III. 2MASS Facilities and Operations


1. Facilities

a. Telescopes

The 2MASS project was 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 moved continuously in declination at approximately 57´´ s-1, while tracking in hour angle at the sidereal rate. The articulated secondary executes a sawtooth motion, which freezes the image of the sky on the focal plane during the frame exposures.

The northern telescope is located at the Whipple Observatory 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 was operated by the Smithsonian Institution under contract to the University of Massachusetts.

The southern telescope is located at the Cerro Tololo Inter-American Observatory 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 was operated by the AURA, Inc., 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., Baltimore, MD. The telescope control system software was provided by Comsoft, Tucson, AZ. After the completion of the 2MASS data-taking, the University of Massachusetts donated the telescopes to the host observatories.

Images of the telescopes and observatories can be found here.

i. Optics

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 spherical high density polyethylene spacers. The length of these spacers was chosen so that the thermal expansion of the spacers cancels the thermal expansion of the mirror cell. Both the primary and secondary mirrors were fabricated from Corning ULE glass.

ii. Position Encoding

Heidenhain GMBH encoding 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. One arcsecond (1´´) on the sky corresponds to 5.9 µm on the right ascension drive surface and 3.0 µm on the declination drive surface.

iii. Focus Stability

The primary-secondary mirror spacing is fixed by invar rods. The residual thermal expansion coefficient of the primary-secondary distance is approximately 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 searched for an index mark on the Heidenhain tape and then moved to a software-determined set point. The focus was automatically adjusted for changes in telescope temperature (northern and southern telescopes) and elevation angle (northern telescope prior to August 1999). In August 1999 the radial positioning restraints of the northern telescope primary mirror were changed to be identical to those of the southern telescope (plugs with spherical ends). After this modification the northern telescope did not require an elevation-dependent focus correction.

iv. Control System and Telescope Drives

The telescope control system is a DOS-based program called PC-TCS. 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.

v. Pointing Accuracy with Software Correction

Pointing corrections are made within the PC-TCS 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.

[Last Updated: 2003 Jan 08; by R. Stiening]


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