an astrophysical research site...
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All of the telescope sites which Steward Observatory has tested extensively within the 150-acre Mt. Graham Astrophysical Research Area, around Emerald Peak, provide excellent astronomical seeing. Several years of data show that the typical image size for a large telescope located 25 meters above the ground is about 0.6 arc-seconds; this compares favorably with the image quality at the Multiple Mirror Telescope on Mount Hopkins in Southern Arizona. The table below shows that the Emerald Peak sites are also competitive on a worldwide basis (other data provided by the observatories concerned).
Comparative Image Sharpness
Studies have demonstrated that two atmospheric layers contribute most of the image blurring or seeing. A high level (10km) layer contributes about 0.5 - 0.53 arc seconds and a thin (20m) boundary layer at roughly tree height contributes most of the remainder and accounts for local seeing differences. By suitable design of the telescope, the effects of this boundary layer can be minimized. This has, of course, been accommodated in the LBT design.
The Mt. Graham site thus combines the familiar advantages of existing southwestern sites with very good image sharpness, low light pollution, an altitude high enough for infrared work and an existing paved access highway. It is essentially unique in its astronomical qualities at least in the continental United States. |
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Mount Graham was
selected from a survey of 280 potential mountain sites on the basis of astronomical
considerations such as clear skies, low light pollution, low atmospheric water vapor, and
ease of access and support. Mount Graham is in the Sonoran desert climatic zone along with
established sites such as Kitt Peak, Mount Hopkins and Mount Lemmon; its intrinsic
climatic properties are those which have made the desert southwest so productive for
astronomy for almost a century. Mount Graham does, however, have the additional advantages
of greater altitude and very low light pollution. Unlike rival sites outside the
continental United States, it has excellent logistics: sufficient area for several
telescopes, a pre-existing paved highway, good connections by air and road, and a high
level of local technical support. 
The only astronomical site characteristic not considered in the initial survey
was image sharpness (astronomical seeing) since there is no generally accepted method of
doing this other than by making in situ measurements. The image sharpness on Mt. Graham
has, therefore, been the subject of a ten year study carried out by Dr. Richard Cromwell,
Dr. Nick Woolf, and others. These studies first identified the Emerald Peak region as the
best general area on Mount Graham for an observatory, because it provides good image
sharpness and space for telescopes. In the second study phase, measurements were made of
seeing differences between various locations within the 150-acre Mt. Graham scientific
research area. 
Among the
Emerald Peak sites, the site of the 1.8-m Vatican Advanced Technology Telescope (VATT),
the smallest of the telescopes of the MGIO, yields the poorest image sharpness (median
0.63 arc-seconds at a height of 25 meters). A request to place the LBT, the largest
telescope in the first phase of the MGIO, at the best of the potential sites for the
telescope (peak 10298, median seeing 0.55 arc-seconds at 25 meters) was turned down by the
U.S. Fish and Wildlife Service because they felt the Phase I telescopes should remain
clustered. The next best site, Emerald Peak (10477), with a seeing of 0.59 arc-seconds
median at 25 meters) was then requested. The request was approved in December 1993. While
peak 10298 was preferred on the basis of image quality alone, the LBT will be fully
competitive at Emerald Peak, which better satisfied other important criteria besides image
quality. 
The VATT is now operational at Emerald Peak and
measures of image sharpness at that telescope are beginning to provide some "real
life" calibration of the test data. The VATT which is placed ten meters above ground
is now regularly achieving image sharpness around 0.75 arc-seconds even without thermal
control of the mirror (still to be installed). After adjustment to 25 meters, the
predicted image sharpness for LBT may be slightly better than shown in the table above 1.