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Magellan Telescopes

1. General

 

The Walter Baade Telescope is the first of the twin 6.5 meter Magellan telescopes to be completed. The two telescopes are located 60 meters apart on an isolated peak (Cerro Manqui) at the Las Campanas Observatory. The telescopes are an alt-azimuth design. The principal foci are f/11 at the two Nasmyth locations and f/15 in the Cassegrain position, although at present only the f/11 focus is implemented on the Baade Telescope. In addition, three auxiliary f/11 are provided on the center section. An ADC corrector will be available for f/11 use to provide unvignetted fields up to 24 arcmin. Platforms on either side provide access to the instruments at the principal Nasmyth ports.

 

The Magellan primary mirrors were cast and polished by the Steward Observatory Mirror Lab. The mirrors are made of borosilicate glass with a lightweight honeycomb structure.

The f/11 focus is a Gregorian configuration, which was selected in order to optimize the performance of the collimator optics in the wide-field spectrograph (IMACS) currently under construction. The Gregorian secondary is also considerably less aspheric than the corresponding Cassegrain secondary, and is easier to test.

 

 

The telescope and enclosure are designed to minimize image degradation due to thermal effects. Separate ventilation systems for the enclosure, telescope structure, and primary mirror maintain surfaces within the dome at the outside air temperature during night time observing.

Active controls are incorporated in the telescope optics. The mirrors have position control for alignment. These are active during observing. Figure control of the primary mirror is used to correct low-order aberrations in the optical system. In addition, the secondary mirror has a tip-tilt mechanism for fast guiding.

 

Full information on the active optics system at Magellan can be found here

 

 2. Optical Design

 

The optical configurations of the telescope at f/11 are shown in Figure 1 and Figure 2. The f/11 configuration is used at the two Nasmyth and three auxiliary ports on the optics support structure (OSS) and is intended primarily for observing in visible wavelengths. The tertiary mirror rotates on a turret to direct the beam to the selected port.

Each of the f/11 foci can be configured for wide-field use by inserting a corrector in the beam. The corrector includes atmospheric dispersion compensation (ADC). Use of the corrector shifts the optimum focal surface approximately 115 mm towards the tertiary mirror.

The following sections give the optical specifications.
 

 

2.1 Primary Mirror

 

 
Diameter: 6,502.4 mm
Focal Length: 8,128.0 mm
Shape: Paraboloid
Coating: Bare Aluminum
Focal ratio (ref.): 1.25
Entrance stop distance above primary vertex: 325 mm
Entrance stop diameter: 6,478.4 mm

 

2.2 f/11 Narrow Field

 

Transmissive elements: None
Final focal ratio: f/11.0
Nominal corrected field of view: 6 arcmin
Unvignetted field of view: 24 arcmin
Central obscuration (area): 7.4%
Secondary mirror coating: Bare Aluminum
Focal length: 71,526 mm
Scale: 0.347 mm/arcsec
Focal surface curvature (concave away from M2): 1,519 mm
Distortion at 3 arcminute field angle (at 500 nm): 0.005%

 

2

 

Figure 1. An Optical Diagram of the f/11 Narrow Field Configuration

 

 

2.3 f/11 Wide Field with ADC

 

Final focal ratio: f/10.97
Unvignetted/corrected field of view: 24 arcmin
Vignetting @ 15 arcmin field angle: 9%
Central obstruction (area): 7.4%
ADC cut-off (50% transmission): 330 nm
Focal length: 71,089 mm
Scale: 0.345 mm/arcsec
Focal surface curvature (concave away from M2): 1,231 mm
Distortion at 12 arcminute field angle (at 500 nm): 0.12%

 

3

 

Figure 2. An Optical Diagram of the f/11 Wide Field Configuration

 

 

 

References

The following is a tiny selection of available papers related to the construction and operation of the Magellan Telescopes:

The Magellan Telescopes (Shectman & Johns 2003)

Optical design of the Magellan Project 6.5-meter telescope (Shectman 1994)

Instrumentation at the Magellan Telescopes 2008 (Osip et al. 2008)

First-generation instruments for the Magellan telescopes: characteristics, operation, and performance (Osip et al. 2004)

Magellan Telescopes operations 2008 (Osip et al. 2008)

 

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