A need has arisen for the a single, agreed-upon, list of values with key numbers that we will use consistently across all presentation for Final Design Review and beyond (esp. as they relate to data volumes, etc.).
Below is that list. Please cross-check your slides and make sure they're consistent with these numbers.
This list must be updated on CCB or DM/TCT actions that cause the underlying numbers to change. Robert McKercher is responsible for keeping track.
- LSST Science Requirements Document (SRD), LPM-17
- LSST System Requirements (LSR), LSE-29
- Observatory System Specifications (OSS), LSE-30
|Parameter||Value||Notes||Formal Source Document(s)|
Fiducial number of visits per pointing in the main survey.
|825||Given by the SRD||LSR-REQ-0098, Nv1Sum|
Fiducial main survey area
|Given by the SRD||LSR-REQ-0098, ASky|
|Estimated total covered area, including special programs||25,000 deg^2||LSE-81 cell G92|
|Single exposure 5-sigma point source depth|
u = 23.9
g = 25.0
r = 24.7
i = 24.0
z = 23.3
y = 22.1
|Design spec from the SRD.||SRD|
|10-year idealized 5-sigma point source depth|
u = 26.1
g = 27.4
r = 27.5
i = 26.8
z = 26.1
y = 24.9
|Cadence dependent; this is an illustration from the SRD from a plausible OpSim run assuming the above single-exposure depths.||SRD|
Number of visits collected over 10 years
|2.75 million||Computed from OSS-REQ-0190|
Number of images collected in 10 years
Simply equal to 2.75 * 2
|Number of visits per night||"about a 1000"|
This is 2.75 million / 10 year / 300 days/year = 920 visits/night, rounded up to 1000.
|Derived from OSS, SRD|
Number of calibration exposures
Number of data collected per 24 hr period
|"about 20 TB"|
This is the number of exposures per night + calibration exposures per day(= !2450), with 8.2 GB per exposure (562*2098 pixels including overscan × 16 amplifiers × (189 science CCDs + 4 wavefront sensors) × 18 bits/pixel / 8 bits/byte).
Considering that the in-memory form and the compressed form are both varying physical representations of the "true" logical information, we use 18 bits/pixel.
|Derived from OSS, SRD; LSE-81 for pixel size upper bound|
|Number of data for 10 year mission||"about 60PB"|
10 years x 298.35 observing nights/year
= 1.5M total calibration images
= 8.2GB x 7M total images = 57.4 PB of raw data = 56.2 PB
Per-24 hour period
Standard visit exposure duration
A visit consists of two exposures. Specified in the SRD.
Time to take a single exposure
|Expected median slew time between visits||4.8 s|
Derived from the current baseline cadence (see Observing strategy white paper, section on baseline cadence minion_1016). The mean value for minion_1016 is 6.8 s. Similar, but not identical, values are expected for future cadence realizations.
Requirement: 5 sec (OSS-REQ-0289)
|Derived from minion_1016 analysis|
|Time to take one visit in normal survey mode||Median: 39 s|
Mean: 44 s
Computed from two 18-second units, with the 5-second slew overlapped with the 2-second readout of the second exposure, thus 18 + 18 - 2 + 5 = 39 seconds (and the mean should be 44 seconds, as the mean slew time is 10 seconds). (Slew times taken from OSS-REQ-0289, not from simulation.)
|Derived from OSS|
|Estimated number of objects in DR11||37 billion|
|Estimated number of single-epoch sources in DR11||7 trillion|
Comes from the DM sizing model (LDM-141, db2 sheet), by dividing F17 (number of sources in DR11) by 1.28 (the technical margin factor, derived by dividing D17 by 37 billion above).
(this is a reasonable answer to the question "how many rows are there in your database")
|Estimated number of forced measurements in DR11||30 trillion|
|Estimated numbers for DR1 release|
Objects: 18 billion
|Average number of alerts per night||"about 10 million"|
The science estimate used to be 2M per night. However, this did not include the Galactic plane where we'll have significantly more variability. Also, the SRD requires us to support ~10k/visit, which roughly translates to ~10M/night; before it was interpreted as the "peak", but we didn't have a strong justification why. We're now treating it as an average, so the number of alerts per night flows from that.
The 2 x NNN nomenclature is designed to convey that the data will be transmitted through two independent links; if one gets cut, the other one continues to operate. DM and Camera baselines can operate with a only a single link being up.
x00 - y00 range indicates lower end (some links down) - upper end (all links operating). Any link would go to zero if all links are down, but that is deemed < 0.1% probability.
NOTE: As of 20171220 La Serena - Santiago still negotiating for 100 Gbps lower end, 75% likely
|Data and compute sizes|
|Number of Data Releases||11|
DR1 will include the first 6 months of data
|Date of DR1 release||time of ops start + 12 months|
A common misconception is that the first data release will be released 6 months after the start of the survey. It will be released 12 months after the start of the survey. It's because we collect data for 6 months, and then it takes 6 months to process it.
|<9e- rms, at nominal 2 second readout time________________ __|
This top level noise budget includes all sources internal to the camera system that contribute to the base noise in each pixel, including readout noise, residual noise from dark current, additional noise in the electronics, etc... The camera read noise requirement per exposure is derived from the OSS requirement of 12.7e- per visit and the standard definition of two exposures per visit.
LSE-59 Camera Subsystem Requirements
|CCD Blooming Full Well||Pixel charge capacity shall be no greater than 175,000 electrons.|
Defined as the point at which the detector response (volts out divided by mean per pixel integrated photo-generated charge) saturates when illuminated by a flat field.
|CCD-008 in LCA-128|
Etendue (A \Omega)
|319 m^2 deg^2|
This value comes from doing the numerical integration of (effective aperture * dFOV) over a 1.75 degree radius in 0.02 degree increments, taking into account the vignetting profile from the optical design. Note that this is purely a property of the optical system. It doesn't include focal plane fill factor.
|Field of View||3.5 deg (9.6 deg^2)||From LSST baseline optical design||LSE-11 sec 1.3.1|
|Primary Mirror Diameter||8.4 m||From LSST baseline optical design||LSE-11 Table 7|
|On-axis effective clear aperture||6.54 m|
M1 clear aperture diameter * sqrt(on-axis vignetting ratio)
M1 clear aperture diameter = 8.36 m
on-axis vignetting ratio = 0.61
|Area-weighted effective aperture over FOV||6.49 m||Diameter of the circle whose area is equal to (319 m^2 deg^2)/(9.6 deg^2)||Document-7512|
|Final f-ratio||f/1.233||From LSST baseline optical design. This varies with band slightly. u= 1.232, g/r/I = 1.233, and z/y = 1.234.||LSE-11 Table 5|