Proving the outstanding capabilities of IACTs in high time resolution optical astronomy

M. K. Daniel (for the VERITAS Collaboration); T. Hassan

REVIEW 4 major objections 5 minor 24 references

Proving the outstanding capabilities of IACTs in high time resolution optical astronomy

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read This paper argues that asteroid-occultation shadows observed by IACT-class telescopes can become a routine source of direct stellar diameter measurements down to 13th magnitude, increasing the number of measured K-star radii by 50 percent.

desk verdict A plausible but under-derived feasibility projection: the one-per-week rate and the 50%-vs-double K-star claim don't follow from the plotted forecast, though the technique is real and the paper is honest about its limits. read the letter →

arxiv 1908.03393 v1 pith:CIQFAZNF submitted 2019-08-09 astro-ph.IM astro-ph.HEastro-ph.SR

T. Hassan , M. K. Daniel (for the VERITAS Collaboration) This is my paper · ORCID

classification astro-ph.IMastro-ph.HEastro-ph.SR

keywords asteroidoccultationsstellarangulardiametersimagingatmosphericCherenkovtelescopeshigh-speedopticalphotometryK-typestarsdiffractionfittingsub-milliarcsecondresolutionVERITAS

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Imaging Atmospheric Cherenkov Telescopes are built to catch nanosecond gamma-ray flashes, but their large mirrors and fast photodetectors also make them competitive high-speed optical photometers. This paper argues that the asteroid-occultation diffraction-fitting technique, already demonstrated with VERITAS, can be turned into a routine source of directly measured stellar diameters. The central projection is that a single observatory with a high-speed visible-band photometer reaching 13th magnitude could increase the number of directly measured K-star diameters by 50%, using only a few minutes of observing time per week. The paper works through the predicted event rate, the current hardware limitations, and the upgrades needed to make the measurements precise enough to matter.

What carries the argument

The load-bearing object is the asteroid-occultation diffraction pattern: when an asteroid's shadow crosses the telescope, the recorded ingress and egress light curves encode the star's angular diameter as a modulation of the diffraction pattern, which is fit to a point-source versus uniform-disk model to recover the stellar radius. Two supporting mechanisms carry the argument: a PMT current-monitoring photometer that records the DC light level at up to 4,800 Hz, turning each IACT into a millisecond high-speed photometer with roughly 20 times lower scintillation noise than a standard 50 cm occultation telescope, and an event-rate forecast from the Occult software and the Gaia DR2 catalogue that determines how many occultations per year are visible from a fixed site as a function of star magnitude. The contrast of the diffraction pattern is what limits sensitivity, and the paper shows it can be increased with a narrower optical bandpass.

What would settle it

Run a year-long monitoring campaign at a fixed site with a photometer that reaches 13th magnitude in millisecond integrations, attempt diffraction fits on every predicted high-probability occultation, and compare the recovered angular diameters with interferometric diameters for the few stars measured by both methods. A realized rate well below one usable event per week, or systematic disagreement in the recovered diameters, would falsify the central projection.

Watch

Extended reading notes

Core claim

The paper claims that asteroid occultations are a practical, routine method for directly measuring stellar angular diameters, with the first IACT detections already producing the highest angular resolution visible-band stellar diameter measurements ever taken. Using the Occult prediction software with the Gaia DR2 catalogue, the paper forecasts that a single fixed observatory with a 13th-magnitude high-speed photometer should detect about one usable occultation per week. Since 12% of those occultations are expected to involve K-type stars, a few years of sparse observations would increase the census of directly measured K-star radii by 50%. The paper also shows that the current VERITAS readout is limited to roughly 11th to 12th magnitude by datalogger resolution and night-sky background, and proposes hardware changes—narrow filters, beam splitting, and aperture stops—to reach the needed sensitivity and precision.

Load-bearing premise

The projected expansion of K-star radii rests on the forecast that a fixed site will actually detect about one usable 13th-magnitude asteroid occultation per week; if weather, prediction uncertainties, or faint-star signal-to-noise bring the realized rate down, the 50% increase disappears.

Editorial extensions

If this is right

A fixed-site 13th-magnitude high-speed photometer would deliver about one directly measured stellar diameter per week, using only minutes of observing time per event.
Because 12% of predicted occultations involve K-type stars, a few years of monitoring would raise the number of directly measured K-star radii by roughly 50%.
The technique extends direct diameter measurements to stars far below the magnitude limits of interferometry, at sub-milliarcsecond resolution.
Narrow-band filters and multi-wavelength beam splitting increase diffraction-pattern contrast by 5-7%, which should help bring relative diameter errors below 10% and toward the sub-3% level needed for exoplanet host-star work.
Predicted events let observatories schedule only high-probability occultations, so the program competes for almost none of the night.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The paper leaves implicit that its yield estimate is a lower bound: as Gaia astrometry sharpens asteroid ephemerides, more occultations, especially by smaller asteroids, become predictable, so the 50% K-star increase should grow.
A natural extension of the proposed multi-band beam splitter is to fit the wavelength-dependent diffraction patterns simultaneously, separating the stellar photosphere radius from limb-darkening rather than treating them as convolved.
If the sub-3% relative-error target is met, the same few-minutes-per-week program could anchor host-star radii for transiting exoplanet surveys, turning a niche technique into a calibration product.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The new content here is not the occultation technique—that was already demonstrated in the Nature Astronomy paper—but the quantitative promise that a single IACT with a high-speed photometer reaching 13th magnitude could substantially expand the sample of directly measured K-star radii. That promise is worth taking seriously, but the paper doesn't actually show its work. The abstract says a 50% increase; the conclusions say double. Those are different numbers, and neither is derived from the event-rate forecast in Fig. 3. The 'one measurement per week' is asserted, not computed from the plotted rates, and the 12% K-star fraction appears with no source or calculation. Since the selection is V-band and K stars are red, while IACTs are most sensitive in the UV/blue, the effective K-star rate could shift. The paper also concedes that relative errors below 10% are not yet demonstrated, which is honest but further weakens the headline claim.

What the paper does well: it is straightforward about the current state of the technique, the hardware suggestions (narrowband filters, multiwavelength beam splitting, stopping down the Winston cones) are concrete and physically sensible, and the external forecast via Occult+Gaia DR2 is a reasonable starting point. The argument that Gaia will make predictions better and thus the rates conservative is plausible. For a conference proceedings, this is a useful feasibility sketch rather than a rigorous projection.

The soft spots are exactly where the stress test lands: the central arithmetic isn't pinned down, and 'detectable' means path probability >20%, not successfully scheduled and measured events. IACTs have a primary gamma-ray program, and the paper doesn't discuss how occultation targets would fit into that schedule, though the few minutes per week requirement suggests it could be feasible. In a journal submission, I'd want the rate derivation and the K-star fraction made explicit and the abstract/conclusion numbers reconciled. As it stands, the claim is plausible but not yet demonstrated.

I'd send this to a referee if it were submitted as a journal paper—the idea is important enough, and the prior detection gives it real weight—but the referee should ask for the arithmetic. I'd cite the Nature Astronomy paper, not this proceedings, but it's a fair contribution to the IACT science case.

Referee Report

4 major / 5 minor

Summary. This conference proceedings paper argues that Imaging Atmospheric Cherenkov Telescopes (IACTs), specifically VERITAS, can serve as high-speed optical photometers and that asteroid occultation observations with these telescopes can directly measure stellar angular diameters with sub-milliarcsecond resolution. The authors summarize the recent VERITAS detection of two asteroid occultations, describe the diffraction-fitting technique, characterize the current VERITAS photometric sensitivity, and propose hardware upgrades (narrowband filters, multiwavelength beam splitting, and aperture optimization). The central projection is that a single fixed-site telescope with a high-speed visible-band photometer reaching about the 13th magnitude could substantially increase the number of directly measured K-star diameters, stated as a 50% increase in the abstract and as a doubling in the conclusions. The paper also lists the limitations of the technique, including the need to demonstrate relative errors below 10%.

Significance. If the event-rate forecast and the projected K-star fraction are reliable, this technique would open a new, relatively inexpensive path to direct stellar-radius measurements for faint (V ~ 11-13) stars, a regime largely inaccessible to optical interferometry. The resulting sample could test stellar evolution models and improve the accuracy of transiting exoplanet radius measurements. The paper's strengths include the physically sound diffraction-fitting framework, the use of measured VERITAS sensitivity rather than purely simulated performance, and the honest acknowledgement that sub-10% relative diameter errors remain to be demonstrated. The prior detection is published in a peer-reviewed venue, and no fitted parameter from that work is recycled into the forward projection, so the central feasibility argument is not circular. The main weakness is that the headline yield rests on an incompletely specified event-rate forecast and an unquantified spectral-type fraction.

major comments (4)

[Abstract and Section 5] The abstract states that a 13th-magnitude photometer 'could increase the number of directly measured K stars diameters by 50%,' while Section 5 concludes that such a telescope 'could double the amount of K stars with directly measured stellar diameters in a few years.' These are quantitatively different claims (50% vs. 100% increase). Since this is the headline result, the authors must reconcile the two numbers and show the arithmetic connecting the event rate, the K-star fraction, and the current sample size of directly measured K-star diameters.
[Section 5 and Figure 3 (left)] The 'one measurement per week' rate is not derived from the plotted event rates in Figure 3 (left). The figure shows events per year for two selection criteria, but the text does not quote the plotted counts, nor does it explain how the plot translates to approximately 52 usable events per year. The text explicitly excludes weather and Moon duty cycle, and it equates 'detectable' (path probability >20%) with a successful measurement, ignoring scheduling constraints and prediction errors. Because the projected K-star yield is directly proportional to this rate, the authors should provide a step-by-step derivation of the weekly rate, including assumed duty cycle, weather fraction, and success probability, with uncertainties.
[Section 5] The statement that 'for stars brighter than 14th magV, 12% of the expected asteroid occultations will be on K-type stars' is given without a source, a calculation, or a definition of the K-type selection. This fraction is load-bearing because it converts the total event rate into the K-star yield. The selection is made in V-band magnitude while the IACT sensitivity is discussed in the B band, which could bias the effective K-star fraction given that K-type stars are red. The authors should derive this 12% from the same Gaia-based catalogue used in Figure 3 and state the spectral-type classification criteria.
[Section 5] The paper acknowledges that relative errors better than 10% 'need still to be demonstrated' and that <3% errors are required for impact on exoplanet radii. Given that the abstract describes the measurements as 'usable for population studies,' the feasibility claim requires more than a detection-rate forecast. The authors should provide a quantitative signal-to-noise estimate for a typical 13th-magnitude occultation event, including the expected diffraction-pattern amplitude, sampling rate, and background noise, or explicitly reframe the central claim as a projection of detections rather than of usable diameter measurements.

minor comments (5)

[Abstract] There is a typo: 'mainly emitte' should be 'mainly emitted.'
[Figure 3 caption] The caption does not specify the processing steps applied to the Occult software output (e.g., minimum asteroid diameter, minimum star brightness, or the exact path-probability threshold) or the uncertainties on the plotted rates. Adding this information would make the forecast reproducible.
[Figure 2 caption] The caption says 'Errors for this work are the 68% confidence level,' but the text does not describe how these confidence intervals were computed for the VERITAS points. Please state the method (e.g., chi-square or Markov-chain Monte Carlo).
[Section 5] The claim that 'about only 10 stars accessible to the current generation of interferometers over reasonable observation times' lacks a citation. A reference or a quantitative basis for this number would strengthen the comparison.
[Section 4] The phrase 'the size of the star is determined by the wavelength-dependent surface of last scattering of the photosphere' is vague; clarifying whether this refers to limb darkening or a wavelength-dependent stellar radius would help the reader understand the multiwavelength proposal.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the projected K-star yield combines external Gaia/Occult forecasts and a peer-reviewed prior VERITAS detection; unsupported premises are correctness issues, not circularity.

full rationale

The paper's derivation chain is empirical and forecast-based rather than definitional or self-referential. The feasibility argument starts from the measured VERITAS occultation light curves (Figure 1, from ref. [6]) and the measured current-monitor limiting magnitude (Figure 3, right), then combines an external occultation-rate forecast computed with the Occult software and the Gaia DR2 catalogue (Figure 3, left) with two stated assumptions: 'one measurement per week during the night time' and '12% of the expected asteroid occultations will be on K-type stars.' No parameter in the headline projection is fitted to the quantity that is then predicted; the '13th magnitude' sensitivity is a proposed upgraded capability, not a value recycled from the data used to forecast occultation rates. The only load-bearing self-citation is ref. [6], the VERITAS Nature Astronomy detection of two asteroid occultations; that is a peer-reviewed, externally validated measurement with published light curves and diffraction fits, so it supplies independent evidence rather than a self-referential premise. Several premises are asserted rather than demonstrated, notably the 12% K-type fraction and the derivation of the weekly rate from Figure 3 (left), and the abstract's 'increase by 50%' conflicts with the conclusion's 'double the amount of K stars.' These are transparency and internal-consistency weaknesses, not circularity: none of the paper's central claims reduces by construction to its inputs, and no fitted parameter is renamed as a prediction. The paper also explicitly flags that 'relative errors better than 10% need still to be demonstrated,' further showing that its own feasibility claim is not presented as already proven by its inputs.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

No new entities are introduced. The paper builds on the existing VERITAS current-monitor hardware and external catalogs.

assumptions (3)

domain assumption Occult software predictions using Gaia DR2 provide an accurate forecast of observable asteroid occultation rates from a fixed site.
The rate forecast in Fig. 3 (left) and the 'one per week' and '12% K-type' numbers rest on this external tool; uncertainties are not quantified in the text.
standard math The diffraction pattern of a uniform disk is an adequate model for extracting stellar angular diameters at the reported precision.
The technique is inherited from lunar occultation work; the paper relies on this model when discussing improvements, e.g., Fig. 4 and Fig. 5.
domain assumption Scintillation noise scales as D^{-2/3}/sqrt(t), making 12-m IACTs about 20 times quieter than 50-cm occultation telescopes.
Section 3 uses this standard scaling to justify the sensitivity at millisecond sampling.

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Cite this review

Pith. "Pith review of Proving the outstanding capabilities of IACTs in high time resolution optical astronomy." pith.science (2026). https://pith.science/paper/CIQFAZNF

@misc{pith2026190803393,
  author       = {Pith},
  title        = {Pith review of: Proving the outstanding capabilities of IACTs in high time resolution optical astronomy},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CIQFAZNF}},
  note         = {Machine review of arXiv:1908.03393}
}

abstract

Imaging Atmospheric Cherenkov Telescopes (IACTs) are very-large telescopes designed to detect the nanosecond-timescale flashes produced within extended air showers. Because IACTs are sensitive to the Cherenkov light (UV/blue) and use photodetectors with extremely fast time responses, they are also able to perform simultaneous optical observations. The large reflecting areas of these telescopes (larger than 100 m$^2$) makes them well-suited to studying fast optical transient phenomena with timescales ranging from seconds to milliseconds to nanoseconds, and the unique optical design provides a wide field of view monitoring capability with a modest point spread function. VERITAS, with its recently upgraded PMT current monitoring instrumentation, was able to provide the first detection of asteroid occultations with an IACT, resulting in the highest angular resolution measurements for stellar diameters ever taken in the visible band range. Here we explore the feasibility of using this technique to significantly expand the number of stars with directly measured stellar radii, usable for population studies to test stellar evolution modelling or transiting exoplanet radius measurements. A single observatory with a high-speed visible-band photometer with a sensitivity reaching the 13$^{th}$ magnitude could increase the number of directly measured K stars diameters by 50%.

Figures

Figures reproduced from arXiv: 1908.03393 by the authors.

**Figure 1.** The light curves of the ingress (left) and egress (right) of the (201) Penelope / TYC 278-748-1 occultation [6], with the best-fit diffraction pattern (red line) and theoretical point-source model (dashed blue line). The combined (averaged) residual with respect to the point-source (grey empty squares) and best-fit (black filled circles) models are shown in the bottom panels. 2. The asteroid occultation diffraction … view at source ↗

**Figure 2.** The angular size as a function of distance for all stars with direct angular size measurements [7]. The VERITAS asteroid occultation measurements are marked with black stars; all other occultation measurements by dark red circles; amplitude interferometry measurements by downward pointing red triangles; intensity interferometry by upward blue triangles. Note marker colors were chosen to also indicate the optical wa… view at source ↗

**Figure 3.** Left: Number of predicted asteroid occultation events per year as a function of the occulted star V magnitude observable from a fixed location. Orange dots refer to any asteroid shadow passing less than 300 km away from the site while green squares refer to the number of predicted occultation events with a probability of the shadow actually passing over the site (detectable occultation) larger than 20%. It was calcu… view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4: Differences between the diffraction pattern produced over different optical bandpass when assuming a point-like source (top figures) and an extended uniform disk source (bottom figures). Note the bottom-right figure shows the relative flux difference with respect to t…

**Figure 5.** Figure 5: Diffraction pattern over different optical bandpass produced by a point-like source (left figure) and an extended uniform disk source (right figure). The optical bandpass used is a flat filter 100-nm wide (50 nm in case of the dashed lines). Same parameters used as in …

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

Weekes, T. C. 1996, Space Science Reviews, 75, 1

work page 1996

[2] [2]

Focal Plane Instrumentation of VERITAS

VERITAS Collaboration: T. Nagai, McKay, R., Sleege, G., et al. 2007, arXiv e-prints, arXiv:0709.4517

work page Pith review arXiv 2007

[3] [3]

Lacki, B. C. 2011, Monthly Notices of the RAS, 416, 3075

work page 2011

[4] [4]

A., Antoranz, P., et al

Lucarelli, F., Barrio, J. A., Antoranz, P., et al. 2008, Nuclear Instruments and Methods in Physics Research A, 589, 415

work page 2008

[5] [5]

2017, International Cosmic Ray Conference, 301, 807

Hassan, T., Hoang, J., López Moya, M., et al. 2017, International Cosmic Ray Conference, 301, 807

work page 2017

[6] [6]

2019, Nature Astronomy, 3, 511

Benbow, W., Bird, R., Brill, A., et al. 2019, Nature Astronomy, 3, 511

work page 2019

[7] [7]

2016, VizieR Online Data Catalog, 2345,

Duvert, G. 2016, VizieR Online Data Catalog, 2345,

work page 2016

[8] [8]

1997, IAU Symposium, 189, 45

Richichi, A. 1997, IAU Symposium, 189, 45

work page 1997