ETX TECHNICAL TIPS Last updated: 28 March 2004

Sturdy home made wedge for the ETX

Date: 3/26/04, 14:13
From: Christoph Jansen (CJansen@uebersetzungsbuero-jansen.de)
Sturdy polar wedge for the ETX 125 made of plywood

I built my own polar wedge from plywood, which I bought as scrap in a
DIY store. On using this wedge for the first time I found that it is
perfectly stable. Now being able to use the "Scheiner" method for polar
alignment, I enjoyed having mars in my 9.5 mm plossl eyepiece (about
200x magnification) for more than 4 hours without any correction.





I can now place a planet or the moon in the field of my webcam and
record AVIs without any correction, even using focal lengths of 6 m.
This is far more than I hoped to achieve and it underlines the power of
the ETX if one cares to give it the best operating conditions one can.
Here are examples of Mars and Jupiter taken in this configuration:





Jupiter on Mar 26 2004 at 21:35 UT, f=4000 mm, 2x TeleVue Barlow, 680 of
740 images used; Mars on Sep 29 2003 at 21:52 UT, f = 5500 mm, eyepiece
projection with 9,5 mm Meade 3000 plossl, 400 of 500 images used. Both
images with ETX 125 and ToUCam Pro at 640x480 resolution, 10 frames/s,
stacking and processing done with Registax 2.0. More pictures under:
http:people.freenet.de/Astroseiten/

For anyone tring to make this kind of wedge himself and to whom the
picture above is not sufficient to do so, here is a description of how I
built it.

I used:

1 base board, 20 cm x 38 cm, 15 mm thick

1 altitude board, 20 cm x 25 cm, 15 mm thick

2 (identical) arms of the same thickness as the boards with an empty
space for the bolts, max (outer) radius is about 20 cm

2 threads to be screwed into the base board (the tripod adapter is fixed
to the board with these threads)

2 threaded bolts to be screwed into the sides of the base plate; one end
with a thread for the knobs below and the other end with a thread for
wood

2 knobs fitting on the threads of the two bolts, used to fix the
altitude setting

2 steel washers fitting under the knobs

1 hinge 10 cm x 2,5 cm (4 inch x 1 inch)

8 screws to fix the hinge

4 screws and 4 wooden dowels with 4 mm diameter to fix the arms to the
altitude board

2 screws for screwing the ETX to the alt board, in my case 1/4 inch in
diameter and 1.25 inch in length; this may vary with the plywood's
strength

2 screws for fixing the tripod adapter to the base board

2 steel washers fitting under the above screws' heads

Further needed: wood glue, paint, two spare nuts fitting the threaded
bolts

The overall cost for the material was about 20 Euros. As I had no wood,
none of the screws, no dowels and no glue, I had to buy all of this. So
you may find a lot of the needed accessories in your stock which makes
it a lot cheaper, probably lowering costs to virtually nil. This kind of
wedge can of course easily be used for all other ETX's, you may just
have to adjust the sizes of the material used.

Cut the boards to size and construct the arms. For the arms you only
need a pair of compasses. My arms have an inner radius of 15 cm and are
5 cm wide, giving an outer radius of 20 cm. The empty space has a center
radius of 17,5 cm and is 1 cm wide, so radii of 17 and 18 cm are used to
draw it. The arms cover about 65 degrees, which is sufficient for
latitudes of up to 25 degrees. To cut out the empty space in the arms, I
started by making a cut in the middle of their top. As this part of the
arms is later fixed with dowels and screws this doesn't affect
stability.

I drilled the holes to fit the threads for the adapter into the base
board and then screwed the threads into them. The first hole is 10 cm
from the back edge of the board, the second is 10,2 cm (4 inches) apart,
but this may vary with the adapter used.

Two holes to fit the screws that would later hold the ETX were drilled
into the altitude board. As I don't know if changes were made to the
ETX's base plate, one better measures the distance between the holes
oneself.

Now all wooden parts were painted leaving out the inner top of the two
arms which was later glued to the altitude board.

The hinge was now fixed to the base and altitude boards in that way,
that their front edges are side by side. With the sizes I used for the
boards, the hinge is now well over the middle of the tripod adapter.
This is about where the center of gravity of the scope will later be,
but this of course depends on the preferred observation point's
latitude. At lower latitudes you may have to shift the hinge further to
the base boards front for proper equilibrium.

Now the arms were fitted to the sides of the altitude board, which is a
bit tricky, at least if your are used to doing such constructions as
little as I am. If you want to do this point straightforwardly, you have
to embed the hinge into the wood. You may then simply construct the
position of the arm from the used radii. Lacking the proper tools to do
so, I just screwed the hinge on top of the boards, which causes an angle
between them.

This means that you can't just go ahead and fix the arms in the right
place by measuring. So you have to do it by trial and error. Start out
by marking a dot on the base board. Its position is given by using the
radius of the arm's empty space measured from the hinge. Now hold the
arm to the alt board in that way that the dot at the center of the empty
space and move it up and down. If the dot on the base board does not
stay inside the empty space, try setting another dot in a better
position. This way I soon found the right position for the arms.

Now mark the position of the arm on the alt board and at the last dot's
position drill a hole into the side of the base board into which the
threaded bold will be screwed. Mark the same positions of arm and hole
on the other side of the boards and try if it fits as good as on the
first side. If it doesn't (which was the case for me, I'm really not a
good carpenter), correct it. Before continuing, take some sandpaper and
remove the paint from the base board where the arms will be glued to it.

Now drill four holes into each arm. Two of them are 1 mm wide for screws
and will go right through the arms; the two others are 4 mm wide for the
dowels and are only drilled into the arms' inner side, just deep enough
to glue the dowels into them. Place the holes alternating by diameter:
1mm, 4 mm, 1 mm , 4mm. Drill the corresponding holes into the alt board.
Now first glue the dowels into the arms, put some glue onto the arms'
sides and into the holes for the dowels. Fix the arms to the board and
screw them tight.

Put two nuts on each of the threaded bolts, screw them tight against
each other. Now you may use a wrench to screw the bolts into the base
board (of course through the empty space of the arms, you won't be able
to wrestle the arms over the bolts later). Remove the nuts and screw
the knobs on the bolts. There you are, your wedge is finished.

Before using it I painted it once more and roughly adjusted its altitude
to my location which is round 53 north. As I said above, the increase
in performance by now only having to track one axis (after careful
alignment) amazed me and leaves me quite happy with this new accessory.
I feel that now I really have the ETX I ever wanted.

I hope this encourages some people to make their own wedges, it really
is worth the effort.

Regards,

Christoph Jansen

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