Callisto 7" Scope

Page Created: 2000 July 26
Page Last Updated: 2003 August 11

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 20 Mirror returned from coating. Started assembly by siliconing 3 globs on the bottom of the mirror and 3 angle brackets on the outside edges and more silicon. Found new balance point (using an eyepiece in as well) and mounted 4" bearings.

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:

only the 32mm Koenig and other short eyepieces could focus. My 9mm and 12mm meade would not. The whole secondary assembly will be lowered about 0.5"

The mount is still too short (even after the redesign) and low to the ground. Plans to build a standard dobsonian mount box are in the works to bring the scope higher.

The 90 degree side mount eyepiece assembly works but if you can build one on a 45 degree, go for it.

The mirror came back with a note that it needs more polishing on the outer edges. I noted something to this effect with the laser pointer test (ie it reflected off the top surface more on the edges than it did in the centre.

The 4" plastic bearings (sewer pipe caps) do not have enough friction. It was suggested to replace them with a 8-9" half circle of wood.

The main RA bearing has too much friction. Suggested improvements include coating the bottom circle with arborite or other hard, slippery surface.

The 3 feet (with two adjustable) design is too wobbly. I think I'll add another adjustable foot and spread out the stance.

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

1999 Feb 18 Completed a new base and mount for the deck post. Essentially it consists of a wooden sleeve that slips over the 4x4 post buried in the ground in the middle of the rear deck. The sleeve has a flat surface with 3 nylon bearing dohickies out at a 9" diameter circle. A second piece, the actual mount, rests on top of this iwth a single bolt to holt it down. This is the standard Dobsonian mount part. It is 24" off the top of the post, allowing for much easier viewing using the small 7" scope. 99feba36 The deckmounted scope. 99feba37

1999 Feb 21 Completed new field mount (24" off ground) still too short.

1999 March 02 Completed new (the 4th!) leg assembly. Pictures will soon follow. In essence the last set of feet were 3 1x2x8 tied in to a 1/2 thick plywood base with teflon/nylon contact supports. Then the above mentioned 24" mount sat onto of this 2" base. So to reproduce the deckpost height I built new legs. Take two 1/2 plywood pieces cut into a triangle. Seperate them by 4" and attach together so you get a two layer platform. Cut holes in the bottom for 3 legs and create pockets under the top platform for the legs to rest against. I designed this so the legs are at 45 deg angles. The legs are about 30 inches long, when plugged in, raise the top of the new base to about 24" above ground. Looks stable.

1999 March 09 Legs are probably not steep enough (stance is too great). They are only 1x2's and look like they might snap if someone leans on the scope or base.

1999 May? Rebuilt legs out of 2"x2" wood and carved up the base platform to accept the larger sizes.

2000 May 21 - named the scope after one of my favourite kick-ass characters on TV and touched up the web page.

2000 July 26 - Some thoughts to rebuild the telescope box itself using 1/2" or 1/4" plywood rather than the 3/4" stuff... very heavy! Next time the aluminum trusses will go inside the box as on the outside they interfere with the motion of the scope. Lightening the box will move the balance point farther out again so the box will have to be taller. Only can I get this done before Starfest? (<21 days or so)