Craig's Pages




Background

Right now, this is more of a blog / photo site than anything real. The idea is to show the construction of the Sphere-O-Scope. Another write-up of the project can be found on Cloudy Nights.

I've got nice setups I use for astrophotography, but have wanted a grab-and-go, visual-only setup. The 4" TMB APO on a Unistar Deluxe fit the bill well, but is only 4". The person I bought the TMB from had an 8" PortaBall from Mag1 Instruments. I've always been intrigued by this design. It takes the idea of a minimalist approach to the limit, by making a Dob's mirror box be the mirror box, the altitude and azimuth bearing all in one. In fact, it goes one better by being a ball-joint, really, and not having two separate axes. This makes for a free-wheeling mount! (BTW, if you know the Astroscan, marketed in the 80's by Edmund Scientific, it's that on steroids).

This design requires a section of a sphere, ideally a bit more than a hemisphere (about 240-270 degrees worth of a sphere) to work. Building a sphere isn't fun and that's why I think this design hasn't been more popular with the ATM crowd. A few folks have done it. In Sky and Telescope, Randall Wehler built one by using the top from a round Weber grill as the hemisphere in a March 2003 article. This got me thinking and looking around for spheres.

On the Portaball group, Adam Eckhardt had the breakthrough I was looking for. Acrylic lighting diffusers. Know those big globes that sit atop large outdoor lighting fixtures? Bingo! He'd even built a scope with one and had a source -- www.lightingdiffusers.com. $36.50 (including shipping) later and I had a 16" sphere with a 5" hole in it and the project was started.

One other goal came into this project. I wanted to do it inexpensively. I have several nice scopes. A Meade LX-90 and a TMB 4" APO. Toss in a few mounts, some nice eyepieces and I've got a bit of cash into this. Since I didn't really need this scope (don't tell my wife), one goal was to try to do this well but inexpensively. A few things really helped:

1. 8" f/5 primary, secondary, mirror cell, and spider donated to the cause by a friend (thanks Matt!)
2. Salvaged focuser from an old Meade MTS-SN8
3. A cheap source for the sphere
4. All wood, hardware, Teflon bearings, etc. from Home Depot

Total cash outlay has been about $100. The most expensive part was the sphere ($36) followed by the Rigel QuickFinder ($35) and then the plant stand ($16). After that, you're talking stock plywood, paint, a bit of aluminum and some steel (to make the Z brackets), and nuts, bolts, wingnuts, etc.

Design

Building a spherical-scope all comes down to one thing -- balance. Balance comes down to one thing -- torque. For this scope to work, its center of gravity has to be the center of the sphere. If it is, then the scope will be perfectly balanced. Place a ball on a flat surface and it doesn't roll. Make one side of the ball out of lead and the other out of wood and it'll roll until the lead side is on the bottom. The first ball doesn't roll because it's center of gravity is the middle of the ball. The second does because it isn't.

So, if the scope is bottom heavy, it'll want to point straight up. (Remember Weeble Wobbles?) If the scope is top heavy, it'll want to aim at the horizon. Therefore, we must get the center of gravity in the center of the sphere.

The way we do this is to calculate the torque. We can treat the scope's parts as if they fall along a single line from the bottom of the sphere to the top of the spider. Each piece has a mass and a distance from the middle of the sphere. The torque that each piece puts on the point in the middle of the sphere is equal to its mass times the distance. So a 1 oz weight that is 16" away puts as much torque on the center of the sphere as a 1 lb weight that is 1" away. Since the mirror will be in the sphere, it'll only be about 6" away from the center of the sphere. Since the spider, secondary, focuser, eyepiece, finder, etc. are all at the other end of the scope, you could easily be talking 3-4' away. That's 6-8 times further away and 6-8 times as much torque. This leads to the big constraint. Everything at the top end of the scope needs to be as light as possible!

Bolding that last point doesn't do it justice. Really, the whole scope comes down to how light you can get the upper tube assembly (UTA). In my design, every 1oz by the focuser means about 6 oz of weight by the mirror. I've already had to add a 5 lb counterweight even with my design. One focuser I looked at would have made me add 4 lbs more counterweight to offset the increased torque. Needless to say, I didn't use that focuser.

Diagram


This is a scale drawing of the scope aimed at the horizon. It's an 8" f/5 version using a 16" sphere as the base. By the time batteries and fans are added, it's pretty close to perfect balance.

Version 1

Here are shots of the scope near final assembly with most of the bits are there. In it, you can see the basic design. It's using 3 1" thin-wall Al box tubes for struts, a short length of Sonotobe for the UTA, and the 16" acrylic (1/8" thickness) sphere for the LTA. The LTA has been chopped to give a 12.5" hole. In that hole sits a piece of 3/8" ply, cut in a ring (9.5" ID). This is the "upper ring". Down by the bodged-together mirror cell is the "lower ring", made from 5/8" ply. Between the two are pieces of 1" maple that keep the two rings parallel.




Here's a close-up of the LTA (sphere) part where you can see the two rings and the spacers. The truss tubes go through square holes in the upper ring, slide along the maple spacers, and actually seat in square end caps that are affixed to the lower ring. A few bolts with wingnuts through the truss tubes and the spacers hold things in place.

You can also see the mirror cell setup. I salvaged the actual holder from an existing mirror cell and fashioned Z-brackets that go on the bottom of the holder, up the sides, and then extend out over the lower ring. A bolt passes through them there and with springs below them and wingnuts above them, we've got an adjustable cell.




Here's a closeup of the focuser. It came from a Meade MTS-SN8 -- the 80's precursor to the new LXD55/75 line of scopes. It's a helical 2" focuser that uses a spring-loaded ball bearing in the helical groove. The slick thing is you can pull it or push it for large adjustments and then turn it for fine tuning. I have no idea why this design isn't in use more. It's nice for a visual setup. It did need to go on a diet, however, as the 17 oz starting weight was just way too much.

Here we are looking a lot more assembled. All the bits are in place now except for the fans and their controls. This shot shows the overall design. BTW, the "mount" is a plant stand from Home Depot. It's not the most stable thing in the world, but you can see the beginning of work on that (threaded rods being added to stabilize.) Right now, damping times at 150x are on the order of 4-5s. Tracking is smooth, but I do want it more stable. The bearing surface consists of 6 small Teflon pads attached to nylon zip ties that are attached to the plant stand.


Finally, a few closeups of the LTA in semi-final form. One neat thing about using a clear acrylic sphere is that you can paint the inside white first and then lay down black paint. The net result is that it looks white from the outside and black from the inside. Here, you can see the mirror mount again, a 12V rechargeable battery, and a stack of washers used as a counterweight to offset the rotational torque applied by the eyepiece, focuser, & Rigel finder. You can also see the main counterwieght (5 lbs), the bolts that the cell rides on and adjusts with, and the end caps that the truss tubes sit into here in the first shot, as the mirror and truss tubes aren't mounted here.




Well, there you have the basic design. It comes in at weighing 21.5 lbs for the scope and 5.5 lbs for the plant stand mount, yielding 27 lbs total. That actually ends up being less than the TMB 4" on the Unistar Deluxe setup by just a few pounds. Also, I should note that it's astoundingly easy to move around the yard, owing to the low center of gravity.

Right now, the balance isn't perfect by math, but the static friction is enough that it doesn't matter in the slightest. With the main set of eyepieces, I can swap them out in any position and the scope remains steady. The one that is somewhat problematic is the B&W Optik 32mm. Most of my eyepieces are in the 6-7 oz. range and this one is a whopping 20 oz. I do get to remove the 2" - 1.25" adapter to save 1.5 oz... In any case, this eyepiece works, but only when the scope is 30 degrees or so above the horizon.

Sadly, the weather has prevented any real comparisons or tests on the scope. Most of that will, of course, be due to my nicely donated optics and not the basic idea of a Sphere-o-scope.

Version 2

While usable certainly, the above version had a few flaws. First, the balance wasn't quite right. Lay the scope on a hard surface and put a scale on the top end and it shows I was actually 13 oz off of balance. A few things hadn't entered into my equations and a few things weren't exactly where I'd thought they'd be. Also, the secondary was too large, my focal point was a bit further out than I'd like and the base was usable, but a bit bouncy and had a bit more static friction than I'd ideally like. Peter Smitka of Portaball fame was nice enough to give me a few tips (like using a scale to push the scope up or down to calculate the static friction) and ideas to improve upon things (like methods for cooling and thermal management).

UTA diet plan
Moving to a smaller 1.5" secondary and actually getting the focal plane were I want it mean moving the UTA out further. This will only make the fact that I'm already 13 oz overweight on the UTA even worse. So, the UTA needs to go on a diet.

* A 1.5" enhanced secondary and a spider from Meridan Telescopes clock in at 6 oz and give me a 3.5 oz savings from my last setup.
* Removing the mounting plate that the focuser used saves another 4 oz, getting it down to 10.5 oz.
* My UTA ring of Sonotube clocked in at 12 oz in the first version. That's actually pretty decent for 8" long (less than the Mag1 Portaball's actually, but it has nice rings to keep it looking good over the years). But, I figured I could do better. The new one should clock in at just under 5 oz.

The new UTA is made of balsa wood. It's really an amazing substance. I went around weighing all sorts of options for a dual-ring style design or a single-ring UTA design. All sorts of materials went on the scale, from thin ply, to masonite, to thin-wall aluminum angle (making a hexagon), to expanded PVC, to polystyrene and all sorts of things.

Balsa beats them all. Balsa weighs 7 lbs per cubic foot. A cubic inch of balsa weighs 0.065 oz. Per inch, this is about a quarter as much as an inch of thin-wall aluminum box tubing that I'm using for my trusses (which, aside from 0.05" x 4" of Al are air). Two feet of that is about 1.5 oz. Making a ring (9" ID, 12"OD) out of 1/8" plywood would clock in at 4.4 oz. Making the same ring out of 9/16" balsa clocks in at 1.44 oz.

OK, but balsa isn't the strongest material in the world when thin. 9/16" balsa could split along the seams, right? Well, it's a lot less likely to do so if it's balsa plywood. Yup, take 3 layers of 3/16" balsa sheet, alternating each layer by 90 degrees, glue it all up with thinned-down wood glue and you get balsa plywood -- light and stiff.

New UTA

The new UTA consists of two rings of balsa plywood separated by 1" square balsa supports. The UTA itself clocks in at a mere 5 oz, down from the 12 oz of version 1's Sonotube.

I dropped down to a 1.5" secondary, making a smaller obstruction and saving about an ounce. I also fashioned a new secondary holder out of PVC. One version of the new holder had a central spring that made height adjustment easy, but I realized that when designing the secondary holder specifically for this scope, I could get away without needing to easily adjust the height.

All told, the UTA assembly with spider and secondary now clocks in at a mere 10 oz, down about 12 oz from version 1! I also opted out of a 4 oz plate that had been used to mount the focuser, so the whole thing clocks in at 20.5 oz. I'd say the diet worked.

Here is a pic of the new UTA assembly.


And a pic showing the scope with the new UTA and the new base. Still need to add the fans...



The results? Well, this is pretty much a done product. I still need to add the fans to help with cooling as the winter months approach, but other than that, it's done. It's had a few nights in the yard and one at a dark site to show its stuff and done remarkably well.

The new setup is balanced perfectly. I can sit it on a hardwood floor with an eyepiece in there and it'll stay where I put it. The new base is very solid and smooth and a big improvement over the cheesy plant stand. There's still enough friction in there to take my big 32mm BW Optik without sliding and yet not so much that movement is impaired.

It's light, portable, and 8" of scope. All in all, a sucessful project!

Too bad I dropped the UTA on the asphalt one day....