The Stockton Astronomical Society’s Wheelchair Telescope
The Bruce Orvis Memorial Telescope
The Large Aperture Low Eyepiece Telescope [LALET] will be an 18” f/15 classical cassegrain telescope that can be easily used by wheelchair-bound individuals at our public star parties and schools. Lalet was originally the name for this telescope, but James Marino donated the 18” mirror blank to be used in this telescope on the basis that it would be named after Bruce Orvis, who donated much land in the San Joaquin area for public use. We are gladly agreeable with this, and so his name will be attached to this telescope
Many times we have had wheelchair-bound individuals at our public observing sessions or school star parties where those individuals are restricted to using telescopes that have an eyepiece at their height. If there are no telescopes at that event with lower eyepieces, they end up not seeing through the telescopes. If there are telescopes at their height level, then they are usually smaller aperture telescope possible showing inferior or low light-level views.
The Bruce Orvis Memorial Telescope [BOMT] will be a larger aperture telescope accessible by wheelchair-bound individuals enabling them superior views of planets as well as the fainter objects in the sky, such as nebulae and galaxies. BOMT will have a classical cassegrain design, a concave paraboloidal primary mirror, a hyperboloidal secondary mirror, and a flat elliptical tertiary mirror shooting the focus out the side of the tube. The telescope will be an alt-azimuth fork mount design, and the focal path will be coaxial with the altitude bearing. The altitude bearing will never change in height from the platform upon which the telescope sits, and will be set up to be at the height comfortable for wheelchair-bound individuals. The focus will also extend away from the scope body enabling the person to view through the eyepiece without leaning over or driving the wheels into the scope.
BOMT will have a Dobsonian mount, similar to those like Obsession, Coulter and other Dobs. However, instead of a focusing cage at the top, there will be a secondary mirror cage shooting it back toward the primary mirror, and the light path will then be intercepted by the tertiary mirror projecting the image through the axis of the altitude bearings. Either the scope will be driven by a Stellar Cat system or underneath the groundboard will be a Poncet drive system, which can be reset every hour or so. The altitude of the eyepiece will change by less than a few inches as the telescope rocks over on the Poncet. The Poncet will be sitting on a box whose height will be optimized for the height of a wheelchair-bound individual’s eyeball.
BOMT will also be usable at school star parties where students are shorter. When it is used at high altitude star parties where no wheelchair-bound individuals are present, it can be mounted on a higher platform for standing individuals.
Design Parameters:
Aperture: 18”
Primary Focal Ratio f/3.75
Secondary Magnification 4×
Cassegrain Focal Ratio f/15
Cassegrain Focal Length 270”
Unvignetted field of view 1”
Secondary Mirror Diameter 4.25”
Secondary Hyperboloid Schwartzchild Constant -2.778
Secondary Radius of Curvature 42.93”
Distance Between Mirror Vertices 51.4”
Secondary Obstruction 24%
Optical Start Date January 1st ,2013
Magnificatons: 27mm PanOptic = 254×
31mm Nagler = 221×
20mm Nagler = 343×
13mm Nagler = 528×
Contributors:
Primary Mirror Blank James Marino
Secondary Mirror Blank, Optical Design and Fabrication Jeff Baldwin
Optics and Manufacturing SAS ATM Group
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