Selection Criteria

Written by Dr. Gordon Spear

Selecting a site for an astronomical observatory within the Galbreath Wildlands Preserve requires specific criteria. The site should have...

1. The maximum elevation possible.
2. A clear horizon in all directions.
3. A reasonable absence of costal or marine layer fog.
4. Dark skies without interference from artificial manmade illumination.
5. Reasonable proximity to a road for construction and ultimate maintenance.
6. Good astronomical "seeing", indicated by small, stable stellar images.

The higher the elevation, the less atmosphere present to degrade the viewing.

While a clear horizon is essential, the top of a hill or ridge can be acceptable even if there are trees present which may obscure parts of the horizon. Most of the preserve is forested, and the preserve has a history of selective logging. Thus, removing a few trees from the crest of a hill will not significantly alter the ecology of the preserve.

The absence of fog is an essential criterion. Costal fog prevents many nights from being useful for astronomy at the Sonoma State University campus. While the fog is less prevalent at our remote site at the Pepperwood Natural Preserve north of Santa Rosa, it is still a significant factor there in minimizing nights which can be used for astronomy.

A dark sky with minimal artificial sky brightness is an essential criterion. Satellite data suggests that the entire Galbreath Preserve is in a pocket of dark sky removed and sheltered from man-made light pollution. The preserve has a Bortle rating of 2-3 on a scale of 1 (no artificial sky brightness) to 9 (urban centers dominated by artificial illumination). Throughout the Galbreath Preserve the artificial illumination contributes only about 10% to the total observed sky brightness. The western parts of the preserve are slightly darker than the eastern locations since they are further removed from the nearby town of Cloverdale and the northern edge of the Santa Rosa plain.

Since the preserve has been selectively logged, old logging roads are present. Ideally, an observatory site would be near one of the logging roads.

The higher the elevation, and the higher the telescope above the surrounding terrain, the better seeing, or sharp stellar images, can be expected. Terrain the allows laminar air flow, rather than turbulent flow provides the best seeing. High wind speed tends to encourage turbulent flow.

Preliminary Results

Beginning in the winter and spring of 2006, the preserve began to be explored to identify possible observatory sites. This exploration included traveling the one main road through the preserve as well as numerous overgrown trails that remained from the logging activities.Even sites without direct road access were considered if a road could easily be extended or constructed to reach the site.The focus was on higher elevations, hills and ridges. Ultimately two primary sites were identified.

The first site identified is an obvious viewpoint in the eastern part of the preserve. This site sits directly on the main road and commands a magnificent view, especially to the north. We have named this site the Great Eastern Overlook (although it is also known locally as Windy Ridge).

A View to the North at the Great Eastern Overlook

The build site at the Great Eastern Overlook. The road is in the foreground.

There are two clear disadvantages for the Great Eastern Overlook site. While the view to the north is spectacular, the view includes several properties outside the Galbreath Preserve. These properties contain a number of structures which include barns and houses. These structures are occasionally lit and the properties have the potential for continuous development which can include outdoor lighting. This would not be a prime location to construct an observatory.

The second disadvantage is the wind. Wind is common and strong throughout the year. Valleys to the north and west of the preserve apparently funnel the prevailing wind off the ocean into the preserve and over Windy Ridge. The wind could influence possible construction techniques and could restrict the number of nights an observatory could safely be open for viewing. More importantly, the wind flow over the ridge seems to be clearly turbulent, and this would degrade astronomical seeing at all times.

The second site identified is in the somewhat wilder Western Highlands of the preserve. This region is more isolated, less developed, and less visited than the southern and eastern parts of the preserve. The mean elevation is slightly higher than in other regions. While an old logging road exists, it was initially passable only by all-terrain vehicles. When logging operations were terminated in the western part of the preserve, earth berms several feet high were constructed across the road to prevent washouts. Many of these berms were removed as part of the road maintenance that occurred during the spring and summer of 2006. Most of the roads through the Western Highlands are now passable by 4WD vehicles.

Approximately half way along the road through the Western Highlands there is a high ridge that runs east-west for about 1/8 of a mile and is a few hundred feet wide. The views from this ridge are also magnificent. We now call this location Observatory Ridge.

A View to the South from Observatory Ridge

The Build Site Along Observatory Ridge

The Observatory Ridge site in the Western Highlands is approximately 200 feet higher than the Eastern Overlook site. The views from Observatory Ridge involve mostly lands that are part of the Galbreath Preserve. No structures of any kind are visible from the site and it is not likely that development will occur on any property that is visible from the site. The winds are rarely in excess of 3-5 mph and only occasionally exceed 6 mph for short periods during the daytime hours.

Even though the Observatory Ridge region has been selectively logged, the top of the ridge has a number of trees that will need to be removed to achieve a clear horizon. We have been advised that removing the trees and leveling the top of the ridge to achieve a reasonable space for construction would be a simple matter that would not adversely affect the terrain or the ecology of the region.

By the end of the summer of 2006, the preferred site for the observatory was clearly the Observatory Ridge site in the Western Highlands. The Site Validation phase of the project then began in earnest.