Atmospheric Transparency at Candidate Sites for Sub-Millimeter-Wave Telescopes: Comparison
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Please note this is a comparison between Version 1 by Igor Zinchenko and Version 2 by Jason Zhu.

Radio astronomical observations at millimeter and submillimeter wavelengths are a very important tool for astrophysical research. However, there is a huge area in northeastern Eurasia, including the whole Russian territory, which lacks sufficiently large radio telescopes effectively operating at these wavelengths. 

  • radio astronomy
  • radio telescopes
  • telecommunications
  • millimeter and submillimeter waves

1. Introduction

Radio astronomical observations at millimeter and submillimeter wavelengths are a very important tool for astrophysical research (e.g., [1]). They provide a unique opportunity for detailed investigations of the interiors of the cold dense interstellar clouds of gas and dust, which represent cradles of new stars. The emission peak of these clouds lies in this band. Millimeter and submillimeter waves are very rich in the spectral lines of various molecules, atoms and ions, which can serve as diagnostic tools of physical conditions and chemical content. At these wavelengths, the highest angular resolution can be achieved, which is very important for studies of compact objects, in particular active galactic nuclei. These studies are facilitated by a lower interstellar scattering in comparison with longer radio wavelengths. Bright examples of such a study are recent images of the “shadows” of supermassive black holes in the centers of M87 and Milky Way galaxies [2][3]. These results were obtained with the Event Horizon Telescope (EHT), which is a global VLBI network of sufficiently large millimeter-wave observatories operating at the 1.3 mm wavelength. Nowadays, there are about 10 such observatories in the world. The success of the EHT stimulates the project of its extension, known as the next-generation EHT (ngEHT) [4]. The concept of ngEHT includes observations at 3 and 0.8 mm wavelengths, in addition to the 1.3 mm band. This emphasizes the importance of all these bands for astronomy. Many locations in the world are considered as candidate sites for new EHT telescopes (e.g., [5]). There is a huge area in northeastern Eurasia, including the whole Russian territory, which lacks such facilities, although many years ago construction of a 70 m radio telescope intended for operation at short millimeter wavelengths started on the Suffa plateau in Uzbekistan [6][7] (it was frozen after the USSR collapse), and now there are new relevant projects and proposals [8][9][10][11].
The main obstacle to ground-based radio astronomy observations at short millimeter and submillimeter wavelengths, in addition to technical challenges, is the atmospheric opacity, caused primarily by water and molecular oxygen. The observations are possible only in the so-called “atmospheric transparency windows”—the bands of relatively high transparency between the strong spectral lines of these molecules. The main windows discussed here are those centered at ∼90 GHz, ∼140 GHz, ∼225 GHz and ∼350 GHz. They are usually referred to as the 3 mm, 2 mm, 1.3 mm and 0.8 mm windows, respectively. However, even in these windows, the opacity can be quite high. Radio astronomical observations at the sea level are possible only in the 3 mm and 2 mm windows. At higher frequencies, high-altitude locations should be used.
While the oxygen absorption is stable and can be rather easily evaluated, the water content is highly variable. Water in the atmosphere is present in two forms—water vapor and liquid water (the latter one mainly in clouds). The amount of water vapor is usually characterized by the PWV (Precipitable Water Vapor) parameter, which is the vertically integrated amount of water vapor in the atmosphere. It is usually measured in millimeters. The amount of liquid water is parameterized by the Liquid Water Path (LWP), measured in g m−2 or in μm (e.g., [5]). The zenith opacity (optical depth) of the atmosphere in dependence on frequency (𝜈) is related to these parameters by the following expression:
𝜏(𝜈)=𝜏O2(𝜈)+𝛽(𝜈)PWV+𝛾(𝜈)LWP,    

where 𝜏O2(𝜈) is the molecular oxygen contribution to this opacity, 𝛽(𝜈) is the specific absorption coefficient per PWV unit and 𝛾(𝜈) is the specific absorption coefficient per LWP unit. The suitability of a site for radio astronomy observations is primarily characterized by the opacity statistics in atmospheric windows or by the PWV statistics, which are related to each other under clear sky conditions, although the LWP statistics are also important.

Both 𝜏O2(𝜈) and 𝛽(𝜈) are determined by the vertical distributions of the atmosphere physical parameters (pressure and temperature), molecular oxygen and water vapor. They can be derived empirically for a certain site or calculated using the existing models of the atmosphere in conjunction with spectroscopic databases (e.g., [12][13]). Then, nowadays, global dynamic models of the atmosphere with a high spatial and temporal resolution are available (see below). The dependencies of microwave absorption by molecular oxygen and water vapor on physical parameters and altitude in the atmosphere were analyzed many years ago [14]. In the paper in [15], the dependencies of 𝜏O2(𝜈) and 𝛽(𝜈) on altitude for the 1.3 mm window were calculated. The dependence of the molecular oxygen optical depth on altitude (h) is well described by the exponential function:
𝜏O2(𝜈)=𝛼(𝜈)𝑒−ℎ/ℎ0,    

where 𝛼(𝜈) is the O2 optical depth at the sea level and ℎ0 is the characteristic height, usually adopted to be 5.3 km [16], although it can be somewhat different in different seasons [14].

2. Degradation of the Telescope Sensitivity Due to Atmospheric Opacity

Atmospheric opacity leads to the deterioration of the telescope sensitivity in any case. However, it is worth obtaining numerical estimates of this deterioration. In particular, this can help better understand the acceptable value of opacity in various conditions. For this purpose, researchers estimate the quantity, which can be called a “degradation factor”:
𝑅D=𝑇∗SYS/TRX,  

where 𝑇RX is the receiver noise temperature and 𝑇∗SYS is the system temperature “above the atmosphere”, i.e., calculated from the system temperature at the receiver front-end taking into account the attenuation in the atmosphere. Neglecting antenna losses and background emission,

𝑇∗SYS=𝑇RX𝑒𝜏/cos𝜃+𝑇a(𝑒𝜏/cos𝜃−1).    
In the case of no opacity, 𝑅D=1. The plot of the degradation factor 𝑅D in dependence on the receiver noise temperature 𝑇RX and zenith opacity 𝜏 for the zenith angle 𝜃=45∘, assuming 𝑇a=250 K, is presented in Figure 1. For example, if the degradation factor of 1.5 is considered to be acceptable and the receiver noise temperature is 𝑇RX∼100 K, then the required zenith opacity is 𝜏≲0.1. It is worth noting that the integration time required to achieve the same sensitivity varies as 𝑅2D. In the case of 𝑇RX≫𝑇a, which happens at very high frequencies, the degradation factor approaches 𝑒𝜏/cos𝜃. Then, 𝑅D=1.5 implies 𝜏≈0.29 at 𝜃=45∘.
Applsci 13 11706 g005
Figure 1. The degradation factor 𝑅D (gray scale) in dependence on the receiver noise temperature 𝑇RX and zenith opacity 𝜏 for the zenith angle 𝜃=45∘, assuming 𝑇a=250 K. The curves correspond to 𝑅D=1.2, 1.5 and 2 (from bottom to top).

3. The Effect of Cloudiness

The coefficient 𝛾 in Equation (1) is approximately 2.5×10−3 and 3.5×10−3 μm−1 at 230 and 345 GHz, respectively, with weak temperature dependence [17]. This means that LWP of 100 μm (which is equivalent to 100 g m−2) contributes about 0.25 to the opacity at 230 GHz. The median values of LWP for different cloud classes are from ∼10 to ∼40 g m−2 [18][19]. In [5], the LWP statistics are presented for the existing and candidate EHT sites. The median values are well below 100 μm for most sites. Therefore, in most cases, the opacity in clouds at 1.3 mm is ≲0.1 and cannot fully prevent radio astronomical observations.
However, the problem is that clouds are usually very inhomogeneous, which leads to spatial and temporal fluctuations of opacity and sky brightness. Their influence can hardly be sufficiently suppressed, even by the usual beam-switching technique. As a result, these fluctuations make observations of weak sources practically impossible, especially in continuum. Spectral line observations are less affected because the fluctuations are synchronous in all channels and can be subtracted at the data reduction. However, a more frequent calibration is needed.

4. Comparison of the Candidate Sites for Millimeter-Wave Telescopes in Northeastern Eurasia

Long-term monitoring of atmospheric opacity with the MIAP-2 radiometers has been performed at several sites in Russia and Uzbekistan (the RT-70 radio telescope construction site on the Suffa plateau, the site of the BTA telescope in Caucasus and the Badary observatory). Among these sites, the best conditions for millimeter-wave astronomy have been observed on the Suffa plateau. However, these conditions are far from being excellent and hardly allow for regular observations at wavelengths 𝜆≲2 mm. At 1.3 mm, only episodic observations are possible in winter, when the monthly averaged value of zenith opacity at this wavelength drops to ∼0.3 (somewhat lower estimates of the opacity for this site are obtained in [5]).
Promising results have been obtained in the short summer measurements on the Muus-Khaya peak in Yakutia. However, long-term monitoring at this site is needed. Short measurements on the Terskol peak are not conclusive.
Investigations with various methods (e.g., [9][20][21][22][23][24][25][26]) reveal other promising sites in Eurasia with better conditions in comparison with the Suffa plateau. In the paper in [9], based on the NASA GEOS-FPIT model, the eastern Pamirs and Tibet are shown to be the best places. According to [23], based on the ERA5 reanalysis, very good conditions exist at the Ali 1 site in Tibet (PWV ∼0.4 mm in winter) and at Muztag-Ata in the Chinese Pamirs (PWV ∼0.7 mm in winter). Comparable conditions exist in the Sayan Mountains. The Khulugaisha peak, in terms of its characteristics, is close to the sites of Tibet and Pamirs (PWV ∼0.6 mm in winter). There are promising sites in Altai and Dagestan, in particular the Khorai and Kurapdag mountains [23]. The Terskol peak is also rather good in terms of PWV [20][27] but not so good concerning cloudiness. The Aktashtau peak in Uzbekistan (3383 m) located near the RT-70 construction site has PWV statistics similar to Terskol [23]. With PWV ≲2 mm, astronomical observations at least in the 1.3 mm and 0.85 mm atmospheric windows can be quite efficient. Even lower PWV values of ∼0.5 mm make observations in the higher-frequency windows possible.
It is worth noting that in rugged terrains, the spatial resolution of the global models can be insufficient to characterize atmospheric conditions on certain sites (e.g., on local peaks). This emphasizes the importance of local measurements with radiometric systems or GNSS devices. High-resolution weather prediction and recording on the cloudiness near the observation site could be promising in this respect, too.
In this consideration of the candidate sites for new millimeter- and sub-millimeter-wave telescopes, researchers have taken into account only the atmospheric opacity. However, there are other criteria which should also be considered. One of them is the stability of the atmosphere [28][29]. Enhanced instability can lead to strong phase fluctuations [30] and anomalous refraction (e.g., [31]). This factor is still poorly investigated.
A new large millimeter-wave telescope would be an efficient part of the global VLBI network (EHT). The estimates show that from this point of view, the Caucasus region is the most effective with the existing EHT configuration (Andrey Lobanov, private communication). However, the situation will change if a large millimeter-wave telescope is built in eastern Asia. In this case, places like the Pamirs and Tibet will have an advantage.

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