Long-Wave Infrared Multispectral Imager for Lunar Remote Sensing: Optical Design and Performance Evaluation
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
This manuscript presents an optical system for a high-resolution LWIR multispectral imager designed for lunar remote sensing. The payload achieves an IFOV of 0.04943 mrad from an orbital altitude of 100 km. This specification corresponds to a ground resolution of 5 meters. It represents a significant performance enhancement compared to existing payloads. The article provides detailed design methods and results for the optical system . It also includes comprehensive analyses of the thermal background, stray light, and detection sensitivity. The logic is clear and the arguments are well-supported. The research demonstrates high academic value and engineering potential. Several points should be addressed to further improve the quality of the paper.
- Add Tolerance Analysis for Engineering Feasibility
The current design results satisfy all required indicators. However, manufacturing and alignment errors significantly impact the performance of off-axis systems. A quantitative tolerance analysis should be added to the manuscript. This analysis should specify the allowable ranges for mirror surface errors, decenter, and tilt. It may also define the required detector focusing accuracy to prove the engineering feasibility of the system.
- Refinement of Optical Efficiency Analysis
Table 5 lists the total optical efficiency for each channel. These values show a wide numerical range. A detailed analysis regarding the allocation of efficiency is required. The basis for the transmittance and reflectance values of each component should be stated clearly. The author should explain how material absorption or coating difficulties affect the efficiency of the 14.3 μm channel. This will make the sensitivity evaluation results more convincing.
- Identification of Opto-mechanical Materials
The article evaluates athermalization performance within an operating temperature range of 0°C to 40°C. Section 3.3 should explicitly identify the types of mechanical materials used for the housing during simulation. The substrate materials for the telescope mirrors should also be specified.
- Formatting and Language Revisions Standardization of Technical Acronyms
The manuscript contains many technical abbreviations such as LWIR, FSM, and VOx. All acronyms must be defined at their first occurrence in the text or listed in a definition table. The author should not mix full names and abbreviations in different sections. Terminology must remain uniform throughout the entire paper.
- Standardization of Figure Formats
The font size for coordinate labels in Figures 5, 6, 8, and 9 is too small. These labels can be unified with other annotations in the paper. All axis numbers and units must be clear and must not overlap.
- Consistency of Tense Usage
Tenses are mixed in the descriptions of the design process . Section 3.1 uses the present tense at the beginning but switches to the past tense at the end. The author should use the present tense consistently for all design methods and result evaluations.
- Revision of Long Compound Sentences
The introduction and performance analysis sections contain many long sentences joined by commas. This structure weakens the impact of the technical points. Complex causal logic should be split into independent short sentences to improve the reading experience.
- Verification of Reference Validity
The current reference format does not match published standards . The author must check the bibliography against the specific requirements of the journal Photonics. All DOI links must be valid and citations must be accurate.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors describe the need for the LWIR based high resolution imager for lunar mineralogy by proposing a push broom imager with an off-axis Gregorian configuration. The details on the design process have been nicely written; however, it misses a few pointers which might be essential to prove that the design is fabricable.
- Tolerancing: Please provide the sensitivity analysis of the axial positioning of the the detector given the F/# is very small.
- F-theta is reported to be <2.5%. For a push-broom system for high precision mapping, 2.5% might be quite high and can lead to barrel or cushion kind of effects in the spectral data. Please detail what calibration methodology do you plan to incorporate.
3. Thermal management: You describe the thermal stability of the optics but have not considered. The perfromance can degrade faster with non-uniform expansion of the optomechanical holder. Please comment on this.
Author Response
Thank you very much for taking the time to review this manuscript. We fully agree with your review comments. In response to your comments, we have added detailed descriptions of tolerance analysis, distortion calibration method, and thermal management strategies. We corrected errors in the method section, supplemented key technical details, improved performance analyses, and revised non-standard expressions and figures. Please find the detailed responses below and the corresponding revisions in the re-submitted files. Newly added content and sections in the revised manuscript are highlighted in red for easy identification. For modifications to existing content, given the extensive number of changes, we have updated it directly.
Comments 1: Tolerancing
Please provide the sensitivity analysis of the axial positioning of the the detector given the F/# is very small.
Response 1: Thank you for pointing this out. We agree with this comment. Tolerance analysis is a critical step in demonstrating the manufacturing and assembly feasibility of an optical system design, especially for this F/1.0 system. We have added tolerance analysis content in Section 3.4.
Table 3 presents the tolerance ranges assigned to each component and the telescope module after alignment with an interferometer, based on manufacturing capabilities and assembly experience from previous projects. A Monte Carlo simulation was performed with 1000 rays for analysis. The evaluation criterion is the MTF value at the Nyquist frequency. The analysis results are shown in Table 4. For channels 1–4 and channels 5–8, more than 90% of the samples achieve an MTF greater than 0.34 and 0.20, respectively. And the worst-case results still meet the specification requirements.
The above analysis results demonstrate the manufacturing feasibility of the system. For details, please refer to Section 3.4 in the revised manuscript.
Comments 2: Distortion Calibration Methodology
F-theta is reported to be <2.5%. For a push-broom system for high precision mapping, 2.5% might be quite high and can lead to barrel or cushion kind of effects in the spectral data. Please detail what calibration methodology do you plan to incorporate.
Response 2: We sincerely appreciate and agree with this valuable comment. For a push-broom system for high-precision mapping, the 2.5% f-theta distortion is relatively large. This is mainly due to the satellite envelope constraint that limits the distance between the primary and secondary mirrors to no more than 450 mm, which increases the off-axis distance. Figure 2 presents the relationship between f-theta distortion and telescope angular magnification. During design, we performed a comprehensive trade-off among distortion, primary mirror size, and rear optics aperture. The angular magnification was finalized at 2.5, leading to 2.5% f-theta distortion that cannot be fully compensated by the rear optical path.
Accordingly, we have supplemented a detailed distortion correction scheme in the revised manuscript. In the phase of ground calibration, distortion data are acquired at 0.1° angular steps across the full FOV using a collimator and precision electric rotary stage. A fourth-order binary polynomial model with an FSM x-axis coupling term is established to compensate for distortion deviations caused by dynamic image motion compensation.In the on-orbit calibration phase, fixed natural features such as lunar craters and rock mounds are employed to update the model coefficients regularly.
Based on the experience of previous on-orbit payloads, this integrated calibration strategy effectively restricts the full-field distortion error within 0.2 pixels. It guarantees the geometric imaging accuracy required for high-precision lunar mineral detection over the entire service life.
The above content has been added in Section 3.3 and highlighted in red for easy review. Thank you again for your valuable comments and suggestions.
Comments 3: Thermal management
You describe the thermal stability of the optics but have not considered. The performance can degrade faster with non-uniform expansion of the opto-mechanical holder. Please comment on this.
Response 3: Thank you for the valuable comment. Since the system operates over a wide temperature range, optical approaches such as refractive-diffractive hybrid systems are adopted. And an integrated opto-mechanical thermal management design is implemented based on the aforementioned optical athermalization optimization.
In terms of materials, suitable components are selected based on the athermalization design principle shown in Equation 6. The detailed material selections have been listed in the article. In terms of structure design, thermal insulation and flexible support are applied to reduce thermal stress. In terms of thermal control, the system combines closed-loop active temperature control, multi-layer insulation cladding and high-emissivity coating. The overall temperature gradient is limited to ±1.5 ℃ and the temperature stability to ±0.1 ℃.
The strategies listed above can effectively suppress imaging degradation caused by non-uniform thermal expansion. It ensures the system stability and detection accuracy. The above content has been added in Section 3.3 right after Figure 4. Thank you again for your valuable comments and suggestions.
Once again, Thank the reviewer’s kindness and carefulness, We sincerely appreciate it. We have revised our manuscript according to the comments and suggestions, which we believe should make great improvement for our manuscript.
