
in March.  
  Chris Stubbs emphasized the fact that the observations with a
cadence optimized for one scientific program will naturally be of
great interest and use for other programs as well.  He will take the
lead in trying to make this statement quantitative.  It has been
suggested, for example, that the NEA program might do best in a
decoupled mode, observing at quadrature in evening and morning
twilight.  Pan-Starrs is heading to a model in which a series of
different surveys to different depths and cadences; Tony Tyson
suggested that the higher etendue of the DMT design would allow a
smaller number of observing modes.   

  Speaking of Pan-Starrs, Dave Monet and Al Harris both took part of
the Conceptual Design Review for Pan-Starrs, in which roughly a dozen
2-3 page scientific cases covering a variety of cases were presented.
This is similar in spirit to what I'm calling for above.  Neither Dave
Jewitt nor Nick Kaiser took part in the present meeting, but perhaps
they can tell us whether these reports could be made available to the
LSST SWG. 

  Gary Bernstein described the need to <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.69.html>go
deep in single exposures</a> to look for KBO's down to 26th
magnitude; this is much of the motivation for the second item in the
strawman plan described above.  There was a brief discussion whether
this could be included in an initial static LSST survey.  It was not
clear that this would be useful for a survey of novae, which need
well-sampled light curves every night or two for an extended period.

  We also briefly discussed the possibility of follow-up of
interesting objects such as faint novae on other telescopes.  Many of
us have thought of the LSST as being its own follow-up telescope, but
for sufficiently rare objects, this may not need be the case.  This
is one of the desiderata mentioned above, and definitely needs to be
made as explicit as possible.  Do we want to advocate, for example,
that the LSST should be a joint system of a large-aperture telescope
and several small telescopes?  We didn't talk about this, but the
Pan-Starrs approach of distributed aperture would give the flexibility
to use one or more of the small telescopes for follow-up work. 

  Andy Connolly told us about a planned meeting of the NOAO-based LSST
data management group in early April, to discuss astrometric and
photometric calibration (Andy will send out details to the mailing
list soon).  This is an issue that affects us deeply, and definitely
has an impact on cadence decisions, but which we have not discussed
properly yet.  The desiderata above include requirements on
relative and absolute astrometric and photometric calibration, which
is absolutely crucial. 

  Stellar population studies are one that might push the photometric
calibration hard.  Kem Cook argued elevating these up to one of the
formal science drivers of the project.  We had a somewhat inconclusive
discussion about whether a very long list of science drivers (as
opposed to cool science that comes from the survey) is a good thing or
not.  In any case, there will be a get-together in Tucson immediately
after our March 18-19 meeting of folks interested in the stellar
population question; see the <a
href=http://astro.princeton.edu/~dss/LSST/lsst-general/msg.68.html>posting</a>
on their thoughts thus far. 

  We finished the meeting with an update on the tsunami question.  In
brief, there has been some major rethinking in the field recently due
to an improved understanding of the breaking and run-up on shore of
the short-wavelength tsunami that an asteroid impact would cause; they
seem to be quite a bit less severe than previously thought.  However,
there are still some major controversies in the field, and tsunami
community is meeting on March 16 in Houston to hammer some of these
things out.  David Morrison and Al Harris will attend this meeting,
and report to us a few days later in Tucson. 

  A plot of number of annualized deaths vs. size of impactor shows a
rapidly increasing curve to about 1 km, below which the impactor does
not send up a substantial amount of dust and ash (i.e., no "nuclear
winter" scenario), and the danger drops dramatically; tsunamis being
the most substantial remaining danger.  The exact position of this
transition is not well-known (depends on type of impactor, where it
lands, etc.), but is of order 1 km.  As we expect to have a 90%
complete catalog of 1 km NEA's by the end of the decade, most of what
LSST does in terms of improving our understanding of the risk is
finishing up the last 10%, rather than improving statistics of 200-300
meter objects.  

  All such risk analyses count just the deaths caused by the immediate
impact, and not, for example, secondary effects to the world economy
(it is fair to say that the societal impact of the World Trade Center
attacks extend far beyond the deaths of 3000 people, horrific though
that was, in the form of a depressed economy, wars in Afghanistan and
possibly Iraq, etc.).  

  Al Harris quantifies risk another way: by asking for the expected
interval between impact events that cause x number of deaths.  Thus
impacts that cause more than 1 death are of order once every 2000
years, while impacts that cause more than 1000 deaths are once every
250,000 years. 

  We happily finished the meeting with a more upbeat topic, namely the
agreement of all to add Zeljko Ivezic of Princeton as a member of the
Science Working Group.  He can be reached by email at
ivezic@astro.princeton.edu, and by phone at 609-258-1692.  

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