Questions and Answers
Q: I'd like to make a scientific contribution to this project. Who should I contact?
A: You can contact us by emailing to "contact" at "lsst.org". The LSST Project solicits your input on the technical and scientific choices that we face. Share your views with us! Please make specific reference to technical documents or presentations when appropriate, so we can be sure to route your comments to the relevant people. Consider also browsing our team page:
Q: Why an 8-meter mirror with a 3 degree field? Couldn't a smaller telescope or an array of smaller telescopes do the same science in a somewhat longer time?
A:
Some of the science can't be done at all with a smaller telescope, or
group of small telescopes. For instance, the near-Earth object survey is
looking for things that won't sit still for a long exposure.
An exposure longer than 10 or 20 seconds becomes ineffective, and so
finding the vast majority of NEOs
which are small and faint requires a telescope that can collect a lot of
photons in 10 or 20 seconds. Similarly, longer integrations on a smaller
telescope will not help characterize faint transient objects lasting only
seconds. In an array of smaller telescopes, longer exposures would be
required (to reach sky-noise limit) and multiple Gigapixel camera costs
would be prohibitive.
Some of the science can be done on a smaller telescope in a longer time,
but consider the numbers:
The speed with which you can survey an area of sky for
objects of a given faintness is proportional to throughput
(collecting area times field of view in m^2 deg^2).
The LSST enables totally new windows on the universe because it has
such a high throughput. For the lensing project, the total time would
increase from 5 years to 50 years if the LSST were shrunk to 4 meters
and a 2-degree field of view. There is real value in being able to
complete the project in less than several generations!
Q: Why not a space mission?
A: It's true that the weak lensing and NEO projects would benefit somewhat from the higher angular resolution available in space. But since all the science goals can be achieved from the ground, we must weigh the incremental benefit against the drawbacks. The biggest of these, apart from expense, is that the required data rate will be several hundred MB/s—orders of magnitude greater than for HST or even NGST. Entirely new communication systems would have to be developed to handle this data rate, and/or major compromises in the science would have to be made to reduce the data rate to manageable proportions. A space-based telescope of 2-meter or smaller aperture would require hour-long exposures, thus missing much of the LSST science. LSST key science requires that we reach low surface brightness in a short time, in addition to reaching 24th stellar magnitude in less than 20 seconds, leading to a very low f-number and 8-m class aperture. The science that drives the need for the Large Synoptic Survey Telescope requires ultra deep wide-field imaging at optical wavelengths—a mission best achieved on the ground at a superb site.
Q: Why do you need such accurate measurements of the shear power spectrum?
A: Outside of the huge and rare mass overdensities of rich clusters of galaxies, the mass contrast is much lower, giving rise to shear values below one percent. Here, in the vast volume between the rich clusters, is where most of the dark matter and dark energy of the universe exists. Current observations are suggesting that most of the energy density of the universe is in some unknown form. Weak gravitational lensing will measure both the geometry of the universe and the growth rate of structure—two related probes of the equation of state of this dark energy.