| The third stage in the evolution of my
solar equipment replaced the refracting telescope
described here with a Newtonian
reflector. The goal of this change was to achieve greater
resolution than was possible with a 4.5 inch aperture
system. Since I wanted to approximately double the
resolution I needed to double the aperature. This was not
feasible with a refractor because of cost. A refractor
also requires either front aperture filtration or dealing
with a very hot prime focus image. Larger aperture is
relatively inexpensive in a reflector and the use of an
uncoated primary mirror provides enough decrease in the
concentrated light to make it managable. I chose an open
truss tube structure to minimize the build-up of thermal
gradients in the air surrounding the mirrors.
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| The primary mirror is an 8", f/10 (80" focal length), 1/20l, unaluminized, full thickness, Pyrex paraboloid. The mirror cell is painted white to minimize its heat-up while observing and, hence, stresses that would deform the figure of the mirror. Others have used black mirror mounts on the thinking that reflected light from the mount would reduce the contrast in the image by an intolerable amount. I have not found that to be true. The path back to the camera detector requires many reflections off of surfaces that are painted black. There is also a field stop that restricts the entrance angle of any scattered light an a narrow band optical filter that restricts the wavelength of light that can reach the detector. | |
| Uncoated Pyrex has a reflectivity of about 5% and the reflectivity of the coated secondary is probably not much better than 85% since it is several years old. Consequently, the flux at prime focus is only about 5 times greater than normal incidence sun light. This flux is still more than adequate to do permanent and instant damage to ones eyes! This telescope design is not intended for visual use and such a telescope should never be used visually. No where on this telescope is there an eyepiece. The entire instrument package is electronic (video) as described here. | |
Page Last Updated: 27 June 2001