Cassegrain Focus Research Articles

Design and development of a stand-off Raman brassboard (SDU-RRS) for the spectroscopic study of planetary materials

Raman spectroscopy has emerged as a crucial mineral analysis technique in planetary surface exploration missions. Nonetheless, the inherently low Raman scattering efficiency of planetary silicate materials makes it challenging to extract enough Raman information. Theoretical and experimental studies of the remote Raman scattering properties of planetary materials are also urgent requirements for future lunar and planetary explorations. Here, Shandong University Remote Raman Spectrometer (SDU-RRS) was developed to demonstrate the feasibility of lunar remote Raman technology and conduct preliminary research on remote Raman scattering properties. SDU-RRS utilizes a pulsed 532 nm laser, a non-focal Cassegrain telescope, a volume phase holographic grating, an intensified charge-coupled device, and the time-gating technique to detect weak-signal silicate minerals. The spectral resolution obtained with atomic emission lamps was <4.91 cm−1, and the wavelength accuracy was <1 cm−1, across the spectral range of 241–2430 cm−1. SDU-RRS can detect natural augite within a feldspar-olivine-augite matrix at a concentration of 20 % at ∼1 m under ambient lighting conditions. A series of experiments were conducted to evaluate the influence of measurement conditions and physical matrix effects on acquired Raman signals, either qualitatively or quantitatively, on geological materials. The study indicates that the transmission of Raman-scattered light conforms to Lambert’s cosine law, and a linear correlation exists between Raman intensity and laser power. The study also evaluated the impact of grain size, surface roughness, porosity, and shadow-hiding effects. Reducing grain size decreases Raman intensity and broadens Raman spectra. These characteristics are essential for achieving definitive mineralogical information from granular materials by remote Raman spectroscopy in lunar and planetary explorations.

Propagation properties of the partially coherent radially polarized twisted beam through optical system in turbulence atmosphere

In this paper, the transmission properties of the partially coherent radially polarized twisted (PCRPT) beam propagating in the turbulence atmosphere are investigated. The analytical formulas for the components of the cross-spectral density matrix for the PCRPT beam passing through the optical system in the turbulent atmosphere are developed using the Collins integral and aperture function. Research results indicate that modifying the variable parameters and dimensions of the optical system can control the near-field and far-field distributions of the beam, while providing a more flexible choice of receivers for the PCRPT beam in the receiving plane. By utilizing the Cassegrain reflector system and adjusting the optical system parameters, it is possible to achieve collimated transmission of the PCRPT beam and significantly enhance the beam transmission efficiency in turbulent atmospheric conditions. The derivation process and the research results presented in this paper can be expanded to analyze the application of optical systems to control high-dimensional beam field variations. The envisioned utilization of the results obtained from this research investigation pertains to the fields of beam shaping and optical communications.

Magnetic Fields beneath Active Region Coronal Loops

We examine the hypothesis that multipolar magnetic fields advected by photospheric granules can contribute to heating the active chromosphere and corona. On 2020 September 28 the Gregor Infrared Spectrograph (GRIS) and HiFI+ instruments at the GREGOR telescope obtained data of NOAA 12773. We analyze Stokes profiles of spectral lines of Si i and He i, to study magnetic fields from the photosphere to the upper chromosphere. Magnetogram and EUV data from the Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly instruments on the Solar Dynamics Observatory spacecraft are coaligned and studied in relation to the GRIS data. At coronal loop footpoints, minor polarity fields comprise just 0.2% and 0.02% of the flux measured over the 40″ × 60″ area observed in the photosphere and upper chromosphere, centered 320″ from the disk center. Significantly, the minority fields are situated ≳12″ from bright footpoints. We use physical arguments to show that any unresolved minority flux cannot reach coronal footpoints adjacent to the upper chromosphere. Even if it did, the most optimistic estimate of the energy released through chromospheric reconnection is barely sufficient to account for the coronal energy losses. Further, dynamical changes accompanying reconnection between uni- and multipolar fields are seen neither in the He i data nor in narrowband movies of the Hα line core. We conclude that the hypothesis must be rejected. Bright chromospheric, transition region, and coronal loop plasmas must be heated by mechanisms involving unipolar fields.

Temperature mapping methods for thermoelastic analyses of the ARIEL spacecraft payload module

The ARIEL mission is a European space project that aims to detect exoplanets with a spacecraft orbiting around the L2 point of the Sun-Earth system. The main payload consists of a Cassegrain telescope composed of mirrors that reflect and concentrate the incoming light from the deep space observations to finally guide it to the detectors. As in many other space missions, a dedicated complex assessment is established during the design phase to evaluate the impact on the optical performance caused by thermoelastic effects, which involves the coordinated work of the thermal, structural, and optical engineers. Despite that well-known and standardized processes and tools are established separately in each involved area, there is a lack of standardization about the way of exchanging the data between them, where additional calculations are required in some cases. This work focuses on the temperature mapping, which is the intermediate step between thermal and structural analyses, where temperatures are transferred to the structural model. The main difficulty of this process is related to the differences in modelling methods and approaches between both models, being necessary the development of an adequate algorithm to find the most accurate transfer of temperatures. This paper shows two different options for temperature mapping, detailing the proposed flowcharts. One of these methods requires the performance of an additional thermal conductive analysis, where a new improved procedure has been implemented in this work to solve some computational issues that made its application for large models difficult or even unfeasible. Both temperature mapping methods have been applied to the payload module of the ARIEL spacecraft, comparing the output results in terms of temperatures, stresses, forces, and displacements to evaluate their differences.

The dependence of the magnetism of a near-limb sunspot on height

Context. The physical parameters of the sunspot are not fully understood, especially the height dependence of the magnetic field. So far, it is also an open question as to which heights the He I 10 830 Å spectral line is formed at. Aims. Our aim is to investigate the magnetic and dynamical properties in the atmosphere above a sunspot, from the photosphere to the chromosphere. We analyzed the photospheric and chromospheric magnetic field properties of a stable sunspot in AR 12553 on June 20, 2016 using spectropolarimetric observations obtained with the GREGOR Infrared Spectrograph (GRIS) at the 1.5-meter GREGOR telescope. Methods. A spectral-line inversion technique was used to infer the magnetic field vector and Doppler velocities from the full Stokes profiles. In total, three spectral lines were inverted and the variation of the magnetic properties was qualified using the average values of the radial circles. The sunspot is located close to the solar limb, and thus this allows us to make a geometrical determination of the height of the spectral line He I 10 830 Å. Results. We find the height of helium spectral line to be 970 km above the photospheric spectral lines directly from observation at a stable sunspot. The total magnetic field strength decreases with height over the sunspot; the rates are −0.34 G km−1 for the umbra and −0.28 G km−1 for the penumbra. The inclination increases with increasing height in the umbra, but decreases in the penumbra. In the umbra, the vertical component (Bz) decreases with height, while the horizontal component (Bhor) remains almost constant. In the penumbra this is reversed, as Bz remains nearly constant over height, while Bhor decreases. We also observe fast velocities with 30 km s−1 in small chromospheric patches on the central side of the spot. Conclusions. The key parameters depending on height in the sunspot are the Bz component of the magnetic field for the umbra and the Bhor component of the magnetic field for the penumbra. The observation revealed supersonic downward velocities in and near the outer penumbra.

Multi-purpose InSTRument for Astronomy at Low-resolution: MISTRAL at the Observatoire de Haute-Provence

Multi-purpose InSTRument for Astronomy at Low-resolution (MISTRAL) is the new Faint Object Spectroscopic Camera mounted at the folded Cassegrain focus of the 1.93m telescope of the Haute-Provence Observatory (OHP). We describe the design and components of the instrument and give some details about its operation. We emphasize in particular the various observing modes and the performance of the detector. A short description of the working environment is also provided. Various types of objects, including stars, nebulae, comets, novae, and galaxies, have been observed during various test phases to evaluate the performance of the instrument. The instrument covers the range of 4000 to 8000 AA with the blue setting, or from 6000 to 10000 AA with the red setting, at an average spectral resolution of 700. Its peak efficiency is about 22$&lt;!PCT!&gt;$ at 6000 AA . In spectroscopy, a limiting magnitude of rsim 19.5 can be achieved for a point source in one hour with a signal-to-noise ratio of 3 in the continuum (and better when emission lines are present). In imaging mode, limiting magnitudes of 20-21 can be obtained in 10-20mn (with average seeing conditions of 2.5 arcsec at the OHP). The instrument is very user-friendly and can be put into operations in less than 15mn (rapid change-over from the other instrument in use) if required by the science (e.g. for gamma-ray bursts). Some first scientific results are described for various types of objects, and in particular, for the follow-up of gamma-ray bursts. While some further improvements are still under way, in particular, to facilitate the switch from blue to red setting and add more grisms or filters, MISTRAL is ready for the follow-up of transients and other variable objects, in the soon-to-come era of the Space-based multi-band astronomical Variable Objects Monitor (SVOM) satellite and of the Rubin telescope, for instance.

An analytic expression for the field dependence of three-order Zernike aberrations in decentered and/or tilted optical systems

The double Zernike polynomials can describe the aperture dependence and field dependence of non-coaxial optical system aberrations simultaneously, and characterize the complex optical aberrations, which are of great value in the optical system design and alignment. In this paper, the specific analytic forms of all third-order double Zernike aberrations in decentered and/or tilted optical systems are derived based on the nodal aberration theory. This paper realizes the orthogonalization of the third-order aberrations with different aperture dependence and field dependence. The calculation process of the third-order double Zernike aberrations is presented. The ray tracing of the Cassegrain telescope verifies the analytic derivation of the double Zernike aberrations.

1064 nm rotational Raman polarization lidar for profiling aerosol and cloud characteristics

The vertical profiles of aerosol or mixed-phase cloud optical properties (e.g. extinction coefficient) at 1064 nm are difficult to obtain from lidar observations. Based on the techniques of rotational Raman signal at 1058 nm described by Haarig et al. [ Atmos. Meas. Tech. 9, 4269 (2016)10.5194/amt-9-4269-2016 ], we have developed a novel rotational Raman polarization lidar at 1064 nm at Wuhan University. In this design, we optimized the central wavelength of the rotational Raman channel to 1056 nm with a bandwidth of 6 nm to increase the signal-to-noise ratio and minimize the temperature dependence of the extracted rotational Raman spectrum. And then separated elastic polarization channels (1064 nm Parallel, P and 1064 nm Cross, S) into near range (low 1064 nm P and 1064 nm S) and far range detection channels (high 1064 nm P and 1064 nm S) to extend the dynamic range of lidar observation. Silicon single photon avalanche diodes (SPAD) working at photon counting mode were applied to improve the quantum efficiency and reduce the electronic noise, which resulted in quantum efficiency of 2.5%. With a power of 3 W diode pumped pulsed Nd:YAG laser and aperture of 250 mm Cassegrain telescope, the detectable range can cover the atmosphere from 0.3 km to the top troposphere (about 12-15 km). To the best of our knowledge, the design of this novel lidar system is described and the mixed-phase cloud and aerosol optical properties observations of backscatter coefficients, extinction coefficients, lidar ratio and depolarization ratio at 1064 nm were performed as demonstrations of the system capabilities.

An Investigation of six contact binary systems and a semi-detached one

We present a photometric analysis of six W Ursae Majoris contact binaries and one semi-detached system using multifilter CCD observations. The data were obtained with the 0.25 m Schmidt Cassegrain telescope at the observatory Stazione Astronomica Betelgeuse (SAB) in Magnago, Italy, and the 30-cm Ritchey Chretien Astrograph at the IRIDA observatory (IO) of the NAO Rozhen in Bulgaria. We applied the Wilson–Devinney code to model the light curves and derive the orbital and physical parameters of the systems. Four of the contact binaries belong to the W sub-type and two to the A sub-type, and all are in thermal contact. 2MASS J23194851+3603503 has a very low mass ratio q = 0.093 and a fill-out factor of f = 71.4%, whereas the other contact binaries have mass ratios between 0.2 and 0.38 and fill-out factors between 8% and 39%. 2MASS J04525351+6246069 is a semi-detached system with q = 0.125 and a temperature difference of 540 K between the components. We also estimated the absolute elements of all seven eclipsing binaries.

Photothermal spectroscopy on-chip sensor for the measurement of a PMMA film using a silicon nitride micro-ring resonator and an external cavity quantum cascade laser

Abstract Laser-based mid-infrared (mid-IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC-QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm−1), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin-coated on top of a silicon nitride (Si3N4) micro-ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si3N4 transducer coated with PMMA, as a proof-of-principle for an on-chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid-IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid-IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on-line/in-line monitoring in several industrial processes.

Design of a compact multispectral telescope consisting of a Cassegrain telescope and a concave elliptical grating.

In this paper we design a high-performance multispectral telescope with a concave elliptical grating for a field of view of 3° in the VNIR spectral range of 0.48-0.82µm, at an altitude of 760km from the ground, with total length of 140mm, which has a small volume and a simple structure. The paper reports on the MTF, spot, and field curvature diagrams, which show that it can achieve spectral and spatial resolutions of 25nm and 5.5m, respectively, with good image quality (MTF value for all wavelengths is higher than 0.2 at Nyquist frequency of 217 cycles per mm) and has the least possible aberrations, without the need for any lenses.

Optical systems with translational invariance.

The leading order monochromatic aberrations are investigated for optical systems, which obey single-plane symmetry and translational invariance. These aberrations are classified from the symmetry principles for the wave aberration function. Fermat's principle is applied for a system made of generic cylindrical surfaces, and a complete set of the aberration coefficients required for calculation of the aberrations for any paraxial ray is obtained. To demonstrate the applied value of the analytical results obtained, the criteria of compensation of the leading aberrations for the cylindrical analog of the Cassegrain telescope were found.

An alternative, versatile, high‐tolerance design of a modified Richter–Slevogt camera, using standard glasses

AbstractPrime‐focus catadioptric astrographs have been used for a long time in various astronomical applications. The prime advantage offered by them is the capability to produce remarkably wide fields of view, and hence generate a huge amount of data in relatively less observation time. An emerging application of such wide‐field astrographs is in the form of telescope arrays. While this has been implemented mostly, using commercial refractive lenses, low‐cost catadioptric objectives can be used as an alternative for wide‐field or high‐contrast array applications. Commercial catadioptric systems are generally available as modifications of Schmidt and Maksutov systems, that too, mostly in the Cassegrain configuration. Here, we present a low‐cost alternative prime focus camera design of Richter–Slevogt configuration, which is in turn an extension of the Houghton correctors. The Richter–Slevogt design has the potential for a very high performance due to several degrees of freedom. The presented one is a 150 mm aperture, system, providing 3.5° (circular) diffraction‐limited FOV (strehl ), using only standard glasses, N‐BK7 and F2. We present the performance analysis, tolerance sensitivity, and statistical (Monte‐Carlo) analysis for this design. Potential applications of the system, other than as an array are also briefly discussed.

Comparison of the Optical Efficiency of Two Designs of the Maksutov–Cassegrain Telescope

The research aims to develop the best possible design for the widely used Cassegrain telescope. The system consists of two models of different designs: (a) the telescope consists of a Maksutov lens, a spherical primary mirror, and a secondary mirror attached to the lens; (b) it consists of a Maksutov lens and a spherical primary mirror, plus a non-lens attached secondary mirror located between the lens and the primary mirror. The image was evaluated and analyzed using the analysis tools in Zemax software. The results of the two designs showed that the telescope whose secondary mirrors are not adjacent to the Maksutov lens produces high quality image that is almost free from aberration, and then comes the telescope whose secondary mirrors are adjacent to the Maksutov lens in terms of image quality. In the first design, the tangential and sagittal axes in the MTF function of the remaining angles (0.1, 0.2, 0.3, 0.4, and 0.5) contain two curves, which show how the loss of symmetry has changed the value of the function for the two axes. In the second design, the sagittal and tangential axes are identical in the angles of incidence (0, 0.1, and 0.2), and the transverse and sagittal axes of the remaining angles (0.3, 0.4, and 0.5) contain two curves. In PSF for the first design, there are several pretty high peaks on the surface of the image at the angle of incidence (0, 0.1). Since the shape is regular, the point spread function indicates that there isn't an aberration, but in terms of the angles (0.2, 0.3, 0.4, 0.5), we observe a gradual loss in intensity and the appearance of elevations on both sides of the picture. In the second design, at the angles (0, 0.1, 0.2, 0.3), we observe in Figure 5 that there are several high peaks free from aberration.

Bessel light beam for a surgical laser focusing telescope-a novel approach.

As the demand for CO[Formula: see text] laser surgeries continues to grow, the quality of their main instrument, the laser micromanipulator, becomes increasingly important. However, in many surgery systems, a large ratio of the laser power is wasted due to the reflection from the mirror of a telescopic system, like a Cassegrain telescope, back to the laser side, which not only decreases the system's efficiency but can also damage the system itself. In this article, we introduce a new design of the micromanipulator telescope for CO[Formula: see text] laser surgery, which employs a Bessel beam to improve the system efficiency. As in the propagation of a Bessel beam, the power of the light beam can be transferred from the center to a ring shape, the whole power reflected from the first mirror can reach the second mirror and no power goes back to the second mirror hole. The micromanipulator telescope design and optimization are carried out using Zemax Optics Studio, and the integration of the Bessel beam into the system is implemented using MATLAB. Our simulation results show that by applying the appropriate Bessel beam, the system efficiency can reach more than 96%, and the normalized peak irradiance can increase by 40 to 73% for various working distances. In addition to increasing the system efficiency and normalized peak irradiance, resulting in a sharper surgical blade, the use of the Bessel beam enhances the depth of focus, making the system less sensitive to depth misalignment.

Standoff Deep Ultraviolet Raman Spectrometer for Trace Detection.

We developed a state-of-the-art, high-sensitivity, low-stray-light standoff deep-ultraviolet (DUV) Raman spectrometer for the trace detection of resonance Raman-enhanced chemical species. As an excitation source for Raman measurements, we utilized our recently developed, second-generation, miniaturized, diode-pumped, solid-state neodymium-doped gadolinium orthovanadate (Nd:GdVO4) laser that generates quasi-continuous wave 228 nm light. This 228 nm excitation enhances the Raman intensities of vibrations of NOx groups in explosive molecules, aromatic groups in biological molecules, and various aromatic hydrocarbons. Our DUV Raman spectrograph utilizes a custom DUV f/8 Cassegrain telescope with an ∼200 mm diameter primary mirror, high-efficiency DUV transmission gratings, custom DUV mirrors, and a custom 228 nm Rayleigh rejection filter. We utilized our new standoff DUV Raman spectrometer to measure high signal-to-noise ratio spectra of ∼50 μg/cm2 drop-cast explosives: ammonium nitrate (AN), trinitrotoluene, pentaerythritol tetranitrate as well as aromatic biological molecules: lysozyme, tryptophan, tyrosine, deoxycytidine monophosphate, deoxyadenosine monophosphate at an ∼3 m distance within 10-30 s accumulation times. We roughly estimate the average ultraviolet resonance Raman (UVRR) detection limits for the relatively homogeneous drop-cast films of explosives and biological molecules to be ∼1 μg/cm2 when utilizing a continuous raster scanning that averages Raman signal over ∼1 cm2 sample area to avoid quick analyte depletion due to ultraviolet (UV) photolysis. We determined 3 m standoff UVRR detection limits for drop-cast AN films and identified factors impacting UVRR detection limits such as analyte photochemistry and analyte morphology. We found a detection limit of ∼0.5 μg/cm2 for drop-cast AN films on glass substrates when the Raman signal is averaged over ∼0.5 cm2 of sample surface using a continuous raster scan. For a step raster scan, when the probed sample area is limited to the laser spot size, the detection limit is approximately tenfold higher (∼5 μg/cm2) due to the impact of UV photochemistry.

A high-precision and efficient adaptive alignment method for Cassegrain dual mirror optical system based on machine learning algorithms

The Cassegrain telescope is widely used in aerospace exploration equipment, characterized by compact structure, complex optical path, and high imaging quality. However, due to the difficulty in establishing an accurate correspondence between the relative pose and imaging quality of a multi mirror group with real machining errors, the current Cassegrain telescope assembly process is very difficult, with blind operation, time-consuming, and low accuracy. This article proposes a high-precision adaptive alignment method for Cassegrain dual mirror optical systems based on machine learning algorisms, and an experimental adaptive alignment system with uncoupled degrees of freedom is developed. Firstly, the overall architecture of the adaptive alignment method is proposed consists of detection, calculation and alignment modules. In the detection module, the Zernike polynomial coefficient of wavefront aberration of the optical system is detected online through the interferometer, meanwhile the pose coordinates of the secondary mirror is accurately fed back through a 6-DOF nanoscale micro mechanism. In the calculation module, machine learning algorithm is applied to build a nonlinear mapping model between the Zernike coefficient and the pose coordinates of the secondary mirror. In the alignment module, the pose coordinates of the secondary mirror can be forced to adjust to the target position. Then, during the real alignment process, the Zernike coefficient test data of the optical system alignment process is monitored in real time, and the nonlinear mapping model is employed to calculate the pose coordinates and then the misalignment of the secondary mirror. Finally, the alignment module is driven to execute the pose correction according to the calculated misalignment value, realizing a high-precision adjustment of the Cassegrain dual mirror optical system. Experimental results shows that the average alignment time cost can be dramatically reduced from around 7 days using the current manual alignment method to just 30 minutes using the proposed adaptive alignment method for achieving a current standard alignment accuracy of wavefront aberration root mean square (RMS) less than 0.1λ, which greatly improves the assembly efficiency. This study proposes a new method for high-precision and efficient alignment of optical systems based on artificial intelligence and contributes to the efficiency improvement for optical assembly.

Photometric analysis of Type Ia Supernova 2023jvj

The study of supernovae – large explosions at the end of a star’s life that release immense energy and various elements into space – is important in the fields of stellar evolution and the expansion of the universe. In the large cosmological time scale, supernovae allow for a short glimpse into the universe’s evolution. Upon the discovery of Supernova (SN) 2023jvj, located in an anonymous galaxy in the Boötes constellation, our team launched an investigation to further knowledge of transient objects and their role in the cosmos. In this study, we conducted a photometric analysis of SN 2023jvj to determine its type and distance. We hypothesized SN 2023jvj would be a Type Ia supernova based on reports that there were no hydrogen spectral lines. We observed SN 2023jvj and collected photometric data over four weeks using the 12-inch Meade Schmidt Cassegrain Telescope (SCT) and 16-inch Ritchey-Chretien Telescope (RCT) at the Leitner Family Observatory and Planetarium (LFOP). Images were additionally collected remotely from Australia and Utah via the iTelescope Network. Using this data, standard Sloan green and Sloan red magnitudes were found through color calibration and used to generate a light curve (a plot of supernova brightness over time). SN 2023jvj was then determined to be a Type Ia supernova — a standard candle — with a distance modulus of 35.36. Therefore, SN 2023jvj is approximately 1.246e8 parsecs away from Earth. Our findings have a broader impact on how astronomers analyze the relationship between the brightness and distance of transient objects in the ever-expanding universe.

Designing Cassegrain Telescope System with Best Obscuration Ratio of Secondary Mirror

Designing Cassegrain Telescope System with Best Obscuration Ratio of Secondary Mirror

Optical Pupil Shift Correction Method for Large Ground-Based Optical Telescopes

Cassegrain telescopes with larger apertures suffer significant optical pupil shifting errors caused by gravity and temperature gradients. This study constructs an optical model of a large Cassegrain telescope and a method to calculate and compensate its shifting error. First, the optical structure is simplified by incorporating only the most relevant telescope components, and the system optics is modeled using ray tracing. A computer-aided mounting-based method for calculating the misalignment error of primary and secondary mirrors is proposed, with the spatial position change of secondary mirrors as the input. Next, a compensation method based on the coma-free point theory of the Cassegrain system is proposed, with the main mirror’s optical axis as the reference. Finally, using a 4 m aperture telescope as an example, the gravity- and thermal-deformation-induced shifting error is simulated. Based on this simulation, the secondary mirror position is adjusted using a secondary mirror-adjustment mechanism Hexapod platform to compensate for the misalignment error. Both gravity and temperature are major causes of shift in the pupil position, with a maximum shift of 18 mm. After compensation, this shift is controlled within 1 mm. The modeling method and the simulation process mentioned in this research can also be used in the other relevant fields.