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arxiv: 2606.01769 · v1 · pith:Q52WDIIJnew · submitted 2026-06-01 · 🌌 astro-ph.IM · astro-ph.EP

Design and development of Fabry-Perot based wavelength calibration system for PARAS-2 spectrograph

Shubhendra Nath Das , Kapil Kumar Bharadwaj , Abhijit Chakraborty , Kevikumar A. Lad , Jajaendra Siva Seshu Vera Prasad Neelam , Rishikesh Sharma , Nikitha Jithendran , Vishal Joshi This is my paper

Pith reviewed 2026-06-28 12:58 UTC · model grok-4.3

classification 🌌 astro-ph.IM astro-ph.EP
keywords Fabry-Perot etalonwavelength calibrationradial velocityPARAS-2 spectrographxenon arc lampinstrumental driftetalon stability
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The pith

Fabry-Perot etalon with xenon lamp achieves theoretical 10 cm/s wavelength stability for PARAS-2 spectrograph

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

The paper reports development of a Fabry-Perot etalon calibrator paired with a xenon arc lamp for the PARAS-2 spectrograph. It produces more than 10,000 stable lines across 62 echelle orders with free spectral range from 0.16 Å to 0.49 Å. Temperature is held to 0.002 °C RMS and pressure to 5×10^{-4} mbar RMS. These controls yield a calculated wavelength stability limit of 10 cm/s, presented as a lower-cost option than laser frequency combs. Measured radial-velocity drifts of 40-70 cm/s are ascribed to arc-lamp wandering.

Core claim

The central claim is that the Fabry-Perot etalon wavelength calibrator, when operated under the reported temperature and pressure control, reaches a theoretical stability within 10 cm/s and therefore constitutes a reliable alternative to laser frequency combs for high-resolution spectroscopic calibration.

What carries the argument

The Fabry-Perot etalon illuminated by a xenon arc lamp, which generates a dense comb-like spectrum whose line positions remain stable when temperature and pressure variations are limited to the measured levels.

If this is right

  • The system supplies a dense, uniformly spaced grid of lines for determining stellar line positions and tracking instrumental drifts.
  • It covers the full range of 62 echelle orders needed by the PARAS-2 spectrograph.
  • The observed 40-70 cm/s relative drifts are attributed to xenon arc-lamp instabilities rather than the etalon itself.
  • The design is positioned as a practical substitute for laser frequency combs in high-precision radial-velocity work.

Where Pith is reading between the lines

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

  • Mitigating the arc-lamp wandering could bring measured performance closer to the 10 cm/s theoretical floor.
  • The same environmental-control approach could be applied to other echelle spectrographs that need affordable dense-line calibration.
  • Systematic tests that isolate lamp output from etalon transmission would clarify the dominant error source.

Load-bearing premise

That temperature and pressure stability alone set the wavelength drift limit once the arc-lamp contribution is subtracted, with all other mechanical, optical, and illumination effects remaining negligible.

What would settle it

A long-term measurement of actual line-position drift exceeding 10 cm/s while temperature and pressure stay within the stated RMS bounds would falsify the stability claim.

Figures

Figures reproduced from arXiv: 2606.01769 by Abhijit Chakraborty, Jajaendra Siva Seshu Vera Prasad Neelam, Kapil Kumar Bharadwaj, Kevikumar A. Lad, Nikitha Jithendran, Rishikesh Sharma, Shubhendra Nath Das, Vishal Joshi.

Figure 1
Figure 1. Figure 1: Schematic of the FP cavity showing the incident angle ( [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Simulated transmission function of FP etalon for polychromatic light incident upon [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The spectra of the Xe arc lamp taken from the Newport website ( [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The transmission response function of the bandpass filter used in the Xe arc lamp [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: (a)Top view of the SolidWorks design of the Xe arc lamp assembly. (b) The top view of [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The Zemax optical design of the FP wavelength calibration system illustrates the [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: (a) The FWHM of each spectral line increases with the increasing diameter of the input [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: (a) Similar to Figure [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: (a)The fabricated aluminium housing (b) Vacuum enclouser for the FP wavelength [PITH_FULL_IMAGE:figures/full_fig_p012_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: CAD cross-sectional view of the FP system enclosed within the vacuum chamber [PITH_FULL_IMAGE:figures/full_fig_p013_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Variation in the spectral lines of the FP due to changes in pressure (left panel) [PITH_FULL_IMAGE:figures/full_fig_p014_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: RV variation due to (a) change in divergence angle and (b) change in decentering. [PITH_FULL_IMAGE:figures/full_fig_p014_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: The schematic of the FP system inside the temperature bath, controlled by the [PITH_FULL_IMAGE:figures/full_fig_p016_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: The FP vacuum enclosure (Figure [PITH_FULL_IMAGE:figures/full_fig_p017_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: The temperature stability data measured by the MicroK sensors on 20 March 2025 [PITH_FULL_IMAGE:figures/full_fig_p017_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: The vacuum stability data measured from just before starting the observation till [PITH_FULL_IMAGE:figures/full_fig_p018_16.png] view at source ↗
Figure 18
Figure 18. Figure 18: Finesse variation with wavelength measured using a 50 [PITH_FULL_IMAGE:figures/full_fig_p019_18.png] view at source ↗
Figure 17
Figure 17. Figure 17: A snippet of the FP spectra taken by PARAS-2 spectrograph by illuminating both [PITH_FULL_IMAGE:figures/full_fig_p019_17.png] view at source ↗
Figure 19
Figure 19. Figure 19: Instrumental drift measured with PARAS-2 using UAr (upper panel) and the FP [PITH_FULL_IMAGE:figures/full_fig_p021_19.png] view at source ↗
read the original abstract

Precise wavelength calibration is essential for high-precision radial velocity (RV) spectrographs, necessitating a stable calibrator that provides a dense grid of uniformly spaced lines to accurately determine stellar line positions and monitor instrumental drifts. In this work, we present the development of a cost-effective Fabry-Perot (FP) etalon-based wavelength calibrator designed to overcome the limitations of conventional sources such as hollow cathode lamps (HCLs) and iodine cells. This FP calibrator, combined with a Xenon (Xe) arc lamp assembly, has been integrated with the PARAS-2 spectrograph on the PRL 2.5m telescope at Mount Abu Observatory. Operated under controlled temperature and pressure conditions, the system generates a dense, comb-like spectrum covering 62 echelle orders with more than 10,000 well-defined and stable spectral lines, enabling precise measurement of instrumental drift. Initial results show that the free spectral range (FSR) varies from 0.16 \AA~near 4000 \AA~to 0.49 \AA~ near 7000 \AA, with a value of 0.3 \AA~around the central wavelength of 5500 \AA~. The estimated finesse ranges from 9 near 4000 \AA~to 19 near 6900 \AA, with an approximate value of 17 at 5500 \AA. The temperature and pressure stability tests demonstrate RMS variations of $0.002 ^\circ\mathrm{C}$ and $5\times10^{-4}$ mbar, respectively. Based on these values, the theoretical stability of the FP wavelength calibrator is estimated to be within 10 cm/s, establishing it as a reliable alternative to Laser Frequency Combs (LFCs) for high-resolution spectroscopic calibration. We present an initial assessment of the RV stability of the FP calibrator, yielding 40-70 cm/s of relative drifts, which are up for further investigations. The observed excess over the theoretically estimated limit is likely attributable to instabilities arising from arc wandering in the xenon arc lamp.

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

2 major / 1 minor

Summary. The paper describes the design and integration of a Fabry-Perot etalon wavelength calibrator paired with a Xe arc lamp for the PARAS-2 echelle spectrograph. It reports a dense grid of >10,000 lines across 62 orders with measured FSR (0.16–0.49 Å) and finesse (9–19), environmental control yielding 0.002 °C and 5×10^{-4} mbar RMS stability, and a derived theoretical RV stability limit of 10 cm/s that positions the system as a cost-effective alternative to LFCs. Observed relative drifts of 40–70 cm/s are reported and attributed to arc-lamp instabilities.

Significance. If the mapping from environmental RMS values to the 10 cm/s limit is placed on a firm, reproducible footing with a full error budget, the work supplies a practical, lower-cost calibration approach suitable for moderate-aperture RV instruments. The explicit reporting of line counts, FSR/finesse curves, and on-sky drift measurements provides concrete benchmarks that other groups can use when evaluating FP calibrators.

major comments (2)
  1. [Abstract] Abstract (stability estimate paragraph): The conversion of the measured temperature (0.002 °C RMS) and pressure (5×10^{-4} mbar RMS) into a theoretical RV stability of 10 cm/s is asserted without any derivation, partial-derivative chain, coefficient table, or uncertainty propagation. Because this number is the sole quantitative basis for claiming the FP system is a “reliable alternative to LFCs,” the absence of the intermediate steps (e.g., d(nL)/nL expressed in terms of ΔT and ΔP for the specific spacer and fill gas) renders the central performance claim unverifiable from the given data.
  2. [Abstract] Abstract (drift assessment paragraph): The observed 40–70 cm/s relative drifts are stated to exceed the 10 cm/s theoretical limit and are attributed entirely to Xe arc-lamp wandering, yet no quantitative error budget or residual analysis is supplied that isolates lamp contributions from other potential terms (mirror coating aging, mounting creep, illumination non-uniformity, detector effects). Without this separation, the claim that the FP etalon itself already meets the sub-10 cm/s target remains untested.
minor comments (1)
  1. [Abstract] The phrase “which are up for further investigations” is awkward; “which warrant further investigation” would be clearer.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which highlight areas where additional detail will strengthen the manuscript. We address each major comment below and will revise accordingly.

read point-by-point responses

  1. Referee: [Abstract] Abstract (stability estimate paragraph): The conversion of the measured temperature (0.002 °C RMS) and pressure (5×10^{-4} mbar RMS) into a theoretical RV stability of 10 cm/s is asserted without any derivation, partial-derivative chain, coefficient table, or uncertainty propagation. Because this number is the sole quantitative basis for claiming the FP system is a “reliable alternative to LFCs,” the absence of the intermediate steps (e.g., d(nL)/nL expressed in terms of ΔT and ΔP for the specific spacer and fill gas) renders the central performance claim unverifiable from the given data.

    Authors: We agree that the mapping from the measured RMS temperature and pressure stabilities to the quoted 10 cm/s theoretical RV limit lacks an explicit derivation in the current text. In the revised manuscript we will add a new subsection (or appendix) that presents the partial-derivative chain for the optical-path-length change d(nL)/nL, the relevant coefficients for the Zerodur spacer and air fill gas, the conversion to velocity units, and a basic uncertainty propagation. This will render the 10 cm/s figure reproducible from the reported environmental data. revision: yes

  2. Referee: [Abstract] Abstract (drift assessment paragraph): The observed 40–70 cm/s relative drifts are stated to exceed the 10 cm/s theoretical limit and are attributed entirely to Xe arc-lamp wandering, yet no quantitative error budget or residual analysis is supplied that isolates lamp contributions from other potential terms (mirror coating aging, mounting creep, illumination non-uniformity, detector effects). Without this separation, the claim that the FP etalon itself already meets the sub-10 cm/s target remains untested.

    Authors: We acknowledge that the present attribution of the 40–70 cm/s drifts rests on the known behavior of Xe arc lamps and does not yet include a full quantitative error budget that isolates every possible systematic term. The manuscript already describes these measurements as an “initial assessment” that is “up for further investigations.” In revision we will expand the relevant section with a preliminary error-budget table listing the dominant known contributions and will explicitly state that dedicated follow-up tests (e.g., with a more stable illumination source) are required to confirm that the etalon itself reaches the theoretical limit. revision: partial

Circularity Check

0 steps flagged

No circularity: stability estimate derived from independent environmental measurements without reduction to fit or self-citation

full rationale

The paper reports measured RMS temperature (0.002 °C) and pressure (5×10^{-4} mbar) stabilities, then states that these yield a theoretical FP stability within 10 cm/s. No equations, partial derivatives, or coefficient tables are supplied that would make the 10 cm/s figure tautological with the inputs. No self-citations, uniqueness theorems, or ansatzes are invoked to justify the mapping. The observed 40-70 cm/s drifts are separately attributed to the Xe arc lamp, leaving the theoretical limit as an external estimate rather than a redefinition of the same data. This satisfies the default expectation of a non-circular derivation chain.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the standard Fabry-Perot transmission formula relating cavity spacing, refractive index, and wavelength, plus the assumption that environmental control directly limits wavelength drift without additional optical or mechanical terms.

axioms (2)
  • standard math Fabry-Perot transmission peaks occur at wavelengths satisfying the standard etalon equation mλ = 2nd cosθ
    Invoked implicitly when FSR and finesse are calculated from observed line spacing.
  • domain assumption Wavelength drift is linearly proportional to changes in cavity optical path length induced by temperature and pressure
    Used to convert the reported 0.002 °C and 5×10^{-4} mbar RMS values into the 10 cm/s stability figure.

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Reference graph

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