Subject: Minutes from 2002 Oct 20,21 SWG meeting
From: strauss@astro.Princeton.EDU
Submitted: Mon, 28 Oct 2002 14:21:03 -0500 (EST)
Message number: 17
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Minutes of the Princeton LSST SWG meeting
October 20,21 2002
Minutes by Jeremy Mould, Dick Shaw, and Michael Strauss
In attendance:
David Morrison
Nick Kaiser
Dennis Zaritsky
Dave Monet
Alan Harris
Dave Jewitt
Alan Stern
Gary Bernstein
Fiona Harrison
Mike Shara
Steve Larson
Peter Garnavich
Andy Connolly
Daniel Eisenstein
Jeremy Mould
Dick Shaw
Abi Saha
Chris Smith
Wayne van Citters
Matt Johns
Absent: Chris Stubbs and Kem Cook
Science Working Group Charge
NSF needs a science case of the very highest standard in order to fund
a project of this scale. And a fully united community. The NSF will be
represented by either Wayne Van Citters or Eileen Friel at SWG
meetings. The LSST SWG is not the group which should propose for
funding to implement LSST itself, but it will determine if there is a
science case for it, and if so, what the case is.
NOAO is undertaking to prepare a fully costed proposal for the decadal
survey concept of LSST (i.e., a single 8.4m wide-field imaging
telescope, referred to below as the DMT to distinguish it from other
programs like Pan Starrs). This will level the playing field with the
already funded Pan Starrs. The science all these facilities (i.e.,
DMT, Pan Starrs, and other proposed projects such as VST, VISTA, and
SNAP) can do needs to be compared quantitatively. Jeremy Mould
stressed the Design Reference Mission which enables the funding
agencies to evaluate what each is getting out of LSST. NOAO staff are
available to support the SWG.
Sidney Wolff noted that the LSST project also has the benefit of a
data management group, an NEA requirements group, and a
detector/camera group. These groups have been meeting in Tucson over
the last year or so, where there have also been several meetings to
discuss possible science goals for the LSST. In this context, Alan
Harris has been contracted by NOAO to carry out asteroid simulation
work in the LSST context. Gary Bernstein mentioned the SNAP mission's
simulation capacity.
Next followed a series of presentations of some of the broad science
goals of the LSST:
*************Near Earth Asteroids
David Morrison emphasized that the political purpose of LSST was to
protect the Earth by finding asteroids that are going to hit. The
scientific goal of adding to statistical information is not
politically interesting; people want to know, "Is there an asteroid
which will hit us?", not "what are the chances?". However, we still
phrase the goal as finding 90% of the threatening asteroids; comets
make up 10% of the threat and are much less predictable (being on
*much* longer orbits). In any case, the aim is to find such a
threatening object decades in advance, so that ways can be found to
deflect it.
David added that if Congress accepts a case for extending the survey
to find objects larger than 300 meters (there is confidence that the
sample of >1 km objects will be 90% complete by the time the LSST sees
first light), Congress will demand annual progress reports.
There is a qualitative difference between 300 m and 1 km impacts.
Objects larger than 1 km would blow a hole in the atmosphere, and
cause a 'nuclear winter' which could cause world-wide crop failures;
this could be a civilization-threatening event. The damage from a 300
m impact would be more localized, although if it fell in the water,
the tsunamis would be devastating.
There is a committee convened by NASA, the 'sub-kilometer
committee', whose job it is to answer the question of the rationale
for searching for objects smaller than one kilometer.
For reference, an object < 50 meters in size is unlikely to
penetrate the atmosphere.
Alan Harris said that NASA has commissioned a report on whether
another threshold exists below 1 km. Predicted annualized fatalities
seem fairly flat in this region. The report is due June 2003. NASA
Langley will examine the relative cost of earth and space detection,
using 2, 4 ,8 meter telescopes on the ground. Scientifically the
distribution is interesting all the way down to H = 30 mag. There is
a possibility of doing a *statistical* study of NEA's all the way down
to 5 meters.
Note that these small objects have rotational periods of minutes.
Given their typical large aspect ratio (2:1), their brightness varies
on minute timescales by up to a magnitude.
Exposure times of 10-20 secs prevent image blurring of 300 m objects
at 24th mag.
**********Kuiper Belt Objects
Solar system exploration is a NASA mandate. David Jewitt indicated
that the structure and population of orbital resonances in the Kuiper
Belt can tell us about early times in the solar system. Of order 700
KBOs are known now, of which 200-300 have good orbits; there are of
order 100,000 objects > 100 km in diameter between 30 and 50 AU (for
scaling, a 10 km object with standard albedo is about 28th magnitude
at 40 AU). A few objects per sq degree at 24th mag will map the
dynamical structure. Gary Bernstein noted that if 26th mag can be
reached with coadded scans (although there will need to be some
algorithm development to be able to do that), even more orbits are
sampled. The survey could reach at least to 100 AU, perhaps much
further for Pluto-sized objects.
Jewitt sees complete sky coverage as important, but colors as less
interesting. These objects have featureless linear spectra, and so
color information is gained by getting the longest possible wavelength
baseline. Pan Starrs taking 30 secs to reach 24th, compared with
LSST's 10 seconds is not thought to be a weakness. Additional
performance would be attainable through a super Pan Starrs.
Note that LSST is recommended by both the McKee-Taylor and Belton
reports, in part to do KBO science.
*************Optical Transients
Generally, the question is "what is the population of optical
transients?" Specifically, Fiona Harrison pointed to the following
areas of extreme interest.
Orphan afterglows (10^-3 deg^-2 @ 24th); the stats constrain the
collimation angle
GRB-like events (note the connection to GLAST, EXIST)
Hypernovae
Catastrophic mergers (note the connection to LISA)
Mike Shara added that novae were expected at 1-10 per day and would
be a foreground problem, unless spectroscopic followup were arranged.
Of order 10-20% of long GRB's are truly dark optically; are they
extincted? No short GRB's have been associated with afterglows. The
visibility of afterglows probably depends on opening angle and viewing
geometry. This has been modelled by many, e.g., Totani and Panaitescu
2002, Astrophysical Journal, Volume 576, Issue 1, pp. 120-134.
Of course, almost by definition, LSST opens a parameter space that
hasn't been explored before, and thus the potential for discoveries of
an unanticipated nature is high.
Fiona emphasized that telling these various classes of objects apart
will be difficult without follow-up spectroscopy, which by necessity
will need to be carried out on a 8-meter-class telescope or larger.
Also needed are colors, access to archival data, and coherent light
curves.
Several high-energy missions are worth keeping in mind in this
context:
SWIFT
GLAST
EXIST
*************Supernovae
The science case is two-fold: detailed study of the population of SN
at low redshift, and use of SN as a cosmological probe.
Peter Garnavich described the ideal SN survey as UBVRI every 3-5 days
with followup in the rest blue bandpass. Early detection is
important! Among the scientifically interesting questions:
Is the explosion in Ia's deflagration or detonation?
What are the masses of the white dwarf progenitors?
How do age and metallicity affect luminosity?
A second parameter for SNIa standard candles
A possible bright x-ray flash in SNIIs
Does a SN explode before GRBs go off ?
SNIas behind clusters (magnification)
LSST shold be able to find essentially *every* SN in the region of sky
it surveys to z=0.2, and of order 10,000 per year with z<0.1. Should
allow a definitive measurement of the SN luminosity function. These
also can be used to study large-scale peculiar velocity flows.
For cosmology studies, the best case systematics are 0.02 mag,
justifying 70 SN per z-bin, or 100 SN per z-bin taking account of
subtypes. To get good light curves at z=1, one needs to go to roughly
i=25. Spergel & Starkman claim that the contribution to minimizing
the error in the determination of w plateaus beyond z = 0.7. If w
changes as z > 1, SNAP is the way to follow it (see the SNAP
presentation below). But w and Omega_matter are covariant in this
analysis; we need to assume that we'll know Omega_matter
independently, e.g., from CMB and cluster work. In any case, it will
be difficult to find and use SN at z>1 from the ground; at z=1, the B
band is at 9000 A, and you really don't want to go any bluer than
that.
*********Weak Lensing
Tony Tyson pointed to great improvements in control of systematics
using stars to determine the PSF. Because of source shape variance, he
believes 0.5 arcsec seeing is fine. The LSST weak lensing experiment
should be designed to cover 0.3 < z < 0.8 and as wide an angle as
possible. Photometric redshifts are absolutely crucial for this
work! Gary Bernstein remarked that there was no obvious power at
low multipoles, and Dan Eisenstein added that the advantage of wide
angle was reduction of the error bars due to cosmic variance.
Joe Hennawi and David Spergel have been carrying out detailed
simulations of the ability of the LSST to distinguish cosmological
models by weak lensing. There are two signals to try to measure: the
cosmic shear, and the mass function of clusters as a function of
redshift. The latter in particular is sensitive to w.
Measuring weak lensing shear on large angular scales (>> 1 degree)
will be scientifically very interesting, but difficult; the signal is
down from that on smaller scales by an order of magnitude.
*********Astrometry and Galactic Science
Dave Monet said that LSST was the natural extension of the series
5 mas Hipparcos
30 Tycho
50 UCAC
250 USNO-B => 50 mas LSST
50 mas is limited by atmospheric refraction, for a single LSST
image. Multiple images should shrink this number considerably.
Scientifically it offered a distance limited survey with parallaxes of
a few mas uncertainty at 10 pc; this would be the first complete
inventory of the nearby universe. Indeed, for nearby stars, one
should have reliable parallaxes after only six months of observations.
The cadence you need for this comes naturally in most suggested
implementations of LSST. There was also discussion of using these
data for studies of the structure of the Galactic halo: proper motions
(a few mas per year?) should be measurable for most halo stars.
Pan Starrs plans to use Orthogonal Transfer CCD's; their astrometric
properties need investigation.
Note that GAIA will only go to 18-20th magnitude.
***********Other science areas
These include main belt asteroids and comets, planetary transits
around other stars, short timescale events, variable stars, galactic
structure, strong lensing arcs, AGN variations, large scale structure
at z = 4, and clusters of galaxies at z = 1.5.
************Study Groups
For each of these science drivers, a detailed science case needs to be
made. We formed ourselves into a series of study groups to do this,
as listed below.
Key general questions to be addressed by all study groups include:
What is the optimum observing strategy (including cadence,
passband(s), and single-exposure integration times) to maximize the
return for this area of science? How does the science return degrade
with departures from the optimal observing strategy?
(see also the list posted at
http://www.astro.princeton.edu/~dss/LSST/lsst-general/msg.16.html)
--NEAs (questions include:
To what size should we aim for completeness?
Do we need colors?
How should we include observing Conditions [e.g., seeing,
moonlight, airmass] in simulations?
How many observations are needed to define orbits?
-Alan Harris
-David Morrison
-Dave Jewitt
-Steve Larson
--KBOs + distant planets (questions include:
what cadence is needed to determine good orbits? A model is
needed of the sizes and orbit distribution]
-Dave Jewitt
-Gary Bernstein
-Alan Stern
--Variable Universe (includes variable stars and microlensing)
[questions include: what region of temporal parameter space need to be
covered for transient studies?]
-Fiona Harrison
-Mike Shara
-Dennis Zaritsky
-Tony Tyson
-Abi Saha
-Peter Garnavich
-Kem Cook
--Supernovae [Questions include: what kind of follow-up is
needed (e.g., photometric & spectroscopic? How will the various SN
sub-types be studied?]
- Peter Garnavich
- Gary Bernstein
- Chris Stubbs
- Fiona Harrison
- Chris Smith
- Dan Eisenstein
- Nick Suntzeff
--Weak Lensing (questions include: which are the optimal filters
for photometric redshifts? What can be achieved by a not-quite optimum
system ? What would be the effect of going shallow in one filter, but
deep in another?)
- Tony Tyson
- Gary Bernstein
- Dan Eisenstein
- Nick Kaiser
- Chris Stubbs
- Andy Connolly
--Astrometry [Questions include: what are the break-points for
accuracy for each science goal?]
- Dave Monet
- Denis Zaritsky
- Nick Kaiser
- Alan Harris
- Jeremy Mould
- Chuck Claver
The general tasks for these study groups are to:
-Describe the science goal for LSST in this area in some
quantitative detail.
-List the requirements on the instrument and the data management
system to accomplish the goal.
These should be baselined on an ability to reach R = 24.0 for a 3
sigma detection of point sources in 30 sec. [Editor's note: shouldn't
this be 5 sigma?]
Action items:
-Sidney Wolff will provide a roadmap of LSST information already
present on the web. She will also provide a summary of the LSST
workshops that have been held in Tucson over the last year.
-Michael Strauss will circulate a generic list of desiderata for the
science goals (already done; see
http://www.astro.princeton.edu/~dss/LSST/lsst-general/msg.16.html)
-Michael Strauss will form a group to consider the other science
drivers not allocated to these 6 groups.
-Jeremy Mould will provide a contact admin assistant at NOAO to
help set up telecons.
There will be a progress telecon in a month and a face to face SWG
meeting on the Sunday or Friday of the Seattle AAS, whichever
maximizes attendance. Action item: Chris Stubbs to find us a room at
the AAS.
---LSST Public Outreach and scientific followup
Mike Shara drew our attention to LSST's great potential for education
and outreach. Jeremy Mould offered to address the question of
spectroscopic followup. These can be taken up at a future meeting.
---The DMT implementation of the LSST
The current Angel-Livermore design has a flat focal plane, and a
very uniform PSF across the focal plane. There are aspherics on 5
surfaces, but the convex secondary is close to a sphere. The camera
concept has 2.3 Gpx with a 2 sec read and 0.2 arcsec/10 micron
pixel. Components for the ideal detectors exist. They may be CCDs or
CMOS. Fermilab could lay out the focal plane. A detector shopping list
has been prepared for vendors to try out against. LLNL has designed a
shutter (useful if CMOS is not selected). By 2017 LSST will have a 15
Pbyte database, and a 1 Tbyte catalog.
---Pan Starrs
Nick Kaiser said that the advantages of multi aperture included
telescope cost, rapid construction, dynamic range, scalability,
reliability, multicolor capability, slow optics, and low profile, [but
not necessarily all of these simultaneously]. This is now an approved
project, with 4 1.8 meter telescopes. IfA/MHPCC/SAIC/LL would have it
operational in 2006, and it would be a pilot project for LSST. Its
etendue A Omega of 54 m^2 degrees^2 is a quarter of DMT's 250, "the
same ratio as the cost." OTCCD pixel size would be 0.3 arcsec, but the
resolution could be better with sub-pixel dithering. Read noise of 2-3
electrons was assumed at 1 MHz. Time to reach 24.0 in R+V is 31
secs. [Editor's note: is this 3-sigma? The on-line Pan Starrs
documentation,
http://poi.ifa.hawaii.edu/poi/documents/poi_book.pdf gives a 5 sigma
exposure time of 63 seconds to go to 24.0]
Survey rate is 6000 sq deg/night. To reach the decadal survey
requirements, Pan Starrs could exploit economies of scale in
telescopes and detectors, and increase the numbers of telescopes.
[Cost break-down is roughly $10M for telescopes, $12 for detectors,
and 40% ($16M) for software. These numbers are likely to change a bit
after a CoDR in April, 2003, and CDR on 30 Sept. 2003. A contingency
plan to scale back the project, should that be necessary, would be to
fill onlyl half of the detector plane. Kaiser emphasizes that
detector costs scale not as number of pixels, but as the physical area
of silicon; thus at a fixed focal ratio, a smaller telescope has a
cheaper wide-field camera.]
There was some discussion, without a clear conclusion, on the
scientific need to go to 24th magnitude in 10 seconds or 30 seconds;
this clearly needs resolving by this group.
--SNAP
Gary Bernstein summarized the science case for SNAP. It is a mission
noted that SNAP was a mission to measure the time evolution of the
equation of state of the universe (i.e., w). It is a 2 meter space
telescope with a 0.7 sq degree FOV. Half the focal plane is IR
detectors, half are optical detectors (with a total of 9 passbands)
and there is a spectrograph. The goal is 2000 well characterized SNe
to z = 1.7. There is also a weak lensing survey to 28th mag at 5? and
a guest program. Launch (if funded) is 2010 and duration 4 years. DoE
funds are sought for a 2 year design phase. SNAP's strengths are
widefield NIR, photo-zs to 10 or beyond and a stable PSF. SNAP's weak
points are NEA's, limited mission sky coverage, long exposures, and
lack of U band. In the lensing area SNAP complements LSST by providing
a more distant lensing screen. Together SNAP and LSST probe the
evolution of structure from higher to lower z.
---Conclusions
The push now is to describe some real science programs for LSST and
turn them into requirements. M. Strauss will post a summary list of
desiderata, after merge-sorting and simplifying. A significant
question is whether requiring more than 10 yr to achieve the science
goals is a hard or a soft constraint. (Perhaps, but probably not; may
have more to do with practical issues such as funding prospects,
advances in telescope & detector technology, etc.) Near-earth Asteroid
discovery is an inverse exponential (that is, the last, say 10% of the
asteroids are appreciably more difficult to discover than the first
10%), so achieving the science goals in <10 yr would be really hard,
and having 15 yr would help a lot with completeness. The SWG would
like to see posted on the Web all engineering docs that have been or
will be generated, along with a road-map/summary so that SWG members
can come up to speed quickly on the technical issues. There was some
preliminary discussion of LSST project science deliverables, which
will be needed in part to justify the program, to ensure adequate QA
of the data products and other deliverables, and to enable adequate
funding to support the crew of young astronomers that will be needed
to staff the LSST.
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