A Low-Cost Portable SDIMM for Daytime Atmospheric Optical Turbulence Measurement in Observatory Site Testing: Primary Results from Ali Site

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents the design, implementation, and validation of a low-cost, portable Solar Differential Image Motion Monitor (LP-SDIMM) tailored for daytime atmospheric turbulence measurements at astronomical sites. By leveraging a simplified DIMM optical configuration and computational image analysis, the LP-SDIMM achieves reliable Fried parameter (r₀) estimation without the need for complex wavefront reconstruction optics. The system’s compactness ( 60 cm length), light weight (<20 kg), and low cost (< ¥30,000 RMB) make it well-suited for mobile deployment across remote, high-altitude locations. Comparative tests with a conventional SDIMM demonstrated high measurement consistency, with a root mean square error <0.2 cm and a correlation coefficient of 0.92, confirming the system’s accuracy and reliability. This work is interesting and can be published in Photonics. Here are some suggestions.

1. The font in the picture is too small and the lines are too thin, making it hard to see clearly when printed.
2. English needs further refinement.
3. The number of references cited in the introduction is too small and needs to be further increased.

Author Response

1. The font in the picture is too small and the lines are too thin, making it hard to see clearly when printed.

Author's Response:

We have enlarged and bolded the text and lines in the figure(s) to improve clarity.

2. English needs further refinement.

Author's Response:

We appreciate the reviewer’s feedback on the language quality. The manuscript has been carefully revised by a native English-speaking colleague/professional editing service to improve clarity, grammar, and academic style. Significant refinements include:

  Restructuring ambiguous sentences for better readability;

  Correcting grammatical errors and improper word choices;

  Ensuring consistent terminology and formal academic tone.

3. The number of references cited in the introduction is too small and needs to be further increased.

Author's Response:

To address the reviewer’s concern about the insufficient number of references in the Introduction section, we can revise the introduction by adding relevant citations and expanding on the background. Below is a suggested revision, incorporating additional references and ensuring that the introduction provides a more thorough context for the study:

Increased the Number of References: In the introduction, we have added several key references relevant to our study, including foundational works on DIMM and SDIMM methods, as well as more recent advancements in daytime turbulence measurement techniques (e.g., MISOLFA and PDSL). These additional citations help to provide a more thorough background and clearly demonstrate the relationship between our work and previous studies.

Expanded the Research Background: We have expanded the introduction to provide a deeper context on atmospheric turbulence measurement, discussing both traditional and modern methods in more detail. This ensures that the introduction not only gives a comprehensive overview of the field but also highlights the novelty and significance of our work.

Cited Additional Relevant Literature: We have now referenced studies such as Liu et al. (2001), Tokovinin et al. (2002), and Song et al. (2018), which provide crucial context for our research. These references further strengthen the background and place our study within the broader scientific landscape.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The abstract, introduction and conclusion all line up around a single purpose: to present a lightweight, inexpensive Solar Differential Image-Motion Monitor (LP-SDIMM), describe its design, and report the first daytime-seeing statistics from the 5 050 m Ali site in Tibet. The paper explicitly states the engineering motivation (portability for multi-site surveys), the technical solution (retain DIMM optics, remove slit optics, recover r₀ with image-processing), and the headline results (Ali daytime r₀ 1.5–12 cm, mean ≈ 4.1 cm). Consequently the main objective and conclusions are easy to grasp. LP-SDIMM offers a practical route for routine daytime turbulence monitoring at remote, high-altitude sites. Because reliable daytime r₀ data remain scarce, the instrument—and especially the 85-day Ali data set—will interest site-testing teams planning solar or high-resolution nighttime facilities. The impact is therefore moderate but concrete: it lowers the cost-of-entry for comparative surveys and supplies fresh statistics from a newly favoured plateau site. Overall, the article is still worthy of publication. However, there are some suggestions as follows:

1.Overall skeleton is sensible (Introduction → Principles → Implementation → Results → Conclusion), but execution is uneven. Cross references such as “In section2, first outlines…” lack the space after the word section, and some headings are missing from the table of contents.

2.In the article, the abbreviation for the instrument is LP-SDIMM, but in the Field deployment section, "PL-SDIMM" is repeatedly used.

3. Figure 5 caption lines 176-181. Figures and captions are over-long and sometimes duplicate text. Figure 5 caption is 11 lines and repeats numbers already in the paragraph.

4. Mean/median reported at lines 194-198. Results quoted without uncertainty bars or percentiles in the main text. Histogram gives quartiles but the text cites only the mean 4.09 cm.

5. Only brief citations [20, 21] without discussion. No comparison with other portable daytime seeing monitors (e.g. MISOLFA, PDSL).

6. Spelling / typos “r0 meadian = 2.9 cm” in Figure 5 text.

7. Intro lines 33-38 run 46 words without a full stop.

Comments on the Quality of English Language

This paper is expressed clearly in English.

Author Response

Reviewer 2:

1. Overall skeleton is sensible (Introduction → Principles → Implementation → Results → Conclusion), but execution is uneven. Cross references such as “In section2, first outlines…” lack the space after the word section, and some headings are missing from the table of contents.

Author's Response: 

We sincerely appreciate the reviewer’s valuable feedback on the manuscript’s structure and formatting. In response, we have made the following improvements:

Revised Logical Flow – The paper has been reorganized to ensure a more balanced and cohesive presentation, particularly in Section 4, which now includes dedicated subsections (4.1 and 4.2) for clearer exposition.

Enhanced Discussion Section – As suggested, we have expanded the Discussion to provide deeper insights into the implications of our findings and their relation to prior work.

Corrected Cross-References – All in-text citations (e.g., “Section 2”) now follow proper formatting with spacing after “Section.”

Updated Table of Contents – Missing headings have been added to ensure full consistency between the outline and the manuscript body.

 

2. In the article, the abbreviation for the instrument is LP-SDIMM, but in the Field deployment section, "PL-SDIMM" is repeatedly used.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: LP-SDIMM.

 

3. Figure 5 caption lines 176-181. Figures and captions are over-long and sometimes duplicate text. Figure 5 caption is 11 lines and repeats numbers already in the paragraph.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: The LP-SDIMM was calibrated against a reference SDIMM at the Yunnan Observatory over a 3-day campaign in March 2024, as shown in Figure 3. Comparative analysis showed excellent agreement between the two instruments, with a mean bias of only 0.08 cm in $r_0$ measurements and a root mean square error (RMSE) of less than 0.2 cm across the measured range of 1-15 cm.

To further validate the daytime measurements, we performed simultaneous observations with a collocated solar theodolite, which yielded a strong Pearson correlation coefficient of 0.92. Statistical analysis of the comparative data revealed that the LP-SDIMM produced a mean r0 of 2.94 cm (median = 2.92 cm), while the reference SDIMM showed a mean of 2.91 cm (median = 2.9 cm), demonstrating the reliability of our portable system.   

4. Figure 5 presents representative results from a single day of comparative testing (March 12, 2024). The left panel shows the time series of r0 values from both instruments, displaying close tracking throughout the day. The right panel provides a histogram comparison of the measurements, visually confirming the consistency between systems. Additional validation tests under various seeing conditions confirmed the LP-SDIMM's robust performance across its operational range. 

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: Comparative r0 measurements between LP-SDIMM and reference SDIMM on March 12, 2024. Left: Time series; Right: Histogram distribution (statistics detailed in Section 3.4).

 

5. Mean/median reported at lines 194-198. Results quoted without uncertainty bars or percentiles in the main text. Histogram gives quartiles but the text cites only the mean 4.09 cm.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: The LP-SDIMM The LP-SDIMM provided continuous daytime r0measurements with a median of 3.87 cm (mean: 4.09 cm; IQR: 3.32–4.56 cm), indicating moderate variability driven by solar heating (Figure 7). Data coverage was balanced between morning (60.83%, 09:00–14:00) and afternoon (39.17%, 14:00–19:00), with 95% of values falling within 2.8–5.1 cm. The instrument's precision (SD=0.2 cm) matches commercial systems at lower cost. 

 

6. Only brief citations [20, 21] without discussion. No comparison with other portable daytime seeing monitors (e.g. MISOLFA, PDSL).

Author's Response:

We thank the reviewer for their insightful feedback regarding the need for a detailed discussion of the cited references [5, 21] and a comparison with other portable daytime seeing monitors, such as MISOLFA and PDSL. To address these concerns, we have revised the manuscript as follows:

In the revised manuscript, we have added a more detailed discussion of the relevant works, particularly those by Ikhlef et al. (2016) and Shikhovtsev et al. (2024), to provide a deeper understanding of their methods. We explain how the estimation method used in these studies compares to the approach used in our work, and we highlight the similarities and differences in the methodology.

MISOLFA (Ikhlef et al., 2016) uses wavefront segmentation and spatio-temporal monitoring for detailed characterization of atmospheric turbulence. While it provides high-resolution measurements, it requires complex calibration and larger system configurations, which limits its portability.

PDSL (Song et al., 2020), which utilizes solar limb brightness fluctuations, shares similarities with our approach but requires a larger telescope aperture for accurate measurements. In contrast, LP-SDIMM simplifies the optical design by removing the need for wavefront reconstruction optics, making it more portable and cost-effective for mobile deployment across multiple sites.

We have also expanded the discussion of the  retrieval method used in our work, which is similar to the method used in PDSL. We have clarified that while both methods use the covariance of wavefront angle-of-arrival fluctuations at different separation angles on the solar limb to estimate the turbulence profile, LP-SDIMM simplifies this process by eliminating the need for complex wavefront reconstruction systems, improving efficiency and accessibility for field applications.

 

7. Spelling / typos “r₀ meadian = 2.9 cm” in Figure 5 text.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: Statistical analysis of the comparative data revealed that the LP-SDIMM produced a mean $r_0$ of 2.91 cm (median = 2.9 cm), while the reference SDIMM showed a mean of 2.94 cm (median = 2.92 cm), demonstrating the reliability of our portable system. 

 

8. Intro lines 33-38 run 46 words without a full stop.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: For daytime seeing measurements, the observational target transitions from point-source stellar images to the extended solar disk, introducing inherent complexities. Building on DIMM principles, Liu[6] developed a method using a focal-plane slit and downstream wavefront segmentation to generate two parallel slit images of the same solar limb in a conjugate plane, enabling DIMM-analogous daytime seeing measurements.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Review of Photonics manuscript  3736000 titled “A Low-cost Portable SDIMM for Daytime Atmospheric Optical Turbulence Measurement in Observatory Site Testing: Primary Results from Ali Site” by Jingxing Wang, Jing Feng, Xuan Qian, Yongqiang Yao, Mingyu Zhao, Kaifeng Kang, and Tengfei Song

The manuscript presents a version of a differential motion system specifically designed for day-time seeing measurements using the solar limb.  

The manuscript is well written and concise. It has few typos that I would encourage the authors to do a careful editing.

For example the subscript of equation 2 I suspect should be l and not i.

Line 46 planeSong is missing a space.

From the technical point of view my biggest concern is the temporal resolution.

While the authors do a good job in describing the spatial resolution and their algorithm to retrieve r₀ there is very little discussion of the temporal resolution. Their camera has a 36 Hz frame rate this means that they are sampling the turbulence around 3Hz. This is definitely too slow and may cause an under estimation of r₀ due to time averaging.

 

Comments on the Quality of English Language

The English is adequate, but a good editing would help.

Author Response

Reviewer 3：

1. For example the subscript of equation 2 I suspect should be l and not i.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to:

 

2. Line 46 planeSong is missing a space.

Author's Response: 

Following the reviewer's suggestion, we have modified the wording to: A small wedge prism inserted into one of the segmented sub-wavefronts induces a fixed wavefront tilt perpendicular to the slit axis, generating two parallel slit images of the solar limb in the conjugate image plane Song et al.[9].

 

3. From the technical point of view my biggest concern is the temporal resolution. While the authors do a good job in describing the spatial resolution and their algorithm to retrieve r0 there is very little discussion of the temporal resolution. Their camera has a 36 Hz frame rate this means that they are sampling the turbulence around 3Hz. This is definitely too slow and may cause an under estimation of r0 due to time averaging.

Author's Response:

To address this concern, we have made the following improvements: To enhance temporal resolution, the images were cropped to a 400 × 400 pixel region around the solar limb, increasing the sampling rate to 120 Hz, which was adequate for capturing the rapid dynamics of atmospheric turbulence. The system was remotely controlled, with real-time image quality monitoring enabling adjustments to exposure settings to account for fluctuations in solar brightness. Automated quality control algorithms identified and flagged images with excessive noise or cloud interference, ensuring that only high-quality data were processed.

Author Response File: Author Response.pdf