I agree with Zeljko's assessment and comments. However, I am a little less agreeable on the very last points that Tim and Zeljko have made:
> > one can do photometry of sequences of stacks, each of which is > > made by N individual exposures, where N = 4 or 10 or whatever. I > > suppose there might be some interesting science there, but I do have > > my doubts. > > It may help with objects that vary only on long time scales, but I agree > that at this time this is not a strong driver. We cannot assume that there is no compelling science in a scenario that has yet to be investigated very thoroughly and for which LSST can (and should) provide definitive all-sky data (i.e., to find long-term variable phenomena in very faint objects --> e.g., brown dwarfs, L/T dwarfs, rotating Oort cloud objects, stochastic accretion of infalling gas on the most distant massive black holes = baby QSO's ... and who knows what?). So, I do agree that this is not a strong driver, but let us not exclude the possibility. - Kirk > Date: Fri, 16 Jun 2006 13:05:30 -0700 > From: Zeljko Ivezic <[EMAIL PROTECTED]> > Subject: [LSST-data] the number of LSST stars > To: LSST Data Management <[email protected]> > > Kem Cook wrote: > > > This sums to 2,000,000,000 stars in the database for single > > exposure depths. > > This seems too low by a factor of a few. The USNO-B1.0 catalog > lists 10^9 objects to V=21 (whole sky, about 10% are galaxies). > Dividing by 2 for the sky coverage, and multiplying by 10**(0.3*3.5) > (0.3 for a moderate counts slope, 3.5 for the depth) gives about > 6 billion. It could be factor ~2 or so more if the counts are > steeper than 0.3. > > > I conservatively assume that with 10 years of co-additions, we will see > > about twice as many stars. > > As you said, this estimate may be too conservative. According to > the SRD, we'll reach r=27.8, or about 3 mag deeper. > Using the same slope as above (0.3), this means that the final > coadded catalog could include about 10 times as many stars as > the single epoch catalog. That is, we could easily end up with > a catalog containing several tens of billions of stars, i.e. > many more than 4 billion. > > Unless, of course, we exclude the galactic plane. But why > would we ever do that??? The Milky Way mapping is one of the > four main science drivers! > > > Now, there is a choice which needs to be made. We can release light > > curves for all detected stars, or we can only release light curves for > > stars which are detected in the difference images (ie variable stars). > > > We will have to set some limit (3 sigma? 5 sigma?) for > > detections in the difference images which will be greater than a > > detectable signal in the folded light curve of a periodic variable. > > It's not obvious how to do that. One can't simply set rms level > because there are many stars which are usually steady and then > spend a small fraction of time with a very different brightness > (e.g. eclipsing binaries, eruptive stars, etc). rms cut is not a > good way to fins them. I would argue that we can and should provide > at least photometry (with error bars!) and positions for _all_ > observations of _all_ point sources (detected in any single visit). > > > Tim Axelrod wrote: > > I'm inclined to say that unless an object has a useful SNR in a single > > image, it will not have a lightcurve, but only static properties. > > This sounds reasonable, since photometric errors will already be > 0.20 mag at the detection limit in single visits. > > > In the > > middle one can do photometry of sequences of stacks, each of which is > > made by N individual exposures, where N = 4 or 10 or whatever. I > > suppose there might be some interesting science there, but I do have > > my doubts. > > It may help with objects that vary only on long time scales, but I agree > that at this time this is not a strong driver. > > > I hope this helps, > > Zeljko > _______________________________________________ > LSST-data mailing list > [email protected] > http://www.lsstmail.org/mailman/listinfo/lsst-data ----------------------------------------------------------------------------- Dr. Kirk D. Borne NASA Goddard Space Flight Center, SSDOO Program Manager, QSS Group Inc. and George Mason University, Associate Research Professor, College of Science <mailto:[EMAIL PROTECTED]> Tel. +1-301-286-0696 Fax: 301-286-1771 Staff page: http://rings.gsfc.nasa.gov/~borne/ US Virtual Observatory: http://www.us-vo.org/ Large Synoptic Survey Telescope: http://www.lssto.org/ _______________________________________________ LSST-data mailing list [email protected] http://www.lsstmail.org/mailman/listinfo/lsst-data
