CBP : Collimated Beam Projector

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Preliminary measurements of the StarDice response

at the per mil statistical uncertainty level

LPNHE team : Marc Betoule, Sébastien Bongard, Jérémy Neveu, Thierry Souverin

Harvard team : Christopher Stubbs, Sasha Brownsberger, Elana Urbach

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Rubin/LSST-France @ Annecy - 17/05/2022

Slides from Thierry Souverin

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Introduction

What is a CBP ? Why ?

• What ? CBP, for Collimated Beam Projector, is a device able to shoot a known quantity of photons at a known wavelength and in a parallel beam → mimic a monochromatic star of known flux

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• Why ? Photometric calibration is a dominant uncertainty (~1%) for the dark energy parameter constraint

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→ The CBP aims at measuring the telescope filter transmissions across the visible spectrum to decrease this uncertainty to ~0.1% (StarDICE, VRO, AuxTel, ZTF, etc.)

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1. Instruments

General recipe for CBP

1. A tunable monochromatic light
2. An optic device able to recreate a parallel beam from a point source

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CBP configurations

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CBP response measurement

StarDice transmission measurement

Laser

Integrating sphere w/ monitoring instruments

CBP configurations

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CBP response measurement

StarDice transmission measurement

Laser

Integrating sphere w/ monitoring instruments

Integrating sphere

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Photodiode

Laser

Spectrograph

Integrating sphere

Pinhole of variable size

CBP optics

Two instruments in the integrating sphere, to monitor the input light :

1. A spectrograph to monitor the laser wavelength
2. A photodiode to monitor the flux quantity

How do we measure our responses ?

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CBP response R_CBP [𝛾.C⁻¹] at any given wavelength

StarDice response R_SD [ADU.𝛾⁻¹]

• Q_SC : solar cell charges [C]
• Q_PD : photodiode charges [C]
• Q_SD : stardice charges [ADU]
• 𝜖_SC : solar cell quantum efficiency [C.𝛾⁻¹]
• e = 1.6x10⁻¹⁹ [C]

How do we measure our responses ?

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R_SD

Spectrograph calibration

• Spectrograph calibrated with a Hg-Ar

⇒ Calibration precision around 1Å

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• Difference between detected and requested wavelength < 2nm → this difference is very stable in time and is always the same

⇒ Allow detection of possible light contamination

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Allows angtröm precision and detection of possible contamination

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Monitoring photodiode

• Photodiode plugged to the integrating sphere and connected to a electrometer
• Monitor the total charge collected in the photodiode Q_PD in Coulomb

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PD total charge Q_PD in Coulomb

CBP output with a Solar Cell

• Large solar cell calibrated with a NIST photodiode
• Measure the photons at the output of the CBP

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Solar Cell

• Large solar cell calibrated with a NIST photodiode

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SC total charge Q_SC in Coulomb

StarDice telescope

Andor camera : CCD 1024x1024 pixels

• Find spot position on camera
• Aperture photometry with dark subtraction

⇒ Measure Q_SD the photons collected by the camera in ADU

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II. Measurements

CBP measurements

• Measure CBP response for [350-1100] nm range
• Shoot in Solar Cell with a 5mm pinhole
• Measure StarDice response for [350-1100] nm range
• Shoot at a given position in StarDice for every filter with 75µm, 2mm and 5mm pinhole
• Measure the uniformity of the StarDice mirror (“pupil stitching”)
• Shoot in StarDice at 8 different positions on the mirror with 5mm pinhole
• Measure the uniformity of the StarDice focal plane
• Shoot in StarDice telescope at 16 different positions on the focal plane (CCD)

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1. CBP response

CBP response

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Solar Cell measurement with 5mm pinhole

SNR

CBP response

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Solar Cell measurement ; 5mm pinhole

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b. StarDice response

StarDice transmission 5mm

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We don’t normalize by the CBP response yet

Image for 5mm pinhole for light at 841nm

• Statistical precision below 0.08%

StarDice filter transmissions (75µm)

• Precision below 0.4% for every filters
• Wavelength resolution high enough to see the slopes of the filter edges

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Mirror : radial positions (75µm)

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• Expected blueshift of the filter borders depending on the radius

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Different radial positions on the mirror

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c. StarDice response

systematics

StarDice systematics

• Study of the focal plane uniformity (16 positions) :

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• Study of the mirror uniformity (4 positions) :

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Conclusion

Conclusion

• Reached unprecedented statistical precisions on the measurements :
• CBP response : <0.07%
• StarDice filter responses : ~0.1%
• Future and ongoing analysis
• Modeling effects : fitting the fringing, photometry and astrometry of the ghosts…
• Detailed estimate of systematic errors : check stray light, mirror uniformity…
• Next steps:
• Writing a paper and building a portable CBP for on-site measurements (with Kélian Sommer)
• Transpose our experience to the RO CBP
• Slides credits to Thierry Souverin

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Thank you

CBP response : scattered light

Solar cell is moved backward (~16cm)

• Nearly no chromatic effect ; under 5‰

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StarDice grating transmission (75µm)

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• Uncertainty around 0.2% for 1st order

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Grating → disperse light to observe absorbing rays

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c. Focal plane uniformity

Focal plane uniformity (75um)

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4x4 positions grid on the CCD

UV + visible

IR

Spline → Mean curve of all the positions

Focal plane uniformity (75um)

[350-900] nm range

• UV : Relative difference of 4% between all the positions
• Visible : Each positions are behaving the same way with a non-uniformity of around 2%

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Focal plane uniformity (75um)

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[900-1100] nm range

• All positions are oscillating the same way in the infrared
• Fringing caused by the depth of the CCD depending on the wavelength

→ There is another fringing less intense depending on the position on the CCD

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Fringing depending on position

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d. Mirror uniformity

Mirror : quadrants positions (75µm)

⇒ Same behaviour for each quadrant

Different quadrant positions on the mirror

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Mirror : quadrant positions (75µm)

Different quadrant positions on the mirror

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Mirror : radial positions (75µm)

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CCD

Window

Main spot

Ghost

Mirror : radial positions (75µm)

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⇒ Same behaviour for each radial position

• We can see a significant difference between the positions in the blue light

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Different radial positions on the mirror

Fringing