A Great Big Astroscan

Designed and built by Jerry Oltion

Featured in the September, 2011 issue of Sky & Telescope Magazine

 

Jerry with Big & Little AstroscansMy first homemade telescope was the trackball, a Newtonian reflector with a round base. When I would show it to people at star parties, they would often ask me if I'd gotten my inspiration for it from the Edmund Scientific Astroscan. I hadn't, but I had been aware of the Astroscan's round-bottomed, bright red, pot-bellied scope, and the more people mentioned it the more I considered making a trackball version of one.

At the Oregon Star Party a couple of years ago, I bought a used Astroscan from someone who felt that I really needed one (thanks, Jeff!). That gave me the template to work from while I built my own. It also gave me a real appreciation for why the Astroscan is such a popular telescope. It's a great little instrument! It's simple to set up and easy to use, and it provides a crisp, wide-field view of the Moon, star clusters, planets, even galaxies. The Astroscan view of the Andromeda Galaxy is spectacular, because you can fit the whole galaxy in the field and there's enough brightness to see detail within it.

The original Astroscan has a 4.25" mirror and a 10" diameter ball. I decided to double the scale, more or less, by using an 8" mirror and a 20" ball. I already had an 8" mirror blank that was ground to about f/4.5, so it was relatively easy to reduce that to the same f/4.2 ratio as the Astroscan. I did that while I was gathering parts for the rest of the scope.

For my previous trackball scopes I'd made the ball out of fiberglass over a kids' rubber ball, but this time I decided to save myself a lot of work and buy a polycarbonate globe from 1000Bulbs.com. Unfortunately, when it arrived I discovered that it was too flexible, so I wound up having to fiberglass it anyway, but at least I was able to put the fiberglass on the inside of the ball this time, saving myself a lot of sanding. I could keep the smooth polycarbonate outer surface, and the rough fiberglass surface on the inside acted as a nice light baffle when I painted it black.

 
Here are some of the materials I started with. The 20" polycarbonate ball already has the fiberglass inside it. The wastebasket will soon have long skinny triangles cut out of it to make it cylindrical. The 8" mirror is already parabolized and coated. The frying pan will become the mirror cell, and the PVC pipe will become the focuser draw tube. The pane of glass was to become the optical window that sealed the tube and held the secondary mirror, but it degraded the image too much, so I wound up making a curved spider instead.

I like to make mirror cells out of frying pans. The aluminum is easy to work with and it conducts heat well, so the mirror seems to cool faster than it would with a plywood cell. I cut the excess material out of the sides, then bend the entire thing flat, then bend the tabs back upright. That proves easier than trying to work with the curved surface of the frying pan's sides, and I haven't had one break from metal fatigue yet.

You can see how I clamp the mirror in place. The wooden blocks are wide enough that they won't twist sideways and dump the mirror out when the scope is aimed at the horizon (or riding on its side in a car seat on a bumpy road). The other bolts hold the mirror cell to the feet that I glue to the inside of the ball. You can see how they're curved to match the inner surface of the sphere about 1/4 of the way up from the bottom.

When this photo was taken I had planned to collimate the scope by reaching down from above; hence the rubber grips at the top. That proved too difficult, so I took the rubber off and left the bolt heads exposed, and I reach down inside the ball with a socket on a long stick to collimate. That works really well, since I can do that while looking through the eyepiece at a bright star and get it spot-on.

 

Here's me looking through the top cylinder. I've already cut out the skinny triangles and turned the tapered wastebasket into a cylinder. You can see one of the glue seams running along the inside on the bottom-right. I cut rings out of 1/8" PVC sheet to reinforce the cylinder and provide light baffles.

Having seams in the side of the cylinder adds to the consistency of the reproduction, since the original Astroscan has seams, too.

 

I figured the focuser would be the make-or-break feature of this scope. The original Astroscan's focuser is a molded part of the upper cylinder, and it drives the drawtube with a rubber roller on a straight shaft between the two knobs. It has a disctinctive "cushy" feel that's part of the Astroscan experience, and it allows surprisingly precise control of the focus. I knew I would have to reproduce the feel as well as the shape.

Fortunately, as complex as it may look, the focuser can be made from two flat pieces of plastic: one bent in a wide U shape, the other bent into a shallow L. I used the same 1/8" PVC sheeting that I used for the cylinder supports/light baffles above. It bends nicely with a little heat, it bonds well with superglue, and it sands smooth enough that the seams are nearly invisible.

I didn't go with a perfect double scale on the focuser. I wanted to use 2" eyepieces, and the original Astroscan uses 1.25" eyepieces, so the proper scale-up factor was only 1.6. That seemed a little small, and I figured the focuser would look noticeably undersized at that scale, so I compromised with 1.8 scale and that seems to have worked out. The finished product looks so much like the original that I can't notice any scale discrepancy.

 Inside the focuser

This is the inside of the focuser. I decided to make mine a Crayford style, riding on bearings rather than simply skidding along on plastic or even Teflon pads. I found the perfect bearings from a model helicopter's rotor shaft, and made mounts for them out of the same PVC sheeting I used for the rest of the focuser. They rest on screws tapped into the focuser body (with thin plates of galvanized tin glued on for additional strength), so they can be adjusted up and down for precise alignment of the draw tube.

The drive shaft is simply a stiff wire (the bail of a 5-gallon plastic bucket) with a piece of gas line slipped over it for grip. It provides the same feel and the same precise focus control as the original Astroscan. With the bearings instead of skids, it was a little too easy to move, though, so I clamped a clothes pin to the shaft to provide more tension.

The knobs are the lids off of big methanol carbouys that Kathy uses at work. They're not a perfect match for the original knobs, but they're as close as I could find, and much better than what I was able to make by hand.

The focuser housing attaches to the top of the scope with the two keyed slots you see here. I have bolts sticking out of the side of the scope that engage the slots, so you just set the focuser in place and pull it downward to lock it in place. I made a place for a third slot on top, but two proved sufficient so I never finished the top one.



The top cylinder attaches to the ball the same way as the focuser. There are no bolts to loosen or tighten; you just stick the keyed slots in the top part over the bolts protruding from the ball and give the top a twist to lock it in place.

I was worried that the collimation wouldn't stay put with this kind of system, but it holds very well, even when I transport the scope over bumpy roads.



Top end with curved spiderThe original Astroscan uses an optical window to seal the tube and to hold the secondary mirror. I couldn't find a piece of glass flat enough to serve, so I made a curved spider instead. That provided the same diffraction-spike-free view, and left the tube open for better cooling and ease of collimation. I was dissappointed that I couldn't duplicate the Astroscan design completely, but this is actually a better design choice for utility, so I'm not that disappointed.

Here's the top end showing how I mounted the spider. The plastic tube was too thin to mount the spider vanes directly to it -- it would dimple outward where the springs pushed on it -- so I used wooden blocks that I shaped to match the curve of the tube to spread out the force a little. I shoe-gooed them to the tube and mounted the spider to the blocks.

The spider is made from steel cargo strap. The central hub is another carbouy cap like the ones for the focuser knobs. The mirror is mounted on a 1" wooden dowel that fits up inside the cap, and I tapped threads into the plastic cap for collimation. I prefer four collimation screws rather than three: it's a lot easier to figure out which direction the mirror is going to go when you turn one of the screws. (The downside being that you can't turn just one screw; you have to turn its companion an equal amount to keep the pair tight.)


Astroscan bases comparedThe original Astrocan sets on a metal base with felt pads for the ball to slide against. I wanted mine to track with my trackball system, but I wanted it to look as much like the original as I could make it, so I recessed the rollers into the legs (made of plywood). When the ball is sitting on its base, you can barely see the rollers. I mounted the switch and the variable speed control inboard a ways so they weren't obtrusive. The batteries and motor are mounted underneath.

I couldn't join the plywood legs in the center the way the original was done, so I mounted them to a circular disk that I painted black so the legs would stand out. I also mounted actual legs to the scope (see the top photo) to bring it to a comfortable viewing height. Those legs are removeable with bolts that attach inside the legs so they don't show.

Dust coversThe dust cover is another distinctive feature of the Astroscan. The early models said "Edmund" on them, so I made mine with my own name: Oltion. (You should be glad I wasn't modeling mine off one of the later editions that say "Astroscan" on the cover. I doubt if I could have resisted making mine say "BigAss-troscan.") I couldn't make a stamped cover like the original, but I was able to have an awards shop laser-cut the lettering into a piece of PVC plastic, which I mounted to a raised handle I made from more of the same plastic, which is in turn mounted to a disk made out of more of the same.

The finders are both made of the same stuff. I didn't have an original finder, so I made one based on a photo of one I found online. I figured I'd use this one for show and use something else when I was actually using the scope, but there's no need. It's surprising how accurate and easy to use a little peep-sight finder can be!


Eyepiece partsBig and little eyepiecesI couldn't find an eyepiece that matched the Edmund RKE's classic look well enough, so I made one. Surplus Shed sells lens sets for various focal lengths, so I bought a set for a 26mm "modified Kellner." RKE stands for "Reverse Kellner Eyepiece," so I figured I would be making a pretty similar design with that set, and it turned out to be true. I had to make spacing rings to hold the lenses apart (the four rings in the middle of the photo) and slide the stack inside a "Clear Care" contact lens solution bottle, which then slid inside an AOsept solution bottle. I used a piece of wrapping paper tube for the 2" base, and bought a replacement rubber grip ring for a Nagler eyepiece and painted it orange to match the original Edmund RKE.  The result is an incredibly lightweight (3-ounce!) eyepiece that works surprisingly well and looks remarkably like the original.

Astroscan in piecesHere's the scope in pieces. The mount is upside-down to show how the batteries and motor and gearing fit into the legs. You can see the mirror down inside the ball and curved spider in the top section.

The scope is surprisingly light: 20 pounds. The mount, with the legs attached, is another 12.

Some other notes on the Big Astroscan:

The original Astroscan uses an iron counterweight around the mirror to keep the scope balanced. I used lead shot mixed with epoxy and poured into a puddle below the mirror. I used 3.75 pounds (counting the weight of the epoxy), which turned out to be almost a pound too much when I use my light eyepiece. So I made some counter-counterweights to hang from the front of the tube. I used round lead fishing weights and painted them red, of course.

The mirror doesn't cool well down inside the mostly-enclosed ball, so the first half hour or so of observing time gives me fuzzy views unless I pre-cool the mirror. Fortunately, that's a simple matter: I stick a couple of ice packs next to the mirror while I'm setting up, and by the time I'm ready to observe, the mirror (and probably more importantly, the counterweight) is cool. Once it's cool, it gives good views all night, even as the temperature continues to drop.

How good is the view? Surprisingly good!  It's an f/4.2 scope, so it has some coma even in the best eyepiece, and my little homemade eyepiece has some distortion of its own, but even so, the view is crisp and clean and pleasing. The homemade eyepiece gives me 30x, so it's not pushing the scope's mirror much, but the optics are nice and clean even at 100x or 200x. I can split the double-double in Lyra with room to spare.

One of the most significant improvements that this scope has over the original, besides the doubling of its size, is that it tracks. That's worth an extra inch of aperture right there, since I don't have to nudge it along every few seconds. I can sit down, get comfortable, and gaze into the eyepiece for as long as I want to, concentrating on what I'm seeing rather than fussing with the scope all the time. Plus, since it's a trackball, there's no Dob's hole or equatorial contortions to worry about: I can just point the scope wherever I want to, orient the eyepiece however I like, and observe.

For more on the trackball mount, click here: Trackball Telescope.

I had a lot of fun building this scope, and I'm having even more fun using it. It's such a silly-looking thing that I can't help but smile while I'm using it, and it turns out to be a fine telescope in its own right. You can't ask for better than that.

How to contact me

email graphicI'd love to hear from people who are interested in this scope design. Please feel free to email me at the address on the right. (Sorry you can't click on it or copy and paste it; it's a graphic file to thwart spambots that search the internet for addresses to send junk mail to.) I have no idea how much mail this idea will generate, so I can't guarantee a response, but I'll do my best to answer everyone who writes with a genuine question or comment about the design.

 

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