Onboard Filter

Note to self: At least one more cut has recently been added. - Tracy?

The onboard filter is a series of cuts designed to reduce the event trigger rate, so that fewer background events that are not of interest will be passed to the ground in order to preserve bandwidth for events of scientific interest. The onboard filter is also applied during the digitization phase of ground processing in order to determine the nature (class ???) of the triggered events that are being passed through. Cuts are sequenced to conserve readout/processing time, i.e., those cuts that can be made with the fewest readouts and/or least processing time are applied first.

Gamma Filter Configuration Parameters Files (GFC_DB, EFC_DB)

The gamma filter can be configured in four separate ways:

Filter parameters control how energy, track, and tile data retrieved from the CAL, TKR, and ACD are used as inputs into the gamma filter algorithm during execution of the 15 gamma tests. These quantities are specified in CDM database files in the GFC_DB package. (See table: Gamma Filter Configuration Parameters.)

Performance parameters are used to set limits on certain CPU-intensive calculations – namely, the number of projections that the gamma filter is allowed to find in its track-finding algorithm. Since the track-finding code is used by the GRB code as well, a separate projection limit is defined for the GRB processing.

Note: In B2-0-0, the limit for normal gamma filter operations is 100 projections; for GRB operations, 50 projections.

Sampler parameters control how the filter allows a subset of events that would normally be rejected to pass through into the science data stream; this sampled stream allows for analysis of filter behavior. Two kinds of sampling are provided: simple sampling (with events passed through at a specified rate) and countdown sampling. Parameters affecting the sampling rate are defined in CDM database files from the EFC_DB package; in the case of the gamma filter, a sampler configuration is associated with a particular instance of the Gamma filter in the GFC_DB package.

Results parameters define how statistics on events that pass the various filters are sampled, collected and included in the science data stream. Sample sizes for this analysis are settable using CDM database files from the EFC_DB package.

Note: Output of this data into LSFdatagrams is not implemented in B2-0-0.

Gamma Filter Configuration Parameters (GFC_DB)
Test	Test Type	Test	Description	Configuration Parameter Used
1	Trigger discriminator	Veto tile response; no CAL LO	If any ACD tile near the top of the instrument ("veto tile") responds to the event and the CAL low-energy (CAL LO) trigger discriminators all read zero, the event is rejected.	N/A
2	Trigger discriminator	ACD splash veto (Pass 0)	If the ACD gets too many hits (or if a relatively small number of hits cluster in a particular “splash” pattern) and the CAL high-energy (CAL HI) trigger discriminators all read zero, the event is rejected.	splashCount splashMap
3	Energy-level	Veto tile response; CAL energy < 350 MeV	If a veto tile responds to the event and the CAL detects energy of less than 350 MeV, the event is rejected.	N/A
4	Energy-level	Any ACD tile response; CAL energy < 10 MeV	If any ACD tile responds to the event and the CAL detects energy of less than 10 MeV, the event is rejected.	N/A
5	Energy-level	ACD splash veto (Pass 1)	If the CAL detects energy of less than 40 GeV (this is almost always the case) and the ACD gets too many hits, the event is rejected.	topsideEmax topSideFilterEmax splashEmax splashCount splashMap
6	Energy-level	TKR Tower match with ACD top or side tile	If a responding ACD top or side tile shadows a Tower with the "3-ina-row" tracker trigger, the event is rejected.	Atf.shadowed atf.emax
7	Energy-level	No connection between CAL energy and TKR (pass 0)	If there is any CAL energy and no tracker Tower has hits in more than four of its bottom six planes, the event is rejected.	zbottom.emin
8	Energy-level	CAL layer 0 energy < 1 percent of total	If the energy detected by layer 0 (the top later) of the CAL is less than 1% of the total energy, the event is rejected. This test is only executed of the event energy falls within a configurable range (emin, emax).	Layer0RatioLo
9	Energy-level	CAL layer 0 energy > 90 percent of total	If the energy detected by layer 0 of the CAL is more than 90% of the total energy, the event is rejected. This test is only executed of the event energy falls within a configurable range (emin, emax).	Layer0RatioHi
10	Energy-level	CAL crystal ratio	If the number of CAL crystals with less than 1% of the energy exceeds 20% of the total number of hit crystals, the event is rejected.	logEtotRatio
11	Track projection	No projections	If no projections are found, the event is rejected.	Tkr.geometry zeroTkremin
12	Track projection	TKR/ACD matching	If a track projection points at a responding ACD tile, the event is rejected.	row2Emax row01Emax topEmax
13	Track projection	Track(s) project into skirt region	The "skirt region" lies between the CAL and the lower row of ACD tiles. Thus, it is outside of the area covered by the CAL, but inside the active area of the instrument. If a track projection points into this region, the event is rejected.	skirtEmax
14	Track projection	CAL energy < 350 MeV; fewer than 2 tracks	If the energy level detected by the CAL is less than 350 MeV and there is no evidence of two or more tracks, the event is rejected.	twoTkrEmin
15	Track projection	No connection between CAL energy and TKR (pass 1)	If there is any CAL energy and no projection points at that energy, the event is rejected.	N/A
16	High-energy event	CAL energy > X MeV	If the energy exceeds the specifies amount, the event is PASSED, regardless of other veto conditions. Currently 20000.	Epass
17	Low-energy event	CAL energy < X MeV	If the energy fails to reach threshold, the event is VETOED, regardless of other veto conditions. Currently 5.	Eveto

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Last updated by: Chuck Patterson 03/12/2009