1993 Baseline Telescope Description

J. M. Hill, Steward Observatory

Large Binocular Telescope Project


Technical Memo

UA-93-01

June 10, 1993
http://medusa.as.arizona.edu/lbtwww/tech/ua9301.htm

Abstract

History

Primary Mirrors

Baseline Focal Stations

Other Focal Stations

Secondary Complement

Other Optics

Instrument Envelopes

Telescope Structure and Enclosure

Figure 1

Figure 2

Figure 3

Figure 4

Abstract

The baseline telescope contains those focal stations and features which are deemed scientifically important enough to have a major impact on the optical or mechanical design of the telescope. The Large Binocular Telescope uses two 8.4 m, F/1.14 parabolic primaries. This provides a collecting area equivalent to an 11.8 meter circular aperture. Two pairs of secondaries provide a wide field optical focus at F/4 and an infrared Gregorian focus at F/15. The F/15 focus is reimaged to provide an additional phased focus for interferometry. The telescope structure and drives will be designed to have a lowest resonant frequency of at least 8 Hz.

History

The optical parameters and the mechanical design of the Large Binocular Telescope (LBT, former Columbus telescope) have been significantly refined and evaluated since the previous baseline telescope memo. The baseline telescope contains those focal stations and features which are deemed scientifically important enough to have a major impact on the optical or mechanical design of the telescope. Because of the importance of these features, we anticipate that they would all be present before the telescope finishes its first decade of operation. Note that ``baseline'' does not imply the ``minimum'', ``maximum'' or ``first light'' telescope. The original baseline telescope described in UA-87-20 was adopted following the Yerkes SAC meeting in October 1987. At the Tucson SAC meeting in January 1988, the baseline was revised as described in UA-88-04. Baseline memo UA-88-12 was issued after the June 1988 SAC meeting. UA-89-08 refined details of secondary sizes and locations. Based on action of the SAC in May 1990 and approval of the Council in October 1990, the current telescope uses two 8.408 m, F/1.14 primaries. This provides a collecting area equivalent to an 11.8 meter circular aperture.

The SAC recommended at its January 1993 meeting the following changes to the secondary configuration: The wide field F/5 focus was removed from the Cassegrain focal station and moved to an F/4 trapped focus above the primary. This increases the field of view to 60 arcminutes. The F/15 infrared secondary was changed to a Gregorian design. This change was made primarily to accommodate use and testing of an adaptive secondary. The F/33 secondary was removed but not excluded from the baseline anticipating that interferometry would used reimaged beams from a bent F/15 focus. Other changes included moving the tertiary height to 2.25 meters; increasing the F/15 back focal distance to 2.50 meters and reducing the primary hole diameter to 0.89 meters.

While this should be the final baseline description. There will certainly be one more detailed iteration before the optical dimensions of the telescope are frozen completely, since we need to include the final design of the wide field optical corrector and the beam combination optics. Note that the telescope mechanical drawings in this memo represent work in progress and are not precisely the final design.

The error budget specifies that the telescope and its optics will produce images to match an r0 = 45 cm atmosphere. Hill (1990, SPIE 1236,86) reviews the error budget and the detailed performance specifications.

Primary Mirrors

Baseline Focal Stations

Other Focal Stations

 View Figure 1 here

View Figure 2 here

View Figure 3 here

Secondary Complement

Other Optics

Instrument Envelopes

Telescope Structure and Enclosure

 View Figure 4 here