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arxiv: 2606.05782 · v1 · pith:ELEGBFPQnew · submitted 2026-06-04 · 🌌 astro-ph.IM

Two Fabry-Perots and two calibration units for CARMENES

Sebastian Schafer , Eike W. G\"unther , Ansgar Reiners , Johannes Winkler , Michael Pluto , J\"org Schiller This is my paper

Pith reviewed 2026-06-27 23:54 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords Fabry-Perotwavelength calibrationCARMENESspectrographetalonradial velocityinstrumentationM dwarfs
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The pith

Two temperature-stabilized Fabry-Pérot units supply dense line grids for CARMENES wavelength calibration and drift monitoring.

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

The paper presents the design of separate calibration units and Fabry-Pérot etalons for the visible and near-infrared arms of the CARMENES spectrograph. Each Fabry-Pérot is illuminated by a halogen-tungsten lamp and produces thousands of regularly spaced emission lines that are used both to establish the wavelength solution and to track instrumental drifts over a night. The units incorporate active temperature and pressure control, and the design targets an internal stability better than 10 cm/s per night. Hollow-cathode lamps in the same calibration units provide absolute reference lines. This combination is intended to support the high radial-velocity precision required for the CARMENES exoplanet survey of M dwarfs.

Core claim

The central claim is that two independently optimized Fabry-Pérot units, one with 15 GHz free spectral range for the 520-960 nm channel and one with 12.2 GHz for the 960-1710 nm channel, together with Th-Ne, U-Ar, U-Ne hollow-cathode lamps, furnish the wavelength calibration and nightly drift measurements for CARMENES, with the etalons engineered to maintain internal stability better than 10 cm/s through temperature and pressure stabilization.

What carries the argument

Temperature- and pressure-stabilized Fabry-Pérot etalons that generate ~17,900 useful lines in the visible and ~9,700 in the near-infrared for wavelength solution and drift tracking.

If this is right

  • The visible Fabry-Pérot supplies approximately 17,900 emission lines across 520-960 nm.
  • The near-infrared Fabry-Pérot supplies approximately 9,700 lines across 960-1710 nm.
  • Hollow-cathode lamps (Th-Ne, U-Ar, U-Ne) and flat-field lamps operate alongside each Fabry-Pérot for absolute calibration.
  • Separate units for each spectrograph arm allow independent optimization of line density and stabilization for the two wavelength ranges.

Where Pith is reading between the lines

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

  • The line densities imply that interpolation between calibration lines can be kept small enough to support sub-10 cm/s radial-velocity work.
  • Having independent calibration hardware for the visible and infrared channels reduces the risk that a single failure affects the entire instrument.
  • The same stabilization approach could be adapted to other dual-arm high-resolution spectrographs that require nightly drift monitoring at the 10 cm/s level.

Load-bearing premise

The temperature and pressure stabilization choices will in practice deliver the claimed internal stability of better than 10 cm/s per night.

What would settle it

Direct measurement of the residual nightly wavelength drift in the CARMENES spectrographs when using these Fabry-Pérot units, if the observed scatter exceeds 10 cm/s over multiple nights.

Figures

Figures reproduced from arXiv: 2606.05782 by Ansgar Reiners, Eike W. G\"unther, Johannes Winkler, J\"org Schiller, Michael Pluto, Sebastian Schafer.

Figure 1
Figure 1. Figure 1: Transmission spectrum of an ideal FP with [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The VIS FP, made by SLS Optics Ltd. We use two LOT halogen lamps with stabilzed power sources as light sources for the FPs. An additional shutter is placed between the lamp and a Thorlabs RC08SMA-P01 silver coated off axis parabolic mirror that is used to couple the light into the respective input fibers for the two FP units. The shutter is used to maintain a constant count rate on the CCD detector of the … view at source ↗
Figure 3
Figure 3. Figure 3: The optical bench (yellow), the Al block holding the FP (blue) and the o [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Both FP units (NIR in red and VIS in blue) assembled in the calibration room at Calar Alto with temperature [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Histogram over all pressure measurements in 2016 and 2017 of the VIS FP in log P [mbar]. The requirement to [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Histogram over all 5-minute running mean temperature measurements in 2016 and 2017 of the VIS FP. The [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Schematic layout of fiber-system of CARMENES. [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Drawing of the octagon with the lamps in the rods. At the centre of the octagon is the rotating mirror, which is [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Schematic layout of the calibration unit. [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: The two calibrations units on Calar Alto. [PITH_FULL_IMAGE:figures/full_fig_p011_10.png] view at source ↗
read the original abstract

The wavelength calibration and nightly drift measurements for CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs) are provided by a combination of hollow cathode lamps and two Fabry-P\'{e}rot units. CARMENES consists of two spectrograph, one for the visible part of the spectrum (520-960nm) and one for the near infrared (960-1710nm). Each spectrograph has its own calibration unit and its own Fabry-P\'{e}rot. The calibration units are equipped with Th-Ne, U-Ar and U-Ne hollow cathode lamps as well as a flat field lamp. The Fabry-P\'{e}rots are optimized for the wavelength ranges of the spectrographs and use halogen-tungsten lamps as light sources. The Fabry-P\'{e}rots have a free spectral range of 15GHz for the visible and 12.2GHz for the near infrared which translates to $\sim$17,900 useful emission lines for the visible spectrograph and $\sim$9,700 for the infrared. These lines are used to compute the wavelength solution, and to monitor the instrumental drift during the night. The Fabry-P\'{e}rot units are temperature and pressure stabilized and designed to reach an internal stability of better than 10\,cm/s per night. Here, we present the designs of both Fabry-P\'{e}rot units and the calibration units.

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 / 2 minor

Summary. The paper describes the design and implementation of two Fabry-Pérot etalons (one for the visible arm, one for the NIR arm) together with their dedicated calibration units for the CARMENES spectrograph. It specifies the free spectral ranges (15 GHz visible, 12.2 GHz NIR), the choice of halogen-tungsten lamps, the sets of hollow-cathode lamps (Th-Ne, U-Ar, U-Ne), and the temperature/pressure stabilization approach whose goal is an internal stability better than 10 cm/s per night for wavelength solution and drift monitoring.

Significance. If the described stabilization and optical choices deliver the stated performance, the work supplies a concrete, reusable reference for dense-line calibration systems on high-resolution echelle spectrographs. The explicit line counts (~17 900 visible, ~9 700 NIR) and the dual-arm architecture are useful benchmarks for similar instruments.

major comments (1)
  1. [Abstract and design sections] The central design claim is that the temperature/pressure stabilization will achieve <10 cm/s nightly internal stability, yet no quantitative thermal model, pressure-control specification, or error budget appears in the manuscript to show how the chosen hardware meets this target. This is load-bearing for the paper's purpose as a calibration-system design document.
minor comments (2)
  1. [Abstract] Abstract: 'two spectrograph' should read 'two spectrographs'.
  2. The manuscript would benefit from a short table summarizing the key parameters (FSR, line count, lamp types, stabilization method) for both arms side-by-side.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive report and for recognizing the potential utility of this work as a reference for dense-line calibration systems. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract and design sections] The central design claim is that the temperature/pressure stabilization will achieve <10 cm/s nightly internal stability, yet no quantitative thermal model, pressure-control specification, or error budget appears in the manuscript to show how the chosen hardware meets this target. This is load-bearing for the paper's purpose as a calibration-system design document.

    Authors: We agree that the manuscript would be strengthened by an explicit quantitative error budget. The current text states only that the units are 'designed to reach' the target stability; it does not derive this figure from the hardware parameters. In the revised version we will insert a new subsection (in the Fabry-Pérot design section) that supplies the thermal model, pressure-control tolerances, and error budget showing how the chosen stabilization hardware is expected to deliver <10 cm/s nightly internal stability. revision: yes

Circularity Check

0 steps flagged

No significant circularity; design description only

full rationale

The manuscript is a hardware design paper documenting optical, mechanical, and stabilization choices for Fabry-Pérot and calibration units. It contains no equations, no fitted parameters, no predictions derived from data, and no load-bearing self-citations or uniqueness theorems. The stability claim is explicitly a design target rather than a derived result, so no reduction to inputs occurs. This is a self-contained descriptive document with no derivation chain to inspect.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical model, free parameters, axioms, or new physical entities are introduced in the abstract.

pith-pipeline@v0.9.1-grok · 5821 in / 952 out tokens · 21496 ms · 2026-06-27T23:54:53.921634+00:00 · methodology

discussion (0)

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