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