The effect of telescope aperture, scattered light, and human vision on early measurements of sunspot and group numbers
Pith reviewed 2026-05-24 23:09 UTC · model grok-4.3
The pith
Degrading modern HMI images to simulate old telescopes reveals how aperture, scattered light, and 3% contrast threshold alter sunspot and group counts.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Degrading HMI white-light observations to match historical telescope apertures with added scattered light and imposing a 3% intensity contrast threshold for detection results in changes to sunspot appearance and counts; the paper derives functional dependencies from these simulations to normalize early measurements to a common standard.
What carries the argument
Degradation of modern HMI white-light images by reduced aperture, added scattered light, and application of a fixed 3% contrast detection threshold to model historical detectability.
If this is right
- Smaller apertures and higher scattered light reduce the number and area of detected sunspots and groups.
- The 3% contrast threshold excludes small or faint features even when the telescope resolves them.
- The derived functional dependencies directly quantify how aperture, scattered light, and contrast affect sunspot number, group number, and area.
- These functions can be applied to adjust disparate historical datasets to a shared reference.
Where Pith is reading between the lines
- The model could be applied to specific known telescopes to estimate systematic biases in particular observer records.
- Individual differences in observer contrast sensitivity could be added as a variable to refine the corrections.
- The same degradation approach might be tested on other historical features such as faculae if suitable modern proxies exist.
- Digitized drawings from the period could serve as an independent check on whether the functional dependencies hold.
Load-bearing premise
That degrading modern HMI images accurately reproduces how sunspots appeared and were detected through 17th-19th century telescopes by human observers.
What would settle it
A direct comparison between the model's predicted sunspot and group numbers from degraded images and the actual recorded counts from a specific historical telescope with known aperture and scattered-light properties would falsify the derived functions if they do not match.
read the original abstract
Early telescopic observations of sunspots were conducted with instruments of relatively small aperture. These instruments also suffered from a higher level of scattered light, and the human eye served as a "detector". The eye's ability to resolve small details depends on image contrast, and on average the intensity variations smaller than $\approx$ 3\% contrast relative to background are not detected even if they are resolved by the telescope. Here we study the effect of these three parameters (telescope aperture, scattered light, and detection threshold of human vision) on sunspot number, group number, and area of sunspots. As an "ideal" dataset, we employ white-light (pseudo-continuum) observations from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory, and we model the appearance of sunspots by degrading the HMI images to corresponding telescope apertures with an added scattered light. We discuss the effects of different parameters on sunspot counts and derive functional dependencies, which could be used to normalize historical observations of sunspot counts to common denominator.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper models the effects of telescope aperture, scattered light, and a fixed ~3% human visual contrast threshold on sunspot and group counts by degrading modern HMI pseudo-continuum images to simulate historical observations. It derives functional dependencies from these simulations intended to normalize early sunspot records to a common standard.
Significance. If the derived functions prove accurate, they could enable systematic correction of instrumental biases in pre-20th century sunspot data, strengthening long-term solar activity reconstructions. The systematic use of high-resolution HMI data as a starting point is a strength, but the absence of validation against historical counts or error quantification limits assessed impact.
major comments (2)
- [Abstract / modeling approach] Abstract / modeling approach: The normalization claim rests on the degradation pipeline (aperture blur + scattered light + fixed 3% threshold) reproducing historical detectability, yet the manuscript supplies no validation metrics, error bars, or direct comparisons with actual historical sunspot/group counts from overlapping epochs.
- [Abstract / modeling approach] Abstract / modeling approach: The fixed 3% contrast threshold and isotropic scattered-light model are presented without sensitivity tests or justification against variable human contrast sensitivity (size-, luminance-, and adaptation-dependent) or historical optical aberrations beyond aperture and scatter; this assumption is load-bearing for the functional dependencies.
minor comments (1)
- [Abstract] Abstract: The phrase 'derive functional dependencies' is used without indicating their mathematical form, range of applicability, or example values.
Simulated Author's Rebuttal
We thank the referee for their review and recommendations. We address the two major comments point-by-point, indicating where revisions will be made to the manuscript.
read point-by-point responses
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Referee: [Abstract / modeling approach] Abstract / modeling approach: The normalization claim rests on the degradation pipeline (aperture blur + scattered light + fixed 3% threshold) reproducing historical detectability, yet the manuscript supplies no validation metrics, error bars, or direct comparisons with actual historical sunspot/group counts from overlapping epochs.
Authors: The referee correctly identifies that our manuscript does not provide direct comparisons to historical counts or error bars on the derived functions. The work is intended to provide the functional dependencies from the simulation pipeline as a basis for future normalization. We will revise the manuscript to include error quantification from the model variations and a section discussing validation approaches using available historical records where overlap exists. This will be a partial revision as full validation may require additional data not in the current scope. revision: partial
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Referee: [Abstract / modeling approach] Abstract / modeling approach: The fixed 3% contrast threshold and isotropic scattered-light model are presented without sensitivity tests or justification against variable human contrast sensitivity (size-, luminance-, and adaptation-dependent) or historical optical aberrations beyond aperture and scatter; this assumption is load-bearing for the functional dependencies.
Authors: We note that the 3% contrast threshold is a commonly cited average value for human vision in astronomical contexts, and the scattered light is modeled isotropically as a simplification. However, we agree that sensitivity tests would be beneficial to assess the impact of these assumptions. In the revised manuscript, we will add sensitivity analyses varying the contrast threshold and exploring different scatter models to justify or refine the functional dependencies. revision: yes
Circularity Check
No significant circularity; derivation uses external HMI data and independent degradation model
full rationale
The paper takes public HMI pseudo-continuum images as an external benchmark dataset, applies a forward degradation model (aperture blurring + isotropic scattered-light addition + fixed 3% contrast threshold), and extracts functional dependencies from the resulting counts. No equation reduces an output quantity to a parameter fitted on the same target historical records; the normalization functions are generated by the simulation rather than by construction from the inputs they are meant to correct. No self-citation chains, uniqueness theorems, or ansatzes imported from prior author work are invoked as load-bearing steps in the abstract or described approach. The central claim therefore remains independent of the quantities it seeks to normalize.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Human eye does not detect intensity variations smaller than approximately 3% contrast relative to background
discussion (0)
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