The Role of the Large Millimeter Telescope in Black Hole Science with the Next-Generation Event Horizon Telescope

Round 1

Reviewer 1 Report

This is a paper to highlight the role of the Large Millimetre Telescope (LMT) in the context of the upgraded Event Horizon Telescope (EHT) array, known as new-generation EHT or ngEHT. 

The paper has two clear parts, one describing the present status of the antenna developments as it would be in a conference report or a technical memo, and one addressing synthetic imaging and its reconstruction with the future array. 

While the first part is genuine, I have problems to know whether the second part will not be repetitive with other similar works in the context of the ngEHT effort.  Given the scope of the paper, the main point would be discussing whether the presence or absence of the LMT changes the figures of merit of image reconstruction, or other aspects of the analysis. In similar works, usually the scenario "with the LMT" vs "without the LMT" is presented, and I do not see this clearly explained.  The paper focus in the reconstruction in general, out of what the title and abstract were promising.  In this sense, this part might be put in a separate publication, and focus on the LMT enhancements, or be much better explained in title, abstract, and introduction.  As said, the paper does not show well the added value of having the LMT in an array (which is obvious, considering the filling in the (u,v) plane south of the Arizona telescopes and the impact in the detection of extended emission in M 87* and also in AGN jets with extended emission.

Formally the paper is well written, with some minor things which can be fixed with language editing, it offers the necessary information to follow the explanation, as well as references (which could be more comprehensive, but are fair enough to follow the text), or missing accents (Volcán, Sánchez, Gómez).

The ngEHT, as far as I know, has also considered observing at 3 mm wavelength (there was a workshop last Summer in Harvard on this), this component is not addressed in the paper.  The present operations of the GMVA jointly with ALMA providing complementary science (see recent publications on 3C273, OJ287, or Sgr A*), the simple existence of the VLBA and the GBT at this wavelength in the same continent as the LMT, and the plans of the ngVLA to observe down to 130 GHz (matching the KVN), providing an excellent opportunity for VLBI millimetre science, are not properly discussed in the paper.

The paper should be consistent in nomenclature: it jumps from the band capital letter (e.g., W-band) to wavelength (3.5 mm) to frequency (86 GHz), making the reader confused.  Unification in style is very desirable, see, e.g., the paragraph around line 108.

A formal aspect: I do not know whether the journal allows not to define acronyms in the text given the table at the end, but check it to be consistent.  Some acronyms are missed (JPL, SIS, RTD, 

Some comments on the text.

Title: non-uniform selection of capitalising of nouns (e.g., role vs Science)

Line 45: Fix J1924–2914 (Southern Source).  Additional sources were observed, both as PI observations (OJ 287, NGC 1052) and as calibrators, only in 2017.  For 2018 and later there are additional targets, as far as one can see in the EHT webpages and related press releases.

Figure 2 and related discussion: the "small" antenna should be specified, how small, how is the efficiency, or at least provide the used SEFD.  

Table 2 is a bit content-less, is this really needed?  More specs would be useful, elsewhere, this can said in the text.

Figure 5 has too small fonts and unclearly labeled axes.  Fix.

Arrays in the performance simulation section 3 can be tabulated, reading lists of antennas in the text is tedious.  The initials around line 245 are really confusing.  NA is Namibia?  Countries are labeled like USA states.  

For the telescopes with solar avoidance or only operating by night by technical or logistical reasons, this should be clearly indicated in the uptime plots, or skip these from the observations.  The use of color in Fig. 7, for instance, is confusing.  PB and LM have two color codes.  

 

Author Response

Point 1:
While the first part is genuine, I have problems to know whether the second part will not be repetitive with other similar works in the context of the ngEHT effort.  Given the scope of the paper, the main point would be discussing whether the presence or absence of the LMT changes the figures of merit of image reconstruction, or other aspects of the analysis. In similar works, usually the scenario "with the LMT" vs "without the LMT" is presented, and I do not see this clearly explained.  The paper focus in the reconstruction in general, out of what the title and abstract were promising.  In this sense, this part might be put in a separate publication, and focus on the LMT enhancements, or be much better explained in title, abstract, and introduction.  As said, the paper does not show well the added value of having the LMT in an array (which is obvious, considering the filling in the (u,v) plane south of the Arizona telescopes and the impact in the detection of extended emission in M 87* and also in AGN jets with extended emission.

Response 1: We have assumed that upgraded LMT participation and related upgrades will be essential to the operation of the ngEHT array, particularly when ALMA is not co-observing. This includes the anchoring of small dishes using the LMT-50m, as well as phase stabilization of 345 GHz observations using the dual-frequency capabilities developed at LMT and to be deployed at future ngEHT sites. However, an analysis of what happens to the performance of the ngEHT when LMT is not participating in the array, with all calibration consequences accounted, for is quite complicated to properly address in simulation and beyond the capabilities of our current software, so this is left for future work.

Point 2:
The ngEHT, as far as I know, has also considered observing at 3 mm wavelength (there was a workshop last Summer in Harvard on this), this component is not addressed in the paper.  The present operations of the GMVA jointly with ALMA providing complementary science (see recent publications on 3C273, OJ287, or Sgr A*), the simple existence of the VLBA and the GBT at this wavelength in the same continent as the LMT, and the plans of the ngVLA to observe down to 130 GHz (matching the KVN), providing an excellent opportunity for VLBI millimetre science, are not properly discussed in the paper.

Response 2: This is an excellent point and LMT is able to participate in 3mm VLBI with GMVA. However this work was completed prior to the ngEHT 3mm workshop, and 3mm capabilities are not yet formally in the ngEHT reference design. Most of the joint science with 3mm can be done using nearby (but not strictly simultaneous) observations. There is other work, in preparation and appearing on the same timescale as this paper, which discusses the role of 3mm for ngEHT and covers many of the topics brought up by the referee.

Point 3: The paper should be consistent in nomenclature: it jumps from the band capital letter (e.g., W-band) to wavelength (3.5 mm) to frequency (86 GHz), making the reader confused.  Unification in style is very desirable, see, e.g., the paragraph around line 108.

Response 3: Fixed

Point 4: A formal aspect: I do not know whether the journal allows not to define acronyms in the text given the table at the end, but check it to be consistent.  Some acronyms are missed (JPL, SIS, RTD, 

Response 4: I have defined the acronyms listed in this point as well as some other acronyms that were missing their in-text definitions. 

Some comments on the text.

Point 5: Title: non-uniform selection of capitalising of nouns (e.g., role vs Science)

Response 5: Fixed.

Point 6: Line 45: Fix J1924–2914 (Southern Source).  Additional sources were observed, both as PI observations (OJ 287, NGC 1052) and as calibrators, only in 2017.  For 2018 and later there are additional targets, as far as one can see in the EHT webpages and related press releases.

Response 6: Typo was fixed. The sources mentioned are ones that the EHTC has published to date, though indeed there are other scientific AGN targets for the array.

Point 7: Figure 2 and related discussion: the "small" antenna should be specified, how small, how is the efficiency, or at least provide the used SEFD.  

Response 7: The small antenna diameter is shown as the x-axis of figure 2. We highlight that even a 6 m antenna with a local opacity of 0.6 can be used with LMT and still be able to achieve the targeted performance. We have added additional details to the figure caption including the assumed aperture efficiency for the small dish (0.5) and the LMT (0.37).

Point 8: Table 2 is a bit content-less, is this really needed?  More specs would be useful, elsewhere, this can said in the text.

Response 8: Table 2 was removed since the information is included in section 2.1. We wanted to provide a quick reference for the specs of the instruments however we agree that it looks sparse and the information is provided in the text. Table 2 was removed from the manuscript and merged with Table 1. 

Point 9: Figure 5 has too small fonts and unclearly labeled axes.  Fix.

Response 9: The figure has been updated with a bigger font and clearer axes labels.

Point 10: Arrays in the performance simulation section 3 can be tabulated, reading lists of antennas in the text is tedious.  The initials around line 245 are really confusing.  NA is Namibia?  Countries are labeled like USA states. 

Response 10: We have updated the figures to show the same nomenclature as Raymond 2021 where these sites are defined. In addition, a table with the nomenclature was added. 

Point 11: For the telescopes with solar avoidance or only operating by night by technical or logistical reasons, this should be clearly indicated in the uptime plots, or skip these from the observations.  The use of color in Fig. 7, for instance, is confusing.  PB and LM have two color codes.

Response 11: Some telescopes (SMA and SMT for example) do skip occasional observations due to solar avoidance, but this is scan, season, and source dependent. The plots are meant to be representative of typical coverage during the part of the year. The colors distinguish EHT and ngEHT observations. LMT has two colors due to additional daytime observations for ngEHT which were not available in previous EHT observations, while PB has a small amount of additional coverage due to mutual visibility contributed by ngEHT stations (GB).

Reviewer 2 Report

The paper details the status and upgrade of the LMT, as well as the improvement in the performance of the observational network by the addition of the LMT to the EHT and ngEHT, which has already made landmark achievements in black hole imaging. The next generation of EHT (ngEHT) is moving towards more ambitious goals. ngEHT requires increasing the number of telescopes to improve UV coverage, increasing the receiving bandwidth to improve image sensitivity and dynamic range, increasing the frequency of observations to obtain higher resolutions, improving the performance of snapshot images, etc. LMT's prime location and large aperture make it a core member of the EHT. The LMT has undergone some major upgrades to meet the requirements of the future ngEHT. A detailed description is given in the text, including the dual-frequency receiver, and thermal stabilization.

Some additional detailed information would have made the paper more informative for the readers. 

Section 2.1 - It would be interesting to see how the test results compare with other receivers in operation.

Section 2.2.3 - "The LMT has installed OOF software on the antenna and began testing during the first half of 2022." please add some description of the test results 

Section 2.2.4 - "Simulation studies suggest that LASERS will be able to remove low spatial order thermal deformations of the primary to a level of about 20 microns rms over the surface." A plot or table would be more convincing. 

The font for the X/Y axis in Figure 5 is too small.

 

Some minor comments can be found in the attached PDF. 

Comments for author File: Comments.pdf

Author Response

Point 1:
Section 2.1 - It would be interesting to see how the test results compare with other receivers in operation.

Response 1: Currently, the dual frequency receiver is still in development so we don’t enough data to compare to other instruments. 

Point 2:
Section 2.2.3 - "The LMT has installed OOF software on the antenna and began testing during the first half of 2022." please add some description of the test results 

Response 2: Figure 6 was added to address this comment. This figure shows an example of OOF test results at the LMT during the daytime when surface deformations are significant

Point 3:
Section 2.2.4 - "Simulation studies suggest that LASERS will be able to remove low spatial order thermal deformations of the primary to a level of about 20 microns rms over the surface." A plot or table would be more convincing. 

Response 3: Figure 7 was added. It shows a Monte Carlo simulation of LASERS, assuming that the surface is deformed from its parabolic shape along the optical axis using Zernike Polynomials.

Point4:
The font for the X/Y axis in Figure 5 is too small.

Response 4: Fixed

Point 5:
Some minor comments can be found in the attached PDF.

Response 5: Fixed

Reviewer 3 Report

This is an interesting and well written paper.

I am happy to recommend it for publication as it is.

Very minor typos/suggestions are:

- Fig. 1: Caption: Remove 1st "is" after "Top"  => "Top: The total intensity..."

- p.3, l.92: explain abbreviation "UMass"

- Fig. 6-8, left: Abbreviations for instruments is given with two letters, but   with three (or more) in the text (cf. also p. 17,  l. 408).

Author Response

Point 1:
Very minor typos/suggestions are:
    - Fig. 1: Caption: Remove 1st "is" after "Top"  => "Top: The total intensity..."
    - p.3, l.92: explain abbreviation "UMass"
    - Fig. 6-8, left: Abbreviations for instruments are given with two letters, but with three (or more) in the text (cf. also p. 17,  l. 408).

Response 1:
Figure 1 caption was updated with the suggestions from the reviewer.
UMass was replaced with the University of Massachusetts (UMass).
An explanation of the meaning of the abbreviations for the telescopes in figures 6-8 was added to the caption of figure 6. A note was added to the other figures to refer to figure 6.  

Reviewer 4 Report

I am recommending this article for publication. Only minor suggestion I have is to specify in figures 10+11 at which frequency (230 or 345 GHz)) the images are generated.

Author Response

Point 1:
I am recommending this article for publication. Only minor suggestion I have is to specify in figures 10+11 at which frequency (230 or 345 GHz)) the images are generated.

Response 1:
We believe the reviewer is referring to figures 9 and 10. If so, the images of the models and reconstructions shown are at 230 GHz. This information was added to the figures’ captions.