The ASTRI Mini-Array: A New Pathfinder for Imaging Cherenkov Telescope Arrays

Round 1

Reviewer 1 Report

The paper presents in a concise and clear way the status of the ASTRI mini-Array. I have only some minor comments that could improve the clarity of the presentation.

1. I could not find the information about the date when the ASTRI Mini-Array is supposed to start operations. It would be a useful information for the reader. Also what is the status of the funding? Is it completely funded, also during operations?

2. Figure 3: are the PSF D80 data are based on simulations or on ASTRI results?

3. Figure 4: internal labels should be bigger

4. Figure 12: same comments, labels are small

5. line 408: a reference to a section is not correct

6. line 66: you refer to a "Paper II", but I cannot find the reference of this paper

7. Section 2.2.1: I understand that the size of one pixel corresponds to the PSF D80. This means that the PSF is under sampled right? Is it a technological limitation of the SiPM? A compromise wrt to the consumption/heating/complexity?

Minor English editing issues

line 93: remove "with us"

line 94: The aim...

line 95: allow, rather than allowed ?

line 101: required rather than requested

line 106: same -> some

line 196: remove "or were"

line 233: heart

line 312: the degree symbol is not exponent

Author Response

1. Based on the currently availavle information on the status of subsystems production the ASTRI Mini-Array should start operation in the second half of 2025. Funding, including operations, is secured and available. I will add few lines in the conclusions
2. Yes they are simulations. Data obtained during the optical validation are in reference 14
3. OK done
4. I can only increase the figure dimensions
5. It was a typo from a previous version in which a separate section describing that part was included then I decided to merge it into this same section
6. That is a typo paper II is reference 4
7. No, it is not related to SiPM limitation even if smaller pixels implies a larger number of them with a great increase in the complexity of the electronics. The reason for having D80 contained in one pixel is that Cherenkov astronomy does not work with point-like sources but with extended ones which is the footprint of the atmospheric shower on the detectors. A gamma-ray originated shower produce an ellipse shaped footprint that spans several pixels, so it is not actually necessary to have the PSF properly (like in optical astronomy) sampled to recover from the ellipse all the geometrical information necessary for the analysis.
8. I have implemented all the minor comments

Reviewer 2 Report

The paper describes the status of the ASTRI mini-array in different aspects, mostly managerial and technological. The article is well written and deserves to be published.  I have some question for the Authors and some suggestion in order to clarify some points:

1. Figure 4 the question is related to the magenta curve. As it is a fraction of photons falling outside M2, maybe the y scale is not m^2 but fraction of 1 ? 

Line 211: D80 is not defined in the text, only in the caption of fig 3. It is the diameter of 80% of the collected energy? Could the author mention at some point the definition in the text? And report in the list of acronyms  at the end of the paper?

Fig 3 can the author explain why the D80 has a minimum at around 3 degrees of axis?

Line 237 without going into details, I would like to ask if any pipe-up effect would occur within each pixel. If so, and if you work on peak detection, would not be affected on pile-up ?

Line 295: if the analysis will be done off-line after the transfer at SSDC, there is no real-time detection for synergy with other telescopes. Is this right? Will you implement any type of on-line synergy between observatories, space telescopes at later times? 

Line 408  need to correct the "see Section ??"

Furthermore, there are minor corrections to be implemented, for better quality of the manuscript:

Line 57 and line 35:  dual mirror --> dual mirrors

Line 37: (Canary Islands,Spain) --> (Canary Islands, Spain) 

Line 68: to monitor simultaneously --> simultaneous monitoring

Line 99: what it is  --> what is

Line 100: most of the times -> most of the time

Line 103: as forerunner --> as a  forerunner

Line 139: correspond to a linear size --> corresponds to a linear size

Line 144: in contained --> is contained

 Line 189: because of its dimensions -->  due to its dimensions.

Line 193: but regard the handling procedures -->  regarding the handling procedures

Line 255: every time the input signal exceed --> every time the input signal exceeds

Line 333: the possibility to use the window --> the possibility of using the window

Line 397: The only a real-time  analysis is performed --> Real-time  analysis is performed

Line 411: proceeding or move to a more appropriate --> proceeding or moving to a more appropriate

Line 434: Remote operations translates --> Remote operations translate

Line 529 performance others led also to simplification --> performance, others also led to simplification

line 229 : within  requirements --> within the requirements

no further comments to share only with the Editor

Author Response

1. Actually, the calculation is done via ray tracing so at every step we have the number of photons lost due to vignetting or, as in the case of the magenta curve, because they fall outside M2. So, at the end, this can be expressed either as the loss of the initial number of photons or as the loss of geometrical area.
2. Actually, D80 is the diameter containing 80% of the photons from the ray tracing analysis. I changed all the reference to D80 in the text accordingly. 
3. This is related to the polynomial optimization of the Schwarzschild-Couder design. The Schwarzschild-Couder design has the minimum in the center of the field of view and then the D80 gets worse going offsite so that the FoV is defined by the radius at which the requirement is no longer satisfied. The overall effect of polynomial optimization is to have a flatter behavior of the PSF along the field of view making the size at center and at the border very similar with the minimum in between. The 3 degree was then a result of the optimization of the design.  
4. I guess the pile-up effect you are referring to is that occurring when different microcells in a pixel fire in rapid sequence due to photons impinging on them. If this happen during the time interval when the peak detector is up then there is no effect as the peak detector integrates the signal produced inside the pixel
5. Right now the interaction is the other way around. If any other ground-based o space telescope/observatory will trigger some ToO due to real-time detection the ASTRI Mini-Array can respond. On the other hand it is correct to say that we will not trigger any ToO.
6. OK corrected. It was a typo from an older version
7. All the minor corrections were implemented

Reviewer 3 Report

I have reviewed the submitted paper titled "The ASTRI Mini-Array: a new pathfinder for Imaging Cherenkov Telescope Arrays". The paper is well written and of interest to warrant publication in Universe. I have only a few comments that should be addressed before I can fully recommend the paper for publication.

Comments:

The paper will benefit from a short description of the different potential science cases that ASTRI will be targeting. There is some scatter information in the paper, but I think a concentrated paragraph will be it easier for a non-IACT person to understand the importance of the array and Cherenkov arrays in general.


introduction: (Canary Islands,Spain) --> (Canary Islands, Spain)

Figure 1: the labels are a bit small. I would suggest the authors enlarging them if possible.

section 2.3 : average temperatures ([28]) --> average temperatures [28]

Figure 12: same comment as Fig.1

section 3.1:  (see Section ??)

Author Response

I have added the following lines to section 1.1

During the first 4 years of operations the ASTRI Mini-Array will be run as an experiment and not as an observatory. After this initial period the ASTRI Mini-Array will gradually move towards an observatory model. The scientific program during the first observing years will be devoted to the following Core Science topics: the origin of cosmic rays, the extra-galactic background light and the study of fundamental physics, gamma-ray bursts and multi-messenger transients. Detailed description of science that the ASTRI Mini-Array will perform can be found in (reference 7).

I have implemented all the other comments

Round 2

Reviewer 2 Report

The author has replied to all the questions / comments that I have raised. Furthermore the text has been improved. I recomment the publication of this paper.

Reviewer 3 Report

I thank the authors for implementing my comments.  I have recommended the paper for publication.