Telescope Construction
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The first photograph shows most of the components for the base of the mounting. These were cut from flat steel sheet and square tubing. The tubing formed two sides of an "A" frame that carried the main base plate. A large "U" channel was used to hold the polar axis bearings. This was fixed at angle by steel plates fixed to the main base plate. Any lateral motion of the polar axis "U" channel assembly was prevented by bolting supports to each side of the vertical plates (not shown). The lower part of the photo shows the fork and its 11/4" stainless steel polar axis shaft.
The second photograph shows the assembled base.
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In order to provide a light but rigid fork, thin sheet steel was cut and formed into three "U" sections. Two for the fork tines and a shorter section for attaching the fork to the polar shaft. Additional plates were welded into the sections to provide extra stiffness. A large boss allowed the shorter fork base section to be bolted to the polar shaft.
Side sheets were finally welded onto the "U" sections to provide a rigid box section fork assembly.
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Two quarter inch steel plates were welded into the tops of the fork tines. Self aligning bearings were mounted on studs on each plate to hold the declination shafts. This allowed the telescope together with the declination shaft and bearings to be removed in once piece from the fork.
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A home made drive system was constructed from surplus gears and a 6V DC motor. The main polar axis drive wheel was machined from an aluminium disc, it's teeth were cut using a tap. The drive electronics were designed and built by myself as part of a degree project when I was at university. In order to achieve a constant tracking rate, a closed loop control system was employed. A tachometer circuit measures the angular velocity of one of the gears in the drive. Any deviation from a preset level results in an error signal being applied to bring the angular velocity back to the preset level. This works fairly well in practice, but doesn't take into account any errors in the drive gears that are not within the loop i.e. the final worm and drive wheel.
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Learning from the Mark I telescope, a larger and thicker walled plastic drainpipe tube was used to hold the optical components. This photograph shows the contents of my father's garage (circa 1980). Various bits of telescope can be seen around the place. My father can be seen cutting a hole in the tube for the focuser. The assembled telescope mounting is in the foreground. On the table is the prototype for the drive electronics. In the extreme bottom right hand corner on the garage floor is another smaller fork mounting. This was a prototype mounting for a 5"x4" sheet file camera.
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Again learning from the Mark I telescope, the mirror cell was totally redesigned. The mirror rests on 3 small thin felt pads within its open cell. As much of the cell as possible was cut away to allow the glass to cool quickly to ambient air temperature. In the photograph, the cell is directly on the table top, within its cell holder. The mirror cell orientation can be adjusted using 3 push-pull bolts attached to a cell holder.
A low profile focusser was constructed from a old projector focussing mount. This has very fine screw threads and provides fine non-rotating focus control. The focusser is mounted on a plate that is fixed on the inside of the tube. Eyepieces and cameras to be placed closer to the secondary mirror and this allows a smaller secondary mirror to be employed.
Also shown in this photograph is a large homemade 5"x4" sheet file camera. This was intended to be used to photograph comets, but to date has not yet been tested. The lens is a 100mm F4 projection element, but I think its optical quality is questionable.
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A classic spider design was used for the secondary mirror. Three small screws allow the orientation of the mirror to be adjusted. The internal tube attachment of the focusser can also be seen in this view.
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This photograph shows the completed telescope tube. Also shown in this picture is the declination axis drive motor. This drives a long tangent arm attached to the declination axis to provide north/south slow motion control. An inexpensive 60mm refractor provides a good wide field finder telescope when used at low magnification. This mounted on the tube directly above the eyepiece focusser. Below the main tube are additional brackets that may be used to hold cameras and other equipment.