september 16, 2008lsst camera f2f1 camera calibration optical configurations and calculations keith...
TRANSCRIPT
September 16, 2008 LSST Camera F2F 1
Camera Calibration Optical Configurations and Calculations
Keith Bechtol
Andy Scacco
Allesandro Sonnenfeld
September 16, 2008 LSST Camera F2F 2
Objectives
• Efficiently calibrate pixel response over entire camera focal plane to level ~ 0.1%
• Identify ghosting effects• Model camera optics – ZEMAX
Propose two calibration techniques
1) “Artificial Star” (Scacco and Sonnenfeld)
2) “Headlight” test beam
September 16, 2008 LSST Camera F2F 3
• Load standard LSST optical deck
• Consider only the camera– Three lenses
– Filter
– CCD surface
Optical Deck
September 16, 2008 LSST Camera F2F 4
“Artificial Star” Calculations
Reference Photodiode
Photodiode Array
(or Telescope)
Not To ScaleL1
L2
Filter
L3
FPA
30 m (Approximate FWHM of LSST PSF at 0.6 arc-sec seeing.)
Optical Source
14 – 23.6 degrees
Reflectivity R ~ 0.3%.
300 m(4cm away)
(Not all reflections shown.)
September 16, 2008 LSST Camera F2F 5
Sequential ZEMAX Model
0.384 micron wavelength Gaussian beam at an angle
0
20
40
60
80
100
120
140
160
180
-400 -300 -200 -100 0 100 200 300 400
Distance from center of focal plane in mm
Spo
t rad
ius
in m
icro
ns
0
14
19
23.6
14
19
23.6
Angle in degrees
AzimuthalRadial
Focal Plane Spot Size
Distance from FPA Center (mm)
Focused pin-hole beam from quartz lamp and monochromator (length of optical source 1.4m).
Scacco and Sonnenfeld
“Artificial Star” Calculations
September 16, 2008 LSST Camera F2F 6
• “Headlight” test beam parallel to optical axis• Run ZEMAX in non-sequential mode• L1, L2, and L3
– Quarter-wavelength magnesium-fluoride AR coating
• CCD treated as reflective surface– Scatter fraction = 0.33 (n = 3.6 for Si)
– Lambertian angular distribution (scattered intensity is proportional to the cosine of the angle with surface normal)
– Quarter-wavelength magnesium-fluoride followed by half-wavelength of lanthanum-oxide AR coating
Methodology
Lambertian scattering
September 16, 2008 LSST Camera F2F 7
• Scan test beam over pixels in series of exposures• Each pixel traces out beam intensity profile• If the spatial profile of the test beam intensity does not
change significantly over the characteristic size of the beam at the focal plane, we can compare the response of nearby CCD pixels
General Strategy
What is the optimal test beam size?
How should we scan the test beam?
September 16, 2008 LSST Camera F2F 8
1 cm Beam Intensity Profile
Center of focal plane (0,0) 5 cm offset from center (5cm,0)
Test beam intensity profile at focal plane – radial slices
Notice rapid change in beam intensity profile!
Significant features on 0.1% level
September 16, 2008 LSST Camera F2F 9
10 cm Beam Intensity ProfileCenter (0,0)
Offset 125 cm (125cm,0)
Offset 250 cm (250cm,0)
Test beam intensity profiles at focal plane – radial slices
Intensity profile of larger test beam is more stable
September 16, 2008 LSST Camera F2F 10
A Comment on StatisticsIntensity fluctuations readily apparent in 20 minute simulation
Fortunately, we can do much better with a real test beam
Full well potential ~ 100000 eˉ
High QE
Collect N ~ 40000 photons in single exposure
σ = sqrt(N)/N ~ 0.005
With multiple exposures, can reach 0.1% level accuracy
September 16, 2008 LSST Camera F2F 11
10 cm Beam Scoring PlaneCenter 10 cm diameter beam
over a grid of positions
(250,250) (125,250) (0,250)
(250,125) (125,125) (0,125)
(250,0) (150,0) (0,0)
Test beam positions in cm
Plot incoherent irradiance (W / cm²)
on log scale
September 16, 2008 LSST Camera F2F 12
Ghosting Analysis
No filter
L1 L2 L3
September 16, 2008 LSST Camera F2F 13
Ghosting Analysis
No filter
L1+L2 L1+L3 L2+L3
September 16, 2008 LSST Camera F2F 14
Wavelength Dependence
LSST camera range 400 - 1000 nm
AR coatings are wavelength dependent
Optimize for 700 nm light
400 nm test beam 700 nm test beam 1000 nm test beam
September 16, 2008 LSST Camera F2F 15
1. Produce test beam several cm in diameter
2. Scan outwards in radial direction
3. Fit shape of beam intensity profile
4. Scan in concentric circles
Suggested Procedure
Beam intensity profile changes continuously while going outwards
Step 1: Step 4:
Take advantage of azimuthal symmetry
Relative calibration possible independent of
exact model results
September 16, 2008 LSST Camera F2F 16
• CCD surface most challenging element to model– Scoring pattern strongly dependent on CCD surface properties– Observe ghosts to understand CCD reflection
• Use ghost patterns to determine relative positions• Include diffraction in simulations• Use test beams of various wavelengths to parse QE
from pre-amp gain
Future Directions