1. Face-to-Face IDT Meeting Session 1 Observation Strategies and EGRET Experience S. W. Digel
March 19, 2002
SLAC

2. EGRET Experience Observing constraints (from S. Hunter)
x-z plane within 15° of the sun for radiators
Moon, Jupiter, Saturn not in FOV of star cameras (45 or 90° off x-z plane)
Atomic O was not a constraint (i.e., could look in direction of orbit)
Major constraint: OSEE needed two targets
Scheduling software (constraint checker) written by R. Kroeger (NRL)
Typical viewing period ~2 weeks inertial pointing (with pointing accuracy requirement comparable to GLAST)
Observing time was lost during earth occultations (trigger modes below horizon disabled)
All-sky survey, but very non-uniform exposure
COMPTEL z
EGRET
OSSE
BATSE x

3. Considerations for LAT Observing Strategies
Constraints:
28.5° inclination orbit, SAA perimeter
Slewing profiles
Maximum angle from zenith
Others?
Degrees of freedom:
Define where GLAST spends its time within this circle:
orbital plane
‘rocking’
zenmax
­ 105° - 55°
‘zenith pointed’

4. Uniformity of Exposure?
GLAST Science Req. Doc. (433-SRD-0001) says must have <±20% uniformity of exposure on time scale of 7 days, not including effects of SAA.
Goal is <±10%.
[Don’t tell anyone: SRD doesn’t specify the energy]
What is uniform at one energy might not be as uniform at a different energy.
(AO-Era, don’t take literally)
What are the scientific drivers for uniformity of exposure?
As opposed to, say, uniformity of sensitivity
To not miss the bright AGN flares? (How often do they occur and how bright are they?)
To avoid exposure biases in likelihood analysis?

5. Beyond Uniformity of Exposure
Idealized simulations presented here
Instantaneous step rocking
No accounting for azimuth (location of the sun)
No consideration for possible reorientation for downlink, ToOs
Averaged over an integral number of precession periods (equivalently, a long time), the exposure depends only on declination
Precession period ~55 days
Old news: The exposure can be made remarkably independent of declination for representative energies
The effective PSF, however, cannot.
In the analysis, effective PSF may not apply, but still a useful concept to make discussion of observing strategies semi-quantitative

6. Beyond Uniform Exposure 2
Dist of coverage
Corresponding effective PSFs have angular diameters that vary by 10% or more
Likewise (or even more so), the sensitivity to point sources cannot be made uniform with a simple observing strategy, because the diffuse interstellar emission is much brighter near the Galactic equator than at high latitudes
EGRET (>100 MeV)
Inclination

7. Beyond Uniform Exposure 3
Rule of thumb scaling
Step rocking has the appeal of giving at least some exposure everywhere in the sky every other orbit – for LAT to function as an all- sky monitor, but as we can see, it shortchanges the orbital plane (low declinations) and low Galactic latitudes in terms of sensitivity.
d = 0°
Galactic center and anticenter!
Galactic unident. 3EG sources!
EGRET (>100 MeV)

8. Pointed Observations vs. Scanning
Except for low-b, low-d regions, it will be difficult to make the case for a pointed observation just to increase the exposure (i.e., increase the sensitivity for steady sources)
Good scientific cases do exist for other kinds of pointed observations – e.g., periodicity searches or campaigns for known transient sources
Tradeoff for inertially pointed (or ‘smart’ pointed) observations is how much occultation of the FOV by the earth will be tolerated
Accumulation of exposure for inertially pointed observation of Vela

9. Strategies for Earth Avoidance
Basic: Around the horizon vs. around the orbital pole
One step up: Simulation comparing inertial pointing with a ‘smart’ observing mode that kept the LAT as close as possible to the desired viewing direction with constraints
no part of FOV occulted
slew rate < 15°/min
for illustrative purposes here

10. Issues for Planned Pointed Observations
Likely duration - (guess > weeks)
Scheduling to optimize coverage/efficiency
with respect to the 55-day precession period
multiplexing targets separated by > FOV
What to do in between times that the target is observable?
slew back to be in position to catch the rise (minimum angular rate)
‘rock’ about this great circle segment
slew to (or quite likely just toward) an alternate target
slew to a celestial pole?
How closely to track the source
How far (in zenith angle) to track the source
Lost efficiency to earth occulation
Albedo gamma rays (must reject onboard)

11. More Pointed Issues
How much time would be requirede to equalize sensitivity with typical high-latitude region? (A lot)
Should this be part of the sky survey year?
For sky survey planning: Issues are effect of removing idealizations (finite slewing rates, azimuthal dependence, reorientations/interruptions) and deciding whether we really want uniform exposure