Interference Measurements Across Vacuum and Atmospheric Environments for Characterization of Space-Borne Telescope

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article examines an innovative setup for interferometric testing of a space telescope within a thermal vacuum chamber. The paper's primary objective is to propose a diverging lens within the vacuum chamber, isolate the interferometer, and enable testing under conditions where thermal dissipation and material outgassing could otherwise disrupt measurements. The optical path and the environment within and out of the vacuum chamber are critical.

While the article thoroughly describes the technical challenges—such as interferometric fringe blurring caused by the extended distance between the interferometer and diverging lens—it also offers solutions, including relay optics, to improve signal quality. The optical simulations assess diffraction effects and telescope wavefront variations, underlining the importance of a meticulous alignment procedure to maintain measurement accuracy.

However, the paper could benefit from a more detailed error budget analysis, particularly regarding the stability of auxiliary optics and their comparison with the telescope's optical tolerances. Overall, while the methodology provides valuable insights for enhancing pre-launch testing, further discussion on potential sources of error and stability under prolonged vacuum conditions would strengthen the analysis.

 

In detail:

 

Row 86: Specify the characteristics of the optical windows.

Row 107: Explain why you consider that the optical window maintains its shape in the feasibility test and when installed in the vacuum chamber.

Row 124: The interferometer's pupil was changed, which caused the blurred interferogram.

Figure 3 and text: Clarify if the interferogram results from a specific type of subtraction.

Row 129: Describe the ASAP software and expand the acronym.

Row 135: Why choose a 1 mm aperture for the pinhole?

Row 274: What is the positioning accuracy of the CMM?

 

In the interferogram, add a scale and color bar with limits.

 

It would also be mandatory to define the accuracy of the stability of the internal auxiliary optics under vacuum and on the hexapod. An error budget is not provided but would be necessary for this type of experiment, and it is not compared with the optical tolerances of the telescope under investigation. Additionally, a description of the interferometric stability of the auxiliary optics under vacuum and the optical window's stability is necessary to adequately describe the environment.

Author Response

Thanks to the reviewers for giving constructive comment and suggestion on the manuscript. The reply to each comment and corresponding revision are listed as attached. In addition, both the experimental results and WFE measurement budget are also provided in the revised manuscript. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript reports a novel architecture of interferometer whose light path travels across vacuum chamber and atmospheric environment, with a successful implementation of interferogram observed from the prototype testbed. This manuscript presents a solid work on the experimental effort of solving the problem of building an in-situ optical measurement for large aperture within limited room. The study is within the scope of the journal. The work is wonderful except lacking of theoretical statement of the novelty for the work, with rather a writing style of technical notes. There are several issues to be addressed in the work:

(1)   If the aperture is 400 mm for the M1 of the paper, the phase aberration of the atmosphere should be considered even for Earth observation, the quiv. Freid length is 1.8r_0, where r_0 is the value of Fried length for ground telescope viewing to space, the solution should be involved.

(2)   The co-tuning of three hexapods may introduce cumulative errors in the optical path. It would be beneficial to specify the tilt control precision achieved with this testbed.

(3)   It would be better to include an acronym table in the paper and the format should be extensively refined.

(4)   The quantitative results should be included in the conclusion section.

Author Response

Thanks to the reviewers for giving constructive comment and suggestion on the manuscript. The reply to each comment and corresponding revision are listed as attached. In addition, the WFE measurement budget is also provided in the revised manuscript which is related to the tuning accuracuy of three hexapods. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I thank the authors for the corrections made, and the article is now nominated for acceptance for publication.