3  Telescope Specifications

3.1) Historical Introduction
3.2) Optical Specifications
3.3) Mechanical Specifications
3.4) Geographical Coordinates

3.1  Historical Introduction

The 1.3-m telescope was built by Tinsley of California. The instrument was first erected in July 1969 at Stinchfield Woods near Dexter, Michigan, some 15 miles from Ann Arbor.

The Dexter site was deteriorating even then - city lights, haze, cloudiness - and when Al Hiltner came in 1970 he tried to use the telescope and to get others to use it, but soon realized the site was poor. Incidentally, the telescope was mounted in a TALL dome structure, like the Mayall 4-m, into which was built a Coude tunnel. The mounting was hollowed out for a Coude mirror, but no further work was done to provide for a feed or spectrograph.

MIT was preparing to launch SAS-3, a Small Astronomy Satellite, to observe celestial X-ray objects. MIT went to NASA and proposed NASA build them an optical telescope. NASA responded by encouraging them to join up with other groups who had telescopes. Al Hiltner and Delo Mook were working on optical counterparts of X-ray objects, especially Sco X-1, and Al was talking about the inutility of the 1.3-m. They all got together and decided to discuss moving the 1.3-m to Kitt Peak in a joint way.

As all the plans were knitted together Delo Mook moved to Dartmouth and so Dartmouth was included. The Consortium was established and the name decided by coin toss. Michigan was to have 50% of the telescope time, so its name would be first. The coin toss was to decide whether Dartmouth or MIT came second or third. Each would have 25% of telescope time.

Money for the move was furnished by McGraw-Hill, Inc. and by the Alfred P. Sloan Foundation. The gifts/grants came in 1974 and the telescope was moved quickly, as the purpose was support of SAS-3, which was due to be launched in 1975. The telescope and observatory became operational on May 4, 1975, just three days before the SAS-3 launch, and began to be used to observe the optical counterparts of X-ray objects (not confined to those discovered by SAS-3).

An additional grant from Don & Lucile Nierling of Phoenix was used for the computer installation, which was added in 1977. The facility was called the "McGraw-Hill Observatory." The asteroid 4432 McGraw-Hill is named after the telescope, which was the site for the first physical observations of this minor planet.

When the consortium decided to build a larger telescope, to which all would contribute equally, the time allocation was changed to 1/3 per institution, and the observatory name changed to the Michigan-Dartmouth-MIT Observatory operating the McGraw-Hill and Hiltner Telescopes. The observatory became just MDM when MIT left the consortium to join the Magellan Telescope project. Michigan relinquished half their 1/3 share. Ohio State and Columbia each joined MDM as 1/4 partners.  Since then, Ohio University has also joined, forming the current operating consortium.

Table 3.1: 1.3-m McGraw-Hill Telescope Optical Specifications1

Measurements in INCHES and MM

f/3.0 Primary Mirror

Diameter of front face 52.0 1321

Clear aperture (after masking) 50.0 1270

Thickness 9.1 230

Diameter of central perforation 14.0 356

Weight 1410 lb 640 kg

Paraxial radius of curvature 300.00 7620

Sagitta 1.04 26.4

f/7.6 Secondary Mirror

Diameter of front face 19.75 501.7

Diameter of front face (illuminated) 19.1 484

Secondary baffle outside diameter 20.25 514.4

Thickness 3.30 83.8

Paraxial radius of curvature 173.95 4418.3

Prime focus intercept 52.64 1337

Separation of mirrors 97.36 2473

Back focal distance (optical) 36.00 914.4

Image scale (arcseconds/mm) 20.72

Unvignetted field diameter 3.4 87

Effective focal length 380 9652

Secondary amplification 2.533 2.533

f/13.5 Secondary Mirror*

Diameter of front face 13.0 330

Diameter of front face (illuminated) 112.3 313

Secondary baffle outside diameter 13.25 13.25

Diameter of central obscuration 20.25 514.4

Thickness 2.12 53.8

Paraxial radius of curvature 86.96 2208.8

Prime focus intercept 33.82 859.0

Separation of mirrors 116.18 2951.0

Back focal distance (optical) 36.00 914.4

Image scale (arcseconds/mm) 11.81

Unvignetted field diameter 6.0 152

Effective focal length 675 17145

Secondary amplification 4.5 4.5

1 Fundamental parameters in bold text taken from Tinsley Laboratories drawing 421399 Rev. C, dated 8/8/68. All other parameters derived.

3.2  Optical Specifications

The 1.3m McGraw-Hill Telescope can be used at f/7.6 and f/13.5* Cassegrain configurations. Scaled optical diagrams are given in Figure 3.1. Full optical specifications are given in Table 3.1.

All three mirrors are made of low expansion Cer-Vit, with a regular aluminum coating.

The primary mirror is 1.32 m (52 inches) in diameter, but the optically worked surface is only 1.27 m (50 inches) and has been masked accordingly.

The primary is supported by a wiffle-tree arrangement. On nights of good seeing the telescope is capable of producing stellar images of 1.0 arcseconds FWHM or slightly better, over a 10 minute integration.

3.3  Mechanical Specifications

The telescope mount is an English cross-axis equatorial which employs worm and worm wheels. The worm wheel on the polar axle has been damaged and it can only be used on the WEST side of the axis. It is possible to crash the telescope into the north pier and great care must be taken when working near the celestial pole, especially at large western hour angles. The telescope can be moved from several locations:

The telescope drive and control systems were both upgraded by Starman Systems, LLC in 2008.  This update has allowed for variability in track rate, tracking capabilities in both axes, and simplified motion control to one motor per axis.  It also serves as a step in the direction of potentially automating operations of the telescope some day.  Most importantly, this upgrade has drastically improved the pointing capabilities of the telescope.
Track rates (arcseconds/s)
Default RA 15.001
Default Dec 0.0
Maximum 25

Guide rates (arcseconds/s)
Default 1.33

Set rates (arcseconds/s)
Default 100

Slew rates (degrees/s)
Default 2.5

3.4  Geographical Coordinates

MDM Observatory is located on the south-west ridge of Kitt Peak, Arizona. The precise position of the 1.3-m McGraw-Hill Telescope is:

   Latitude: N 31o 57'
Longitude: W 111o 37'
Altitude: 1925 m (6317 ft)

*f/13.5 configuration is not currently available.  There are no plans to date for reviving it.

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Updated: 2014Jan14 (Galayda/MDM)