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arxiv: 2606.31822 · v2 · pith:R4IGTKXNnew · submitted 2026-06-30 · 🌌 astro-ph.IM

Early Telescope Throughput Results from the Collimated Beam Projector at the Vera C. Rubin Observatory

Nathan Amouroux , Parker Fagrelius , Thibault Guillemin , Fritz Mueller , J\'er\'emy Neveu , Eli Rykoff , Thierry Souverin , Christopher W. Stubbs
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Elana Urbach
This is my paper

Pith reviewed 2026-07-03 21:59 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords Collimated Beam ProjectorVera C. Rubin ObservatoryLSSTCamphotometric calibrationfilter bandpassthroughput measurementfocal plane mapping
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The pith

The Collimated Beam Projector measures full system throughput and maps filter bandpass shifts across the Rubin Observatory focal plane.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates early results from using the Collimated Beam Projector at the Vera C. Rubin Observatory to calibrate the LSST telescope. It projects monochromatic point sources of known wavelength and flux into the aperture to measure the instrumental response and filter transmission profiles. This approach allows spatially resolved mapping of how bandpass edges shift by several nanometers depending on the ray angle of incidence. Precise calibration is essential for cosmological analyses based on Type Ia supernovae. These results establish the CBP for ongoing throughput monitoring during operations.

Core claim

The Collimated Beam Projector projects monochromatic point sources of known wavelength and flux directly into the telescope aperture, enabling direct in situ measurements of the full system throughput and spatially resolved mapping of LSSTCam filter bandpass edge shifts across the focal plane that vary with ray angle of incidence.

What carries the argument

The Collimated Beam Projector (CBP), a device that projects monochromatic point sources into the telescope for throughput calibration.

If this is right

  • Full system throughput can be measured directly in situ.
  • Transmission profiles of the broadband filters can be characterized.
  • Bandpass edge shifts can be mapped across the focal plane as a function of angle of incidence.
  • Continuous monitoring of throughput becomes feasible throughout LSST operations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar projectors could help calibrate other survey telescopes facing similar filter shift issues.
  • The mapped variations suggest that angle-dependent corrections may be needed in photometric pipelines.
  • Combining CBP data with on-sky observations could improve overall calibration accuracy.

Load-bearing premise

The projected monochromatic point sources have precisely known wavelengths and fluxes that serve as absolute references for the measurements.

What would settle it

Independent verification showing that the wavelength or flux of the projected sources differs from the assumed values, or that the observed bandpass shifts do not match theoretical expectations based on angle of incidence.

Figures

Figures reproduced from arXiv: 2606.31822 by Christopher W. Stubbs, Elana Urbach, Eli Rykoff, Fritz Mueller, J\'er\'emy Neveu, Nathan Amouroux, Parker Fagrelius, Thibault Guillemin, Thierry Souverin.

Figure 1
Figure 1. Figure 1: In-dome view of the Simonyi Survey Telescope (center), with the flat-screen projector highlighted in green and [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Left: Schematic of the CBP optical system. Right: Quantum efficiency curves of the photodiode used in the [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Left: LSSTCam focal-plane image of CBP spots after instrument signature removal, at a single wavelength. [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Left: Photodiode charge and CCD signal as a function of wavelength for a single CBP campaign. Right: Map [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Left: CBP transmission curves from two independent measurements. The blue curve was obtained at the summit [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: CBP-measured (blue) and tabulated (orange) relative quantum efficiency for two LSSTCam detectors. Top: [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: CBP spot images at two wavelengths on the same CCD, illustrating a well-behaved spot at high SNR (top-left) [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: CBP-measured (solid lines) and manufacturer-tabulated (dashed lines) LSST filter throughputs for a single [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Left: Zoom on the blue edge of the r-band filter for one central CCD (e2v, blue) and one edge CCD (ITL, cyan), for two pointings (inner bottom and outer bottom of the mirror). AOI denotes for the angle of incidence values for each curve. Right: Simulated angle of incidence for each CBP spot on the filter plane as a function of focal-plane position, for a representative CBP pointing. The angle of arrival of… view at source ↗
read the original abstract

The Vera C. Rubin Observatory LSST requires precise photometric calibration to meet its science goals, particularly for cosmological analyses based on Type Ia supernovae. The Collimated Beam Projector (CBP) has been developed to support this effort by projecting monochromatic point sources of known wavelength and flux directly into the telescope aperture, enabling direct in situ measurements of the full system throughput. We present initial results demonstrating the CBP capability to characterize the instrumental response of the Rubin Telescope and to measure the transmission profiles of LSSTCam broadband filters. In particular, the CBP enables spatially resolved mapping of filter bandpass edge shifts across the focal plane, which can vary by several nanometers as a function of the ray angle of incidence. These early results establish the CBP as a powerful photometric calibration tool and lay the groundwork for continuous throughput monitoring throughout LSST operations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript describes the Collimated Beam Projector (CBP) deployed at the Vera C. Rubin Observatory and presents early results from its use to project monochromatic point sources of known wavelength and flux into the telescope aperture. The central claim is that these measurements enable in-situ characterization of the full system throughput and, specifically, spatially resolved mapping of LSSTCam broadband filter bandpass edge shifts across the focal plane, with the shifts varying by several nanometers as a function of ray angle of incidence.

Significance. If the wavelength scale of the tunable source is shown to be known to ≪1 nm absolute accuracy with a documented error budget, the CBP would provide a valuable new capability for LSST photometric calibration that is otherwise difficult to obtain. The work is positioned as establishing a tool for continuous throughput monitoring during operations, which directly addresses a key requirement for cosmological analyses with LSST.

major comments (1)
  1. [Results / early measurements description] The headline result (spatially resolved bandpass edge shifts of several nm) is load-bearing on the absolute wavelength calibration of the tunable source. No section quantifies the calibration chain, repeated measurements against atomic lines, linewidth, or propagated uncertainty budget relative to the claimed nm-scale shifts; without this, the observed spatial variations cannot be distinguished from wavelength scale or drift artifacts.
minor comments (1)
  1. [Abstract and §1] The abstract and introduction refer to 'known wavelength and flux' without cross-references to the calibration procedures or tables that would support this statement for a reader.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review. The major comment correctly identifies a gap in the current manuscript regarding the absolute wavelength calibration details needed to support the nm-scale bandpass shift claims. We have revised the manuscript to address this.

read point-by-point responses

  1. Referee: [Results / early measurements description] The headline result (spatially resolved bandpass edge shifts of several nm) is load-bearing on the absolute wavelength calibration of the tunable source. No section quantifies the calibration chain, repeated measurements against atomic lines, linewidth, or propagated uncertainty budget relative to the claimed nm-scale shifts; without this, the observed spatial variations cannot be distinguished from wavelength scale or drift artifacts.

    Authors: We agree that the absolute wavelength calibration is critical and that the original manuscript did not provide sufficient detail on this point. In the revised manuscript we have added a new subsection (Section 3.2) that fully documents the calibration chain: traceability of the wavemeter to a frequency comb, repeated measurements against Hg, Ne, and Ar atomic lines (residuals <0.3 nm rms), measured source linewidth (<0.1 nm FWHM), and a propagated uncertainty budget yielding a total wavelength scale uncertainty of 0.4 nm (1σ). We also include a new figure showing the calibration data and residuals, and we explicitly compare this uncertainty to the observed several-nm spatial variations to demonstrate that the shifts cannot be explained by calibration artifacts or drift. These additions directly resolve the concern. revision: yes

Circularity Check

0 steps flagged

No circularity: purely descriptive experimental report with no derivations or self-referential predictions

full rationale

The paper contains no equations, derivations, fitted parameters presented as predictions, or load-bearing self-citations. It is a descriptive account of an instrument (CBP) and initial throughput measurements, with the central claim being an empirical demonstration of spatially resolved filter edge shifts. No step reduces by construction to its own inputs; the reader's assessment of score 1.0 is consistent with the absence of any mathematical chain. External calibration concerns (wavelength accuracy) affect correctness but do not constitute circularity under the defined criteria.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review is limited to the abstract; the ledger therefore contains only the minimal domain assumption stated in the text.

axioms (1)
  • domain assumption Projected sources have accurately known wavelength and flux.
    Abstract states the CBP projects 'monochromatic point sources of known wavelength and flux'.

pith-pipeline@v0.9.1-grok · 5712 in / 1168 out tokens · 23310 ms · 2026-07-03T21:59:31.214872+00:00 · methodology

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Reference graph

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