Calibration of the LSST Camera

Andy Scacco

LSST Basics

• Ground based

• 8.4m triple mirror design

• Mountaintop in N. Chile

• Wide 3.5 degree field survey telescope

• ~30 Tbits / night of data

• Dark energy / cosmology

LSST Layout

Etendue

• Etendue = FOV * Collecting area• Measures the rate of incoming data

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LSST PS4 PS1 Subaru CFHT SDSS MMT DES 4m VST VISTAIR

SNAPOpt+IR

The point spread function

• Stars are point sources

• PSF is image of a point source

• Combination of atmosphere + telescope aberration

• Measured by the full width at half maximum (FWHM)

• PSF of LSST has a

30 micron FWHM

Atmospheric Seeing

• Atmosphere blurs images

• Instrumental blurring is much less than atmosphere

• Large ground based telescopes need adaptive optics

Camera Design

Focal Plane CCD Array

• We need a 30 micron spot on focal plane

• CCD wells are 10 x 10 microns

• LSST has 3.2 Gpixels

Laser

• TEM00 mode

• Helium-neon / Tunable

• Gaussian beam

• Very good for optics analysis

Monochromator part 1

• Filter / Monochromator

• Pinhole produces Frauenhofer diffraction

• Airy diffraction pattern

Monochromator part 2

• Airy pattern resembles Gaussian

• Second pinhole cuts off all but the central peak

Lens aberrations

• Lenses aren’t perfect

• Astigmatism

is biggest

problem for us

Astigmatism

• Sagittal / tangential rays focus to different locations

Camera ZEMAX Design

Spot size as a function of wavelength for a Gaussian beam with an initial waist radius of 15 microns striking the center of the focal plane at an angle

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wavelength in microns

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Azimuthal component

Radial component

Angle in degrees

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u g r i z YFilter

Radial spot size as a function of wavelength for a 15 micron radial waist Gaussian beam pointed at a 0 degree angle from varying distances from the center of the focal plane

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Wavelength in microns

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Distance from center in mm

Radial spot size as a function of wavelength for a 15 micron radial waist Gaussian beam pointed at a 0 degree angle from varying distances from the center of the focal plane

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Wavelength in microns

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Distance from center in mm

0.384 micron wavelength Gaussian beam at an angle

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Distance from center of focal plane in mm

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Angle in degrees

AzimuthalRadial

0.994 micron wavelength Gaussian beam at an angle

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Distance from center of focal plane in mm

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Angle in degrees

AzimuthalRadial

Testing Schematic

Reference

Photodiode Laser

30 micron spot

Photodiode

Array

Focal Plane

My Other Project…

• Testing a laser sensor system for use in measuring distance very precisely

• It will be accurate enough to be used to measure the flatness of the focal plane of the LSST

Apparatus

Laser displacement sensors

Optical Flat

Optical FlatPrecision movable platform

Data

Data #2

Further work

• Figure out why the correction function differs between the two trials

• Calculate a best fit sawtooth function to subtract from the data to make it more accurate

• Use the sensor with the correction function to measure the components of the LSST

Acknowledgements

• David Burke – my excellent mentor

• Andy Rasmussen – other excellent mentor

• Steve Rock

• The DOE, Office of Science

• SLAC

• Stanford

• All my fellow SLAC-ers

References

http://www.cambridgeincolour.com/tutorials/graphics/airydisk-3D.png

http://navj.wz.cz/061116_025307-70_56_19_226.jpg

http://www.rp-photonics.com/img/gauss_r.png

http://publication.lal.in2p3.fr/2001/web/img344.gif

http://laser.physics.sunysb.edu/~wise/wise187/2005/reports/deb/gauss1.gif

http://cache.eb.com/eb/image?id=3246&rendTypeId=4

References 2• “Large Synoptic Survey Telescope”, Available at http://www.lsst.org

(2007 August 9).• D. Burke, private communication (2007).• “Point Spread Function”, Available at http://en.wikipedia.org (2007 August

6).• “Astronomical Seeing”, Available at http://en.wikipedia.org (2007 August 3).• “Full Width at Half Maximum”, Available at

http://www.noao.edu/image_gallery/text/fwhm.html (2007 August 6).• “Gaussian Beam”, Available at http://en.wikipedia.org (2007 July 25).• A. Sonnenfeld, private communication (2007). • “Airy Disk”, Available at http://scienceworld.wolfram.com/physics/ (2007 July

25).• “Astigmatism”, Available at http://en.wikipedia.org/wiki/Astigmatism (2007

July 25).• “Aberrations”, Available at http://grus.berkeley.edu/~jrg/Aberrations/ (2007

July 25).