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ATM: Boy Scout Ball Scope


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AppleMark
An 8-inch Boy Scout "Ballscope"
Rick Steiner

Background & Design Constraints



As an astronomy merit badge counselor for a local BSA troop, I've often wanted the troop to own a purpose-built astronomical telescope specially suited to use by scouts. What are the important characteristics of a telescope intended for use by Boy Scouts?
  1. It must be portable (must not take up the entire back of a car or SUV when going on campouts)
  2. It must be simple (so that 12 year old boys can see and understand how it works)
  3. It must be easy to use (so that 12 year old boys won't get frustrated trying to point it or look through it!) ? smooth, simple motions are a must!
  4. It must be rugged (no parts to easily break off)
  5. It must be inexpensive (Troops don't have a lot of money. Donations are good, but it should not be so valuable that it's loss or breakage is a major tragedy)
  6. It must be fairly easy to set up (so that older boys can set up and align it with minimal adult help)
  7. It must have enough aperture to be interesting on bright DSOs (M81/M82, M13, etc.), and thus inspire the boys.
  8. It must have optics good enough to resolve detail on the bright planets (Jupiter & Saturn), and really inspire the boys!


Note that this notional troop telescope could (and probably should) be supplemented by the merit badge counselor's personal equipment. The troop telescope, however, should be the constant, familiar utility piece of equipment that the boys can use by themselves. Things a troop telescope doesn't need (in order of decreasing importance):
  1. Go-to - if anyone should be encouraged to learn the night sky, a scout working on an astronomy merit badge should! A green laser pointer in the hands of the counselor is a much better investment for BSA skygazing than any go-to mountÉ the boys will learn a lot more from a person showing them the sky than a go-to display!
  2. computer/TV imaging - BSA astronomy merit badge emphasizes looking at the sky, not at a computer screen. Short time imaging has merit only for boys who have vision problems, and can't look through a telescope. I have not met many of these kids!
  3. fancy mounts - these are likely to baffle a 12 year old. Also, time spent aligning a mount will detract from observing time - remember, 12 year olds have a short attention span. I usually take a fast refractor on a GEM to campouts, but that's not the troop telescope, and it's not for boys to use by themselves.
  4. Clock drive - I'll admit the merit of a driven mount for high power peeks at globulars or planets, but in my experience the benefit isn't worth the hassle. Besides, the boys get a real kick out of aiming the telescope at a planet or DSO, and then seeing it through the scope it's not hard to do, and it makes them feel great! By keeping magnification around 200x, and using a wide-angle eyepiece, the boys will be able to get and keep bright object in the FOV just fine. If the troop telescope has smooth, simple action, then star hopping shouldn't be much of a problem ? especially if they focus on brighter Messier objects.

Design Selection & Prototype


AppleMark
With the above considerations in mind, what would be the ideal design for a troop telescope? Newtonian optics indisputably provide the best value for a given aperture. I briefly considered purchase a small aperture closed optical system (refractor or maksutov) that the boys wouldn't have to collimate, but I kept returning to the Newtonian because of cost and performance. Then, through a couple of very generous donors, the troop acquired TWO 8 inch mirrors (and f7 and an f6)! It was clear that the troop would have a Newtonian telescope!

The f7 mirror came with a complete OTA: cardboard tube, mirror cell, focuser, spider, diagonal, and rotating rings. One weekend, I quickly fabricated a pipe-fitting GEM out of spare parts, complete with cement counterweight, and the troop had a functioning telescope!

I quickly verified the quality of the optics on Saturn, as well as the smooth action of a well-lapped pipe fitting GEM. The rotating rings were very handy, and the eyepiece height was OK for most boys. While this scope was fun, it never made it to a dark sky campout! For 18 months it was passed among various parents, used in their back yards and stored in their garages. It was extremely bulky, the mount was very heavy, and the OTA would not easily fit in a small car. While it was fun, this design really wasn't adequate to the task of being the principal troop telescope!

Reflecting on the failure of the f7 OTA/GEM, I really started to consider what makes a telescope both portable and easy to use. I set an arbitrary goal of fitting the telescope into a 2 foot cube for transportation and storage. I experimented with various folding or ultralight Dobsonian designs, but they all seemed too complex, fragile, or expensive for a troop scope. I had to remind myself that this telescope wasn't for my use, but for the scouts to use, and it really had to be simple!

AppleMark
I was drawn to the simplicity, portability, and ease of use of the Mag-1 Portaball (http://www.mag1instruments.com). I decided to build a small prototype of a "cheap" ball telescope design, using a lighting diffuser as a ball, and cheap surplus optics. The prototype has a 2.5 inch f6 spherical primary, a tiny prism diagonal, and an 8 inch globe diffuser. The mirror cell is a 3 inch round "Altoids" tin. The stand has three small Teflon pads set into the legs for the ball to ride on. The design for this little scope proved so popular with the boys in the troop that I acquired parts for 5 more "kits" (I bought out the inventory of mirrors from www.surplusshed.com), and helped the scouts build them for themselves! Total cost was $20 per kitÉ each boy ended up with his own functioning telescope, and a real appreciation for aligning Newtonian optics!

Full Scale Design & Assembly

Optics


The prototype proved that a light-diffuser based ball telescope is feasible. But, is it scaleable to an 8 inch mirror, without the use of exotic materials? Checking with Mag-1, I notice that their 8 inch f6 uses a 16 inch ball. This encouraged me to focus on using the troop's 8 inch f6 mirror with a 16 inch globe. Generous donors had also provided the troop with two diagonal mirrors, 25mm and 32mm Plossl eyepieces, and a Cheshire collimation tube. There were certainly enough parts to make a telescope!

Overall design


From the prototype, the troop telescope needed to consist of the following parts:
  1. The ball (lower tube) assembly, including the mirror and cell
  2. The stand, with appropriate Teflon pads
  3. The upper tube (focuser, finder, diagonalÉ lightweight but STURDY)
  4. The rod, used to connect the ball with the upper tube

The troop's experience with the f7 Newtonian proved that it is impractical to transport an assembled OTA to a campout. The ball telescope would have to be assembled and aligned for every usage. I knew that this would be a challenge for the younger boys to do by themselves, but I had confidence that the older boys could do it especially those that had built their own 2.5 inch ball telescopes.

Stand


AppleMark
AppleMark

The first thing that had to be built was a stand for the ball. I had to have this in order to work on the ball itself! I wanted the stand to be as low-profile as possible, thus allowing short scouts to get to the eyepiece at zenith. I cut the triangular base from 3/4 inch MDF on a table saw, then cut a beveled circular hole in the center of the triangle with a scroll saw. The legs are 2x2's, dadoed to fit the base, with their tops cut at 45 degrees and fitted with 1/4 inch Teflon plugs. All of this was made from s**** I had lying around.

The spacing of the legs of the stand is tricky. As in a Dobsonian mount, the further apart the Teflon pads get, the stiffer the motionÉ but on the other hand, if the supports for the ball are too close together, the ball runs the risk of lifting out of the stand instead of rotating when trying to turn the scope! This was a problem noted with the prototype, which had the supports two close together. I opted to attempt to position the pads to contact the ball at 45 degrees from vertical.

Ball/Mirror cell


I purchased a 16 inch clear acrylic lighting diffuser from AZ Partsmaster (www.azpartsmaster.com) for $25. Before cutting the ball, I decided to construct a mirror cell and support ringÉ this would tell me how big a hole I would need in the top of the ball.

AppleMark
I could not find a cheap mirror cell that would work adequatelyÉ I needed the mirror to sit as low as possible in the ball. I decided to fabricate the cell myself. It was made from 1 1/2 x 1/8 steel strap and a 3/4 inch slice of 3 inch round s**** steel, purchased from an industrial metal supply house for around $6. I bent the strap in my bench vise to the appropriate shape, hand cut and fitted the cell together. The 3 inch round stock at the back of the cell is heavy, and provides a decent counterweight. 5/16 hex bolts provide the collimation screws.
AppleMark
AppleMark
The cell was designed to just fit into the lower ring. This was made from two rings of 3/4 inch MDF glued together. Holes were bored in the sides of the lower ring to allow ventilation behind the mirror.

The size of the lower ring required cutting an 11-inch diameter hole in the globe. This I attempted with a coping saw, and things were going well until I went just a little too fast, when the acrylic melted and the sawblade stuck in the acrylic. I quickly found out first hand just how fast a crack can propagate in thin acrylic! Ugh! Rather than start over with a new ball (and another $25), I tried using CA glue (superglue) to repair the crack. This stuff is amazing! The crack is still visible, but the structural integrity of the sphere is completely restored. I tell the boys that the cracked ball gives the scope character!

AppleMark
AppleMark
AppleMark
The upper ring on the ball is fabricated out of three parts: a lower ring, beveled to fit inside the ball, slightly bigger than the hole, a middle ring the same thickness as the ball, and an upper ring on the outside of the ball that is larger than the hole.

AppleMark
AppleMark
The lower ring was eased into the ball (by warping the ball just a little), the other rings assembled in place, and all three rings screwed together. The middle ring has a beveled edge and acts as a light baffle. A circular cover was made of plywood that fits neatly in a recess in the upper ring.

AppleMark
AppleMark

Three 1x3 spacers were used to connect the upper and lower rings within the ball. These were initially trimmed for a firm friction fit, and eventually glued and screwed in from the top. Inexpensive pull handles were screwed into the upper ring.

Rod


The rod between ball and upper tube assemblies is made of 1/2 inch steel EMT (conduit), which is very cheap, thus easy to replace if it gets damaged or lost. Prior to final assembly of the ball, a 3/4 inch hole was bored into the upper and lower rings. The conduit was placed in these holes, and a framing square used to alignment of the rings so that the rod was parallel to the axis of the telescope. A wooden split block was screwed into the bottom of the upper ring, and served to clamp the conduit in position.

Upper tube


The upper tube really needed to be lightweight but rugged! I was able to purchase a used helical focuser made of delrin on Astromart for $15. Everything else I could fabricate from s****. The diagonal was made from a 5 inch diameter boy scout carmelcorn can. Note that this is not a full-diameter upper tube assemblyÉ just think of it as a curved-vane spider! Placing the diagonal inside the can, opposite the focuser, puts it on the optical centerline and also protects it from inadvertent damage.
AppleMark
The can and focuser are screwed into a s**** of lightweight 2x4, which was suitably shaped to accommodate the curve of the can. A 3/4 inch hole was carefully bored in the 2x4 to accept the conduit rod. The wood was carefully cut, and a bolt used to provide a means of clamping the upper tube to the rod. The diagonal mirror mount was made of wooden closet rod, cut at 45 degrees and carefully screwed to the inside of the can. The diagonal was temporarily taped to its mount for assembly and alignment. After initial assembly, I ended up using epoxy to fix the diagonal to its mount.

A red-dot finder from www.apogeeinc.com was added to the upper tube.

Once the upper tube was built, it was a simple matter to cut the rod to length. Aiming at a nearby hilltop with a low power eyepiece in the focuser, I slid the upper tube along the side of the rod until the image came into focus. I marked the rod, allowing for the distance the rod must protrude into the clamp of the upper tube, and cut it to length.

Parts List and Cost


Donated Parts
8" f6 parabolic primary mirror
Diagonal mirror
32mm Plossl eyepiece
6mm UltraWide 66 eyepiece (www.scopestuff.com)
3/4 inch MDF
1x3, 2x4, 2x2, closet rod s**** wood
1/4 inch plywood (for box)
5 inch popcorn can
1/2 inch EMT conduit
bulk Teflon (cut into 1/4 inch plugs)
various fasteners & knobs

Purchased parts (estimate $90 total)

Item

Cost ($ est)

Source

16 inch clear globe lighting diffuser

25

azpartsmaster.com

helical focuser (delrin)

15

astromart.com

"Mars red-eye" reflex finder

12

apogeeinc.com

Steel for mirror cell

10

industrial metal supply

Felt pads for mirror cell

3

ace hardware

Drawer handles (2) for ball

6

ace hardware

Hinges, handles, latch for box

10

ace hardware

flat black spray paint

4

ace hardware

Green paint for box

5

ace hardware

Assembly & Use

Storage


AppleMark
AppleMark
I built a removable shelf to fit inside the ball, over the mirror, with cutouts for three eyepieces and the upper tube assembly. I also built a storage box from s**** plywood to house the stand, ball, and accessories. Patches of s**** felt were glued inside the box to keep the ball from getting scratched. The storage box is a 17 inch cube, and can be lifted by one sturdy boyscoutÉ. the portability objective has been achieved! It is important, however, to remember to take the rod, along with the storage boxÉ This was learned the hard way, much to the embarrassment of the author in front of several boys!
AppleMark
AppleMark

Assembly & Alignment


The older scouts have become rather proficient in assembling and aligning the scope. This requires opening the carrying box, lifting out the ball, lifting out the stand and setting the ball on it. The upper tube, eyepieces, and shelf is then removed from the ball, and the rod inserted and tightened down. The upper tube assembly is then positioned onto the rod. A donated Cheshire eyepiece makes alignment straightforward - first position the upper tube so that the crosshairs are centered on the "donut" on the primary, then adjust the primary so that the reflection of the Cheshire is centered.

Counterweight


It became clear during the first field test that more counterweight was necessary. Subsequently, lead shot filled film canisters were hot-glued to the lower ring inside the ball, opposite the rod, until balance was achieved. The scope can now be used at low elevation without danger to scouts or optics.

Eyepieces & accessories


A donated 32mm Plossl provides excellent low power (37x) views. The boys quickly learn to use the red-dot finder, and loudly express their approval when they find their target visible in the eyepiece. It takes just a little prodding to get them to accurately align the finder, with the promise of going to "more power" once it is aligned! An inexpensive 6mm "UltraWide 66" eyepiece provides decent high power (200x) views, with a field of view wide enough that several scouts can take a look before repositioning the scope. This is plenty of power for most objects the scouts will be interested in, so there is really no need for a Barlow.

It's a great feeling to hear the boys exclamations when they position the scope at Saturn and take their first look through the eyepiece!

Impressions


AppleMark
AppleMark

AppleMark
Nothing gives greater satisfaction to an amateur astronomer than sharing his/her passion with others, and especially with young people! This troop telescope has given scouts not only interesting views of some of my favorite objects, but has given them the confidence that they can explore the night sky on their own! The scope is so easy to use that boys are eager to try their hand pointing it at planets and stars, and then quickly progress to wanting to find galaxies and star clusters! This is a great way to get them interested in reading star charts, and taking on the challenge of star hopping to the brighter Messier objects! Armed only with a green laser pointer, the astronomy merit badge counselor can point out bright DSOs, and the scout can find them on their own with this telescope. This is extremely rewarding as the night goes on, and the excitement of discovery builds! This telescope cost less than $100 to build, but the experiences it provides under the stars for both scouts and adults are really priceless!



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