Telescope Mirror Box Details

Telescope Mirror Box details
(Best viewed using

etscape @ 600x800)


	The mirror box carries the mirror cell at the bottom and the telesccope top-end via 8 alluminium trusses. It gets suspended from the mount by means of two Formica cladded plywood disks allowing movement in Declination. Since the mirror cell is made to be quite heavy in contrast to the light-weight top-end, the declination axis is fairly low, allowing the mount to be more compact.
	The "mirror cell" is actually the brake drum of a motor car, modified to support the mirror on a 9-point flotation system. Three "ears", welded on at 120° intervals are each carrying a captured mounting bolt doubling as collimation adjustments. The three alluminium triangles are allowed to freely swivel round their centers of gravity from three bolts locked to the bottom of the cell.
This was achived by rounding the ends of the three M8 bolts (green) forming a nice socket with the countersunk holes in the triangular plates (blue). The M2 bolts (red) tapped into the ends of the M8's keep the plates from falling off with enough clearence for free movement.
Each triangle contains three nylon screws (yellow above) which are positioned to all carry the same fraction of the weight of the mirror when the telescope points at the Zenith. A similar program to this one (29K) by David Chandler was used to calculate their axact locations. Lateral support is done by three lock-able nylon screws at 120° intervals round the side of the mirror. A safety ring over the top (not shown in this view but installed in the next picture) prevents the mirror from falling out when the telescope gets tipped too far over. (The middle part of this mail from my Telescope FAQs Page also explains the construction of the mirror support.)
	This view inside the mirror box shows the mirror cell installed with the only tool required, the collimation adjustment tool (a piece of round bar with a "T" handle on the upper end and a ball-head allen key welded to the bottom end) still in place. This arrangement enables one to adjust the collimation while at the same time watching the effect in the eyepiece draw-tube. Because of the direct feedback only one person is needed to collimate the telescope in a matter of minutes.
Also visible is one truss tube installed with (what I think) a unique way of a truss tube mounting. This is done by sliding the pipe over a rubber plug which is an exact fit inside the pipe. The plug in turn has a piece of threaded rod fitting snugly down a hole through it's center with a nylock nut at the top (for fine adjustment) and an over-center clip at the bottom. Like with the telescope top-end, this system sits inside a hollow, machined at the angle of the trusses to the mirror box.
To install the trusses, they are simply slid over the rubbers and the over-centers clipped up. This causes the rubbers to expand enough to tightly grip the trusses while at the same time pulling them down into the machined hollows. Because of the simple way the truss tubes are gripped at both ends, they are reduced to just eight lengths of identical pipe without any complicated end pieces. (See this mail, this one and this one from my Telescope FAQs Page for more on this system.)
	Once all the truss tubes are installed and the collimation adjusted, a baffle gets clipped into place to prevent as much stray light as possible from reaching the primary mirror. Also visible is the centre mark on the mirror to aid in collimation adjustment when using my homemade auto-collimator which, because of it's double reflection, is quite sensitive resulting in a fairly accurate starting collimation setting. (The last part of this mail from my Telescope FAQs Page have information on producing the centre mark and the construction and use of the auto-collimator.)