Round 1

Reviewer 1 Report

Summary

This manuscript is a technical note on the 3U CubeSat to image the Earth's surface and atmosphere using a harmonic diffractive lens. This work is claimed to be the first space demonstration of this technique.

The manuscript is decently written with room for improvement. I am recommending a major revision before considering it for publication. My comments are given in the following sections. Major comments need to be addressed before the publication, and minor/specific comments are clarifications or typos to be corrected.

Major Comments

1. Why is there only one image processed from CubeSat?. Especially when launched to space one year before. Since you are claiming that this is the first space instrument based on HDL, you should provide more supportive evidence by providing more captures. This comment is very important.
2. It would help if you gave a detailed description of the ground pixel resolution of the instrument. It should also include the field of view (FOV) information. 
3. All the plots you have captured from space should have latitude-longitude information. Currently, it is not visible in the figures. 
4. Please make sure that the figures are high quality and numbering are consistent throughout the manuscript

Specific/Minor Comments

Line #45: What do you mean by the 'lens microrelief features higher'?. You should rewrite the sentence to make it clear.

Line #52: It is 'MOIRE' not 'MOIR.' Please correct this typo where it is applicable.

Line #59: '... announced in 2018.' will be a proper way to close the sentence.

Line #77: Expand what GMSK stands for.

Line #82: Expand UART

Line #166: Expand B-DON

Line #168: Expand BCM

Line #241: Reference for the 'Harmony' software should be provided.

Line #262: It should be 4.2 instead of 3.2

Line #271: It is 'Jetson' not 'Jatson'

Line #279-283: Check if the #' is associated with the parts in the figure and the text matches or not. I can see some inconsistencies.

Line #290: it should be the 'actual orbit of the nanosatellite'

Line #301: Expand PSNR

Line #399: What is the duration of the mission?. Is it still operational?. Give more information.

Line #430: You haven't provided the resolution of the current CubeSat mentioned in this manuscript.

Line #460: It should be section 4.1 and 4.2

Line #466: Gain level should be 4 instead of 1

Line #488: Missing figures 11a,b an

Author Response

We thank the reviewers for their valuable and constructive criticism and recommendations. We have carefully incorporated their suggestions in the revised manuscript (changes are marked yellow), as well as included detailed responses below.

Major concerns

Why is there only one image processed from CubeSat?. Especially when launched to space one year before. Since you are claiming that this is the first space instrument based on HDL, you should provide more supportive evidence by providing more captures. This comment is very important.

Several Images captured over Europe and their CNN-based reconstruction are added in Appendix A, Fig A3. Due to the challenges with our CubeSat control and communication, there are not as much images with the useful content as we originally hoped.

It would help if you give a detailed description of the ground pixel resolution of the instrument. It should also include the field of view (FOV) information.

We have added a paragraph with the explanation of the orbit altitude and FOV information at the end of Section 3.

All the plots you have captured from space should have latitude-longitude information. Currently, it is not visible in the figures. 

We have added latitude-longitude and geographic information to each image in Fig.6 and Fig A3.

Please make sure that the figures are high quality and numbering are consistent throughout the manuscript

Figures 1, 2, 3 were replaced with higher-quality versions.

Minor concerns

Line #45: What do you mean by the 'lens microrelief features higher'?. You should rewrite the sentence to make it clear.

Fixed – as the lens microrelief thickness increased, from about 500 nm up to 10 mkm, these diffractive lenses are called harmonic.

Line #52: It is 'MOIRE' not 'MOIR.' Please correct this typo where it is applicable.

Fixed

Line #59: '... announced in 2018.' will be a proper way to close the sentence.

Fixed, according to your recommendations, thank you

Line #77: Expand what GMSK stands for.

Expanded

Line #82: Expand UART

Expanded

Line #166: Expand B-DON

We mentioned B-dot, the standard algorithm for very small satellites detumbling. We have fixed it and added the reference.

Line #168: Expand BCM

Expanded

Line #241: Reference for the 'Harmony' software should be provided.

We have added reference [54] to the external resource where our lens simulation software was introduced.

Line #262: It should be 4.2 instead of 3.2

Fixed

Line #271: It is 'Jetson' not 'Jatson'

Fixed

Line #279-283: Check if the #' is associated with the parts in the figure and the text matches or not. I can see some inconsistencies.

Fixed, now the description information correctly corresponds to the numbering, thank you

Line #290: it should be the 'actual orbit of the nanosatellite'

Fixed, according to your recommendations, thank you

Line #301: Expand PSNR

Expanded

Line #399: What is the duration of the mission?. Is it still operational?. Give more information.

Added to the first paragraph of the Discussion - A three-year mission was launched on March 22, 2021 and still operational as of April 22, 2022.

Line #430: You haven't provided the resolution of the current CubeSat mentioned in this manuscript.

We have added the information about ground sampling distance (GSD) of the conventional onboard camera (270 m/pixel) in the last paragraph of the Section 3.

Line #460: It should be section 4.1 and 4.2

Fixed

Line #466: Gain level should be 4 instead of 1

Fixed, thank you!

Line #488: Missing figures 11a,b an

We fixed the figure numbering, thank you!

Author Response File: Author Response.docx

Reviewer 2 Report

p. 2
Please outline what the advantages of this type of lenses are. Is it the size-to-focal-length ratio?

p. 3
Include figures of the other units as well. The mainly interesting part is the unit with the diffractive lens. The main unit is farily generic.

p. 4
The images can be improved. Why the viewing angle in the left hand side image? The right hand side image is far too wide-angle, crop the relevant part (the cubesat).

p. 7
Improve resolution of figure

p. 8
Please justify the somewhat unorthodoy shape. You mention "optimized for mechanical loads", hwoever you do not show the analysis. 

I do not understand this: What zero-CTE material did you use? I know ZERODUR, this is basically glass, though. Also, can you actually 3D-print such material? 

p. 10
Please put altitude (570 km) in caption. 
What extend in km is this? Do we see 100 x 100 km or rather 1000 x 1000 km?

Comments for author File: Comments.pdf

Author Response

We thank the reviewers for their valuable and constructive criticism and recommendations. We have carefully incorporated their suggestions in the revised manuscript (changes are marked yellow), as well as included detailed responses below.

1. 2

Please outline what the advantages of this type of lenses are. Is it the size-to-focal-length ratio?

Added at the end of introduction section: This lens is significantly lighter and thinner than a conventional refractive lens of a similar focal length and aperture, as we described in more detail in our previous work [10].

1. 3

Include figures of the other units as well. The mainly interesting part is the unit with the diffractive lens. The main unit is farily generic.

We replaced the Figure 2 with another image, which shows the camera unit assembly, while the camera itself is shown in more detail in Fig 5. And also we have replaced Fig 1 with another image of a better visual quality.

1. 4

The images can be improved. Why the viewing angle in the left hand side image? The right hand side image is far too wide-angle, crop the relevant part (the cubesat).

We replaced the image in this Figure

1. 7

Improve resolution of figure

Since the on-board lens is currently orbiting the Earth, we replaced the image with a higher quality fragment we cropped from the original higher-resolution image we had.

1. 8

Please justify the somewhat unorthodox shape. You mention "optimized for mechanical loads", whoever you do not show the analysis. 

I do not understand this: What zero-CTE material did you use? I know ZERODUR, this is basically glass, though. Also, can you actually 3D-print such material? 

We have removed two references to the lens case to avoid confusion and keep the focus of the paper on the overall CubeSat design, the lens and the image processing techniques.

P 10. Please put altitude (570 km) in caption.

We have added an altitude information in the caption, thank you.

What extend in km is this? Do we see 100 x 100 km or rather 1000 x 1000 km?

We have added a FOV information in the last paragraph of the Section 3.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I still wonder about the "bionic" design of the lens housing (fig. 4 / 5). It does not alleviate the scientific soundnedd and content (both fine!), however it has not been justified really. It seems a little bit coltish. Fine by me!

Minor: Caption of fig 6 shows some artefacts (at least in my PDF).