Birchardville Observatory
Site Always Under Development
This telescope is a 10" diameter Newtonian reflecting telescope using a Dobsonian mount. John Dobson's mount design has a low center of gravity and large bearing surfaces, so it is quite stable.

Although I don't have a picture of the scope broken down here, the diagonal cage fits into the mirror box so that it is quite compact when transporting the scope. This was another idea from Tom Clark.

Click on the picture to see a larger view, and use your browser BACK button to return to this page!

Four years ago, granddaughter Megan was a pixie sized six year old when this was taken. You have a good view of the "back" of the diagonal cage and can see the mounting block for my Rigel Systems Quickfinder.

Diagonal Cage

Looking down the scope's axis into the diagonal cage, you can see the inside of the focuser in the "10-o'clock" position, and the diagonal mount and three vaned spider (the vanes are in the "2-o'clock", "6-o'clock", and "9-o'clock" positions). A discussion of the spider vanes and diffraction is at the right.

That's my (left) hand -- a typical 9 inch hand-span. I usually guide with my right hand in this position. I sized the diagonal cage using Tom Clark's guidelines so that all parts of the cage frame are about 0.5 inches outside the incoming light path.

Down the cage axis

A side view of the telescope mirror box and truss tubes.

I mounted a U.S.Optical encoder on the altitude axis. Another encoder is mounted in the bottom of the rocker box. The encoders are tracked by a Microxxx controller which connects to my laptop's serial port. I use the Earth Centered Universe software package to provide real-time tracking of where the scope is pointing. It is compatible with Equatorial Platform use as well.

Encoder mounting was simple... I screwed a small clamp block to the top of the altitude bearing and drilled a hole in the bearing/clamp combination right on the center of the axis. The encoder shaft clamps in that hole. The encoder's shaft bearing mounts on the 1/8" (3 mm) aluminum bar. No big brainpower needed here.

Altitude Axis with Encoder

This view is of the bottom of the mirror box. You can see the mirror and its cast aluminum cell through the nine-inch hole in the bottom cover. This leaves lots of air flow space but also baffles any light from behind the mirror.

The three collimating bolt handles can stick out about one-half inch below the bottom cover, so I added the two small blocks on each side to support the mirror box without grounding on the collimation bolts when the mirror box is removed from the rocker box.

I store the encoder controller circuitry, connecting cables and the truss tubes (laying diagonally) in the bottom of the rocker box when not in use.

Bottom of mirror box with mirror cell

The short helical focuser is very light weight and does not intrude into the light path.

At the bottom of the diagonal cage you can see the maple block clamps that secure the diagonal cage to the truss tubes.

The truss tubes are only 0.5 inch (12 mm) diameter, but because they are short (only 24 inches (61 cm) long) and because the tubes are under tension or compression, and not flexure loads, they hold the diagonal cage rigidly in position relative to the mirror box.

A laser collimator makes alignment quick and painless (if you heed the warning not to look at the laser beam with your remaining good eye).

Inside the Diagonal Cage

10 Inch F/4 Dobsonian Telescope

Diffraction notes

When you look at astrophotos taken with a Newtonian telescope, you will often see diffraction "spikes", especially around bright stars. I mentioned elsewhere on this site that years ago I had an early form of refractive surgery, Radial Keratatomy, to correct myopia. In this procedure, a surgeon makes small cuts radially near the edge of the cornea (hence the name). Depending on the degree of myopia, the cuts are longer (or shorter). In my case, when my pupils are dialated more than 4-5 mm, the scars from these tiny incisions are within the area uncovered by the opened pupils, and they produce diffraction spikes around bright objects, similar to the spikes the telescope's diagonal mount spider vanes cause.

The diffraction spikes are formed at right angles to the obstruction (spider vane) causing the spike. If two vanes are parallel, only one diffraction spike will be seen, but at a greater intensity as the diffraction from the two parallel vanes will reinforce each other.

It is sufficient, especially for a relatively small scope with a small diagonal mirror, to have three vanes to hold the diagonal mirror mount. In my telescope, two of the vanes are at right angles to each other, producing spikes in the 3-9 o'clock and 12-6 o'clock directions. The third vane is in the direction of the bisector of the right angle, extended out the other side. This vane produces spikes in the 1:30-7:30 o'clock direction. Each of the vanes produces its own spikes, but the spikes are in different directions and they don't reinforce each other. This should improve the contrast when viewing bright clusters, like the Pleides.

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Contact Info:
Name:Dan Janda