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Discovery Channel Telescope – Technical Information


Site testing


The testing station at the Happy Jack site.


Seeing equipment ready for use at the Happy Jack site.

Lowell Observatory began limited testing of potential sites for the planned new telescope in the summer of 2001. Four sites were tested for image quality by the differential image motion technique (DIMM): Lowell's existing dark sky site at Anderson Mesa 15 miles ESE of Flagstaff; Saddle Mountain north of Flagstaff near state highway 180; Hutch Mountain approximately 35 miles SSE of Flagstaff a few miles east of County Road 209; and a small unnamed mountain near the Happy Jack Ranger Station approximately 40 miles SE of Flagstaff immediately adjacent to the same County Highway. The latter two sites gave very similar image quality results and were significantly better than the other two sites in the survey. Because the site near Happy Jack presented far fewer logistical difficulties and environmental concerns than did Hutch Mountain, future site testing activity was restricted to this site.

A temporary use permit was granted by the United States Forest Service (USFS) in 2002 for installation of a “permanent” site testing station at Happy Jack; operations began in January 2003 and were completed in November 2004. DIMM seeing measurements made in the optical at about 2 meters above the ground on more than 117 nights resulted in a median seeing value of 0.84 arcsec, with the average of the first quartile at 0.62 arcsec. In addition to the DIMM measurements, meteorological data have been logged continuously. For a complete discussion of the site testing program at Happy Jack, download the paper by Bida et al. 2004 [251 KB PDF].

Data analysis from the testing program shows that the Happy Jack site offers seeing comparable to the best already developed U.S. continental sites; a “Clear Sky Clock Predictor” developed by Attilla Danko for the Happy Jack site provides further seeing projections. Application was made to the USFS for a long-term Special Use Permit that would allow construction of the Discovery Channel Telescope on the site. The required National Environmental Policy Act (NEPA) process was conducted from October 2003 through October 2004 and involved extensive assessment of the site as well as a 30-day public comment period. In November 2004, the USFS issued the permit for development of the DCT site.




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Primary Mirror

Secondary Mirror

Active Optics System

Optical Coating System

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Prime focus configuration images

Ritchey-Chretien configuration images



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Building design & construction

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Environment in the dome



Site testing

Seeing histogram

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