Camera Calibration Optical Configurations and Calculations

Keith Bechtol Andy Scacco Allesandro Sonnenfeld

September 16, 2008 LSST Camera F2F 1

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 2

Optical Deck

• Load standard LSST optical deck • Consider only the camera – Three lenses – Filter – CCD surface

September 16, 2008 LSST Camera F2F 3

“Artificial Star” Calculations

Photodiode Array (or Telescope) Reflectivity R ~ 0.3%.

(Not all reflections shown.) 300  m (4cm away)

September 16, 2008

Reference Photodiode 14 – 23.6 degrees Optical Source L1 L2 Not To Scale Filter L3 FPA 30  m (Approximate FWHM of LSST PSF at 0.6 arc-sec seeing.)

LSST Camera F2F 4

“Artificial Star” Calculations

Sequential ZEMAX Model 180 160 140 120 100 Focused pin-hole beam from quartz lamp and monochromator (length of optical source  1.4m).

Scacco and Sonnenfeld

September 16, 2008

80 60 40 20 -400 -300 0 -200 -100 0 100 200 300

Distance from center of focal plane in mm

Distance from FPA Center (mm)

LSST Camera F2F

400

5

Angle in degrees 0 14 19 23.6

14 19 23.6

Azimuthal Radial

Methodology

• “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 Lambertian scattering

September 16, 2008 LSST Camera F2F 6

General Strategy

• 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 What is the optimal test beam size?

How should we scan the test beam?

September 16, 2008 LSST Camera F2F 7

1 cm Beam Intensity Profile

Test beam intensity profile at focal plane – radial slices Significant features on 0.1% level Center of focal plane (0,0)

September 16, 2008

Notice rapid change in beam intensity profile!

LSST Camera F2F

5 cm offset from center (5cm,0)

8

10 cm Beam Intensity Profile

Center (0,0)

September 16, 2008

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

LSST Camera F2F 9

A Comment on Statistics

Intensity fluctuations readily apparent in 20 minute simulation Fortunately, we can do much better with a real test beam Full well potential ~ 100000 e ˉ 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 10

10 cm Beam Scoring Plane

Center 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 / cm ²) on log scale

September 16, 2008 LSST Camera F2F 11

L1

Ghosting Analysis

No filter L2 L3

September 16, 2008 LSST Camera F2F 12

L1+L2

Ghosting Analysis

No filter L1+L3 L2+L3

September 16, 2008 LSST Camera F2F 13

Wavelength Dependence

LSST camera range 400 - 1000 nm AR coatings are wavelength dependent Optimize for 700 nm light 400 nm test beam

September 16, 2008

700 nm test beam

LSST Camera F2F

1000 nm test beam

14

Suggested Procedure

1.

2.

3.

4.

Produce test beam several cm in diameter Scan outwards in radial direction Fit shape of beam intensity profile Scan in concentric circles Relative calibration possible independent of exact model results Step 1: Step 4: Beam intensity profile changes continuously while going outwards

September 16, 2008 LSST Camera F2F

Take advantage of azimuthal symmetry

15

Future Directions

• • • • – – 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

16 September 16, 2008 LSST Camera F2F