Hi Tim, I probably wasn't clear enough, but I am not suggesting (nor did we size it) that we have lightcurves for all detected stars in the image stacks. I was just suggesting that the mean properties for all detected stars in the stacks be part of the deep database (as you do too). This is a significant increase in the deep catalogs for the early data releases (because of the larger number of stars now), but becomes much less significant with time.
Unless, I've done my math wrong (again), I have about 100,000 stars/square degree--2,000,000,000 stars over 20,000 square degrees is where this number came from. > Hi Kem, > > Thanks, that is very helpful to have. If I understand what you've > done, the rough averaging of the stellar density gives about 80000 / sq > deg, vs the 10000 I assumed. This underscores the point that the > sizing of the time-dependent database will depend *critically* on > whether we have a galactic plane exclusion zone, and if so, how big it is! > > So I agree with nearly everything, but do have one significant > disagreement as well. First, I am in complete agreement that we want > lightcurves for all stellar objects, not just those above some threshold > in a difference image. Where I disagree (I think) is about extending > this to objects that are detectable only in image stacks. This is > obviously what's needed for the static properties of galaxies (and stars > as well), but seems pretty useless for variability studies. 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. > Obviously this is one extreme strategy, and yours is the other. 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. > > So, I think we have a factor of two or so uncertainty based on how we > answer that question, and probably at least another factor of two > arising from how far into the plane we choose to go. > > Cheers, > Tim > > Kem Cook wrote: > >> Keywords: DataAccWG >> >>Hi All, >> >>I am responsible for the large number of stars which Jacek has in the >>current sizing estimate. >> >>I have had a look at various stellar density studies. A recent >>theoretical model of the Milky Way is given in Robin etal >>(astroph/0401052) which also presents a number of recent observational >>results for comparison (and more-or-less all of the observational results >>have higher stellar densities). I also think Hall etal (ApJS 140:185, >>1996) is relevant. >> >>I am assuming a depth of a single exposure is about 24.5 in r. I have >>estimated the galactic latitude and longitude coverage of the WL survey >>and other surveys (NEA, KBO, SN) from my experience with the LSST >>operations simualtor. I estimate about 6,000 square degrees in the >>galactic latitude range of +/-10-20, about 6,000 degrees in the range >>-20-40, and about 10,000 degrees in the range -40-90. This is >>conservative since I have no fields within 10 degrees of the galactic >>plane. From Robin, I estimate about 300,000 stars per square degree at >>10-20 degrees, 5,000 stars per square degree for 20-40 and 3,000 per >>square degree for 40-90. These are some sort of geometric mean for the >>rapidly declining density as we move away from the plane. For a sanity >>check, Hall etal get densities of 25,000 per square degree averaged over >> a >>range of latitudes from 35 to 63--clearly higher than my Robin etal model >>estimates. This sums to 2,000,000,000 stars in the database for single >>exposure depths. >> >>I conservatively assume that with 10 years of co-additions, we will see >>about twice as many stars. I also assume that the mean properties of all >>detected stars for a data releaese are part of the deep database. >> >>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). >> If >>we only release variable objects, this will decrease the number by about >> a >>factor of 20 (5% variability). This would be the cheap way to do things, >>but might impose some serious loading on the archive site by at least two >>sets of users: low amplitude pulsation studies and planetary transit >>studies. 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. So, >>either we release light curves of all detected objects and let people >>analyze them as they will, or we face the prospect of having people >>generate light curves of all detectable objects in order to do this >>analysis. >> >>This sizing has not explicitly accounted for AGN and QSO variabliity, but >>this is well included in the 5% variability assumption for the 'stellar' >>objects. >> >>I think we may actually detect more stars in single exposures than I have >>estimated based upon the fact that we will likely have nearby, resolved >>galaxies in some of our fields and we will be observing the ecliptic, >>probably as it passes through the galactic plane. I also think that a >>factor of two is conservative going from single exposures to DR20 depths. >> >>cheers, >> >>Kem >> >>_______________________________________________ >>LSST-data mailing list >>[email protected] >>http://www.lsstmail.org/mailman/listinfo/lsst-data >> >> > _______________________________________________ > LSST-data mailing list > [email protected] > http://www.lsstmail.org/mailman/listinfo/lsst-data > _______________________________________________ LSST-data mailing list [email protected] http://www.lsstmail.org/mailman/listinfo/lsst-data
