## 1 Introduction  
The spaceflight environment influences physiological changes in biological molecules, cells, tissues, and organs. Microgravity (\(10^{-3}\) to \(10^{-6}\) of 1 g) is one of the most prominent stressors that organisms experience during spaceflight, and is known to induce epigenetic and structural changes in cells. In humans, long-term microgravity exposure has been associated with conditions such as delayed wound healing, muscle atrophy, bone density loss, cardiovascular deconditioning, immune dysregulation, and ocular changes, making it a serious challenge to extended human habitation in space [1]. As space exploration progresses toward long-duration crewed missions to the Moon and Mars, it is important to understand the mechanisms by which microgravity affects physiology in order to accurately assess health risks and to develop effective countermeasures. The processes of gravity-sensing, signal transduction, and gravity-mediated cell adaptations are still not fully understood, primarily due to the limited spaceflight opportunitiesand the costs associated with them. Microgravity analogs for research on Earth include parabolic flights, high-altitude drops, and sounding rocket tests; however, these platforms offer only a few seconds to minutes of microgravity, require specialized hardware for travel and sustained functioning in the microgravity environment, and are often cost prohibitive. Microgravity simulation devices (MSDs) provide ground-based simulated microgravity (SMG) platforms that enable greater opportunities for gravitational biology research and provide an opportunity to test hypotheses, parameters, and experimental conditions, thereby providing preliminary data before venturing into the true microgravity of space [2; 3; 4]. MSDs function by averaging the gravity vector to zero over time and ensuring that gravity does not act in a constant direction for longer than a few seconds. The random positioning machine (RPM) is a type of MSD that randomizes the gravity vector in three-dimensional space through continuous random rotation along two axes. In many cases, the effects of SMG on samples may be functionally comparable to that of true microgravity if the gravity vector's direction changes faster than the sample's response time to the change [5]. Under these conditions, living objects such as mammalian cells and plants are suitable candidates for microgravity simulation studies [6]. Microscopy is widely used to analyze cellular structures and processes such as cell division, proliferation, cell cycle, and wound healing, and to measure various parameters of tissue growth and development in cells. Many studies on SMG-induced cellular changes require the extrapolation of data deduced from images that are acquired on a microscope in the 1g environment after the samples are exposed to SMG, which fails to completely capture the series of events and mechanisms that lead up to a response. The real-time microscopic monitoring of samples during SMG exposure is beneficial for studying rapid and choreographed cellular processes such as cytoskeleton reorganization and morphological changes [6; 7]. Until recently, the continuous real-time imaging of living specimens was not routinely available on the RPM and other 3D clinostats, and the analysis of the effects of microgravity on biological samples in spaceflight and in SMG were routinely performed a posteriori the fixation of specimens. Microscopes used in the physical sciences for microgravity research can withstand the vibrations of microgravity analogs but have poor imaging performance and low numerical apertures that render them incapable of the high magnification needed for cellular imaging. Digital holographic microscopy (DHM) for full-scale RPMs was recently developed and has successfully detected cell morphology changes and cytoskeletal alterations in real-time for up to 1.5 h [8].