Apache Point Observatory

Apache Point Observatory

August 28, 2009, 11:01 pm
Source: Apache Point Observatory
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Introduction

The Apache Point Observatory (APO) consists of:

  • The Astrophysical Research Consortium's 3.5-meter telescope,
  • the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope, and
  • New Mexico State University's 1.0-m telescope.

3.5-meter Telescope

3.5 Meter Telescope.
(Source:
APO.)

The 3.5-meter Telescope is used with spectrographs and imaging devices to make observations at optical and infrared wavelengths. It is a general purpose telescope for studies of all kinds of astronomical objects, from relatively nearby planets of our own solar system to the most distant galaxies. The construction of this telescope incorporates many innovations which allow it to take full advantage of the dark and transparent sky at Apache Point.

  • Compact and lightweight design: The primary mirror is fabricated using a new spin-casting technology which results in a hollow mirror weighing only 1/5th as much as its solid glass equivalent. This, together with its deeply curved surface, results in a very compact telescope needing only lightweight support structures. Consequently, the telescope can be moved quickly to any point on the sky and then acquire and track any object to very high precision. The telescope has even been used to track missiles.
     
  • Thermal control: The observatory site was selected in part because of its airflow patterns which result in minimal blurring of images due to atmospheric turbulence and temperature differences. This good "seeing" is further protected by drawing temperature-controlled air through the telescope structure and the interior of the primary mirror. Vents and exhausts, plus various forms of thermal insulation, further control temperatures of the telescope and its surroundings.
     
  • Multiple instruments: Several spectrographs and imaging devices are permanently mounted on the telescope; others are maintained in a "ready" state and can be attached in minutes. Light is directed to any instrument by simply rotating the telescope's flat tertiary mirror. This feature allows the telescope to quickly respond to different scientific requirements, changing observing conditions, or unexpected celestial events such as supernovae.
     
  • Remote observing: Most observations are conducted remotely, with the astronomer located at some distant university and operating the telescope by computer commands transmitted over the Internet. The telescope has been operated in this way from many countries, from the South Pole, from aircraft,and through satellite links.

This, together with the ease of instrument changes, allows several astronomers to be scheduled on the telescope each night for different scientific programs. In rapid response to new discoveries, APO astronomers can drop-in to take short "quick-look" observations or execute long-term surveys.

Apache Point Observatory Lunar Laser-ranging Operation (APOLLO)

The Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO, is a project at the Apache Point Observatory in New Mexico. It is an extension and advancement of previous Lunar Laser Ranging Experiment, which uses retroreflectors on the Moon to track changes in lunar orbital distance and motion.

Using telescopes on Earth, the reflectors on the Moon, and accurate timing of laser pulses, by the early 2000s scientists could measure and predict the orbit of the Moon to an accuracy of a few centimeters. This already impressive accuracy (the Moon is typically about 385,000 km away) provides the best known test of many aspects of our theories of gravity. APOLLO improves this even further, measuring the distance between the Moon to an accuracy of a few millimeters. Using this information, scientists will be able to further test various aspects of gravity: do the Earth and the Moon react the same to gravity despite their different compositions? Does the energy content of the Earth and the Moon react to gravity in the same way as Einstein predicts? In general, does Einstein's General Relativity correctly predict the motion of the Moon, or are new theories required?

The APOLLO collaboration built their apparatus on the 3.5 meter telescope at Apache Point in southern New Mexico. By using a large telescope at a site with good atmospheric "seeing", the APOLLO collaboration gets much stronger reflections than any existing facilities. (Strong is a relative term here—APOLLO records approximately one returned laser photon per pulse, as opposed to the roughly 0.01 photon-per-pulse average experienced by previous LLR facilities.) The stronger return signal from APOLLO translates to much more accurate measurements. [1]

2.5-meter Telescope

The 2.5-meter telescope.
(Source: FermiLab/SDSS.)

The 2.5-meter Telescope, named the Sloan Foundation Telescope, incorporates many of the design ideas successfully pioneered on the 3.5-meter telescope. The 2.5-meter also incorporates many innovations of its own, such as a unique movable baffle system which shields the optical components from wind, and the camera and spectrographs from stray light. This telescope is used to conduct the Sloan Digital Sky Survey, a three-dimensional survey of a large part of our universe. It will use two instruments for this mapping project:

  • The imaging camera consists of an array of CCD detectors covered by colored filters which isolate different wavelength bands. As the sky is allowed to drift across the telescope's large field of view, the positions, shapes, and colors in five wavelength bands are recorded. Nearly 100 million galaxies, a million quasars, and a like number of stars will be measured in this way.
     
  • The dual spectrographs use prisms to separate by wavelength the light of stars, galaxies, and quasars. This spectrum contains information about their composition, temperature, and motion. Redshifts due to the expansion of our universe provide distance information.

Spectra of over 600 objects can be acquired at the same time using a system of optical fibers connected to the telescope's focal plane and the spectrographs. Nearly 1,000,000 galaxies, 100,000 quasars, and a similar number of stars will be spectroscopically examined in this survey.

20" Photometric Telescope

20" Photometric Telescope.
(Source:
APO.)

The 20" Photometric Telescope is used during Survey observations to continuously monitor the brightness of the sky, transparency of the Earth's atmosphere, and provide calibration for the 2.5-meter telescope observations.

The 20-inch SDSS photometric telescope is a classical Cassegrain with a parabolic primary and hyperbolic secondary. Such telescopes have small fields so Shu-i Wang at Yerkes Observatory designed a corrector system of two concave-skyward meniscus lenses made of fused silica to expand the field to almost a degree in diameter, the diagonal of the CCD detector. The image size, about 1 arc second FWHM, would not be considered good for most professional applications but the detector pixels are large (about 1.15 arc seconds across) and are a good match for the modified telescope.

The primary mirror is made of Pyrex and has a bare aluminum coating maintained and applied at the National Solar Observatory coating facilities in Sunspot, NM. The secondary, also of Pyrex, has a protected aluminum coating applied by Denton Vacuum. The two corrector lenses are made of fused silica and were manufactured by Coastal Optical. They have broad band antireflection coatings from QSP similar to that on the SDSS 2.5 m telescope first corrector. The second corrector also serves as the vacuum window on the detector cryostat. The CCD is a SITe 2048x2048 device with 24 micron pixels and a UV antireflection coating. [2]

One-Meter Telescope

One-Meter Telescope.
(Source: APO.)

The One-Meter Telescope is operated by New Mexico State University for research and graduate instruction. Utilizing many of the successful design innovations of the 3.5-meter telescope, this telescope is optimized for studying extended objects such as nebulae, galaxies, star clusters, and clusters of galaxies. Fully computer-controlled, it can be operated from its control room at Apache Point or from the New Mexico State campus.

The telescope is an alt-az telescope and was (mostly) built in the early 90s by the AutoScope corporation. Significant work was required to make it fully operational, but it now functions reliably as a robotic telescope. Currently, there is an optical CCD 2048x2048 imager mounted on the Nasmyth port, with a field of view of almost 16 arcminutes on a side.

In 2005, we obtained an NSF PREST grant to improve the telescope and make it operate more reliably. Significant progress has been made, and work is underway for more improvements and to increase capability. As part of the proposal, NMSU is making some time available to the community. [3]

 

Click link below right for more information on the Apache Point Observatory. »

 

Glossary

Citation

(2009). Apache Point Observatory. Retrieved from http://www.cosmosportal.org/view/article/140914

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