GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
Face-to-Face IDT Meeting Session 1
Observation Strategies and EGRET
Experience
S. W. Digel
March 19, 2002
SLAC
Session 1
1
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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
Session 1
z EGRET
OSSE
x
2
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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’
‘zenith pointed’
Session 1
zenmax
105° - 55°
3
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
Uniformity of Exposure?
•
GLAST Science Req. Doc. (433-SRD0001) says must have <±20% uniformity
of exposure on time scale of 7 days, not
including effects of SAA.
(AO-Era, don’t
take literally)
– 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.
• 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?
Session 1
4
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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
 E    Aeff E , T  d
•
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
Session 1
5
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
Beyond Uniform Exposure 2
•
Corresponding effective PSFs have angular diameters
that vary by 10% or more
Dist of coverage
T  A E, E PSF x, y, E, dEd
x, y   
 T  A E, E dEd
2
PSFeff
eff
2
eff
•
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)
Session 1
6
Inclination
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
Beyond Uniform Exposure 3
•
•
Rule of thumb scaling
Fmin 
1
 PSF
ID

Step rocking has the appeal of giving at least some exposure
everywhere in the sky every other orbit – for LAT to function as an allsky 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)
Session 1
7
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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
Session 1
8
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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
Session 1
9
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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)
Session 1
10
GLAST LAT Project
Face-to-Face IDT Meeting, March 19–20, 2001
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
Session 1
11