{"id":"3a8e4ee7-1160-445d-838e-d6c30759fd0f","arxiv_id":"2608.13257","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Optical simulations of the BISOU balloon FTS show spillover below 7.5% and beam pointing stable to within 0.1 degrees across a 20 mm range of mirror motion.","lead":"BISOU is a proposed balloon-borne instrument to measure tiny distortions in the cosmic microwave background spectrum. This paper models its optical system with ray tracing and Gaussian beam simulations, finding low light loss and stable beams when the interferometer mirrors move.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The headline performance numbers are computed with a monomode Gaussian feed (9 mm waist at 90 GHz), but BISOU will use multimode feedhorns; if the real horn pattern differs, the spillover and beam-stability claims are not yet supported.","rationale":"The reader's weakest assumption is the monomode Gaussian feed model, and I agree that this is the load-bearing point because every headline number in the paper is a direct output of that source model. The concern is not that the optical engineering is outside consensus; it is that the model-to-instrument gap is explicitly acknowledged in Section 3.2 and the conclusion, yet all quantitative conclusions currently rest on it. The paper also provides no Zemax/GRASP files, no measured feedhorn pattern, and no breadboard data, so the quoted margins cannot be independently checked at present. The Section 2 OPD/stroke factor-of-two inconsistency is real, but I treat it as secondary because a corrected stroke would still be smaller than the ±10 mm simulated range; it mainly undermines the 'four times required stroke' margin statement. A multimode feed simulation is a decisive and relatively inexpensive check: if the multimode pattern preserves the edge taper and spillover, the design claim stands; if not, the headline numbers would need revision. I therefore keep the reader's CONDITIONAL verdict unchanged.","tokens_in":6776,"tokens_out":6833,"duration_ms":74990,"concrete_test":"Run a GRASP physical-optics simulation of the same optical train with a multimode feed model for the actual horn, for example a modal expansion of the horn aperture with the number of propagating modes set by the aperture area at 90 GHz, and recompute the Section 4.1 total spillover and Section 4.2 beam peak direction and directivity at the nominal and ±10 mm mirror positions. If the total spillover exceeds the quoted 7.5% (or the extreme-position loss changes materially) or if depointing or directivity loss exceeds 0.1° / 0.5 dB, the monomode-based central claim fails; a measured room-temperature feedhorn pattern inserted into the same simulation would provide an empirical cross-check.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 3.2 states that although BISOU will operate with multimode feedhorns, the first modelling treats them as monomode Gaussian beams with a 9 mm waist at 90 GHz, the lowest and least favourable frequency. All quantitative claims in Section 4 — <0.2% FTS spillover, <7.5% total spillover, <±0.1° depointing, <0.5 dB directivity loss — are outputs of GRASP simulations using that single Gaussian source. A multimode horn excites a set of waveguide modes whose combined radiation pattern is generally wider or differently shaped than the fundamental Gaussian; higher-order modes change the edge taper on the FTS mirrors and the primary mirror, so both the absolute spillover and its variation with mirror displacement can differ from the quoted values. The paper itself defers modelling of a more realistic multimode pattern to future work, so the central performance claim is not yet established for the actual instrument. A secondary internal inconsistency also appears in Section 2: the text states OPD = 4z but derives a required stroke of ±2.5 mm from OPDmax = 2 cm, whereas 4 × 2.5 mm = 10 mm, corresponding to OPDmax = 1 cm and Δν = 30 GHz, not the stated 15 GHz. This does not directly invalidate the optical performance claims, but it weakens the 'four times the required stroke' margin used to frame the ±10 mm simulations.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents an optical concept model for BISOU, a balloon-borne polarizing Fourier-transform spectrometer for CMB spectral-distortion measurements. The authors describe a five-mirror FTS fed by a Mizugushi-Dragone telescope, use Zemax ray tracing for the layout, and then use GRASP physical-optics simulations with a 9 mm-waist Gaussian beam at 90 GHz to evaluate spillover and beam stability as the FTS moving mirrors are displaced. The headline results are that FTS spillover is below 0.2% and total system spillover below 7.5% at the nominal mirror position, and that at ±10 mm stroke the beam stays within ±0.1° of the nominal axis with a maximum directivity loss of 0.5 dB relative to the nominal position. The paper concludes that the optical concept is promising and defers multimode-feedhorn modelling and breadboard measurements to future work.","tokens_in":7036,"tokens_out":8533,"duration_ms":85690,"significance":"The study is a useful preliminary validation of the BISOU optical layout and an honest account of a modelling chain that can be extended. Its main strengths are the clear statement of the modelling assumptions, the use of standard simulation tools, and the explicit admission that the multimode feedhorns of the real instrument are not yet modelled. If the quoted spillover and beam-stability numbers survive a realistic multimode feed treatment, they support the feasibility of the proposed optical concept; as they stand, they are conditional single-mode estimates rather than final performance predictions. The work is appropriate for a conference/proceedings audience, but the quantitative conclusions need to be either re-scoped or backed by additional modelling before they can be used as instrument-level requirements.","major_comments":[{"comment":"Section 2 states that a mirror displacement z produces an optical path difference of δ=4z and that, for the target resolution Δν=15 GHz, the required OPDmax=2 cm corresponds to a total mirror stroke of ±2.5 mm. This is inconsistent: if δ=4z, a 2 cm OPD requires z=5 mm, i.e. a ±5 mm stroke, while ±2.5 mm gives OPDmax=1 cm and Δν=30 GHz. The subsequent statement that the explored ±10 mm stroke is 'four times the stroke required' therefore needs to be re-derived; as written the explored stroke is either four times a 2.5 mm stroke (with an OPD consistent with 30 GHz resolution) or twice a 5 mm stroke (with the stated δ=4z relation). Please correct the OPD relation, the stroke value, or the resolution statement.","section":"Section 2, Eqs. (1)-(2)"},{"comment":"The quantitative performance claims in Sections 4.1 and 4.2 — <0.2% FTS spillover, <7.5% total spillover, <±0.1° depointing, and <0.5 dB directivity loss — are all computed with a monomode Gaussian feed of 9 mm waist at 90 GHz, while the instrument is designed to use multimode feedhorns. As the paper acknowledges in Section 3.2, the real horn pattern will differ; multimode patterns are typically wider and frequency-dependent, so the edge tapers, spillover, and beam-wander behaviour could change. The current wording of Section 4.2 presents these numbers as properties of the optical system rather than of the Gaussian model used. I would like to see either a multimode-feed simulation, or a clear re-framing of Section 4 as a preliminary single-mode study with a sensitivity analysis (e.g., varying the waist or edge taper) demonstrating that the conclusions are robust. This is load-bearing because the central claim is that the BISOU optical concept meets its performance targets.","section":"Sections 3.2 and 4"}],"minor_comments":[{"comment":"The symbol δ is used both for the optical path difference (δ=4z) and for the sampling interval in Eq. (2); please use a different symbol for one of them to avoid ambiguity.","section":"Section 2"},{"comment":"The statement that a -35 dB edge taper 'ensures that 98% of the beam power is contained' appears inconsistent with the standard Gaussian-beam calculation, which gives well above 99.9% contained power for a -35 dB power edge taper; please check the value or the definition of edge taper used.","section":"Section 3.2"},{"comment":"The maximum directivity loss is quoted only for the +10 mm position; for completeness, give the corresponding value at -10 mm or state explicitly that it is the same within the quoted tolerance.","section":"Section 4.2"},{"comment":"The phrase 'power loss through the FTS (feed to M1)' is ambiguous; please specify the exact optical path segment and the emission/reception convention used in the simulation.","section":"Section 4.1"}],"recommendation":"major_revision","confidential_remarks":"The requested revisions are within the normal scope of a revision; I see no deeper novelty or integrity problem. The paper is a workmanlike preliminary design study, and the main issue is the need to reconcile the strength of the conclusions with the acknowledged single-mode Gaussian approximation."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's the short version: this is a serviceable optical concept study for the BISOU balloon FTS, useful for the Phase A and for instrument colleagues, but it is not a scientific breakthrough. The specific configuration is new for BISOU: a five-mirror FTS with a parabolic M5 to collimate on the dichroic, a Mizugushi-Dragone Gregorian telescope to kill cross-polarization, and a dichroic split of the band. Those choices are sensible and the paper explains them clearly. The GRASP physical-optics work at 90 GHz is appropriate for the diffraction-limited regime, and the reported beam stability (±0.1° pointing, <0.5 dB directivity loss across ±10 mm stroke) is encouraging, with a noted asymmetry that is worth following up.\n\nThe soft spots are two. First, Section 2 has a clean arithmetic error: the text says OPD = 4z, but then claims a 2 cm OPD needs only ±2.5 mm stroke. With OPD = 4z, ±2.5 mm gives 1 cm OPD and Δν = 30 GHz, not the stated 15 GHz. The \"four times the required stroke\" framing for ±10 mm is off by a factor of two — it's either two times the correctly computed stroke or four times the incorrect one. That does not invalidate the optical results, but it is precisely the kind of error that has to be corrected before design decisions are locked in.\n\nThe more substantive concern is the monomode Gaussian feed. The paper is honest that BISOU will use multimode feedhorns and that they model them as a single Gaussian with a 9 mm waist at 90 GHz. All the headline numbers — <0.2% FTS spillover, <7.5% total, <±0.1°, <0.5 dB — are outputs of that idealized source. Multimode horns generally produce broader or differently shaped patterns, and higher-order modes change the edge taper on M1 and the primary, so the absolute spillover numbers are not yet the instrument's numbers. The authors flag this and promise future work, which is fair, but it means the central quantitative claims are conditional.\n\nThe citation pattern seems honest, drawing on PIXIE and the earlier BISOU design paper. No code or optical prescription is included, so independent reproduction is limited, though the method is standard.\n\nOverall, this is a paper for instrument people, not cosmologists. It deserves a serious referee — it is not a desk reject — but the referee should require the OPD fix and at least a note (or better, a quick multimode sensitivity run) before accepting the numbers as design drivers.","headline":"Workmanlike optical design study for the BISOU balloon FTS; numbers are plausible but the monomode Gaussian approximation and a factor-two OPD error need attention.","tokens_in":7621,"tokens_out":4303,"would_cite":false,"duration_ms":39403,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Simulations show BISOU's five-mirror Fourier-transform spectrometer keeps optical losses below 7.5% and beam pointing stable within ±0.1° across a four-times-overdrive mirror scan.","keywords":["BISOU","CMB spectral distortions","Fourier Transform Spectrometer","Gaussian beam analysis","physical optics","spillover","balloon-borne instrument","optical design"],"falsifier":"Measure the 90 GHz far-field pattern and per-mirror spillover of the actual multimode feedhorn in the cryogenic breadboard, and compare with the predicted under-0.2% FTS loss, under-7.5% total loss, ±0.1° depointing, and ≤0.5 dB directivity variation over a ±10 mm stroke; if the measured losses or beam shifts exceed these bounds, the monomode-Gaussian assumption is the point of failure.","tokens_in":6548,"feed_emoji":"🎈","tokens_out":8212,"duration_ms":80174,"temperature":0.7,"pith_summary":"BISOU is a balloon-borne Fourier-transform spectrometer built to detect tiny deviations of the cosmic microwave background's spectrum from a perfect blackbody. This paper establishes, through Gaussian-beam and physical-optics simulations, that the proposed five-mirror optical chain is efficient and stable enough for that goal. At 90 GHz, the lowest and most diffractive operating frequency, spillover inside the interferometer stays below 0.2% and the full optical system, interferometer plus telescope, loses less than 7.5% of the power. When the scan mirrors are pushed to ±10 mm, four times the stroke required for the target resolution, the beam stays centred within ±0.1° and keeps its shape, with at most 0.5 dB of directivity loss. These numbers are what make the instrument's sensitivity target plausible.","feed_headline":"Balloon FTS optical model keeps losses under 7.5%","feed_subtitle":"Moving mirrors shift the beam by at most 0.1 degrees, a step toward measuring CMB spectral distortions.","key_machinery":"The load-bearing tool is Gaussian-beam propagation: the feedhorn is modelled as a monomode Gaussian source with a 9 mm waist at 90 GHz, and the beam envelope is carried through the optical system with physical optics. The edge taper, the chosen attenuation at each mirror's edge, is the lever that sets spillover, with −35 dB demanded on FTS mirrors to contain 98% of the beam power and −20 dB allowed on the primary to keep that mirror reasonably small. The moving-mirror scan is then tested by recomputing the propagated beam at the extremes of stroke; the Mizugushi-Dragone telescope geometry is what suppresses the cross-polarisation and astigmatism that such offsets would otherwise excite.","core_discovery":"The central claim is that the BISOU optical concept, a five-mirror polarising Fourier-transform spectrometer with a direct view of the internal 2.7 K reference and a Gregorian telescope tuned to the Mizugushi-Dragone condition, performs acceptably at the low end of its band. The 90 GHz Gaussian beam, launched from a horn with a 9 mm waist, is kept at a −35 dB edge taper on FTS mirrors and −20 dB on the primary, so diffraction and spillover stay under control: the FTS loses under 0.2% of the power up to its first mirror and the entire chain loses under 7.5%. As the moving mirrors travel through a ±10 mm stroke, the peak beam direction moves by less than ±0.1°, the beam remains Gaussian, and directivity drops by no more than 0.5 dB at the +10 mm extreme. Because the science signal is a part-per-million-level deviation in sky brightness, a configuration whose losses are small, stable, and mostly independent of mirror position makes the measurement feasible.","pith_inferences":["If the real multimode horn has a broader radiation pattern, the 7.5% spillover budget may grow; replacing the Gaussian source with a measured multimode pattern in the same physical-optics setup is a direct test of whether the mirrors stay large enough.","The ±0.1° pointing shift and 0.5 dB directivity swing over the scan can be turned into an empirical calibration: a short preflight scan could map beam centroid versus mirror position, letting the data pipeline remove scan-synchronous gain modulations.","The asymmetry between positive and negative strokes hints at a residual third-order aberration from the mirror surfaces; if it appears in breadboard measurements, an asymmetric tolerance or a slight refocus could cancel it."],"forward_implications":["At the nominal ±2.5 mm stroke needed for the 15 GHz resolution, the spillover and pointing effects will be even smaller than the simulated extremes, so the design has margin.","The five-mirror FTS with the reference imaged by the first mirror reduces optical mass while preserving the interferometric signal, which helps fit the instrument inside a standard balloon gondola.","A beam whose direction changes by less than 0.1° across a scan provides a well-defined line of sight that can be calibrated in post-processing, so mirror motion should not dominate systematic errors.","The small but visible asymmetry between the +10 mm and −10 mm spillover paths shows where future work on mirror shapes and alignment tolerances should focus."],"supporting_citations":[{"why":"It supplies the PIXIE FTS architecture and mission design that BISOU adapts to a balloon platform.","marker":"[3]"},{"why":"It defines BISOU's science goals and instrument constraints that the optical concept must satisfy.","marker":"[5]"},{"why":"It introduces the polarising Martin-Puplett interferometer that forms the core of the BISOU FTS.","marker":"[7]"},{"why":"It provides the ray-tracing tool used to choose mirror geometries and fix the overall layout.","marker":"[10]"},{"why":"It provides the physical-optics solver used to compute Gaussian-beam propagation, spillover, and far-field patterns.","marker":"[11]"},{"why":"It gives the offset two-mirror condition used to design the telescope and suppress cross-polarisation and astigmatism.","marker":"[14]"},{"why":"It supplies the Gaussian-beam quasioptical formalism behind the edge-taper and beam-size calculations.","marker":"[15]"}],"fun_headline_variants":["BISOU balloon FTS: low losses, stable beam","CMB distortion probe keeps losses under 7.5%","Balloon FTS design: beam moves <0.1 degrees","BISOU optics survive 10 mm mirror stroke"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"Every quantitative result rests on treating the instrument's multimode feedhorns as monomode Gaussian beams with a fixed 9 mm waist at 90 GHz, so a real horn whose radiation pattern differs could change the spillover, beam shape, and pointing numbers.","fun_headline_variants_meta":{"raw":{"variants":["BISOU balloon FTS: low losses, stable beam","CMB distortion probe keeps losses under 7.5%","Balloon FTS design: beam moves <0.1 degrees","BISOU optics survive 10 mm mirror stroke"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000229,"raw_usage":{"total_tokens":1461,"prompt_tokens":909,"completion_tokens":552,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":525,"completion_tokens_details":{"reasoning_tokens":482}},"tokens_in":525,"tokens_out":552,"duration_ms":6150,"temperature":1.0,"reasoning_tokens":482,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T15:13:27.260827+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the 90 GHz far-field pattern and per-mirror spillover of the actual multimode feedhorn in the cryogenic breadboard, and compare with the predicted under-0.2% FTS loss, under-7.5% total loss, ±0.1° depointing, and ≤0.5 dB directivity variation over a ±10 mm stroke; if the measured losses or beam shifts exceed these bounds, the monomode-Gaussian assumption is the point of failure.","supporting_citations":[{"cited_title":"The Primordial Inflation Explorer (PIXIE): mission design and science goals,","cited_arxiv_id":null,"evidence_quote":"It supplies the PIXIE FTS architecture and mission design that BISOU adapts to a balloon platform."},{"cited_title":"BISOU: a balloon project for spectral observations of the early universe,","cited_arxiv_id":null,"evidence_quote":"It defines BISOU's science goals and instrument constraints that the optical concept must satisfy."},{"cited_title":"Polarised interferometric spectrometry for the millimeter and submillimeter spectrum.,","cited_arxiv_id":null,"evidence_quote":"It introduces the polarising Martin-Puplett interferometer that forms the core of the BISOU FTS."},{"cited_title":"Ansys zemax opticstudio,","cited_arxiv_id":null,"evidence_quote":"It provides the ray-tracing tool used to choose mirror geometries and fix the overall layout."},{"cited_title":"Grasp software,","cited_arxiv_id":null,"evidence_quote":"It provides the physical-optics solver used to compute Gaussian-beam propagation, spillover, and far-field patterns."},{"cited_title":"Offset multireflector antennas with perfect pattern symmetry and polarization discrimina- tion,","cited_arxiv_id":null,"evidence_quote":"It gives the offset two-mirror condition used to design the telescope and suppress cross-polarisation and astigmatism."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It supplies the Gaussian-beam quasioptical formalism behind the edge-taper and beam-size calculations."}],"review_version":1}