Transcript Setup_timing_model_description.ppt

PFS setup timing model
Peter Mao
WARNING
• This is a toy model. It is intended to promote
discussion of PFI setup time trades.
• Assume that all parameters and assumptions
made here were pulled from thin air.
Timing diagram (schematic)
close shutter, read out CCD
telescope slew/settle
cobra anti-home, cobra scan to home w/ MC integration
MC readout, calibration, centroiding
calculate distortion map
transfer new
targets
iteration sequence
Cobra move
MC integrate
MC readout, calibration, centroiding
ID centroids, calc SF PFI(x,y)
ASIAA
JPL
Caltech
Other
repeat
done
report final status to OBCP/SOSS
Positioning error model
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Presently, derived from C.Fisher’s 2008 data on Cobra convergence.
The calibrated (red) curve has a functional dependence:
• d = d010-0.6(n)
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with d = distance in microns, interpreted as the 1 stdev distance in a 2D gaussian
distribution.
and n = iteration
Distance to Target
Target #56: [1000,1525]
This is used to calculate
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During Calibration
1. the Cobra move time
2. the MC integration time
3. the number of (un)finished fibers
Post-Cal #1
Post-Cal #2
Post-Cal #3
10000
Distance to Target (mm)
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1000
100
10
1
0
1
2
3
4
5
6
7
Iteration Number
8
9
10
11
12
Cobra move time
• move time should be a function of the distance error. probably a
linear function.
• INPUTS:
– positioning error (d) [microns]
– miminum move time (default: 0.01 sec)
• INTERNAL PARAMETERS:
– Cobra velocity (5000 microns/sec)
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This should really be sorted out in (θ,ϕ) space, not in cartesian coords.
For initial moves, the angular velocity is 2π rad/sec.
When angular errors are smaller (< 10 deg) cobra moves at π/2 rad/sec.
High velocity is likely only used for the first two iterations.
– Assume Cobras are moved in 5 groups.
• FUNCTION
– t = (tmin < d/v) * 5
Cobra move time vs. step #
MC integration time
• integration time should scale inversely with required
accuracy
• required accuracy should be proportional to distance error
(of the next step)
• INPUTS:
– MC allowed position error [function of cobra position error]
• using d/10 for now.
– seeing limit [in microns now, but should be in arcsec]
– integration time at seeing limit [default = 10 sec]
– minimum exposure time (default = 0.1 sec)
• FUNCTION
– C = dseeing * tseeing
– dMC = (dseeing < dcobra/10)
– tMC = (tmin < C/dMC)
MC integration time vs. step # and
positioning error
# finished fibers
• Assume that SF locations are Gaussiandistributed about the desired position, and that
the position error, d, is the standard-deviation of
that distribution.
• INPUTS
– position error [d = d(iteration step)]
– number of science fibers
– error budget allocation for SF location (dmax)
• Nfibers_on = floor(NSF * (1 – exp(-0.5 * (dmax/d(n))2)
• NSF = Nfibers_on + Nfibers_off
Science fiber positioning success vs
step #
Sample output
• Study of fiber efficiency vs.
MC integration time at
seeing limit.
• Based on timing model:
– giving up 27 fibers breaks
even if the MC integration
time at the seeing limit
drops by ~5 seconds.
– up to 5 seconds, the MC
integration time is a
subdominant factor in
observing efficiency.
• Trade can be calculated
for any variable in the
timing model
dBη = 10 log10(η)
η = (# observed)/(max observable)
1 Fiducial fiber = 0.8 Science fibers
Loose ends
• Cobra timing models needs to be updated
• MC timing model needs input from ASIAA
• Setup excecution sequence will evolve during
development
• All parameters need to be vetted.