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arxiv: 2606.31822 · v1 · 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 Souverina , Christopher W. Stubbs
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Elana Urbach
This is my paper

Pith reviewed 2026-07-01 03:06 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords Collimated Beam ProjectorLSSTphotometric calibrationfilter transmissionthroughput measurementbandpass shiftsVera C. Rubin Observatory
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The pith

The Collimated Beam Projector maps filter bandpass edge shifts of several nanometers 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 Vera C. Rubin Observatory LSST needs precise photometric calibration for cosmological analyses such as those using Type Ia supernovae. The Collimated Beam Projector projects monochromatic point sources of known wavelength and flux directly into the telescope aperture to measure full system throughput in situ. Early results demonstrate that this device can characterize the instrumental response and map how LSSTCam broadband filter transmission profiles shift across the focal plane. These shifts reach several nanometers and depend on the ray angle of incidence. The work establishes the projector as a tool for ongoing throughput monitoring during operations.

Core claim

The Collimated Beam Projector enables direct in situ measurements of the Rubin Telescope instrumental response and the transmission profiles of LSSTCam broadband filters. In particular it 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.

What carries the argument

The Collimated Beam Projector, which projects monochromatic point sources of known wavelength and flux into the telescope aperture to measure full system throughput.

If this is right

  • Enables continuous throughput monitoring throughout LSST operations.
  • Supports the precise photometric calibration required for cosmological analyses based on Type Ia supernovae.
  • Characterizes full system throughput including effects from varying ray angle of incidence across the focal plane.
  • Provides spatially resolved transmission profiles of the broadband filters.

Where Pith is reading between the lines

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

  • The same projector approach could be applied to other wide-field telescopes to quantify similar angle-dependent filter effects.
  • Correcting for the mapped shifts may reduce systematic errors in derived supernova distances at the level needed for dark energy constraints.
  • The spatial variation data could be used to test filter manufacturing uniformity across large substrates.

Load-bearing premise

The projected monochromatic sources have flux and wavelength known to sufficient accuracy that any measured deviations can be attributed to the telescope and camera rather than to the projector itself.

What would settle it

Independent calibration of the same filter positions using standard star observations that yields bandpass edge positions differing from the CBP map by more than the expected uncertainty.

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 Souverina.

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 / 0 minor

Summary. The manuscript presents early results from the Collimated Beam Projector (CBP) at the Vera C. Rubin Observatory. It claims that the CBP enables direct in situ measurements of full-system throughput by projecting monochromatic point sources of known wavelength and flux into the telescope aperture, and specifically demonstrates the capability to measure LSSTCam broadband filter transmission profiles with spatially resolved mapping of bandpass edge shifts across the focal plane that vary by several nanometers as a function of ray angle of incidence.

Significance. If the calibration accuracy and repeatability claims hold with supporting quantitative data, the CBP would represent a valuable addition to photometric calibration infrastructure for LSST, directly addressing needs for precise throughput knowledge in cosmological analyses. The in-situ projection approach offers a route to continuous monitoring that is not currently available.

major comments (1)
  1. [Abstract] Abstract: The claim that bandpass edge shifts of several nanometers can be mapped and attributed to angle-of-incidence effects in the LSSTCam filters is load-bearing for the central result. This attribution requires that the CBP monochromatic sources have wavelength and flux known to substantially better than ~1 nm (with corresponding flux precision). No error budget, stability measurements, repeatability tests, or cross-check against an independent wavelength reference is supplied to establish this margin, so measured deviations cannot yet be cleanly separated from projector systematics.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for highlighting the importance of a quantitative error budget to support the central claims. We address the single major comment below and agree that additional material is needed.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that bandpass edge shifts of several nanometers can be mapped and attributed to angle-of-incidence effects in the LSSTCam filters is load-bearing for the central result. This attribution requires that the CBP monochromatic sources have wavelength and flux known to substantially better than ~1 nm (with corresponding flux precision). No error budget, stability measurements, repeatability tests, or cross-check against an independent wavelength reference is supplied to establish this margin, so measured deviations cannot yet be cleanly separated from projector systematics.

    Authors: We agree that the manuscript as submitted lacks a dedicated error budget, stability measurements, repeatability tests, and independent cross-checks for the CBP source wavelength and flux. This omission limits the strength of the attribution to angle-of-incidence effects. In the revised version we will add a new section (approximately 1–2 pages) that presents the wavelength calibration chain, measured source stability over multiple nights, repeatability across repeated scans, and any available cross-checks against a laboratory spectrometer or known atomic lines. These additions will quantify the achieved precision and allow readers to evaluate whether the several-nanometer shifts can be separated from projector systematics. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental results with no derivations

full rationale

The paper reports initial experimental measurements of telescope throughput and filter transmission using the CBP instrument. No equations, derivations, fitted parameters, or self-citations are invoked to support load-bearing claims. The mapping of bandpass shifts is presented as a direct observational capability, not as a prediction derived from prior equations or ansatzes. The reader's assessment of score 1.0 aligns with this; the skeptic's concern addresses calibration accuracy (a correctness issue) rather than any reduction of results to self-defined inputs. The work is self-contained as an empirical demonstration.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is an experimental instrumentation report. No mathematical derivations or new physical models are present.

axioms (1)
  • domain assumption Projected sources have accurately known wavelength and flux.
    Stated directly in the abstract as the basis for throughput measurements.

pith-pipeline@v0.9.1-grok · 5712 in / 1209 out tokens · 43486 ms · 2026-07-01T03:06:27.294388+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

16 extracted references

  1. [1]

    The dark energy survey: Cosmology results with 1500 new high-redshift type ia supernovae using the full 5 yr data set,

    Abbott, D. C. T. et al., “The dark energy survey: Cosmology results with 1500 new high-redshift type ia supernovae using the full 5 yr data set,”The Astrophysical Journal Letters973(2024)

    2024

  2. [2]

    The pantheon+ analysis: Cosmological constraints,

    Brout, D. et al., “The pantheon+ analysis: Cosmological constraints,”The Astrophysical Journal938 (2022)

    2022

  3. [3]

    Measuring the growth rate of structure with type ia supernovae from lsst,

    Howlett, C., Robotham, A. S. G., Lagos, C. D. P., and Kim, A. G., “Measuring the growth rate of structure with type ia supernovae from lsst,”The Astrophysical Journal847, 128 (September 2017)

    2017

  4. [4]

    Improved cosmological constraints from a joint analysis of the sdss-ii and snls supernova samples,

    Betoule, M. et al., “Improved cosmological constraints from a joint analysis of the sdss-ii and snls supernova samples,”Astronomy and Astrophysics568, A22 (2014)

    2014

  5. [5]

    Precise astronomical flux calibration and its impact on studying the nature of the dark energy,

    Stubbs, C. W. and Brown, Y. J., “Precise astronomical flux calibration and its impact on studying the nature of the dark energy,”Modern Physics Letters A30(40), 1530030 (2015)

    2015

  6. [6]

    Decal: A spectrophotometric calibration system for decam,

    Marshall, J. L. et al., “Decal: A spectrophotometric calibration system for decam,” (2013)

    2013

  7. [7]

    Faint white dwarf flux standards: Data and models,

    Bohlin, R. C. et al., “Faint white dwarf flux standards: Data and models,”Astronomical Journal169 (January 2025)

    2025

  8. [8]

    Techniques and review of absolute flux calibration from the ultraviolet to the mid-infrared,

    Bohlin, R. C., Gordon, K. D., and Tremblay, P.-E., “Techniques and review of absolute flux calibration from the ultraviolet to the mid-infrared,”Publications of the Astronomical Society of the Pacific126(942), 711–732 (2014)

    2014

  9. [9]

    The lsst calibration hardware system design and development,

    Ingraham, P. et al., “The lsst calibration hardware system design and development,” 99060O (2016)

    2016

  10. [10]

    Toward 1% photometry: End-to-end calibration of astronomical telescopes and detectors,

    Stubbs, C. W. and Tonry, J. L., “Toward 1% photometry: End-to-end calibration of astronomical telescopes and detectors,”The Astrophysical Journal646(2), 1436–1444 (2006)

    2006

  11. [11]

    Preliminary results from detector-based throughput calibration of the ctio mosaic im- ager and blanco telescope using a tunable laser,

    Stubbs, C. et al., “Preliminary results from detector-based throughput calibration of the ctio mosaic im- ager and blanco telescope using a tunable laser,” in [The Future of Photometric, Spectrophotometric and Polarimetric Standardization],364, 373 (2007)

    2007

  12. [12]

    Precise throughput determination of the panstarrs telescope and the gigapixel im- ager using a calibrated silicon photodiode and a tunable laser: initial results,

    Stubbs, C. W. et al., “Precise throughput determination of the panstarrs telescope and the gigapixel im- ager using a calibrated silicon photodiode and a tunable laser: initial results,”The Astrophysical Journal Supplement Series191(2), 376–388 (2010)

    2010

  13. [13]

    Testing of the lsst’s photometric calibration strategy at the ctio 0.9 meter telescope,

    Coughlin, M. W. et al., “Testing of the lsst’s photometric calibration strategy at the ctio 0.9 meter telescope,” (2018)

    2018

  14. [14]

    Stardice iii: Characterization of the photometric instrument with a collimated beam projector,

    Souverin, T. et al., “Stardice iii: Characterization of the photometric instrument with a collimated beam projector,” (2024)

    2024

  15. [15]

    Mitigation of the brighter-fatter effect in the lsst camera,

    Broughton, A. et al., “Mitigation of the brighter-fatter effect in the lsst camera,”Publications of the Astro- nomical Society of the Pacific136, 045003 (apr 2024)

    2024

  16. [16]

    E., Kirkby, D., and Thomas, D., “batoid.” [Computer Software]https://doi.org/10.11578/ dc.20200708.1(oct 2019)

    Meyers, J. E., Kirkby, D., and Thomas, D., “batoid.” [Computer Software]https://doi.org/10.11578/ dc.20200708.1(oct 2019). 13

    2019