Hi All, I agree with Tim's esimates, but there are details which haven't been fleshed out. There are parameters which don't really add volume to the data, but they are there. The time dependent database needs motion information: parallax, proper motion or orbital parameters. The time dependent objects will also contain added information in terms of the likelihood of blendedness, multiplicity and variability parameters. These data are added on a per object basis and as such, do not significantly increase the volume of data, but should not be forgotten. Presumably, these parameters will be present from the first detection of a time dependent object and will not increase in volume with time.
Kem > Hi Jacek, > > I have created a simple model for how the size of the object database > will grow between data releases (DR). Here are my assumptions: > > 1. Data releases occur every 6 months > > 2. We meet our SRD requirements of 100 visits per field per year > > 3. The database is split into two parts. The first, dominated by > galaxies, contains the static information for every object detected at > that point in the survey, mostly generated by combining the information > in image stacks. I'll call this the 'deep database' The second, > dominated by stars, contains the time dependent information for objects > bright enough to be usefully detected in individual exposures. I'll > call this the 'time dependent database'. > > 4. An object record in the deep database is about 100 bytes: 6 band > magnitude + errors; data quality flags; shape information. > > 5. An object record in the time dependent database is about 10 bytes: > 1 band magnitude + error + data quality flags. > > 6. For the first DR, the limiting magnitude for the time dependent > database is 24.5 (where it remains), while the limiting magnitude for > the deep database is already at about 26.1 from stacking 20 R band > images. So at DR1, there are already about 20 times more objects in > the deep database than in the time dependent. > > Consider first the growth of the deep database. The limiting flux to > fixed signal-to-noise will decrease as 1/sqrt(n_exp), where n_exp is the > number of exposures effectively stacked and used for detection. I > assume that measurement occurs in all bands, but detection occurs only > in the R band. The SRD calls for 40 R band exposures per field per > year, or 20 additional for every DR. The limiting magnitude increases > as 1.25*log (20DR), and we go progressively fainter in the galaxy > brightness distribution. I've taken the galaxy data here from the > Subaru Deep Field, which gives the slope of the cumulative brightness > distribution to be d(logN)/d(mag) = 0.45 in the region of interest. > The size of the deep database then grows as 100 * (20DR)**(0.45 * 1.25) > > The time dependent database grows strictly linearly with the number of > observations in all bands, which is 50 per DR, so it goes as 10 * (50 DR). > > Taking account of the factor of 20 difference in number of objects at > DR1, two attached plots show the growth of the deep database size, and > the growth of both together. The roughly square root growth of the deep > data dominates the first half of the survey, but is then overtaken in > the second half by the linear growth of the time dependent database. > In spite of my many assumptions, which are unlikely to be right in > detail, I think the overall behavior is about right. > > Let me know if you see an error or need more information. > > Cheers, > Tim > > > _______________________________________________ > 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
