Subject: Exposure info for event lists from scanning instruments

From: Jeff Bloch <jbloch at alexis>
Date: Wed, 3 Jan 1996 23:43:41 -0700

The concern I am addressing in this posting is related to high energy
astrophysics experiments operating in a scanning or wide-field monitoring
mode, although this same issue might arise when storing data for scanning
instruments at other wavelengths as well. 

It is straightforward to store under the current OGIP FITS conventions 
event list data from a scanning instrument. Examples of such types of 
datasets would be the EUVE and ROSAT XRT and WFC surveys. Our ALEXIS 
experiment is always in a scanning mode.

In such scanning experiments, effective exposure on a given location on
the sky changes rapidly as a function of time and position due to
vignetting and other instrumental effects. 

If one stores the event data from a sky survey experiment as photon lists,
then in order to generate rate information for an arbitrary source location
from such data one must know how to compute the exposure for that 
location in addition to selecting photons from that location.

Often, custom programs must be invoked to take into account the effects 
of deadtime, vignetting and other effects to compute the exposure. The 
OGIP memos seem to be slanted to encoding this information for one 
location on the sky in the context of pointed observations.

To recover exposure info for wide area monitoring or survey experiments, 
I would like to raise the idea of having "Exposure Token Lists" in 
parallel with the photon event lists. Each exposure token in such lists 
would have the properties of position on the sky, effective exposure per 
a fixed real time interval, and an area around the position for which 
they are valid.

In order to compute a light curve from a dataset consisting of a photon 
event list and a exposure token list, one would first select events in 
the photon list that were within a specified radius of the source 
position and then bin those events in time to get counts per real time 
interval. Then one would select exposure tokens that were nearest the 
source location and add the effective exposure increment value for each 
token in the appropriate real time bin. One would then divide the counts 
time bin array by the effective exposure time bin array to get a 
corrected count rate light curve.

If this proves to be a useful formulation, should a standard exposure 
token FITS file (looking very similar to event lists) be developed?

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Jeffrey Bloch					Office:    (505) 665-2568
Astrophysics and Radiation Measurements Group	ALEXIS SOC:(505) 665-5975
Los Alamos National Laboratory			FAX:	   (505) 665-4414
Group NIS-2, Mail Stop D436			e-mail:	   jbloch at lanl.gov
Los Alamos, NM 87545				pager:     104-8005
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