My own telescope. What a concept. Why not?
I started with a 7" glass blank (pyrex) purchased locally and started grinding it in July of 1998 at a public mirror grinding demo. Got lotsa people to contribute elbow grease in the rough grinding stage.
9810a25 Mirror grinding table setup for winter grinding (ie room to park the car.
9810a26 The rough grinding tool to take the flat pyrex to f4.5 shape. This is cement, epoxy and tiny little washers used in conjunction with 60 grit and 2-4 hours of hard pushing.
9810a27 Tom's concept of mounting the mirror blank to wood to allow for easier flipping of the mirror and the tool on the machine.
1998 October 12
Replaced the side mount pieces with longer/higher ones to provide a scope higher off the ground. This brought the balance point back to the rear of the base. So... I removed the 18"x18" base and replaced it with a 18"x24" base.
The primary mirror box has been completed and is roughly 10" high by 10" square (everything is constructed out of 5/8" spruce plywood). The mirror will be siliconed onto a Y shaped chunk-o-plywood. This in turn will be connected to the cell mount with 3 fine threaded bolts with springs to provide a dynamic tension both ways (like the Douglas telescope). This mount sits on top of two shelves at the bottom of the box.
The two truss pieces are 3" long each and are 5/4" angle aluminum ($17.50). They are being mounted on the outside of the box and will be removable for transport. The top of the truss will be a simple 6"x10" board that will mount the focuser, spider & secondary and a pointing device (to be determined).
The Mark II mount took a lot more fiddling than the Mark I. A dry run through with all of the parts showed the need for side mounted nylon pads to keep the box centred in the mount. With the 2 angle trusses on the outside of the box, more clearance was needed to allow them to swing freely. Having the eyepiece at 90 deg instead of the more standard 45 deg makes looking at the west and south easy but the east impossible (using a left side mounted eyepiece like most scopes). Flipping the scope over seems to be the only way around this. Another problem has surfaced in the 18"x24" base. The 5/8" plywood is not stable enough in the long axis when twisted. May have to find a way to stiffen the whole thing up or replace it with heavier plywood. The weight of the mount has climbed steeply as well with some of these additions. Weight reducing holes will be drilled throughout the side walls and base.
9811a24 The telescope box and truss itself. Higher side panels were needed as the simulated balance point was higher than the box itself!
9811a25 Looking into the box, showing the mirror cell with some air holes to allow for better cooling. More will be put in later.
9811a26 A look at the bottom of the telescope box and it's primary mirror collimination system. The cell rests on wooden shelves and is removable.
The mirror figuring process continues. After approx 1 -1.5 hours of figuring the parabola came into shape. Testing revealed good progress. Then everything went to (*&@#$&*@# with testing results going all over the place. At the moment I am contemplating going back to more standard figuring strokes to clean up all of the localized work done.
1998 Nov 06
The figuring is done and the mirror has been shipped off for coating. Following the suggestion mentioned above, I abandoned the localized figuring and went back to a standard figuring stroke for 30 minutes. After cooling the mirror tested out at between 1/6th and 1/7th wave with 2 measurements each at 90 degrees apart. I will be posting a time summary here of the whole process shortly.
Secondary constructionThe focuser and secondary mirror arrived from Joe O'Neill so I started building the secondary holder over the weekend. It should be completed by Nov 11th, at which time the holes can be drilled into the trusses, a blank mirror put in to balance the whole scope to allow the main bearings to be mounted on the centre of mass. Then we wait for the mirror to return.
1998 Nov 24 The Scope is Complete. First light was at an RASC Kingston Centre Public Observing Session at Murney Tower park today. Very nice. A few observations:
1998 Dec 30 Added a layer of counter top material (formica) to the primary equatorial axis of the mount (contact cement and small screws); moved the entire secondary assembly 1.5" closer to the primary mirror to allow for larger (taller) eyepieces to focus. See the Time & Money Chart summary of the resources involved in building this scope.. For those who want instant gratification: $310 and 36 hours over 5 months