In early 2009 I completed and began using a larger dedicated solar Newtonian reflector with an unaluminized primary mirror.
This telescope is built around an Obsidian Optics 35 cm aperture, f/4.6 (1.6 m focal length), 1/28l, unaluminized, 3.4 cm thick, Pyrex paraboloid primary mirror. Since the mirror aspect ratio is only 1:10.5, versus the 1:6 "full thickness" rule of thumb, I built an 18 point floation cell to support it without degrading the surface figure.
The Strehl ratio of this mirror is 0.997 and I did everything I could think of to preserve the contrast potential that this affords. The biggest step in this direction was to put the entire instrument package at prime focus. One of the advantages of a 35 cm aperture is that even the 6.4 cm diameter instrument only obstructs 3% of the primary. This is much better than I could do with a traditional elliptical secondary mirror, especially given the fairly fast f/4.6 primary beam.
The instrument consists of a reflective field stop, a projection lens, a neutral density filter, a 10 nm interference filter centered on 515 nm (green), and a DMK 31AU03.AS USB 1024x768 pixel CCD video camera.
The second step in minimizing obstructions was to suspend the instrument by a wire spider instead of a normal solid vane spider. This spider is made up of 0.5 mm diameter stainless steel music wire. The front and back sets of wires are separated by 30 cm so the entire assemble is incredibly stiff and is much less prone to vibrations than a normal spider. This is my first experience with a wire spider and I'm surprised that they are not used more often. They are at least as easy to built as a normal spider, they are lighter, and centering and collimation of the secondary/instrument is easily accomplished by making slight tension adjustments to the individual wires.
The downside of putting the instrument at prime focus is that provisions must be made for getting power to and data from the video camera and for focusing the instrument. Focusing is accomplished by a friction wire drive. A stepper motor drives a plastic coated stainless steel wire around the on old Canon SLR lens from which I removed all the optics and to which I mounted the field stop and projection lens.
The upshot of all this is an almost unobstructed aperture as can be seen in the view from the edge of the primary mirror below.
A 60 mm aperture f/11 refractor with a Thousand Oaks Type 2 glass solar filter and my old Pulnix TM-72EX 2/3" interline transfer CCD video camera provides a separate full disk image that I use to get features of interest into the small field of the main telescope.
Despite my best efforts to light-weight this telescope by constructing it out 2 cm square aluminum tubing, the entire assembly weighs about 45 kg. This is more than I was able to easily lift up onto the mount each observing session as I did with its predecessor. Consequently, I had to come up with a way of leaving this telescope on the mount and protecting it from the elements when it is not in use. I did not want a building around the telescope when observing because of the way this degrades the seeing due to heating of the building. My solution was to build a roll off enclosure.
The enclosure opens on the south end and is hinged at the north. A pair of gas springs counter balance its weight much like a hatchback on a car. The 90 kg cover can easily be lifted by one hand.
The base of the enclosure is a heavy table structure with a slot for the pier that is covered by a removable piece when the telescope is stowed.
The entire enclosure is mounted on four steel wheels that run on a pipe track. This makes it very easy to move this 180 kg structure and ensures that it always returns to the same stow position.
Once the enclosure is fully rolled back, I replace the piece that covers the slot for the pier and close the top to provide a dark place for the computer and video monitor that I use for the finder camera. A small door on the north end of the enclosure provides access to the video monitor. The roof overhang on the north end gives very welcome shade while observing. The Leyland cypress make an effective wind screen that doesn't heat up during the day.
Page Last Updated: 7 July 2009