REVIEW 3 major objections 5 minor 2 references
The MICADO first light imager for the ELT: Testing the Lyot coronograph prototypes
T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The three classical Lyot coronagraph prototypes for MICADO behave as designed: measured performance is close to simulation, the flex-blade mounts survive thermal cycling, and the components pass their optical specifications.
desk verdict A solid, honestly-scoped status report on MICADO's coronagraph prototypes; the new bench data are useful, but the validation is internal to the team's own model and lacks error bars. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the classical Lyot coronagraph: an opaque TiN occulting spot deposited at the center of an IR fused-silica substrate and paired with a Lyot stop in a downstream pupil plane, with the focal-plane substrate held in a flex-blade aluminum mount. Three spot diameters produce the three coronagraphs (99.5, 168, and 336.1 micrometers, corresponding to 15, 25, and 50 milliarcsecond inner working angles). The validation is carried by an infrared testbench that reproduces the MICADO interface—an ELT-like pupil with central obscuration and segment gaps, an entrance focal ratio F/D of 17.75, the expected Lyot-stop filtering, and J/H/K filters ahead of an IR camera—so that recorded and simulated images can be compared directly. The comparison of measured throughput curves and images with the simulation pipeline is what carries the claim that the prototypes meet design specifications.
What would settle it
Re-measure CLC15's J-band throughput after proper focusing; if the deviation from simulation remains larger than the bench's uncertainty, the 'close to simulated' claim is falsified. Likewise, the upcoming cold opto-mechanical tests (tasks M3/O4) falsify the mount-validation claim if they show permanent flex-blade deformation or substrate aberrations exceeding the 12 nm RMS measured warm.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that the three CLC prototypes—CLC15, CLC25, and CLC50—perform close to their numerical simulations, and that the supporting hardware behaves as specified. The occulting spots, made of a 300 nm titanium-nitride layer on IR fused silica, reach an optical density above 4, the required value for blocking starlight in the J, H, and K bands. The flex-blade aluminum mount, designed to accommodate the thermal expansion mismatch between aluminum and fused silica, returns to its original position after repeated cycling between 323 K and 80 K to within the few-micrometer precision of the measurement. Measured throughput as a function of angular separation agrees with simulation for CLC25 and CLC50; CLC15 shows a discrepancy in J band that the authors attribute to an improper focusing of that component during the test, to be rechecked after refocusing.
Load-bearing premise
The load-bearing premise is that the testbench's synthetic ELT pupil and the simulation pipeline faithfully represent the real MICADO/ELT interface, so that agreement between measured and simulated throughput would also hold on sky.
Editorial extensions
If this is right
- The three CLCs are cleared to move from prototype validation to fabrication of the final mounts and optical elements, with final verifications planned for March 2025.
- MICADO's high-contrast mode will provide stellar occultation at 15, 25, and 50 milliarcsecond inner working angles across the J, H, and K bands once the coronagraphs are installed.
- CLC15's J-band discrepancy, attributed to improper focusing, should be resolved by a refocused retest rather than by a change to the mask design.
- The infrared testbench can serve the remaining high-contrast components and the final component verification.
Reading between the lines
- If the bench and simulation are faithful to the real ELT pupil, the measured agreement implies the on-sky coronagraphic images will be close to simulated, but unmodeled effects such as segment phasing errors, tip-tilt jitter, or atmospheric dispersion could still degrade contrast; a testable extension is to inject those perturbations into the bench.
- If the upcoming cold measurements keep substrate aberrations at or below the 12 nm RMS warm value, the flex-blade mounting concept could be reused for other cryogenic substrates in MICADO's wheels.
- The CLC15 result is the main open thread: if refocusing fails to recover agreement, the smallest mask is sensitive to alignment in a way the current tolerance budget may not capture.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports the development status of the three Classical Lyot Coronagraph (CLC) focal-plane masks planned for MICADO on the ELT. It summarizes the high-contrast mode architecture, describes the CLC design (TiN occulting spots on fused silica substrates held in flex-blade aluminum mounts), and presents prototype testing: mechanical warm/cold cycling, substrate optical quality measurements, TiN optical density versus thickness, and initial coronagraphic throughput measurements on an infrared testbench that mimics the ELT pupil. The authors conclude that the mechanical mount behaves as expected at warm and cold temperature and that the first measured performance is close to simulated performance, with cold opto-mechanical tests, CLC15 refocusing, and final component verification still pending.
Significance. If substantiated, these results are valuable for the MICADO schedule and for the coronagraph community: they provide concrete evidence that the flex-blade mount survives thermal cycling without permanent deformation, that a 300 nm TiN layer reaches optical density 4 in J/H/K, that the mounted substrate stays at 12 nm RMS surface quality, and that CLC25 and CLC50 throughput curves broadly follow the MISTHIC model. The explicit validation plan in Tables 3 and 4, the reproducible measurement descriptions, and the inclusion of raw comparison images are strengths. The main limitation is that the optical validation is an internal model comparison on a dedicated bench, so the current evidence supports a status report rather than an end-to-end certification of on-sky coronagraphic performance.
major comments (3)
- [Section 4, Performance validation (Figures 8 and 9)] The claim that the measured throughput values 'fit well the theoretical curves' is supported only by visual inspection: Figure 8 has no error bars, no residual plot, and no quantitative fit statistic, and no acceptance criterion is stated. Because this agreement is the central evidence for the paper's main conclusion, the authors should provide either pointwise error bars with a residual metric or a clear pass/fail tolerance against which the curves are judged.
- [Section 4, IR testbench description] The bench is described only as simulating the ELT pupil, but the paper gives no quantitative specification or measurement of that pupil: central obscuration diameter, spider width and orientation, segment gap width, number of segments, or the position tolerance of the pupil mask. This matters quantitatively for CLC15, whose 15 mas inner working angle makes the coronagraphic throughput sensitive to diffraction from segment gaps and spiders; an unquantified mismatch between the bench pupil and the real ELT pupil could shift the predicted throughput by more than the eyeballed agreement in Figure 8. Please state the bench pupil parameters, how they were measured or set, and a tolerance analysis (or an argument from the MISTHIC model) for their effect on the throughput comparison.
- [Section 4, CLC15 J-band discrepancy] The J-band mismatch for CLC15 is attributed to improper focalization, but no focal sweep, refocused image, or independent check is shown to confirm this hypothesis. Since CLC15 is the most demanding component and the paper explicitly flags its degraded result, the authors should provide the refocused measurements or an experimental demonstration that defocus of the stated magnitude reproduces the observed offset in both throughput and image structure.
minor comments (5)
- [Throughout] The paper contains typographical errors that should be corrected in a final version, including 'Ideed' (Introduction), 'of of' (Introduction), 'contrat' (title of reference [7]), and the inconsistent spelling 'coronograph' in the title versus 'coronagraph' throughout the text.
- [References] Reference [8] lists the year as 2032; the correct year appears to be 2021. Please verify all reference details, especially DOIs and conference identifiers.
- [Table 2 and Section 3] The specification labeled 'Substrate transmission quality' is presumably a transmitted wavefront error specification, but the text later reports 'aberrations' of 12 nm RMS; please clarify whether the 15 nm RMS and 12 nm RMS values refer to transmitted wavefront error, surface figure, or another metric, and state the test wavelength for the interferometric measurement.
- [Section 4, Mechanical validation] The description of the cold test is somewhat ambiguous: it is not clear whether the blade-distance measurement after cold cycling is performed at cold temperature or after returning to room temperature. Please state explicitly at which temperature each microscope measurement was made.
- [Section 4, Optical validation] The TiN optical density measurement is said to be made on a complete substrate 'to avoid the diffraction effect of the limited spot size', but the spot thickness measurement in Figure 7 (left) is only described as micrometric; please specify the measurement instrument and its uncertainty for the spot diameter and centring verification.
Assumptions & free parameters
assumptions (4)
- domain assumption The MISTHIC simulation pipeline (refs [7][12]) accurately predicts the coronagraphic PSF of MICADO for the design parameters.
- domain assumption The IR testbench's ELT pupil mask and F/17.75 interface are representative of the MICADO/ELT entrance conditions.
- domain assumption The TiN deposition recipe used on the calibration substrate yields the same optical density on the small occulting spots as on the full substrate.
- domain assumption The thermal cycling profile (four cycles, 323 K to 80 K, >2 h each) is representative of the operational thermal history of the MICADO focal plane wheel.
Cite this review
Pith. "Pith review of The MICADO first light imager for the ELT: Testing the Lyot coronograph prototypes." pith.science (2026). https://pith.science/paper/XDYY2ZUI
@misc{pith2026241113709,
author = {Pith},
title = {Pith review of: The MICADO first light imager for the ELT: Testing the Lyot coronograph prototypes},
year = {2026},
howpublished = {\url{https://pith.science/paper/XDYY2ZUI}},
note = {Machine review of arXiv:2411.13709}
}
read the original abstract
MICADO, the European Extremely Large Telescope first light imager will feature a dedicated high contrast imaging mode specifically designed for observing and characterizing exoplanets and circumstellar disks. Its improved sensitivity and angular resolution, compared to existing instruments will significantly increase our knowledge on these planetary systems. MICADO will include three classical Lyot coronagraphs, one vector phase-apodized pupil plane (vAPP) and two sparse apertures. After rapidly describing the final design of MICADO high contrast mode, we will describe the current state of development of the coronographic components and the testing of the first Lyot coronagraph prototypes.
Reference graph
Works this paper leans on
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[2]
MICADO HIGH CONTRAST MODE In this context, simple Classical Lyot Coronagraphs (CLC), without apodization are selected as a baseline for MICADO. A CLC is an occulting mask in the focal plane and must be coupled with a Lyot stop located in a pupil plane downstream of the focal plane mask. To take full advantage of the high angular resolution of the ELT and ...
work page 2019
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[5]
SPHERE: the exoplanet imager for the Very Large Telescope,
CONCLUSION In this paper, we showed that the development of the three classical Lyot coronagraph (CLC) planned for MICADO, the first-light imaging instrument of the ELT, is well on track. We built a mechanical prototype mount and CLC prototypes. The mechanical mount is behaving as expected at warm and cold temperature. To test the performance of the CLC i...
Reviewed August 12, 2026 · model on record in the stance chip above.
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