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.
Primary Mirror: 1300 mm diameter, radius of curvature 5200 mm, conic constant
-1.000
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.
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: 2018 Jul 16; by R. Stiening]
1. Facilities
a. Telescopes
Images of the telescopes and observatories can be
found
here.
i. Optics
Secondary Mirror: 232 mm diameter, radius of curvature 965.7 mm, conic constant
-1.847
Pointing Accuracy Without Correction
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