arxiv: 2605.11329 · v1 · submitted 2026-05-11 · 🌌 astro-ph.IM

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Optical Design of OPTICAM-ARG: A Three-Channel High-Time-Resolution Camera for the Jorge Sahade Telescope

A. Castro, D. Altamirano, E. Luna, E. Sohn, J. Herrera Vazquez, M. R. Najera, R. Michel, S. A. Cellone

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Pith reviewed 2026-05-13 01:36 UTC · model grok-4.3

classification 🌌 astro-ph.IM

keywords optical designmulti-channel cameradichroic beamsplittersfocal reducerhigh time resolutionCassegrain focussCMOS detectorray tracing

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The pith

The optical design for OPTICAM-ARG uses two dichroics and three dedicated focal reducers to split the beam from an f/8.5 telescope into simultaneous blue, green, and red channels while correcting aberrations.

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

The paper presents the optical layout of a three-channel camera built for simultaneous imaging across 0.35 to 1.00 micrometers at the Cassegrain focus of the Jorge Sahade Telescope. Two dichroics divide the converging beam into three paths, each routed through its own three-lens focal reducer, an interchangeable SDSS filter, and an sCMOS detector. This arrangement produces an effective focal length of 9.1 meters, a plate scale of 22.6 arcsec per mm, and an 8.4 by 8.4 arcmin field of view per channel that matches typical site seeing. Wedge angles on the second surfaces of the dichroics are optimized to reduce off-axis aberrations that arise when the dichroics operate in a converging beam, and the design's image quality is verified by exact ray tracing of RMS spot radii and encircled energy.

Core claim

The design employs two dichroics to spectrally separate the telescope beam into blue, green, and red channels, each followed by a three-element focal reducer that shortens the effective focal length to approximately 9.1 m and delivers a uniform 22.6 arcsec/mm plate scale over an 8.4 arcmin square field. Wedge angles applied to the second surfaces of the dichroics are adjusted during global optimization to counteract off-axis aberrations introduced by their placement in the converging f/8.5 beam. Optical performance is quantified through ray tracing that reports RMS spot radii and encircled-energy values, with the EE50 metric converted to an equivalent FWHM to allow direct comparison against

What carries the argument

A pair of dichroics with optimized wedge angles on their second surfaces, which both divide the incoming beam into three spectral channels and compensate for the off-axis aberrations that appear when dichroics are used in a converging beam, allowing each channel's focal reducer to produce matched fields and plate scales.

If this is right

Simultaneous three-band data collection eliminates the need to interpolate between sequential exposures when studying rapidly variable sources.
The 8.4 arcmin field of view per channel allows the instrument to capture the full typical seeing disk plus surrounding sky under normal conditions at the site.
Interchangeable SDSS filters and fast sCMOS detectors enable flexible selection of bandpasses and high-cadence timing within the 0.35 to 1.00 micrometer range.
The reported encircled-energy metrics indicate that optical blur remains comparable to or smaller than atmospheric seeing, preserving sensitivity for time-resolved photometry.

Where Pith is reading between the lines

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

The wedge-angle correction technique could be tested on other f/8-class telescopes to see whether it reduces the need for additional corrective lenses in multi-channel designs.
If the on-sky FWHM matches the ray-traced predictions, the same conversion of EE50 to equivalent FWHM could be applied to evaluate other instruments under varying seeing conditions.
The uniform plate scale across channels simplifies the co-alignment and cross-calibration of light curves obtained in different bands.

Load-bearing premise

The ray-traced performance achieved with the chosen wedge angles will survive translation to hardware once manufacturing tolerances, alignment errors, and real atmospheric conditions at the telescope are taken into account.

What would settle it

On-sky images of a point source that show measured full-width at half-maximum values in any channel exceeding the ray-traced equivalent FWHM by more than the expected contribution from site seeing.

Figures

Figures reproduced from arXiv: 2605.11329 by A. Castro, D. Altamirano, E. Luna, E. Sohn, J. Herrera Vazquez, M. R. Najera, R. Michel, S. A. Cellone.

**Figure 1.** Figure 1: General layout of the Jorge Sahade (JS) telescope in Cassegrain configuration (𝑓∕8.5) coupled to the OPTICAM-ARG instrument. The main optical elements of both the telescope and the instrument are shown, together with the geometric constraints imposed by the telescope cell. The available length within the cell, 𝑑M1cell, limits the axial extension of the instrument, while the admissible mechanical volume is … view at source ↗

**Figure 2.** Figure 2: Enlarged view of the instrument within the mechanical volume available inside the JS telescope cell. The axial reference from the primary mirror vertex, the distance to the mounting flange 𝑑ref , and the mechanical aperture diameter 𝑑ap are indicated. The available integration volume is shown by a dotted line, while the camera volumes are represented by solid rectangles. Two fold mirrors, denoted as M3 , a… view at source ↗

**Figure 3.** Figure 3: Transmission profiles of the Astrodon Gen2 SDSS filter set (𝑢 ′𝑔 ′ 𝑟 ′ 𝑖 ′𝑧 ′ ; colored curves) and the quantum efficiency (QE) curve of the Andor Marana 4.2B–11 sCMOS detector (black dashed line). design. According to the manufacturer specifications, fullframe readout can be achieved in less than 50 ms (Instruments, 2021), enabling exposure times from microseconds to several seconds. Marana 4.2B–11 incor… view at source ↗

**Figure 4.** Figure 4: summarizes the design sequence, from the definition of instrumental specifications to the final refinement through exact ray tracing, and provides the conceptual guide for the subsections that follow. 3.1. Starting point designs The starting point designs of OPTICAM-ARG are constructed through a progressive formulation of the optical system that integrates instrumental specifications (Section 3.1.1) with g… view at source ↗

**Figure 5.** Figure 5: Spot diagrams from exact ray tracing for the final optical design. The focal reducers were optimized in KrakenOS; ZEMAX® OpticStudio was used for best focus determination, dichroic wedge optimization, and final fine adjustment. For each spectral channel (rows), three image plane field points are shown: the on-axis field (0.00, 0.00) and the off-axis fields (0.00, 0.07) and (0.07, 0.07). Colors indicate the… view at source ↗

**Figure 6.** Figure 6: Encircled geometric energy curves, normalized by the diffraction limit, for the three spectral channels: blue (channel 1), green (channel 2), and red (channel 3). Line styles indicate field positions: solid (on-axis), dash–dotted (off-axis), and dashed (diagonal) [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗

read the original abstract

We present the optical design of OPTICAM-ARG, a multi-channel instrument for the simultaneous acquisition of images in three spectral bands at the Cassegrain focus of an f/8.5 telescope, covering the 0.35 to 1.00 um wavelength range. The converging beam delivered by the telescope is spectrally separated by two dichroics into three channels, blue, green, and red, each incorporating a dedicated three-lens focal reducer, an interchangeable SDSS filter stage, and an sCMOS detector. The focal reducers establish an effective focal length of approximately 9.1 m, a uniform plate scale of 22.6 arcsec/mm, and a field of view of 8.4 arcmin x 8.4 arcmin per channel, consistent with the typical seeing conditions at the site. Operation of the dichroics in a converging beam introduces off-axis aberrations, which are mitigated through wedge angles applied to their second surface and optimized as part of the global design. Optical performance is assessed through exact ray tracing using RMS spot radii and encircled energy metrics, with EE50 values further expressed in terms of an equivalent FWHM to enable direct comparison with atmospheric seeing and to evaluate sensitivity to manufacturing tolerances.

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.

Desk Editor's Note private letter to a colleague

This is a straightforward, well-documented optical design for a three-channel camera on one specific telescope, with concrete prescriptions and ray-tracing results that look internally consistent.

read the letter

OPTICAM-ARG is a practical optical design for a three-band high-time-resolution camera on the Jorge Sahade telescope, and the paper's strength is in laying out the concrete prescriptions and how they handled the aberrations from the dichroics. The new part is the specific three-lens focal reducer setup for each channel that delivers a consistent 22.6 arcsec per mm plate scale across an 8.4 arcmin field, plus the wedge angles on the dichroic second surfaces to fix off-axis issues in the converging beam. They used exact ray tracing to optimize everything and checked performance with RMS spot sizes and encircled energy, converting EE50 to equivalent FWHM for seeing comparison. The tolerance runs are included, which shows they thought through real-world build issues. It does a good job documenting the design choices for this f/8.5 telescope and the 0.35-1.00 um range with SDSS filters. The calculations look consistent internally. The soft spot is that everything is based on simulations and ray tracing. No prototype data or on-sky verification is presented, so we don't know yet how close the actual performance will get to the modeled values once manufacturing tolerances, alignment, and atmosphere come into play. The paper doesn't discuss potential issues like dichroic coating uniformity or detector specifics in much depth. This kind of paper is mainly useful for other groups building similar instruments or for astronomers planning observations with this telescope. It shows clear thinking on the optics without overclaiming. I'd recommend sending it out for peer review.

Referee Report

0 major / 2 minor

Summary. The manuscript presents the optical design of OPTICAM-ARG, a three-channel high-time-resolution camera for simultaneous imaging in blue, green, and red bands (0.35–1.00 μm) at the Cassegrain focus of the f/8.5 Jorge Sahade Telescope. Two dichroics separate the converging beam into three channels, each with a dedicated three-lens focal reducer, interchangeable SDSS filter, and sCMOS detector. The focal reducers are designed to deliver an effective focal length of approximately 9.1 m, a uniform plate scale of 22.6 arcsec/mm, and an 8.4 arcmin × 8.4 arcmin field of view per channel. Off-axis aberrations introduced by the dichroics are mitigated by optimized wedge angles on their second surfaces. Performance is quantified via exact ray tracing using RMS spot radii, encircled-energy (EE50) metrics, and equivalent FWHM, with additional tolerance analysis.

Significance. If the simulated performance holds after fabrication, this instrument would provide a practical capability for simultaneous multi-band, high-time-resolution observations tailored to typical site seeing, enabling studies of variable astrophysical sources. The manuscript's strengths include the provision of full lens prescriptions, specific wedge-angle values, and tolerance runs, which support reproducibility and community use—features that are valuable in an instrumentation design paper. The internal consistency between the reported plate-scale uniformity, FOV, and ray-traced metrics is a positive aspect of the work.

minor comments (2)

[Abstract] Abstract: The abstract states an effective focal length of 'approximately 9.1 m' and a uniform plate scale of 22.6 arcsec/mm but omits the specific optimized wedge angles and quantitative tolerance results (e.g., maximum degradation in RMS spot radii or EE50 under manufacturing errors). Since the full text supplies these values and runs, adding summary numbers to the abstract would improve accessibility and better support the central performance claims.
The manuscript would benefit from explicitly naming the ray-tracing software (and version) used for the exact ray tracing, global optimization, and tolerance analysis to enhance reproducibility of the reported RMS spot radii, EE50, and equivalent FWHM metrics.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive and constructive review of our manuscript on the optical design of OPTICAM-ARG. The assessment correctly identifies the key features of the three-channel camera, including the use of dichroics in a converging beam, the focal reducer design achieving the target plate scale and FOV, and the mitigation of off-axis aberrations via wedge angles. We appreciate the recognition of the manuscript's strengths in providing full lens prescriptions, specific wedge values, and tolerance analysis to support reproducibility. The recommendation for minor revision is noted, and we will prepare a revised version accordingly.

Circularity Check

0 steps flagged

No significant circularity in optical design derivation

full rationale

The paper presents a forward optical design for OPTICAM-ARG using explicit lens prescriptions, dichroic wedge angles, and exact ray tracing to achieve target effective focal length (~9.1 m), plate scale (22.6 arcsec/mm), and FOV (8.4 arcmin). These quantities are direct outputs of the chosen focal reducer geometry and optimization, not predictions that reduce to fitted inputs or self-definitions. No load-bearing self-citations, uniqueness theorems, or ansatzes are invoked; performance metrics (RMS spot radii, EE50, equivalent FWHM) are computed from the stated design parameters. The derivation chain is self-contained as standard ray-tracing engineering without circular reduction.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The design rests on standard optical-engineering assumptions and site-specific telescope properties rather than new physical postulates.

free parameters (1)

Wedge angles on dichroic second surfaces
Optimized as part of the global design to reduce off-axis aberrations introduced by operation in a converging beam.

axioms (2)

domain assumption The telescope delivers a converging f/8.5 beam at the Cassegrain focus
Standard property of the Jorge Sahade Telescope used as input to the design.
standard math Ray tracing in commercial software accurately predicts real optical performance
Invoked when assessing RMS spot radii and encircled energy.

pith-pipeline@v0.9.0 · 5559 in / 1499 out tokens · 71692 ms · 2026-05-13T01:36:22.764175+00:00 · methodology

discussion (0)

Reference graph

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