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THE 182 CM TELESCOPE: THE TELESCOPE

TECHNICAL DATA

Optics

Optical design of the telescope is a Classic Cassegrain with equivalent focal number f/9. The primary mirror M1 (made of Zerodur/ Shott duran 50) has a diameter of 182cm

Primary mirror
The primary mirror M1

The secondary mirror M2

The main characteristics of the mirrors are given in Table 1 below.

M1
M2
Total mass 1500 Kg
Total mass 67.4 Kg
Diameter 1820 mm
Diameter 580.0 mm
Thickness (external ring) 300 mm
Thickness (external ring)  110.3 mm
Diameter of central hole  Ø 383 mm
-
Radius of curvature 10840 mm
Radius of curvature  4592 mm
Focal length 5393 mm
Theorical distance between M1 and M2 3855 mm
F/number   f/3
-
Sagitta 38 mm
Corsa max di M2 ±30 mm

An active system for the movements of the secondary mirror was implemented in 1990. This unit, in addition to the standard focussing, allows one to control both X and Y positions of M2 and tilt angle ALFA, BETA. The whole system is controlled by a dedicated PC.

The mirror is periodically re-coated with aluminium using the vacuum chamber at Cima Ekar.

Mechanics

The telescope has a fork mounting that allows rotation about the Right Ascension axis for 450 degrees in each direction. The limit of pointing for the declination axis is dependent on the altitude of the target. The minimum altitude is fixed at 15 degrees by an hardware limit switch. It is strongly advised that this position never be reached. The pointing accuracy is approximately 2 arcmin and is dependent of the position on the sky. A pointing model is not yet available. Coordinates are read by two encoders with a resolution of approximately 4 arcsec.

The Cassegrain Focus

The Cassegrain Focus (f/9, scale 12.6 arcsec/mm at about 20 cm from the rear main flange) is the main focus of the telescope. Heavy instruments can be mounted, up to approximately 300 kg. The free space between the flange and the fork when pointing to the celestial North Pole is approximately 128 cm. The distance between the rear flange and the focus is approximately 200 cm (CHECK), but it can be raised by greater than 300 cm without serious degradation of the image. Focussing is performed by moving the secondary mirror along the Z axis; this movement is controlled  by a dedicated PC.

The Nasmyth Focus

A 45 degree tertiary mirror can be inserted to deviate the beam to the Nasmyth focus in the East side of the fork, where a photographic camera is available. This focus is not recommended for optimal optical quality but it can be readily used when other instruments are  mounted at the Cassegrain focus.

Dome

The dome of the telescope is on the third floor of the telescope building and has a diameter of 30 m.
The building is equipped with an external elevator to enable maintenance of the dome. Aperture and rotation of the dome are controlled by the TCS main console. In addition to the observing floor the building contains offices, mechanical and electronic laboratories and a small kitchen.

Operations

Pointing of the telescope is performed manually using the main TCS console FOTO CONSOLE. This employs three different speed of telescope movement (Large, Medium and Micro) to set the telescope position and for guiding. The finest setting (micro) is reserved for guiding.
An auto-guiding system is implemented through a guide probe and a TV camera that allows a usable field of view of approximately 30 arcmin. For targets with appreciable proper motion (< 200 arcsec/hr) in the sky a differential guiding can be applied using the guide-probe movements. During observations the dome is automatically rotated such that the dome window is always aligned with the telescope pointing.


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