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Discovery Channel Telescope primary mirror : $3 Million Project Finalised

Lowell Observatory and the University of Arizona’s College of Optical Sciences (OSC) have finalized a $3 million, three-year contract to complete the Discovery Channel Telescope primary mirror. The 4.3-meter-diameter (14 ft.), approximately 6,700-pound mirror is the heart of the DCT. UA optical scientists will polish and figure the mirror in an exacting, delicate process expected to take about three years. If the mirror were the size of the United States, all the imperfections would be polished down to less than one inch high.

“This is a great telescope and a project we’re very interested in,” said Martin J. Valente, director of OSC’s optical fabrication and engineering facility and UA’s principal investigator on the project. “It’s a great opportunity to apply our advanced processing and testing technology and also to show our students, from start to finish, what it takes to actually make a deliverable product."

News Inside News;

Donation-
All of us at Lowell Observatory are grateful for, and tremendously energized by, the $5 million pledge from Mr. and Mrs. John Hendricks and their family toward the completion of the Discovery Channel Telescope. “In recognition of this truly generous gift, the existing Planetary Research Center at Lowell Observatory will be renamed the Hendricks Center for Planetary Studies,” said Lowell Trustee William Lowell Putnam. Mr. Putnam also noted that, while the Observatory remains open to the possibility of an additional university or observatory partner in the project, such a partner is no longer essential to completion of the telescope by 2010.

The Discovery Channel Telescope will increase Arizona’s economic output by more than $576 million over its 50-year expected useful lifetime, according to a study by the Center for Business Outreach at Northern Arizona University.

Overview of Optical Design-
The optical design of the DCT includes two configurations: a prime focus with a 2° FoV camera, and a Ritchey-Chrétien focus with a 30 arcmin unvignetted field and interchangeable instruments. The schematic diagram below shows the optical path for both foci. The wide field prime focus observations are best conducted during the dark of the moon. The R-C focus is best suited for projects that can be done during bright time, or requiring deep UV or near IR wavelength coverage, or spectroscopy. This focus will allow mounting of instruments weighing up to 5,000 pounds. Both foci include atmospheric dispersion compensation.
In September 2005, the 4.3m ULE (ultra low expansion) meniscus primary mirror blank underwent successful completion and acceptance testing at Corning Glass Works in Canton, NY. The mirror blank weighs 6,700 lbs and is just 4 inches thick.

The completed blank was delivered to the University of Arizona, College of Optical Sciences in Tucson at the end of August 2006 for final figuring and polishing, a painstaking process that will take nearly three years. From there, the finished primary mirror will be delivered to the DCT site for installation on the telescope with first light expected by the end of 2010.

Overview of the DCT Facility
The DCT Facility is the first part of the telescope to be constructed on the site located approximately 40 miles southeast of Flagstaff, AZ on USFS land adjacent to the Happy Jack Ranger Station. A Special Use Permit to develop the site was issued by the USFS in November 2004. Construction of the DCT access road and main facility building began shortly thereafter with full completion expected in early 2007.

The facility is being constructed with the following goals in mind.

1. Provide infrastructure to the site, including water and sewage, access road, storm water drainage, electricity supply and a data link;

2. Protect the telescope from the rain and dust in the day and during bad weather;

3. Keep the air in the optical path of the telescope as close as possible to the ambient air temperature during observations while avoiding wind buffeting;

4. Cool equipment to prevent heat transfer into the optical path of the telescope and temperature gradients in critical parts of the system;

5. Provide a stable pier to support the telescope mount;

6. Provide a supporting wall and interface for the rotating dome;

7. Provide facilities for operational and maintenance personnel;

8. Provide service and utilities including lifts and hoists, HVAC, pneumatics, CCTV system, data and communications, electriciy distribution and emergency supply, fire alarm system, and lightning protection system.

Overview of the Site
The Discovery Channel Telescope is located approximately 40 miles southeast of Flagstaff, Arizona at a site known as Happy Jack. The site was selected following extensive testing and subsequent approval by the United States Forest Service in November 2004 for a Special Use Permit to construct the telescope at this location.

The Happy Jack site (lat +34:44:39.2 and long 111:25:20.2) is a partially mined cinder cone of 7746 ft (2361m) elevation perched near the edge of the Mogollon Rim just west of County Highway 209. The paved, year-round highway is adjacent to the base of the mountain and an unimproved forest road provides access to the summit. Commercial power and telephone are available within less than one mile from the site. The site is surrounded by the Coconino National Forest with occasional pockets of sparsely populated private land.

Development of the DCT site began in November 2004 with construction of the access road and clearing the site summit. The formal groundbreaking for the DCT took place on July 12, 2005 at Happy Jack with over 150 invited guests attending. Construction of the facility has been in full swing ever since with completion expected in 2007

Overview of the Prime Focus Camera
The most notable feature of the DCT is its large prime focus assembly (PFA), which includes the prime focus corrector optics and the CCD camera (PFC) consisting of 40 e2v CCD44-82 2K x 4K CCD's; this combination yields a very wide 2-degree field-of-view. A Phase-2 design study was completed at Goodrich Corporation in 2005 that reduced the projected cost significantly through changes in several of the original requirements:

1. relaxation of the image quality requirement to that based on 10% degradation of median seeing rather than 1st quartile average seeing;

2. no U-band image quality requirement (but U-band capability was preserved);

3. modification of the ADC to a tilt/decenter style on element L4;

4. repackaging of the PFA without a secondary mirror
The new PFA design uses fused silica in 4/5 elements and standard glass LLF6 in element 4, and utilizes spherical surfaces on all but two: an ellipsoid on L3 and an asphere on L5. The image quality requirements, including recovery of the partial U-band, are exceeded over the full 2° field to 75° zenith distance.

The figure at right shows the new conceptual PFA structure. From left to right, the main components are the cable wrap, the prime focus CCD camera, the shutter and the filter changing mechanism, the ADC assembly, and the prime focus corrector optics.

In The Images-
1. rendering of the DCT facility, with a partial cutaway of the dome to show the telescope inside
2.A ray trace showing both the prime focus and the RC optical paths of the DCT.
3.Officials from Lowell and Corning inspect the completed mirror blank. Photo courtesy of Corning, Inc., Sept. 2005
4.Aerial view of DCT site at Happy Jack, AZ - May 6, 2006
5.Project location.
6.Conceptual rendering of the Prime Focus Camera.

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