Subject: Minutes of SWG phonecon, 12/09/02

From: Michael Strauss

Submitted: Tue, 10 Dec 2002 21:32:32 -0500 (EST)

Message number: 33 (previous: 32, next: 34 up: Index)

Minutes of the Telecon of the LSST/SWG
December 9, 2002

Attending:
Al Harris
Jeremy Mould
Sidney Wolff
Tony Tyson
Dave Monet
Gary Bernstein
Nick Kaiser
Peter Garnavich
Alan Stern
Fiona Harrison
Andy Connolly
Kem Cook
David Morrison
Michael Strauss
Mike Shara
Daniel Eisenstein
Jon Thaler
(forgive me if we missed your name!)

Michael Strauss opened the telecon by asking Nick Kaiser to summarize
the two recent PanSTARRs meetings.  The first meeting dealt with the
pipelines for PanSTARRS; the second focused on developing a Design
Reference Mission (DRM), with emphasis on the asteroid problem.  There
was a general consensus in favor of an all-sky multi-color survey in
standard pass-bands, followed by narrow-field surveys to deeper
magnitudes.  There was not much demand for UV observations.  Science
of transients was not strongly represented.  Nick plans to make
minutes and summaries of these meetings available to the SWG when they
are prepared.  In particular, he hopes to get out a conceptual design
document by mid-January. 

Several key players in developing NEA observing strategies attended
the PanSTARRS meetings (including our own Al Harris).  There was
substantial discussion about observing strategies that use "sweet
spots" for searching for NEAs.  The "sweet spots" are ahead and behind
the Earth in its orbit, roughly 60-90 degrees from the longitude of
the Sun and within 10 degrees of the ecliptic.  These areas of the sky
are available for observation only within 2-3 hours of dawn and dusk
and are at relatively high air mass (~2).  These locations are
obviously not optimum for deep sky surveying.  Modeling to compare the
efficiency and time to reach various levels of completeness for
various observing strategies continues, but the different modelers
seem to be in good contact and to be comparing their results.  Harris
reported that if one focusses just on the sweet spots (a total of 600
square degrees of sky), it would take 30-40 years to be complete in
NEA's to 300 meters.  On the other hand, he pointed out that a
full-sky NEA survey would find of order 50% of the 50-meter objects
(i.e., comparable to Tunguska) in the 10 years of the survey: far from
complete, but plenty for studying these objects scientifically.
Harris also pointed out that recovery of asteroids will do well with a
uniform cadence of observing a given area of sky once every five days,
with an opportunity roughly once per month of observing a given area
of sky on two successive days.

The possibility of surveying in selected areas of the sky along with
the desire of the asteroid searchers to observe in broadband (i.e.,
broader than the standard filters) raise the question of whether the
LSST survey should carry out different programs sequentially:
e. g. searching for asteroids near dawn and dusk and doing other
surveys through the middle of the night.  Monet noted that the
asteroid sweet spots are well suited for parallax measurements.
However, broadband measurements and relatively high airmass would
introduce problems of atmospheric refraction.

Harris then described the debate currently in progress about the risk
from asteroids smaller than 1 km (larger impacts are considered to be
civilization-destroying events).  We have taken the goal of a complete
NEA catalog to 300m, but there is nothing special about that value,
and no obvious knee in the risk vs. size plot there.  The standard
concern about 300m class impactors is the tsunamis they generate if
they were to hit the deep ocean.  Among the various researchers there
appears to be good agreement about what happens at the impact site in
the deep ocean and on the wavelength of the wave generated; the wave
has a wavelength comparable in size to the crater generated on the
impact, which is of order 10 times the linear size of the impactor.
There is substantial disagreement about what happens when the waves
reach shallow waters and whether or not they break in such a way as to
mitigate damage.  Jay Melosh, for example, was quoted as having said
that the extent of damage of a breaking wave is strongly dependent on
its wavelength.  Impact-generated waves have much shorter wavelengths
(up to 10 km) than earthquake-generated waves (thousands of km),
implying that they will not be nearly as serious as previously
thought.  

Reconciliation of the tsunami calculations will be important input for
the NASA committee currently assessing hazards of NEAs, and their
report is due in late spring of 2003.  Engaging oceanographers to help
resolve the differences seemed a fruitful strategy.

Dave Monet also attended the PanSTARRS meeting.  He stressed that
there is a great deal of work to be done to prepare for both PanSTARRS
and LSST in terms of synthesizing the results of existing astrometric
catalogs.  PanSTARRS will saturate at magnitude 15-16, which is about
where existing catalogs (such as UCAC) start to lose their precision.
Therefore, there will be a great deal of work to be done during the
early stages of the new surveys to establish a grid on non-moving
objects to serve as references.  Since all stars move at the level of
LSST accuracy (i.e., tens of milli-arcsec over 5-10 years), galaxies
or quasars may have to be used (although this may not be a serious
issue, e.g., for weak lensing work).  This will be a challenge at low
Galactic latitudes!

  Other work that remains is to characterize both the astrometric
behavior of the Orthogonal Transfer CCD's that will be used by
PanSTARRS, and to fully understand the degree-scale coherent
astrometric offsets caused by atmospheric refraction.  For both, Dave
is waiting for data from folks at Hawaii to analyze.  

  Monet also stressed the tremendous astrometric science that
PanSTARRS and LSST could do, including looking for astrometric wiggles
from unseen companions.  He emphasized the difficulty of what's
involved: for every star, one wants to solve for proper motion,
parallax, wiggles, and atmospheric refraction.  

Tony Tyson amplified on his e-mail (lsst-general #24) concerning the
requirements for weak lensing science.  He stressed the importance of
controlling the PSF and of characterizing the jitter of the telescope
by building a model that includes all the servo systems, etc.  He
described his experience with current 4-m telescopes and felt that a
factor of 10 improvement in control of systematics was easily possible
for a modern telescope designed with the appropriate set of
requirements.  Michael Strauss asked whether a factor of 10 was
sufficient and whether there was any cliff where the relationship
between scientific return and cost/engineering feasibility changed
slope.  This will have to be determined as the design is developed.

  There was some discussion of whether a decrease in systematic
effects by a factor of 10 necessarily translates in one's ability to
measure shear improving by a factor of 10.  For example, what is the
difference between measuring the PSF shear as 10%+/-1%, and 0%+/-1%?
The point is that if the PSF shear is 10%, it is unlikely to be
accurately spatially uniform, and there will be some scale below which
one can't measure it, giving an irreducible systematic error.  If one
has faith that the systematics are closer to zero, these irreducible
errors are likely to be much smaller (Tony, is that a correct way to
explain it?).  

  Tyson briefly described simulations of the Hubble Deep Fields,
exploring the sensitivity of weak lensing measurements to seeing.  He
described a fairly steady loss of lensing sensitivity, as the seeing
worsens beyond 0.5".  One advantage of going deep with the LSST is
that one is measuring the shape at quite faint outer isophotes of
galaxies, at 1" or more in radius, where seeing has less of an effect.
Gary Bernstein has developed the optimal filter for measuring galaxy
shapes, in which one fits a matched filter of an elliptical Gaussian
to each image. 

  Tony also claimed (for reasons I, Michael, am still a bit unclear
on) that it is important to measure the PSF shear bias on individual
exposures (as opposed to the final stacked image).  This requires a
large A Omega, so you go deep enough to get enough PSF stars to
measure this.  

Andy Connolly stated that he is doing simulations with various filter
sets to determine the optimum choice for determining photo-z's.  He is
looking at standard filter sets along with double-wide filters.  Use
of double-wide filters would again introduce problems of atmospheric
refraction, decrease the hour angle over which observations are
possible, etc.  One real possibility is the inclusion of a Y filter,
centered at 1 micron (i.e., beyond the z filter), which could give an
important lever arm for photo-z's. 

  We briefly discussed the rationale behind the minimum exposure time
(10 seconds?) for variability/transient surveys.  Tony Tyson asked
whether one could set such a timescale by considering the relationship
between the mass, luminosity, and crossing time of a black hole
accreting at the Eddington rate, at a given redshift; Fiona Harrison
will think through the numbers.  We also briefly discussed the use of
color and spectral information to characterize variable/transient
objects; it is clear that variability in a single photometric band is
not adequate to identify objects clearly, a point that Fiona 
already made strongly at the Princeton meeting. 

  If the density on the sky of interesting variable objects is more
than one per pointing (roughly 7 square degrees), then LSST
automatically becomes its own light-curve follow-up telescope.  If
interesting transients occur less frequently than that, then we can
consider the possibility of coming back to certain fields with
recognized transients deliberately, in order to build up the light
curves.  

  For example, Mike Shara described the potential for identifying
novae in stars that have been stripped from galaxies.  About 300/year
should be observable with LSST in extra-galactic space out to M87
(corresponding to 24th magnitude); this is roughly one every 100
square degrees.  It is desirable to have good sampling of the light
curves (once per night over a month) and some limited multi-color
information may be required to distinguish novae from orphan
after-glows.  Because these objects are standard candles, this would
allow one to trace the intergalactic stellar distribution.  Mike will
write up this science in more detail; this is a good test case as a
possible well-defined transient science driver.  *Dwarf* novae in our
own galaxy could be recognized in a survey going only to 21st
magnitude; this would vastly increase our knowledge of these objects.  

This type of science raises two generic questions for developing a
DRM.  Are there certain events (e.g. something that goes from
invisibility to x magnitudes above invisibility) that should trigger
frequent returns to a specific field?  And what is the requirement for
contemporaneous color information?  Both of these types of
observations will necessarily sacrifice sky coverage for more complete
characterization of specific fields.  As the survey continues, our
knowledge of the various types of interesting transient objects will
increase, and the objects we will decide are worthwhile to follow-up
will change; we certainly need to be flexible on the observing
strategy.

There was no report from the KBO or supernovae groups, and a written
report on transients is being developed.

Minutes by Sidney Wolff and Michael Strauss

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