S. Ritz updated 02/04/2003 15:28 (first draft 30 May)
We summarize here the on-orbit calibration tasks. The main purpose of providing this table at this point in time is to integrate the ground and on-orbit calibration activities, so that the completeness of the testing and the verification of the proposed techniques can be assessed. After PDR, we will develop the techniques and software tools to study the tasks in detail.
| Task/Purpose | When/Frequency/Duration | Test to Verify Technique |
| Establish the internal alignment of the LAT using galactic cosmic rays. Requirement: <7(TBR) arcsec. | First done at turn-on, after initial functional check-out,
with dedicated runs: estimate we need ~106 events [see note here].
If done on the ground (30 Hz accumulation), this corresponds to 10 hours
of data taking. It may be possible to do this on-orbit at higher
rate using flight software, but that adds complexity. The above
estimate assumes internal tray alignment
Done continuously at a low level during normal operations with pass-throughs. It may be useful to do dedicated half-day runs quarterly to verify the internal alignment. |
Can be done at any time during LAT I&T when DAQ is operational using sea-level cosmic-ray induced muons. We estimate that the full LAT can be aligned this way in less than one day. |
| Calibrate alignment between LAT and Observatory GN&C system. Requirement: <7(TBR) arcsec. | First done after turn-on, functional checkout, and internal
alignment. The current plan is to do a pointed observation near the
galactic anticenter, where several bright, well-known, separated point
sources will be in the FOV simultaneously. As with EGRET, it may be
helpful to use timing on pulsars to select photons during known peak
phases to reduce background contamination. Current estimate is that
this alignment can be done in ~one week of observation (TBR).
The observation will be repeated at intervals ranging from quarterly to annually, depending on the expected and observed stability and reliability of the alignment. First-year plan will be defined after S/C contractor is selected. |
Done piecewise during beam test PSF studies. Also possible to use mock data challenge to validate the software tools. The error budget for both this calibration and the internal alignment must be established. |
| Calibrate CAL energy scales | Done continuously using galactic CNO flux. See estimates by Grove(slides 19-26) for precision. | Planned CAL tests at GSI heavy ion beams. |
| Calibrate ACD scales to precision specified in the ACD level III specification. | Done continuously using galactic cosmic ray flux and PHA readout. | Done during beam test electron/hadron tests and sea-level cosmic ray muon tests. |
| Verify L1T efficiencies. | Done continuously using redundant triggers, specifically the CAL-LO trigger as a check on the TKR trigger. | Done during beam test electron/hadron tests and sea-level cosmic ray muon tests. Must establish the precision requirement, based on the effective area knowledge error budget. |
| Calibrate systematic offsets with viewing angle ("fish-eye" effect, etc.) | Done after LAT-Observatory alignment after turn-on.
Observe bright, known point source (e.g., Vela) at 4 (TBR) viewing angles
(q, f). Total time: ~1 week.
More detailed understanding of any subtle effects will be obtained during the first year all-sky survey, binned by viewing angle on known sources. Monitored over mission life using known sources. |
Done during photon beam test PSF studies, but does this drive requirements unnecessarily on fixture and beam position knowledge? |
| Verify PSF over FOV | Done after LAT-Observatory alignment after turn-on. The same observations used to calibrate the systematic offsets will be used for this analysis. | Done during beam test PSF studies. |
| Verify background rejection using first-year data, analyzing the extra-galactic diffuse measurement in bins of varying background rates. | Done during first-year all-sky survey. | N/A, but software tools can be validated during mock data challenge. |
| Effective area uniformity map of instrument (e.g., Willis analysis of EGRET data). | Done during first-year all-sky survey. | N/A |
| Verify effective area over energy range (e.g., check for "Kniffen factors") | Done during first-year all-sky survey. Measure known source flux (e.g., Crab) to lowest LAT energies. | Low energy gamma beam tests. |