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preface
For space telescopes, the proportional index of change with diameter is smaller than that of the ground, and in space, most of the manufacturing costs are proportional to the aperture area.
Another reason is that for space telescopes, their weekly (e.g., design, testing) costs are also larger, large observatories cannot be simply defined in terms of technical and scientific requirements, and cost should also be part of design optimization.
Technical specifications of the observatory
The technical specifications of the observatory are the initial design phase, combining scientific requirements, operational considerations and environmental factors into a comprehensive document that defines all technical measures important for the performance of the observatory.
This process can be easily broken down into "levels", from general to specific, using terms and definitions from NASA, which are shown in the following diagram:
In addition to the science-driven specifications that form the backbone of the design requirements, the requirements for post-commissioning of telescopes cannot be ignored, and in science, operational requirements should include issues such as telescope operations, remote observations, and scientific data processing.
Longevity, safety, reliability, and maintainability are also important issues that are often overlooked or only considered informally in telescope design.
If these factors are not taken into account, they can have huge consequences at a later stage of operation. The corresponding requirements should be determined as part of the Level 2 requirements.
Determining the required lifespan of a telescope is not easy, and ground-based telescopes are typically designed to have a long lifespan, around 30 years, and often continue to be used for considerable periods of time.
A good example is the 5-meter-tall telescope on the mountain, designed in the late 2030s and still in operation, therefore, it is unrealistic to design for a lifetime that is too short and does not take into account the actual history of telescope use.
On the other hand, the requirement of unreasonably long service life forces designers to choose materials and components that are not required by technical performance goals, so these factors must be carefully balanced when setting the day-to-day use requirements of the telescope.
Safety requirements arise from the need to protect the people who use and maintain the telescope, the equipment itself, and the environment around the telescope, and hazards should be identified during the conceptual design phase and eliminated as much as possible through appropriate design.
When safer alternatives are not available, guards and safety devices must be incorporated into the design, and warning devices, safety procedures and personnel training are also used to reduce the risk of any residual hazards.
Reliability and maintainability are two concepts that must be considered simultaneously in telescope design.
Reliability refers to the telescope's ability to continue to meet performance specifications in the future, and maintainability refers to the frequency and complexity of maintenance tasks required to ensure its continued performance.
Telescopes have traditionally been designed to be highly reliable, but rarely have clear reliability specifications and do not always undergo formal reliability analysis.
Similarly, maintenance plans are usually only designed during or at the end of construction, and large telescopes have become quite complex, and both issues should be addressed in the design in order to produce a reliable and easily maintained observatory.
Error budget
Error budgets (also known as "performance budgets") are the most useful design tools for any engineering project, and they are absolutely necessary in the design of observatories due to the diversity of error sources and the complex nature of such systems.
The error budget consists of the performance requirements allocated between the various subsystems of the system and the various components of the system, which allows engineers in each discipline to design their own subsystems under the supervision of a systems engineer.
At the beginning of the project, assignments are somewhat haphazard, and then improvements are made as the design progresses through trade-off analysis, detailed studies, and manufacturing tolerances.
Error budget allocations should be made for all major observatory systems (e.g., optical, pointing, thermal, power), and two examples of optical mass and pointing stability are shown in the figure:
Initially, error budgets were obtained by allocating each potential source of error in a "top-down" manner. These will be based on first-order analysis, experience with other projects and sound engineering judgment.
If the size of the contribution is unknown, the allowance will be set as an initial best guess, summarizing error compensation, balancing the budget to achieve the overall goal, and sometimes the project objectives must be adjusted to match the feasibility of the project.
Most components cause irrelevant errors. These methods can be summed using the square root method (RSS) method, however, some sources of error are systemically linked and RSS summation is not appropriate.
For example, the radii and cone constants of the primary and secondary mirrors are correlated and should be balanced separately before entering the total wavefront error sum.
The initial error budget serves as a design guide for setting performance targets for each component in the system, and a component may sometimes perform better than the initial allowance, or it may not meet that goal.
As the performance of each component is better understood, budgets are filled "bottom-up", preferably with calculations or measurements recorded and the basis of each allowance recorded in a budget spreadsheet.
This allows the error budget to serve as an index of the collected engineering documents.
As initial guesswork is replaced by measurements and calculations, budgets are used to predict the overall performance of the system, budgets are periodically rebalanced and allowances are redistributed.
When done right, wrong budgeting can lead to design economics, focus engineering work where it's most useful, and help control project costs.
As observatories become larger and more complex, the error budgeting method described above becomes inadequate because it is empirical and does not provide a deterministic method for finding the optimal one.
This is because the determination of budget components is done in an educated, but "a priori" way, and not directly traced back to the physical characteristics and behavior of the relevant subsystems.
In addition, error budgeting methods necessarily fail to capture the complexity of environmental or operating conditions, often only dealing with nominal modes of operation in the average or worst-case scenario.
The availability of high-fidelity computer modeling provides an ideal solution, and detailed computer models of the entire system can now be developed to simulate actual operating conditions and explore a wide range of trade-offs.
This provides a deterministic solution for optimal configuration based on performance and cost criteria, using traditional optical, mathematical and finite element methods, generating separate models for each relevant system, and then combining these models into an integrated system-level model.
During the design concept phase of the NGST project, one approach to creating an integrated system-level model was based on a set of tools developed by the Jet Propulsion Laboratory.
The first of these tools is IMOS, which stands for "Integrated Modeling of Optical Systems."
IMOS is a collection of functions or subroutines that allow analysts to combine the necessary subsystem models in the context of Matlab computing.
22Matlab is a popular commercial code for general-purpose, matrix-oriented numerical analysis, and it also includes a powerful set of tools for control system analysis.
The tool for systems engineering, developed by the Jet Propulsion Laboratory, is MACOS, which stands for "Modeling and Control of Optical Systems."
MACOS is an analysis code that provides geometric and physical optical capabilities, making it uniquely useful in telescope design applications, and other features include support for segmented and deformable optics, and programming interfaces that allow other code to access all capabilities.
Using these tools, analysts can quickly determine system performance and perform parametric design optimizations under a variety of operating conditions.
In addition, these ensemble models are powerful tools for simulating jitter and wavefront control, as well as for analyzing specific phenomena such as thermal transients, microdynamic capture, and stick-slip events.
Design testability and breadth
Design testability and forgiveness are two important characteristics that should be considered when comparing various proposed designs.
Testability refers to the ability of a given design to be validated before final assembly (or deployment, for space telescopes).
This factor is especially important for large space telescopes that cannot be thoroughly tested before launch, or very large ground-based telescopes that cannot be installed in remote locations.
Given two design choices with the same performance, priority should be given to the one that can be best tested in the store or before launch.
Design exemptions refer to the ability of a given design to recover from design or manufacturing errors, unexpected changes in the environment, or damage or failure during operation.
A robust, flexible approach that works under a wide range of conditions to compensate for potential problems may ultimately be more cost-effective than highly optimized designs that require extensive testing to ensure their effectiveness.
Since the aperture diameter of a telescope is the single most costly parameter affecting an observatory, it is important to understand its impact on scientific performance.
Aperture diameter affects scientific performance in terms of both sensitivity and spatial resolution, and as we can see, sensitivity is best described as the integration time required to achieve a given signal-to-noise ratio.
The dependence of integration time and flux limit on aperture diameter is summarized in the table:
In space, or on the ground in the infrared, where the image is close to diffraction limitation, the gain of background limiting extended source observations due to larger apertures is most impressive.
This is because the increased throughput collected is combined with a reduced image size, which reduces the contribution of the background.
In order to understand how gravity affects or how the natural frequency scale of the telescope relates to the size of the telescope, it is instructive to model the telescope structure as a simple beam of uniform cross-section.
Structural deflection has two origins: point loading due to the weight of the supporting optics and self-deflection due to the mass of the structure itself.
Based on the above example, the table gives the proportional law of various factors affecting the telescope:
Cost model
Until the early 80s of the 20th century, the cost of traditional telescopes followed a well-established power law as a function of aperture diameter.
If the technology used at the time is still in use today, an 8- to 10-meter class telescope would be unaffordable.
But fortunately, a series of technological advances made it possible to build larger telescopes at a lower cost: mastering the aspheric calculation process led to faster primary selection (and therefore shorter telescopes and smaller domes);
Better testing methods and computer polishing transformed the optical calculation process from black art to deterministic science;
The use of stress mirror or stress ring technology greatly reduces polishing time; Use an alt-az bracket instead of a larger one.
summary
Basic astronomical and engineering data, a list of the major telescopes currently in existence, and an extensive vocabulary.
We hope this article will serve as a foundation for astronomers and engineers who will be faced with the need for larger space telescopes, both in space and on the ground, to work on the frontiers of knowledge.
Bibliography:
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[3] Bufton, J. L., Comparison of vertical-profile turbulent structure with stellar observations. Selection, vol. 12, pp. 8, pp. 1785, p. 1973.
[3] Barat, some features of clear atmospheric turbulence in the middle stratosphere, Atmos. Science. , vol. 39, p. No., p. 11, p. 2553, p. 1982.
[4] Bely, Weather and Observations in Mauna Kea, PASP, vol. 99, pp. 616, p. 560, 1987. [25] Marciadri, Three-dimensional mapping of optical turbulence using numerical models of the atmosphere. Astronomy. Vol. 137 (appended), p. 185, 1999.
[5] Roddier, F., "Effects of Atmospheric Turbulence," Progress in Optics, vol. 19, E, edition, North Holland, 1981, p. 281.