here's a link to my old webpage front end

 

Dave's SolidWorks Virtual 16 inch f5.6 Tri-Dob

(Version 2.0 in the making)

The whole enchilada........................................... the ground ring.........................................the flex rocker (right side up)

the flex rocker (upside down)........................................... altitude center trunnion, stepper motor & bracket and bearing mounts (ball & socket truss clamps)

 

entire lower section view of the scope.............................inside the mirror box

...................................(collimation frame, 12-point PLOP designed cell with rectangles, silicone pads and collimation assemblies)

(looks like two of the machine bolts have defaulted to a much larger size! SolidWorks does weird stuff sometimes, but it's probably just me!)

 

upper end of the OTA. The blue spacer is Styrofoam. The Styrofoam idea didn't work well for me--it collapsed under the compression forces of mounting the truss clamps. Note the light cone from the primary for accurate placement of optical components. SolidWorks software is slick.

In real life the Styrofoam was a disaster. It failed to hold up under the forces required to assemble the endring. In other words, it compressed at the joints where the wire spider supports were mounted. Ultimately, I reduced the endring to a single section of 0.5 inch plywood. It works great. I also eliminated at least 1.5 pounds from the endring by replacing the ball and socket joints with a simple aluminum capture plate and a bolt through open aluminum tubes, capturing everything with a simple hand nut. Not shown/modeled here is the light baffle from the original version 1.0 of this scope.

    

focuser (I rebuilt my first Crayford that I donated to my brother's 8" Celestron dob.) We learned that instead of the two cylindrical nylon bearing supports for the focuser shaft, a single rectangle of delrin-like material in it's place works MUCH better. We didn't have nylon on hand, but found some plastic stuff that worked amazingly well in a jam.)

Secondary holder and mirror. (The secondary mirror was by far the most difficult part of the telescope to model in SolidWorks!)

upward looking view of the endring of the OTA. The eyebolt/vertical backplate holder idea seems to be working well with the wire spider. I know the tubing scenario I used previously also worked well (i.e. no flexure at the eyebolts themselves).

Links to zipped SolidWorks files:

Part 1

Part 2

Dave's 16" f5.6 Tri-dob

Version 1.0

(Click images for high resolution versions)

Let's start at the top and work our way down, shall we?

I originally started with a simple 1/8" thick aluminum endring. It wasn't rigid enough, so I wound up reinforcing it with another ring of 1/2" plywood.

There may be a couple of things that are novel about my approach. One of these is the way I suspended my secondary. Note the 0.5" aluminum tubing "pegs" that extend off the endring. They have eyebolts extending perpendicularly through them at the ends. I tapped these six short sections of aluminum tubing so that they capture short sections of all-thread at 120 degree positions on the endring running parallel to the incoming light path. I also tapped the aluminum endplate. The aluminum pegs mate to the ends of the all-thread, above and below the endring. At the extreme ends of those aluminum caps I drilled holes aimed at the secondary mirror. Into those holes I inserted small eyebolts. The eyebolts, in turn, capture the wire that passes to and from the secondary. The aluminum caps are adjustable along the axis of the light path (assuming you can disconnect the eyebolts that hold the wires--which I can), so you can coarsely adjust both the secondary position AND angle in this manner. The secondary is mounted in the traditional way with three springs for fine adjustment of the angle. The eyebolts also allow me to adjust the lateral position of the secondary and center it in the optical light path. Adjusting the tension and centering the mirror was a nightmare until Mel suggested making a "fixture" or jig (a physical connection between the secondary mirror and the endring) to hold the secondary in place until the wires were in place and tensioned. Thank you Mel! Whew! (See the comments/lessons learned below.)

I realized all my efforts to be able to adjust the secondary were overkill when, during my first setup, I had the epiphany that all I needed to do was adjust individual pole lengths. Criminy! Can anyone be that dense?

 I found some nice light extendable aluminum tubes at Home Depot for $10 each. Then down the aisle I found some paint rollers that screwed onto the extendable poles. I lopped off the rollers, cut the wire the desired length, and then bent the wire around a 1/4" bolt in my vice (into loops). At the endring I had to bend the wires to 90 degrees and fasten them to the endring in pairs with a 1/4" carriage bolt and hand nut. At the focuser I did the same thing without the 90 degree bend.

I think my Crayford style focuser is also a bit unique. I used one heavy piece of aluminum angle to mount four small bearings. This angle is mounted to a piece of flat aluminum (1/8" plate). The drawtube rides against these 4 bearings and passes through the plate. The focuser shaft is held in tension against the draw tube by two nylon thumbscrews that pass through another drilled and tapped piece of aluminum angle. The thumbscrews seat into separate cylinders of Teflon. I basically took a piece of Teflon shaft and drilled a hole perpendicular to the shaft the same diameter as my focuser shaft. Then I cut the Teflon in half right through the hole. The thumbscrews seat into shallow holes that I drilled into the butts of the cylinders. It's simple. Pressure on the draw tube is easily adjusted and I can pile plenty of weight on the drawtube without seeing any slippage. The focuser shaft and knobs were salvaged from the Pitney-Bowes copy machine (see the Copyscope page for more on that.)

I also attached my homemade copyscope/finder to the endring with a small piece of aluminum angle.

My secondary mirror is mounted using Mel's wire spider design. However, my attachment points at the secondary post are just simple aluminum plates separated (or spaced) by a length of 0.5" aluminum tube. These two plates and the spacer are held together by tension from the wires. The black aluminum tubing runs through both plates and the 0.5" tubing spacer and is tapped (1/4-20) on the end. This hole is used to securely capture a 2.25" round plate of aluminum. I used a metal circle cutting jig from Harbor Freight to make two of these 2.25" diameter aluminum plates. These are drilled and tapped for the traditional 3-point, spring-based secondary adjustment. The 3.1" minor diameter secondary mirror is glued with RTV to a section of 2.25" aluminum cut at a 45 degree angle. In these pictures the wire runs directly through holes in the ends of the plates. I used machine bolts as set screws to lock the plates against the central shaft.

I used that fluted plastic sign board stuff to make a baffle. Following the advice from another ATMer on the list I used a razor to slit every third flute allowing the piece to be flexible enough to mount directly to the endring. I simply painted this with flat black spray paint and used wood screws to mount it directly to the wooden endring.

...making our way down the OTA...Here's a view down the OTA.

 

I used simple wooden blocks with 1/4-20 carriage bolts and hand nuts to capture my truss tubes at the mirror box. The clamps are fastened to the mirror box with more carriage bolts and wing nuts. Also in these pictures you can see the grooved screen door guide wheels that serve as pivot points for the Virtual Counter Balance springs. I'm currently using string to connect the springs to the mirror box. I'll switch to wire soon, now that I've settled in on the right solutions with respect to weight. Also in these pictures you can see the central altitude fin or bearing. It rides in a triangular block with a groove that's lined with 1/16" Teflon. Epoxy does not work well with Teflon, even if you rough up the surface to be glued. Thus, the wood screws and wrap-around Teflon. The other bearing surface is Ebony Star. It's every bit as good as it's hailed on various ATM websites.

Here you can see the five 5 lb. lead bricks I had to use to balance out the scope...and the 12V muffin fan that blows on the bottom of the mirror to vent air from or into the mirror box. My preliminary results with fans and boundaries of warm air suggests this fan has little effect. The other fan, however, that blows air ACROSS the surface of the primary has RADICAL effects on improving images at the EP (as long as the mirror is not at equilibrium). There's been discussion on the ATM list recently about fans and I concur: images are aided tremendously if the surface layer of air on the mirror is scrubbed away. Just make sure to isolate the fan to avoid vibration. Rubber bands, old mouse pads...anything to dampen the vibration should work. I'm using chunks of a mouse pad.

This is an early on shot during the building of the mirror box and flex rocker. From this picture you can see the 9 rectangular plates for the 12 point PLOP designed mirror cell. (Once again I have Tom Krajci to thank for his input! The 12 point cell and bars instead of triangles are also some of his contributions to ATMing.) I have not glued the mirror in place yet, but I haven't seen any hint of astigmatism or deformation of the image, even at very low altitudes. The lack of astig should hold out even after RTVing the mirror in place. The rectangles are mounted on a 'T' for adjustment and collimation. I have a homemade laser collimator with a soccer ball image that I cast onto the wall when collimating. Collimation is a cinch: put the collimator in the drawtube. I wrapped Duct tape around the PVC tubing/housing to make it fit snugly into the draw tube. Center the red laser dot in the center of the secondary (I actually put a tiny black dot in the center of my secondary. Hissssss. Call me a heathen, go ahead.) Adjust the truss pole lengths to center the red dot on the primary (I used an adhesive donut to identify the center of my primary; one of those reinforcement thingies for paper that's been hole-punched to be put in a binder.). Adjusting the pole lengths is the trickiest part because it's not difficult to get the optical and mechanical axes out of whack. I then push the scope to nearly horizontal and aim it at a wall or sheet or anything flat and preferably light or white. Adjust the three primary mirror collimation screws such that the reflection or shadow of the secondary is centered in the red soccer ball image on the wall. I've learned that the more time I spend tweaking the collimation, the better the views. I don't think it has anything to do with the mirror cooling down, it's just simply improving collimation!

The other two altitude bearings are mounted to the outside of the mirror box and the setup follows Mel's Tri-Dob. Aligning the three components of the altitude bearing to make them concentric has been an ongoing topic of discussion on the ATM list.

Mel suggests: "The variation in altitude rims is as great as impact as any de-centering.
So, just build to normal good 1/32 inch tolerance, and you'll be fine. That error
calculates to a few arcminutes in most situations."

Jan van Gastel is building a large tri-dob and has implemented the use of a jig to make sure his bearing surfaces are concentric.

I drilled lightening holes in the bracket that connects the two altitude bearings. In hindsight I would NOT drill holes in this bracket because it weakened the entire structure. I used aluminum angle to reinforce all the corners and vertices in the mirror box. A rigid mirror box is ESSENTIAL!!--see my comments below.

I was originally going to mount the truss clamps to the bracket between the altitude bearings, but I couldn't find a way to mount them there and still have clearance as the OTA tips down to the horizon. (Ah, the things you can't see in 2D models or paper!) Thus, mounting the clamps to the alt bearings puts quite a bit of demand on the rigid nature of the bearing. Mine flex. They need to be made thicker, but that will require a new flex rocker and larger ground ring inner diameter.

I'm currently redesigning and enlarging the mirror box and flex rocker (which will be motorized using Mel Bartel's Go-To Stepper Motor Drive system) and I will follow James Lerch's advice regarding driving the altitude bearing from the central bearing (or "fin") rather than from the outer altitude trunions. You can refer to James' ATM list comments HERE, which are based on motorization of his 16" binocs. You can see his creation HERE and first light images HERE. (I always have a hard time finding these pages by Google!) There was also some discussion on the Scope-Drive list (July 2005) regarding driving a tri-dob.

Kevin started it:

I was wondering if the Trilateral Dob design by Mel Bartel's could be
adapted for computer operation.
Thoughts?
Thanks in advance,
Kevin.

...and I replied (edited):

I've bounced several questions off Mel and the group and from the responses, it appears that the tri-dob platform can be motorized. Based on my perspective and research the altitude will have to be driven via the central altitude fin (the center alt bearing). Issues related to pointing accuracy and imprecision resulting from non-concentric alt bearings can be remedied in the software, according to Mel. I don't know how "easy" that fix will be, but given Mel's reputation, I have no doubts that problems would be solved. [comments regarding James Lerch's recommendation for driving the altitude from the center alt fin.]

I foresee problems with weight or pressure on the worm at the alt. drive contact unless there is a "trolley" or pivot bearing installed very near the worm/bearing contact point. Mel suggested the "trolley" idea...it's in the archives under "weight on the worm". Alternatively, you could widen the center alt bearing/fin by reinforcing with an additional section of ply, modify the existing contact point to a roller bearing and add teeth to the nascent ply sector. If you make the bearing "adjustable" with respect to vertical, you wouldn't need to build "tensioning" into this worm drive; you would only need rigid worm support.

Driving the azimuth appears straightforward:  Generate the teeth on the outside of the ground ring.(JB Weld, if you like.) Implement some sort of bearing or guide system on the inside of the ground ring (to maintain even contact between ground ring and flex rocker). I've done this already on mine...I mounted three bearings (another tridobism!) to the underside of the flex rocker and they ride against the inside perimeter/surface of the ground ring. I need to figure out how to make one of them adjustable. Install the stepper directly to the flex rocker and aim it tangential to the outside of the ground ring. Find a way to apply tension to the worm and you're golden.

Ultimately, I think both steppers have to be mounted to the flex rocker. I could be way off, though. Early on I kept telling myself that my flex rocker was too flexible to pull this off. But after thinking about it, the flex rocker doesn't have any real restrictions with respect to how thick or "solid" it needs to be at the two positions where stepper motors will be mounted. You could reinforce and build off of the flex rocker as much as you need 'at these two locations' and still maintain the flexure required for the OTA to sit down on the rocker and adjust to a stable three-point stance. And another thought I had was that I could actually just build a whole 'nuther flex rocker and permanently attach the stepper motors to it. Then, when my battery or pc dies, all I need to do is swap out the stepper flex rocker with my original teflon bearing flex rocker, flip or drop out the roller bearings and I'm back to manual steering. That would be pretty nifty and spiffy. As with all ATM projects, she's a work in progress. But I gotta tell ya, there are few things more enjoyable than waltzing a 16" scope out the back door, setting up and collimating in 15 minutes and be staring at the planets and stars just like that! Sweet!
And I owe it all to you fine folks on the ATM list!
Whooohhoooo!
Dumpster Dave

Mel Bartels: "The only comment I'd add ... is that the altitude can be driven 
from a single long thin drive rod that stretches across and underneath
the forward altitude rim fins."

So there you have it! Now, back to construction...

And speaking of the ground ring, here it is in progress.

...and here's the flex rocker. Note that the rocker does not sit symmetrically on the ground ring. In the picture on the right you can see the Teflon azimuth bearing and one of the three bearings that guides the flex rocker as it spins on the ground ring. The bearing rides directly against the inner wood surface of the ground ring. I believe that this surface needs to be lined, probably with sheet metal or aluminum. And I need to find a way to make this bearing adjustable, so that it rides constantly against the ground ring surface (or holds constant tension against the ground ring inner rim). Mel's Tri-Dob uses 4 bearings that ride on the outside of the ground ring. Mine uses three ball bearings that ride against the inside of the ground ring.

Lessons learned & Ideas

There were several lessons regarding the tri-dob (TriDob) design that I didn't fathom until I tried to build it:

1) the inherent triangles created by the trusses to stabilize the OTA don't meet together on the same face of the mirror box which DEMANDS more rigidity in the mirror box. I still have issues with this. I used 1/4" body wall aluminum angle to stiffen the mirror box (again, thinking light was better! Thinking? NOT.), but my mirror box still flexes. I used 1/2" Birch veneer ply. It was expensive and high quality. However, I do NOT like the way the veneer finish chips so easily. It is frustrating to have a nice cut and then have chips fall off when you bump the edge. It is a very brittle veneer finish, so caveat emptor, my friends! The flexure results in temporary miscollimation when pushing the scope around, especially near azimuth. When I release the "steering wheel" (endring) the scope returns to it's collimated state. However, this miscollimation is particularly frustrating when trying to track something manually at very high power.

2) the split altitude bearings MUST be centered to approximately 1/32" (as Mel has recently stated on the ATM list July 2005). This was tricky to implement and required LOTS of measuring and re-measuring. I couldn't think of a way to build a jig to ensure the bearing surfaces revolved around the same center.

3) When I rebuild the mirror box (and I WILL) I'll make it larger, stiffer and deeper. I was trying to conserve weight, but wound up adding 25 pounds of lead to the butt AND using virtual counterweights (Springs, a la Tom Krajci). I will also shift the mirror farther from the center of the alt bearings (i.e. the altitude pivot point) to improve the balance. I'd LOVE to get rid of some lead.

4) Mounting the secondary holder using the wire spider was a nightmare, until Mel convinced me to build a jig to clamp to the endring and physically suspend the secondary in the correct orientation until I could tighten the wires in the spider. The jig is simply a circle of plywood the same diameter as the endring with three posts spaced at 120 degrees around the outer perimeter. I put three screws through my endring into the posts and then mounted the secondary & mirror to the jig through a hole in the center of the jig (the by-product of cutting the circle with a router and circle-cutting jig). You then mount this massive endring on the scope and collimate with the jig in place. Once you've properly positioned the secondary (with respect to distance from the primary and centered in the endring) you just wire the secondary in place. Save yourself some headache and build the jig--you'll be glad you did! I started using Spiderwire (the really tough, non-monofilament, woven fishing line), but it failed due to abrasion where the line met the aluminum blocks of the central hub. I have since switched to 25 gauge wire, but beware you only get one shot with this stuff before it breaks from bending. I suppose I should try some guitar string, as has been suggested on the ATM list.

This is my first attempt at describing my TriDob. If you would like to see more pictures or better descriptions of any particular part of the scope, I would love to hear from you! Drop a note on the ATM list or send me an email.

 

A couple of astrophotos of the moon through the 16" Tri-Dob. I was using a Russell Optics 19 mm Super Wide Angle (SWA) without my 2x Barlow. The camera is a Pentax Optio 5.0. The image has been cropped but not modified in any other way. We tried shooting a globular cluster in Scorpius, but the hand was too unsteady and I didn't know my way around the camera as well then. The image through the scope was tack sharp, so the images here suffer from operator error. Still, I was pleasantly surprised. I'm looking forward to playing with this some more. I even have a retired webcam that I'd like to modify. (So many projects, so little time!!)

   

Feel free to e-mail me at: unspamsmithersscope@yahoo.com (remove the 'unspam')

 

 

CopyScopes

Copyscopes are basically a poor man's finder scope or monocular made from the lens assembly of a copy machine.

If you find yourself in possession of one of these lens assemblies, then you can add an eyepiece and create yourself a very economical and surprisingly good finder scope. You can also add your own crosshairs to an eyepiece if you can figure out the focal point of the EP.

...the exploded view with all the sections cut...........................and the final product.

For this copy scope I used a lens assembly from a trashed copy/FAX/printer. The eyepiece holder and 12.5 mm (0.96") EP were permanently borrowed from my son's Meade 4.5" reflector. (We're upgrading his focuser and scope.) I have not glued these components together yet. The tight fit seems to be more than enough to keep it all together. Application of duct tape works great where needed also. A pipe cutter is superior to a hacksaw if you can afford one!

 

Here's my homemade cross hairs glued in place on a cheesy 0.96" 12.5 mm EP. Good 'nuf for a finderscope. Works tolerably well.
The copy scope below was made with a lens assembly from a salvaged Pitney-Bowes Copy machine (you should have seen me trying to load that sucker into the back of my minivan!). And what a find THAT was...parts galore!

  

...again, the exploded view with all the sections cut......and the final product.

On this one, the eyepiece was eventually replaced with the EP from my son's 4.5" finderscope. It was literally a piece of junk. You couldn't even get it to focus on the moon. Behind the right lens, though, the little EP performs pretty well. This one will live on as the finder on the 4.5" revamp. You can see the brackets for its mount below.

I have plenty of extra 2" i.d. aluminum tubing that I use for my focuser drawtubes. I just hacked off a 4" piece and then cut a notch about 3 inches wide right out of the middle down to the thickness of the body wall on one side. Then I took a piece of aluminum angle and mounted the two together by drilling and tapping holes. Drilled and tapped some holes at 120 degrees apart on the remaining "rings and 'Voila!', finderscope bracket! This is how I mounted my larger copy scope on my 16" Tridob.
On the 4.5" reflector we just modified the existing plastic bracket by adding on larger rings with holes drilled and tapped at the traditional 120 degree spacing.

 

Homemade Foucault Tester (Moving Source)

(no battery, no razor installed, and the wiring has been disconnected at the red superbright LED)

...and here's an image of my "late" 13 inch mirror (polished, not figured) grabbed with a digital camera held at the knife edge.

Laser Collimator

 

There are lots of references on the internet for laser collimators. Here's my two cents worth on an EASY, CHEAP and incredibly useful project. I take no credit for this design either!

Pretty simple instructions:

Buy some 2" PVC pipe (assuming you're building a collimator for a 2" focuser) and a pipe cap (for aesthetics) and a laserpointer (I got one of those shorty key chain jobs with 50 different screw-on pattern thingies off of Ebay for $5.). You'll also need a drill and taps come in very handy. I got mine from Harbor Freight cheap...but the quality is equally cheap, so caveat emptor. I've since replaced the red laser pointer with a green one and will NEVER go back. To be able to see the beam and watch it move as you collimate is just plain cool! Visible even in daylight. Be careful though, it is VERY bright.

Cut the PVC to length and give yourself extra length for a good fit in your draw tube. My draw tube is 4" long and my collimator is 7". Also cut two additional pieces about 3/4" wide. Cut a gap out of the small pieces (>1" on mine) with a hacksaw.

Pinch the two smaller pieces and jam them down into the business end of the 7" PVC section. You want to position these two pieces such that the set screws that grab the laser pass through them (i.e. two PVC body wall thicknesses will improve the ability to center the laser later on. It just gives the screws more to bite into.

Locate the traditional 3 points around the outside of the PVC (i.e. 120 degrees apart) and spot them. Then drill them and tap them to fit your screws. I didn't have enough of the right size machine screws handy (and I was in a hurry) so I used 1" wood screws. The pitch is a bit steep (and they're not the prettiest), but they work fine. (Note: my collimator started shorting out because the wood screws penetrated the paint on the laser pointer and started grounding it out! You might want to avoid this and use nylon machine screws or something to protect your laser. I wrapped mine in a layer of thin rubber.) You also want to drill and tap one additional screw right on top of where ever your laser's momentary on/off switch will be in the finished product. A nylon thumbscrew works real well in this hole. My button fell right smack in between the other two sets of screws. Go ahead and start the screws into the holes. Adjust rotate and position the laser pointer so that you will be able to turn it on using the thumbscrew after inserting the unit into the PVC tube.

IMPORTANT! Find one of the attachments that creates a nice round three-dimensional pattern and put it on the laser pointer before you mount it in the tube. My favorite is a soccerball. I built one for my brother that uses a smiley face :) and it works pretty well, too. You don't need one if you use a green laser.

 

Position the laser pointer in the center of the tube and gently tighten up the machine screws. The next part was time consuming, but not difficult. It's hard to "take a picture" of this, so I'll just try to explain it. What you want to do is find a groove somewhere around your house where you can place the collimator and spin it in the groove. (Meaning you want to put the end of the collimator that goes into the draw tube into a groove and rotate it.) The groove can be as simple as a ledge on the fireplace or the fence on your tablesaw. Basically anything that has two square edges and gives you freedom to rotate the tube will work. Also note that your groove has to point to a flat surface at least 10 feet away. I think farther is better to an extent; in theory, the farther the image, the more accurate the collimator. I cast my image across the garage from the table saw (probably 20 feet). Now that your collimator is powered on and pointed at the wall, all you have to do is adjust the six set screws and rotate the tube until the beam on the wall (the dot) no longer circumscribes a circle. All of the 3D images on my attachments have a center dot in the pattern, so it should work with any attachment you choose. You'll probably find, as I have, that the mechanical axis of the pointer does NOT correspond to the laser pointer's optical axis. A little trial and error will get you to an awesome piece of equipment for collimating. I will never go back!

Here's an image of the finished product using a smiley face attachment.

It's a piece of cake to use!

First Attempted Scope: 6.1" f5.5 truss tube dob

As with many other things in life, such as this website, this scope is a work in progress that will probably never be completed.

The base of this scope was used to build a new mount for my son's Meade 4.5" scope, replacing the original GEM (the thing just plain wore out).

The focuser lives on in my brother's 8" Celestron Dob.

Most Recent Progress Update (7/20/00) (yep, definitely back burnered--thin glass is nothing short of challenging!)

The truss clamps work really well and I may yet implement them on my 16" tridob. I just need to find some larger square aluminum tubing.

I'm going to forego the thin mirror at this point and trepan a thicker blank and start over. 0.5" plate should be fine for a 6" mirror. Once I've started those optics I'll recreate the base for this scope. It was really easy to make and should be easy to recreate. I now have Ebony Star formica, too.

I've built the spider, mounted a temporary rectangular secondary (copy machine first surface mirror), designed and machined the tube clamps for the upper end. Ordered and received the 1/2" dia. .058" body wall aluminum tubing from Texas Towers and cut it to size. A quick and dirty assembly made it quite clear that balance is a major problem.  I haven't decided on clamps for the rocker box yet. I cut and drilled one split block clamp, but disliked the way it looked (too bulky).  I'm still open to suggestions. In the image below you can see that I am using wire wraps as a temporary "clamp" to size things up.

Click on the images to see a larger view:

In the first picture the aluminum center tube from the spider has not been trimmed yet...it's the long black thing protruding from the end of the scope. I borrowed heavily from Gary Wolanski's design when constructing my spider. It is feather light and incredibly stiff. (Still using a cheesy camera...sorry.) I'm not real pleased with my make-shift spider-to-endring connection though (a nut and bolt through aluminum angle), and will tackle that and try to improve on it another day. I was, however, very pleased with the way my truss tube clamps for the endring turned out. The first execution failed because I didn't allow enough space between the two 1/2" holes for the incoming truss poles to allow for the width of the machine screw. Doh! 

A brief explanation: The clamp is made from a 1.5" long section of rectangular aluminum tubing with 1/8" body wall. The height is 1/2" and the width is 1". I drilled and tapped holes for two hex head 10-32 machine screws (In the right-hand pic on the left side of the image with the Allen wrench pointing at them) through the short side of the rectangular tubing and near the "back" wall (also on the left in the pic). Then I drilled and tapped one 8-32 hole perpendicular to these two threaded holes in the center of the long side of the tubing. The 8-32 machine screw can be seen in place just above the screwdriver in the image. Only the hole in the left-hand wall is tapped; the actual hole that the screw is sitting in is a "close fit". Two 1/2" holes were then drilled through the width of the tubing at 7 degree angles for the truss tubes. Finally, I used a hacksaw to cut the clamps in half, lengthwise. I plan on replacing the 8-32 machine screws with hand knobs with 8-32 threaded extensions. The 10-32 machine screws pass freely through both the aluminum angle and aluminum endring and thread into the clamp (both body walls threaded). So the clamps actually serve as "nuts" to the 10-32 machine screws. The clamps were relatively easy to make, are very lightweight and compact, and hold the trusses and spider very rigidly.

Despite my efforts to keep the end ring light, the scope is severely top-heavy! The entire OTA, including the 2X 2" Barlow, 2"->1.25" adapter, 1.25" plossl eyepiece, mirror mount, mirror and altitude bearings (i.e. rocker box) weighs in at 5.75 pounds. I've toyed briefly with Tom Krajci's "virtual counterweight" system of springs and pulleys, but have been unable to find just the right amount of tensioning. I can get the scope to hold position in the range from zenith down to about 45 degrees, or from 45 degrees down to the horizon, but I'm still trying to find that happy medium! No first light yet...truss tubes have been cut 2" too long ala Kreige & Berry. That may be a boon or boondoggle if I have to redesign the rocker box!

Comments and criticisms welcome, as always! Hope you glean something from my ramblings here!

 

Older Update (5/30/00):

Here are a few images of my soon-to-be first telescope (i.e. still under construction), a 6" f5.5 truss-tube style scope.

So far I've routed an aluminum endring and built a 2" Crayford focuser for the upper tube assembly.

 

Why a 2" focuser on a 6" scope!!??  My new barlow is a nice 2" 2x and I wanted to use it on this scope as well as the other 'work-in-progress' 13" scope.  Yes, it will be heavy, but it seemed to be the most frugal way out when I was shopping for a barlow and knew someday I'd want to invest in some 2" eyepieces.

The Crayford focuser resemblesDave Bevel's version. The aluminum used was all salvaged material, except the draw tube, which I purchased from the great guys at Online Metals by mistake at $11 for 9 feet. The machine screws, steel shaft, rubber knobs, and ball bearings were also salvaged (Pitney-Bowes copy machine) or given to me as 'samples' from DynaRoll here in So.Cal.. The 2" to 1.25" adapter was turned out of a polypropylene-like plastic stock by my dad.

Considering that this was my first attempt at metal working, I'm fairly pleased. It's much smoother and "tighter" than the rack-n-pinion focusers on my son's Meade 4500, or my brother'sMeade  8". In fact, this focuser lives on Steve's equatorial mounted 8" reflector. I did all the drilling with my $50 drill press from Harborfreight...cheap and cheap, if you get my drift. The schedule 20 aluminum pipe was too large for the 2" barlow, so I heisted some dark green velvet from my wife's sewing stuff and lined the tube. It made a perfect fit and will probably keep the barlow housing free from scratches to boot.

I wanted to keep the whole telescope very light, low profile, and portable, so I opted for a truss design that would break down quickly and pack-up small. I know it's overkill for a 6" scope, but I figured I would learn a lot from the building process that would help me avoid mistakes with future scopes. The truss tubes, spider, and diagonal are next on the agenda.

So far I have also built the groundboard and rockerbox, but have not finished them. I'm planning on using a 262 point flotation cell made from astroturf and plywood, with holes drilled to help with aeration and cooling. Now also planning to install a muffin fans for speedier cooling/equilibration and improved balance.  The mirror is plate glass, 6.1" in diameter, 7/16" thick, and f5.5 (trepanned from a broken tabletop). All the plywood was "harvested" from a crate consigned to a dumpster at work. Here are some images I took of the current state of affairs:

The first picture is of the triangular ground board (no feet, no teflon bearings yet) with the rocker box off and on its side to the back. I plan on using teflon on formica for the azimuth bearing surfaces also (note the white formica circle on bottom of the rocker box. The two white strips are future formica altitude bearings (yes, the formica was also salvaged.). Second and third pics are two different views of the altitude bearings/mirror mount sitting in place on the rockerbox. Fourth pic shows the collimation screws that connect the mirror mount to the alt bearings.

 

 

The 4.5" Dob was a Meade GEM mounted POJ in the beginning.

Then we made it into a Dob and put a copyscope finder on it:

Got really irritated with the rack and pinnion 0.9625" focuser and lousy eyepieces. Decided to build a wooden 2" crayford.

That was a bust (see the section on Crayfords--Wooden).

Finally decided to just take the primary and start all over.

Came up with the following, and, wow, what a difference in performance!!

2" aluminum/bearing Crayford focuser, oversized tube, oversized rectangular secondary mirror (images are MUCH brighter), felt-lined cradle (easy balance/adjustment and storage), Teflon/Ebony Star bearings on altitude, 4-vane crate banding offset spider.