Tim,
There are two differences that I see. The first is that I was plotting the ratio DR(n)/DR(1) while you are plotting (DR(n)-DR(1))/DR(1). That accounts for your first 4 vs 5 discrepancy.
ok, I am getting the same plot now for deep-db.
The second difference is in how you define the ratio for the total. You are combining the two sizes as a weighted sum to account (I think) for the initial factor of 20 difference between the sizes of the deep and time-dep. But that is already accounted for in your formula for the deep size. So the effect is to wrongly deweight the time-dep size.
If I simply combine the two sizes, the growth of time-db dominates from the very beginning, because of 50 observations per DR, and I end up with x11 growth, not x6 as you did. I'm sorry, but I still seem to be missing something...
thanks, Jacek
With those fixed, we agree. Jacek Becla wrote:Hi Tim,Thanks. I tried to fiddle with a spreadsheet for a while, but couldn't get exactly the same plot. Deep db: you have x5 after 10 years, and I ended up with x4.39. Total: you have x6, and I ended up with x5.04.See attached. Can you spot the different? What functions did you use? Jacek Tim Axelrod wrote: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 year3. 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
db_growth_estimate.xls
Description: MS-Excel spreadsheet
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