Subject: Strawman science programs for the LSST; please respond!
From: strauss@astro.Princeton.EDU
Submitted: Fri, 17 Jan 2003 14:08:02 -0500 (EST)
Message number: 57
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Hello all,
As a number of you mentioned to me privately after last week's
meeting, we need to do some work to jump-start this working group, so
that we don't find ourselves going over the same territory each time
we meet. It is our job as the Science Working Group to put together a
clean and well-defined science case for the LSST, and to use that to
put requirements on the telescope, the survey it carries out, the
software which reduces the data, and the database with which these
data are analyzed. With this in mind, I am going to take the
audacious approach of trying to put together a strawman series of
requirements/observational needs, to give us all something to respond
to. The details of what follows are surely wildly incomplete, wrong,
etc.; please respond in detail to lsst-general. In particular, I have
*not* tried here to put the various programs described here into a
coherent observing plan. That will be the next step, after we've
refined the individual programs a bit better.
-Michael Strauss
Going through our science goals one by one:
**************Weak lensing:
Sky coverage of 20,000 square degrees.
Effective PSF of less than 0.6" in an r-band stacked image with a
5-sigma detection limit of 26 AB.
(Something on stability of PSF)
Photometric redshifts for galaxies brighter than r=25 with an
accuracy (1 sigma) of 0.2 (requires similarly deep photometry in 3
or 4 other bands, although not necessarily with as high image
quality).
Photometric calibration accurate to 2%.
Astrometric stability requirements need to be stated.
**************Supernovae:
Five color (ugriz?) photometry over 10,000 square degrees to r=24,
three (four?) times per month, for a period of one year.
Photometric calibration accurate to 1%.
Five color (grizy?) photometry over 1000 square degrees to r=26,
twice per month.
Follow-up spectroscopy for redshifts of all supernovae discovered,
down to r=24. (Note that this requires an appreciable amount of time
on other telescopes).
**************Variable objects:
Two-color photometry (B and I?) once every two days for a period of
a year, over as much sky as this cadence allows (10,000 square
degrees?). This would be for a nova search. Depth of 24th mag.
Five-color photometry survey to r=24 mentioned under supernovae
above is invaluable for variability survey as well.
A scheme like that suggested by Ivezic at the meeting last week
allows time-variability sampling (in two bands) on timescales of 25
seconds to 15 minutes; this sounds appropriate for the variability
survey.
The Ivezic approach covers roughly 250 square degrees every fifteen
minutes. Thus:
Single-band (r?) photometry survey to r=24 over 250 square degrees
in a region of moderate stellar density, imaged over 15 minutes, and
repeated continuously over a full dark run. Relative photometric
accuracy at the bright end (i.e., where not limited by shot noise) of
0.002 mag error will allow planetary transit searches, and explore new
regions of variability parameter space for literally millions of
stars.
(I continue to worry about the need for follow-up of rare objects.
Are we convinced that LSST is adequate to do its own photometric
follow-up? The need for spectroscopy needs to be fleshed out!)
*********Kuiper Belt Objects:
The LSST has the potential to map out the full dynamical
distribution of these objects. This requires a survey that is not
necessarily confined to the ecliptic plane, going as deep as
possible.
Two-color (g and i?) imaging photometry to 24th mag, two times
separated by 15-30 minutes, once or twice per lunation, over 15,000
square degrees.
To observe fainter KBO's, to 26th magnitude:
Define a region centered on the Solar System's invariant plane 3
degrees wide. Image this twice in a single band for 20 minutes (a
Plutino will move 1 arcsec in that time), separated by one hour. Do
this once per lunation.
(I've limited this to the ecliptic plane, in a single band, to keep
it finite).
*******Near-Earth Asteroids:
One (or two?) band photometry to 24th magnitude over the full
available sky, imaged twice within 15 minutes. Do this three times a
lunation.
(Most NEA's are in orbits similar to that of Earth, and will be
observed at least several tenths of an AU away. Thus they will be
on the meridian, on the ecliptic, at sunrise and sunset. This would
point to a very different type of survey, and would say that for the
bulk of the night, we would not optimize the datataking for NEA's.
The NEA folks will have to fill this in further).
******Galactic structure/astrometric studies:
The telescope should deliver astrometric accuracy in a 20-second
exposure limited by anomalous refraction in the atmosphere (50-100 mas
per coordinate, rms). (The LSST astrometric science discussed thus
far (parallaxes, proper motions, wiggles due to planets) is all based
on *relative* astrometry; it may be that the large coherence length of
the anomalous refraction will allow relative astrometry to be done
much more accurately.)
The systematics should allow this to be beaten down by root N
statistics to 3 mas per coordinate for relative astrometry (I pulled
this number out of a hat).
Enough exposures should be taken (in a single band), to yield this 3
mas number, once every two months (a timescale set by the desire to
measure parallaxes).
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