Last updated: 26 March 2004
Date: 3/24/04, 19:35 From: Jack Fox (firstname.lastname@example.org) I made this polar alt/az wedge for the ETX 70 from the best features for wedges from your Telescope Tech page and the addition of a micro latitude control rod. I hope you can add this to the Telescope Tech page. Maybe some one will find it useful.
Improved Polar Wedge for ETX-70/60
I made this inexpensive wedge by combining the best features of several contributors of wedge designs from the Technical Tips page and adding a micro adjustment rod to it. It works in both the Polar and Alt/Az mounts and fits on the ETX -70/60 tripod. It can also be adapted for a standard camera tripod mount by adding a 1/4" tee nut on the bottom of the base plate.
The design for the base plate is offset to put the weight of the scope directly over the center of the tripod keeping the tripod and scope stable. The wedge has a center thumb screw bolt that attaches to the base plate. You loosen this bolt so that you can rotate the wedge left and right without having to lift and move the tripod. This feature is very handy when aligning the scope in polar mode and keeps the tripod and scope level. I separated the base plate from the wedge with a 1-1/2" thick block of wood. This was necessary to have the two lid supports clear the base plate when rotated.
How to make the Micro Adjustment can be found at: members.home.nl/damhof/astro/modification884.htm or you can buy one from ATOZastro at http://www.atozastro.com/shop/scripts/prodList.asp?idCategory=5.
I bought some paper hole reinforcements at the store and painted them with Glow in the Dark paint. I put them over all the holes in the mount and the 2 mounting holes on the base of the ETX. This makes it easy in the light or dark to mount the scope on the wedge. Following is the plan and list of materials to build the wedge:
Parts List for Wedge
2 7" x 10" x 1/2" plywood (for wedge)
1 7" x 10-1/2" x 1/2" plywood (for base plate)
1 4-1/2"x 5"x 1-1/2" block of wood (wedge spacer)
2 2" x 3/4" x 7/16" wood strips (hinge spacers)
1 2" hinge with screws
2 lid supports (left and right)
5 1/4" tee nuts
3 1/4" thumb screw bolts
4 3/8" washer
2 1-1/2" wood screws
Latitude adjustment kit parts
1 1/4" x 12" threaded rod with 3/32" hole drilled at top
1 1/4" nut and washer
2 1/4" wing nuts
2 3/8" washers
1 5/16" coupling nut
1 4d, 1-1/2" finishing nail
Trace the patterns on to the 3 pieces of 1/2" plywood or use a compass and ruler and draw directly on the wood. Cut the Wedge Spacer block out of 1-1/2" thick board or glue 2 pieces of board together to make this thickness (Fig. D). The radius of the circle is 2-1/4". Carefully cut out the wedge top, bottom, spacers and base plate with a jigsaw or coping saw. The holes for the tee nuts and pivot have to be measured individually depending on how you want the scope to be balanced over the tripod.
This wedge can be used for both the Alt/Az and Polar mount.
Constructing the Wedge Bottom:
Make sure you cut the top and bottom pieces of the wedge identically (Fig. A & B).
Drill a 1/4" hole in the exact center of the bottom piece of the wedge (Fig. A). This is to attach the Wedge Spacer to the wedge and allow it to pivot.
Glue one of the hinge spacers (fig. E) to the 2" x 3/4" hinge tab at the back of the bottom wedge piece.
Next cut the 3/8" x 5/8" slot at the opposite end for the latitude adjustment rod. You can drill two 3/8" holes, one above the other, and use a file or coping saw to square it off. You are now finished with the bottom piece.
Constructing the Wedge Top:
You next place the bottom wedge piece centered on top of the ETX tripod. From the underside of the tripod, with a sharp pencil trace onto the underside of the wedge bottom, the two bolt holes that the tripod bolts enter to secure the telescope to the base. This will mark where you drill your two holes for the tee nuts.
Next cut the 3/8" x 5/8" slot at the opposite end (Fig. B) for the latitude adjustment rod. Drill a 3/32" hole through the center. This is where the 1/4" threaded bolt for the latitude adjustment rod is attached to the wedge.
Drill a 5/16" or 3/8" hole for the two tee nuts. The holes have to be larger than the 1/4" tee nut for them to fit. Insert the tee nuts from the top side of Wedge Top and hammer them in. Countersink the tee nuts using a spade of bit. Putting the tee nuts on top will cause the tripod bolts to pull the tee nuts deeper into the plywood for a secure fit. Glue the second hinge spacer on the underside of the Wedge Top as shown if Fig. B.
Attached the 2 inch hinge to the two hinge spacers so that the two wedge pieces open away from you, like a waffle iron. Make sure you attach the hinge square or the wedge won't close correctly. These hinge spacers allow for the thickness of the telescope thumb screw bolts that attach to the bottom side of the top wedge piece and also allows you to level the wedge using the latitude adjustment rod for the Alt/Az mount.
Attach the 2 lid supports to opposite sides of the wedge. See the photographs on the first page. You may have to slightly modify the lid supports to get them to close properly.
Attach the latitude adjustment rod assembly. Instructions for this are on the web at
members.home.nl/damhof/astro/modification884.htm . I changed the specs to use 1/4" size hardware (see parts list) or you can buy one from ATOZastro at http://www.atozastro.com/shop/scripts/prodList.asp?idCategory=5.
Constructing the Base Plate & Wedge Spacer:
Cut the Base Plate out of the 7" x 10-1/2" plywood. The radius of the circle is 3-1/2".
Cut the Wedge Spacer block out of 1-1/2" thick board or glue 2 pieces of board together to make this thickness (Fig. D). The radius of the circle for the Wedge Spacer is 2 1/4". Drill a 5/16" or 3/8" hole in the center to fit a 1/4" tee nut. This is very important: Countersink and glue this tee nut so that the head of the nut is just below the surface so that the wedge assembly will be level while resting on the spacer. Place the Wedge Spacer on top of the base plate so that the flat 4-1/2" end of the spacer is about 1 5/8" inches from the flat 7" end of the base plate (Fig. C). Trace the outline of the spacer on the base plate for reference. You now need to glue and fasten the Wedge Spacer to the underside of the Base Plate with the two 1-1/2" wood screws. The holes for the screws must also be countersunk.
Now attach the wedge unit using a 1/4" thumb screw bolt and washer in the pivot hole on top of the Wedge Spacer. The washer goes between the top of the Wedge Spacer and the thumb screw bolt.
Now you need to loosen the lid supports and open the wedge to bolt the telescope to the top wedge assembly using two 1/4" thumbscrew bolts and washers. Attach the bolts to the underside of the top wedge piece through the tee nuts and into the holes on the base of the telescope. Close the wedge. Place the entire assembly on top of the tripod. You will need both hands to hold the wedge and telescope while tracing the tripod holes from the bottom side of the tripod to the bottom of the base plate. You will now have to move the assembly so that the center of the telescope, not the Base Plate, is over the center of the tripod. Note the location. Remove the telescope and wedge from the tripod.
Open the wedge to the Polar position to your locations approximate latitude using a protractor and the latitude adjustment rods and lock it in place with the wing nuts. See photos on the first page. Now center the telescope on the tripod again while in this position. Align the base plate so that the flat end is between two tripod legs and the round end is over the single leg. This position allows the latitude adjustment rod to clear the tripod legs while in the closed Alt/Az position. Trace the tripod bolt holes on to the bottom of the base plate. Remove the base plate and spacer from the assembly. Drill a 5/16" or 3/8" holes in the base plate for the two tee nuts (Fig. 3). Make sure you drill the hole that is under the spacer at least 1-1/2" deep so that the thumb screw bolt can thread all the way into the spacer. Countersink the 1-1/2"hole for the tee nut that is on the bottom of the Base Plate under the spacer. Glue and attach the tee nut. For the other tee nut that is at the round end of the Base Plate, countersink, glue and attach the tee nut to the top of the base plate. Re-attach the wedge to the Wedge Spacer and you are done. Paint it black and add the Glow in the Dark hole reinforcers around the mounting holes and telescope base.
You can now Polar align your ETX-70 using Dr. Clays method, except you do not have to move the tripod, just loosen the pivot bolt and move the wedge. Also the ETX 60/70's do not have hard stops to worry about.
I know this is a lot of details, but it took a lot of trail an error to make this work. I wanted to save you the frustration.
Return to the top of this page.
Go back to the Tech Tips page.
Go back to the ETX Home Page.