Dave's First 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 means it has been re-prioritized--lower on the list.

UPDATE:

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 a 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 resembles Dave 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.