Calculates the friction of movement for a Dobsonian telescope

Distance from eyepiece to center of gravity or tube's pivot point =
Weight that the azimuth bearing carries =
Weight that the altitude bearing carries =
Azimuth bearing radius =
Altitude bearing radius =
Altitude bearing angle from vertical =
Altitude angle up from the horizon =
Azimuth coefficient of friction =
Altitude coefficient of friction =



The calculated azimuth axis friction is
The calculated altitude axis friction is

Notes:

Calculates the friction of movement for a Dobsonian telescope.

An essential feature of John Dobson's telescope design is that the telescope moves smoothly and precisely about the sky, staying put once the operator let's go. There is no clamping or locking down. Tracking an object across the sky consists of periodic adjustments by pushing the scope very small distances while looking through the eyepiece. His design depends on an interesting property of Teflon on pebbly Formica, namely that the friction of movement is about the same as the friction that begins a movement. In other words, the dynamic friction equals static friction. Most materials stick worse than they move, leading to jerky motions.

The force to move the telescope should not be so great as to demand a great deal of effort, particularly when making very tiny high magnification adjustments by hand, nor should the force be so slight that wind can move the telescope. Ideally the force in both azimuth and altitude axes is roughly the same so that the axes 'disappear' as the scope is moved to and fro.

Since the force to move the scope in azimuth increases as the scope is pointed upward, the ability to vary the pointing angle is included. The default angle is 45 degrees or halfway up the sky. You can vary this to see how the friction decreases when the scope is pointed horizontally and increases dramatically when the scope is pointed nearly vertical. The key here is to have a favorable static to dynamic friction ratio where the friction to begin the movement is about the same as the friction to continue the movement.

I like a frictional force of two to three pounds (1 to 1.5 kilos). This gives me a smooth and not-too-stiff touch at the upper end of the scope, avoids balancing issues with heavy eyepieces and resists weathervaning from wind gusts.

The altitude friction can be adjusted by varying the bearing angle. The azimuth friction can be adjusted by varying the distance from the pads to the pivot (the azimuth bearing radius) and by varying materials.Bearings can be substituted for one or more of the bearing points with a proportionate reduction in friction. For instance, the azimuth friction can be reduced by two-thirds if two of the three bearing pads are replaced with roller bearings.

The formula and concept is from Richard Berry's article in Telescope Making 8, page 36-.

In the article Berry reports his investigations into frictional coefficients for a variety of materials. Here is his data:

Teflon-Nylon 0.050
Teflon-Formica 0.083
Teflon-aluminum 0.095
Teflon-PVC 0.13
Nylon-aluminum 0.125
Nylon-Formica 0.19
PVC-Formica 0.17
PVC-plywood 0.23
felt-Formica 0.22
felt-aluminum 0.31
plywood-Formica 0.20
plywood-particle board 0.30
metal-oiled leather 0.15

Mel Bartels