The moon being the closest celestial object to Earth has attracted scientists to study its geological structures and understand the physical and chemical properties of its surface, e.g., [1] - [6]. Detailed knowledge and data of lunar topography are of interest for planning lunar robotic and human explorations as well as establishing a potential platform for a range of scientific investigations [7] [8] [9] [10]. Moreover, lunar explorations are essential for fundamental and applied science, such as to evaluate the Earth, solar system and universe and conduct investigations in fundamental physics, astrobiology, and human physiology and medicine [11] [12] [13].

Optical observations have enabled astronomers to study the moon’s surface at a scale far finer than is possible for any other object in the sky. Several topographical features have been identified on the moon’s surface, including craters, domes, mountains, and dark halo and rubble areas, e.g. [14].

When radio telescopes and infrared (IR) detectors were developed in the mid-20^th century, researchers were able to explore the lunar surface to a greater extent than was previously possible [15] - [23]. Additionally, direct investigation by space probes during the last half-century or so through space-based instruments [7] [21] [24] has also contributed to obtaining detailed information about the lunar surface.

Despite thermal, ground-based lunar observations having been conducted during the 1950s and 1960s using larger and professional telescopes, the utilization of this technology in recent times with respect to the technological developments in the field of thermal cameras and other associated accessories is limited. Considering this, we have succeeded in designing and developing a cost-effective and portable prototype for a ground-based infrared (7 - 14 µm) system to study the thermal behavior of the lunar surface and use the flexibility this system which allows for the further applications of infrared remote sensing. The system was designed from theoretical considerations, without practical experience. However, the obtained results are acceptable and will allow us to further investigate and optimize the functionality and performance of the developed system. Being easy to use the developed system can be utilized for student projects and outreach programs and can be considered a useful tool for teaching thermography by qualitative visualization and/or quantitative analysis of a great number of physical phenomena [23] [25] [26] [27] [28].

In this paper, the basic design and the main components of the developed system are detailed in Section 1. Some of the obtained results are presented in Section 2. The conclusion and the future plans are discussed in the last section.

Figure 1 presents a schematic diagram of the IR-optical system. The principal operation of the system is to collect a large amount of light, using the primary mirror from the target object, and then reflect it onto the secondary mirror. This collected light is then focused into a thermal camera by using an intermediate germanium lens. Figure 2 shows the main components of the developed system, which will be briefly discussed in the following subsections.

VarioCam IR, a microbolometer -based camera manufactured by Jenoptik, was used for the developed system. The camera contains a matrix of tiny resistors, which are warmed up by the incoming radiation. Even the slightest change in resistance can be scanned and measured by the camera. A big advantage of using this type of cameras is that the microbolometers technology does not need cooling and they are temperature stabilized. The camera has a focal plane array of 640 × 420 pixels and is sensitive to thermal radiations of 7 - 14 µm, with a temperature resolution of about 0.08 K at 30˚C. The camera delivers an image containing information about the IR intensity, which is stored on a SD card. The technical specifications of this camera are given in the datasheet of the camera (https://www.jenoptik.com/).

The developed system has been tested, calibrated, and used in several practical experiments to ensure its functionality and performances using different targets.

We began the testing state by implementing signature imaging for some well-known targets such as buildings and towers. These experiments allow us to understand the thermal behavior of the studied objects. Moreover, these studies will enable us to understand the atmospheric absorption/emission and its influence on the observations during different atmospheric conditions [29].

Figure 4 is an example shows the thermal and optical images of Alfayslia tower. This tower is part of the Riyadh skyline and is about 7 km from the

observation site. The thermal image clearly shows the structure of the building, where the red spots indicate the warmer parts of the tower.

Figure 5 shows the thermal images of the moon during different lunar phases. It obvious that the temperature distributions on the surface of the moon are

affected by the illuminated part of the lunar surface. Some of the structures of the lunar surface, such as craters and mountains are clearly seen and their temperatures were different from one phase to another [24] [27] [28] [30] [31].

Figure 6(a), shows the thermal image of the crescent observed on the 21^st of January 2015 at 4:37 PM whereas Figure 6(b), captures the illumination of the moon at the time of observation. The astronomical ephemeris of the moon at the time of observation showed that the moon was a 26-h old moon, with an altitude of 17˚47' and an angular separation of 19˚54', phase 3.6 % at air mass of 3.27, which shows the capabilities of the developed system in detecting the thin crescent. The obtained results of the crescent observation can be of interest to those who are looking for alternative techniques of observing the crescent. However, the obtained results are preliminary and more practical sessions are suggested to attain the world record of crescent citing.

Figure 7(a), shows the thermal image of the planet Mercury obtained on January 22, 2015. The illustration of the position of Mercury at the time of observation was obtained from the Sky software and is presented in Figure 7(b). The observation time was 5:10 PM, two minutes after the sunset, the altitude was 13 19, phase 21.1 %, air mass 4.30, and the angular separation from the sun was 17 34. The success of these initial practical experiments demonstrated the principal functionality of the IR telescope.

Additionally, using the developed system, we were able to observe the partial lunar eclipse that occurred on September 27, 2015. Figure 8, shows some of the obtained thermal images during this eclipse.

It is obvious from the images that the highlands (e.g., Moon Alps) and younger craters like Copernicus and Tycho on the moon are brighter than, for instance, darker lava plains. This is partly because the former features absorb less

solar radiation than the latter. At thermal infrared wavelengths, the lunar surface is nearly an ideal black-body radiator, mostly due to the tiny spaces and holes between the dust grains [6] [24] [28] [32] [33] [34]. The thermal properties of the dust layer on the lunar surface are responsible for the large temperature changes during a lunar day [31]. Therefore, lunar eclipses present excellent opportunities to study the changes that happen on the lunar surface when solar

radiations are suddenly cut off by the transit of the Earth [31] [35]. Thermal properties of the lunar surface during different lunar eclipses obtained by the developed system constitute the subject of ongoing project.

A prototype portable infrared (7 - 14 µm) system has been designed, developed, and used for lunar and atmospheric studies. The system is one of the ground-based thermal imaging instrumentations which utilize the advance technology of thermal cameras and associated accessories.

Preliminarily tests have been conducted under real conditions and have shown good results. Thermal observations for the lunar surface were carried out in different phases and showed superior results. Lunar features like the highlands and particularly, the less dark cratered lowlands were clearly identifiable. Up till now, the telescopic system was probably sensitive enough to detect the stored warmth from the past lunation on the bottom of younger craters. Additionally, a 26-h old crescent has been clearly cited without any complications and the thermal image of Mercury is obtained.

The scientific importance of the IR telescope lies not only in finding out more about the thermal behavior of the lunar surface but also uses the flexibility of the system to study more about further applications of IR systems in remote-sensing applications. The system can be used for non-destructive testing from a distance, such as on power lines or very large constructions like bridges and towers. A notable application of the system is that it can be used to learn more about our atmosphere, particularly about aerosols, by exploring the atmospheric absorption and emission in thermal IR wavelengths. Measuring the atmospheric absorption and emission by comparing the signal with the given lunar temperature at different altitudes above the horizon can deliver valuable information about our atmosphere. This can be very important for improving climate simulations, particularly for the subtropical zones. Additionally, the developed system can be used for student’s projects and outreach programs which can offer rich learning and exploration opportunities for students and researchers in different fields. These include detector technologies and optics, involving in some meaningful measurements to understand the thermal properties of the moon and the atmosphere. In such programs students and researchers will involve in the world of experimental thermal imaging, calibration procedures, data analysis and interpretation, and, finally, the sharing and distribution of the scientific results.

Since the actual version of the telescope is a prototype designed without practical experience, based on theoretical considerations. It works well, but there is still room for further enhancements that build on the experiences gained by the first operation of the prototype. There are several modifications which have been suggested for the better performance and functionality for the developed system. These relate to the limitations of the size of the relay lens, diameter of the optical mirror, alignments, and weight of the whole system.

For extended use, the construction of a sturdy permanent pier instead of a tripod would increase the mechanical precision and stiffness by an order of 10. The actual telescope is sensitive within the given limits of the IR camera and the atmospheric range, but this is true only for the circle of the innermost field of view, which covers 2/3^rd of the vertical dimensions of the camera sensor. The majority of the field is affected by vignetting. This is caused by the limited clear diameter of the germanium relay lens. Despite the vignetting, the developed system can be used without problems but only within a limited field of view. The limited diameter is a consequence of the budget limits imposed for the prototype to reduce the economic risk.

More frequent observational sessions are suggested to overcome the environmental problems. These observations will help in establishing a reliable database of atmospheric behavior under different circumstances, which will be useful for the scientific community.

We would like to thank the King Abdulaziz City for Science and Technology (KACST) for supporting this work. Also, we would like to thank the anonymous reviewers for their valuable comments and recommendations.

The authors declare no conflicts of interest regarding the publication of this paper.

Maghrabi, A., Dittié, G., Al-Dosari, A., Al-Mutiri, M., Al-Tlasi, M. and Alshehri, A. (2021) On the Development of a Simple and Portable Ground-Based Infrared Imaging System for Lunar and Atmospheric Studies. International Journal of Astronomy and Astrophysics, 11, 422-434. https://doi.org/10.4236/ijaa.2021.113020