Dob owners: Achieve precise az/el motion with these inexpensive DIY mods.

[Roy G. Biv]

To my brothers on the Magellanic side—

Those of you who own plain-vanilla, un-motorized Dob rigs no doubt find it frustrating — despite the excellent balance and bearing characteristics of your rigs — to make minor azimuth and elevation movements while peering through high-power eyepieces.

Perhaps the simple, inexpensive, and easily made modifications (see photos) I've made to my own Zhumell 10-inch platform will be of interest.

I would be glad to respond to inquiries.

—Roy

[iwagland]

Do you have instructions on what you did?

Tim

[Andrew C]

Hi,

I have been working on an approach vaguely similar to your azimuth device, and a different approach to altitude control.

Would be interested to know how you de-clutch the slow motion attachments when you want to slew the telescope to an object many degrees away.

Cheers,

Andrew

[Roy G. Biv]

Andrew--
Thanks for your interest. For Gross movements in the az plane I simply remove the socket wrench handle and manhandle the rocker box to the desired degree. The bicycle sprocket follows smartly. For the el plane I hold the tube down with one hand and spin up/down the threaded knob.
--Roy

[Roy G. Biv]

Tim--
Thanks for your interest. Here are the instructions:

Making the Dob Base
(Measurements are in inches; use hardwood wherever possible; use deck screws; consider replacing the original rocker box with maple)

Starting from the ground up, construct a 33 x 7/8 circular base fitted with castor wheels. On the upper surface draw a perimeter ring whose inner radius is 25½ and outer radius 28½. Important: mark the exact center of the circular base (you will see why, later).
Next, construct the 33 x 7/8 lower ring having an inside radius of 12½ and an outside radius of 16½ Screw this ring to the circular base.
Next, construct a 33 x 7/8 upper ring having an inside radius of 13½ and an outside radius of 16½. This ring’s greater inside radius provides for a one-inch “shelf” upon which is laid the azimuth ring (described below). Screw the upper ring to the lower ring.
Make three 3x3x3 disk support blocks (disks explained below) and equally space them around the perimeter ring on the circular base. These blocks are then screwed to the circular base from below. Making the Four Disks
(Requires accuracy)

Construct two 22 x 3/8 outer fixed disks (Important: mark the exact center of both disks). Construct one 21¼ x 3/8 inner fixed disk. (Mark center!)
Construct one 22 x 1 movable disk (Mark center!)
Drill a 7/16 hole through the disks centers
Fitting the Bicycle Chain
Buy a bicycle chain long enough (66.75713 inches to be exact) to fit tightly (not snugly) around the perimeter of the inner fixed disk. If you come up short you can sand or trim the perimeter (be accurate!). Sandwich the inner fixed disk (replete with chain) between the two outer fixed disks, insert a 7/16 three-inch threadless bolt through the centers, and screw the three disks to each other (screw holes must be at least 1½ inches from edge of the outer fixed disks so as not to interfere with the furniture sliders (described below).
Attaching Furniture Sliders
Buy a pack of ¾ x 3/16 furniture sliders and screw at least six of them near the perimeter of one of the outer fixed disks.
Joining the Rocker Box to the movable disk
Place the movable disk on a flat surface and shim it until it is perfectly level. Remove the original three-piece rocker box from its base and place it on the movable disk. Take a rigid rod and lay it across the rocker box’s semi-circular axle cutouts. Measure the distance between the inside faces of the rocker box’s side boards. Locate the middle point, drop a plumb line and move the rocker box until the plumb lies precisely over the hole in the movable disk. Drill the rocker box to the movable disk.
The Azimuth Drive Shaft & Sprocket
Place the circular base (along with its outer & inner fixed disks) on a flat surface. Place the moveable disk (along with its attached rocker box) on the circular base and insert the three-inch bolt.
Weld or hammer jam a 2½" bicycle sprocket onto a 24 x ½ steel rod. Slide the wooden lower rod support down the rod. Slide the ¾" locking pulley-wheel down the rod. Slide the wooden upper rod support down the rod. Screw the lower rod support to the left sideboard. Engage the sprocket teeth with the imbedded bicycle chain. Make vertical adjustments using the locking pulley wheel. Finally, screw the upper rod support to the sideboard.
The Elevation Drive Shaft
The support framework for the drive shaft is 1½ x ¾ maple: Cut two 27" pieces which are then attached to the sideboards using bolts and wing nuts, and cut one 14½" cross-piece that freely rotates between the two (see photo #5). Drill a 3/8" hole in the middle of the cross-piece.
Obtain a 3/8" x 30" threaded rod, a 3/8" threaded knob, and a swivel bracket (see photo #6). (I’ll get the nomenclature for that bracket and its related parts ASAP.)
Locate a balance point for the swivel bracket (mine is located 12" from the lower end of the tube) and firmly bolt it to the tube.
Slide the rod down thru the cross piece and thread it into the swivel bracket. Drill a small hole thru the rod where it joins the swivel bracket and insert a pin (to keep the rod from un-threading during use). Drop a nylon washer down the rod followed by the threaded knob. You will probably need to trim the 3/8" rod by a few inches.
Balance: You will need a bungee cord long enough to reach from a wing nut on the left side board, pass over the tube, and connect to a wing nut on the other side.
******

[iwagland]

Roy,

Thank you very much.

Tim

[JethroB76]

Are you able to operate both slow motion controls when at the eyepiece?

[Roy G. Biv]

Thanks for your interest. Yes, I can use both controls simultaneously while peering thru the eyepiec.

Best,

--Roy

[Andrew C]

Hi Roy and thanks - some good ideas here.

As a guide for people who go down this design path, how many degrees of alt (and az) do you move the scope for each full turn of the knob (and handle)?

Obviously people can design different rates by choosing different threads and sprocket sizes. Did you arrive at your choices through trial and error, or would you choose different ones if you were building version 2?

Cheers

Andrew

[pgc hunter]

wow now that is the coolest dob I've seen! You'll no doubt pull the astro chicks at your next star party ;)

[Miaplacidus]

Excellent! The complete dob. I guess Mr Dobson would be proud.

(Er, does it make coffee...?)

[Roy G. Biv]

Andrew—

Thank you.

As for the span of motion, one full turn with the azimuth handle yields about 33 degrees. The elevation knob is very sensitive—yielding about 0.5 degree per full turn.

The choice of sprocket size was pretty much limited to the tooth gap in the bicycle chain. I asked the bike shop owner to sell me the smallest sprocket available. I think the sprocket-to-chain ratio is just about right.

Best,

—Roy

[Roy G. Biv]

Miaplacidus—

I appreciate your remarks.

While this rig won’t make coffee, it has certainly generated comments from passerby in my neighborhood. One bloke good naturedly quipped that Homeland Security probably got calls from jittery folks who saw the rifle scope mounted on top! Dobs sorta look like military artillery anyway, eh?

Best,

—Roy