III. 2MASS Facilities and Operations

1. Facilities

a. Telescopes

The 2MASS project is being carried out with two identical 1.3-meter aperture, open-tube, equatorial fork-mount telescopes. These telescopes have been provided with a Cassegrain focus mount for the infrared cameras and a secondary mirror which is articulated in the declination direction. During survey data-taking the telescope moves continuously in declination at approximately 57"/second, while tracking in hour angle at the sidereal rate. The articulated secondary executes a sawtooth pattern of motion which freezes the image of the sky on the focal plane during the frame exposures.

The northern telescope is located at 2306 meters elevation on a ridge below the summit of Mt. Hopkins, Arizona. (N 31° 40´ 50.8´´, W 110° 52' 41.3"). The northern telescope is operated by the Whipple Observatory under contract to the University of Massachusetts.

The southern telescope is located at 2171 meters elevation on a ridge below the summit of Cerro Tololo, Chile. (S 30° 10' 3.7", W 70° 48' 18.3"). The southern telescope is operated by the Cerro Tololo Inter-American Observatory under contract to the University of Massachusetts.

The telescopes were designed, manufactured, and installed by M3 Engineering and Technology Corp., Tucson, AZ. The optics were figured by Rayleigh Optical Corp., Tucson, AZ. The telescope control system software was provided by Comsoft, Tucson, AZ.

Images of the telescopes and observatories can be found here.

Primary Mirror: 1300 mm diameter, radius 5200 mm, conic constant -1.000
Secondary Mirror: 232 mm diameter, radius 965.7 mm, conic constant -1.847

The primary mirror is supported by flex rods on an 18-point Hindle mount. The primary is positioned radially by temperature-compensated plugs that press against the outer edge of the mirror. Both the primary and secondary mirrors have been fabricated from Corning ULE glass.

Position Encoding:
Heidenhain tapes with 40-µm bar spacing are attached to the declination and right ascension drive surfaces. Software provided by Heidenhain in the encoder interface interpolates between the bars. The least significant bit of the encoder interface is 0.039 µm on the tape. 1" on the sky corresponds to 5.9 µm on the right ascension drive surface and 3.0 µm on the declination drive surface.

Focus stability:
The primary-secondary mirror spacing is fixed by invar rods. The residual thermal expansion coefficient is 6 µm per °Celsius. The focus setting is encoded by a 40-µm bar spacing Heidenhain tape (interpolated to 0.039 µm). The focus setting repeatability is approximately 5 µm. On command the focus mechanism searches for an index mark on the Heidenhain tape and then moves to a software determined set point. The focus is automatically adjusted for changes in telescope temperature (northern and southern telescopes) and elevation angle (northern telescope only).

Control system and telescope drives:
The control system is a DOS-based program called PCTCS. This software has been provided by COMSOFT. The telescope is positioned by friction contact capstans driven by DC servo motors.

Pointing accuracy without correction:
The pointing accuracy without corrections is approximately 30" on the sky. The polar axis of the telescopes is within 30" of the true poles.

Pointing accuracy with software correction:
Pointing corrections are made within the PCTCS control system software. The correction coefficients are determined by analysis using the TPOINT program provided by Patrick Wallace. After correction the RMS pointing error is less than 7" over the range -4.25h to +4.25h and -30° to +80° with the IR camera installed.

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