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Ground-based telescope

Astronomical telescope

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Ground based telescope, different from Space telescope , Yes Astronomical telescope One of. The foundation is built on the ground and will be affected by atmospheric interference.

Chinese name: Ground-based telescope
Foreign name: ground-base telescopes

Features: Built on the ground
Examples: Keck telescope ， Gemini telescope etc.

catalog

▪ Hydraulic bearing
four Driving of ground-based telescope
five Disturbance of ground-based telescope

Basic explanation

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Since Hubble Space Telescope After, put all Astronomical telescope Both are called ground-based telescopes. ( hubble space telescope It is the largest and most accurate astronomical telescope ever launched into space.) To distinguish between Space telescope The original astronomical telescopes built on the ground are called ground-based telescopes.

Relevant ground-based telescope: located at Hawaii Of Keck telescope 、 National Astronomical Observatory Of Gemini telescope , Hawaiian Pleiades Cluster Telescope Etc.

Foundation telescope support

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Equatorial bearing

The "telescope support" is used to support the telescope tube and realize the rotation of the telescope tube in the process of aiming and tracking. telescope Support It can be divided into equatorial support and horizontal support.^[1]

Equatorial bearing

Almost all telescopes developed before 1980 use equatorial supports. The principle is that the telescope tube rotates in the opposite direction of the earth's rotation around an axis parallel to the earth's axis to offset the earth's rotation. The equatorial bearing can realize the movement around the "polar axis" (parallel to the earth axis) and the "latitudinal axis" (perpendicular to the earth axis).

Horizontal bearing

In the horizontal support, the mirror tube is formed by horizontal axis The rotation of (pitch axis) is aligned. It is different from equatorial bearing. Neither of the two shafting supporting the pitching one azimuth support gravity Change direction. Structurally, this is the most robust and simple rack. Significant reduction in quality (and cost). This makes the current pitch azimuth support become a standard structure, even including medium-sized telescopes.

Foundation telescope bearing

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Bearing Used for mirror tube and Support Positioning of. In the horizontal formula, decrease Focal plane Rotate. When selecting bearings, three main technical performances must always be considered: rigidity 、 accuracy And low friction. Two bearing technologies are used for large telescopes: rolling element bearings and hydraulic bearings. Because the large mass needs support, use Air bearing and magnetic suspension It is unrealistic.^[2]

Rolling element bearing

Cylindrical or spherical rolling element bearings are generally available on the market, with sufficient rigidity and precision. The rolling element bearing has been successfully used in the 4m telescope, but its inherent friction makes it unable to meet the needs of larger telescopes.

Hydraulic bearing

Most very large telescopes rely on Hydraulic bearing 。 The hydraulic bearing has almost no friction and does not exist at low speed Nonlinear 。 The hydraulic bearing has high load capacity, compact structure, extremely high rigidity, and the accuracy can be guaranteed to be at least not lower than that of the rolling element bearing.

although Hydraulic bearing Expensive than rolling element bearings. However, because bearings, pumping units and pipeline systems are only a small part of the overall cost, and hydraulic bearings have the advantages of low friction and high stiffness, making hydraulic bearings a better choice.

Driving of ground-based telescope

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Until the 1970s, most ground-based telescopes were used Worm gear The system is driven because of its high reduction ratio and excellent internal precision. Use a constant speed machine Motor Open loop tracking can be realized. Worm gear reducer It can be realized on a single pair of worm gears Speed ratio (for example, 1/720) to complete a very rigid drive. Moreover, due to its inherent irreversibility, the worm gear is the only type of drive that can provide absolute safety when the telescope is out of balance: when the telescope is operating in an unbalanced state, whether it is human error or component damage, it will not "fly".^[3]

Disturbance of ground-based telescope

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The disturbance of ground-based telescope comes from the internal (i.e. generated from the interior of the telescope) excitation and wind.

When the guide star is visible, it is located at Main focus Or the observers in the Cassegrain structure are themselves potential sources of internal incentives. Nowadays, the internal excitation sources are the mechanical movement in the instrument (the movement of the filter wheel), the torque fluctuation of the drive motor, and the friction between the telescope shafts and cables. Since the rotating inertia of the telescope is very large, the mechanical motion usually has no effect. Torque ripple and friction can be suppressed by reasonable design of the control system.

The biggest interference source of ground-based telescope is wind. The wind is very harmful, because its Power spectrum Contains a lot of Low frequency (0.1~1Hz) energy, quite close truss And active mirror system resonant frequency 。

Most observatories are in remote areas at high altitude, where the wind is very strong. In order to improve the observation efficiency, 95% of the observatories make observations when the sky is clear. This leads to the need for telescopes to observe at relatively fast wind speeds（ Mauna Kea Observatory Wind speed up to 20m/s).^[4]

Ground based telescope reduces background thermal radiation

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Ground based telescope itself thermal radiation , is the main source of background thermal radiation received by the detector, sometimes even the whole source. When telescopes and instruments are used in the infrared band, they need to carefully design the parts that can be seen by the detector to reduce their thermal radiation. The main thermal radiation sources are the mirror optical surface, the light shield ring (unless the telescope is designed without shielding), the lens in the instrument and filter , window of low-temperature box, etc.^[5]

Ground based photoelectric telescope detection

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The ground-based photoelectric telescope is a passive detection method, which is a combination of lens telescope and Photodetector It is mainly used for high-resolution imaging and observation of space debris at a long distance. Like all telescopes, photoelectric telescopes also have application limitations that cannot work around the clock (optical invisibility period), such as inability to observe non luminous objects in the shadow of the earth, clouds, fog, air pollution, and cities and full moon Glow and so on will lead to performance degradation, even can not be observed.^[6]

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