Dobsons are alt-azimuthal telescopes, as everybody knows. This may be a disadvantage if you want to observe planets at say 400x. It does work, of course, but it requires some practice and smooth operation of the telescope mount. Nevertheless, it would be nice, if Jupiter would just stay in the middle of your field of view.
There is a relatively simple method to realize an equatorial mount for Dobsonians, particularly if you are interested in visual use only. And the particularly nice thing is: it works equally well even for larger Dobsonians. The answer is the equatorial platform (or tracking platform, Poncet platform, conical bearing mount, or or or .... there are many names to it).
The principle behind it is simple: You replace the groundboard by a table that can be tilted around the polar axis, i.e. the rotation axis of earth. There are basically two methods to do this. I decided to use the one, where the southern bearing consists of an axial (pivot) bearing and the nortern bearing of a segment of a circle around the polar axis. Alternatively, you can make both bearings of circle segments. The principle of the equatorial platform is explained in detail on the internet pages of Chuck Shaw or David Molyned. You can find more links to EQ platforms at the bottom of the page here.
I abandoned the idea of a spindle drive with a threaded rod and switched to a direct drive, which allows easy reset of the platform by simply lifting and moving the upper table. I designed the platform such that the center of mass of the Dobson is on the polar axis. This might be important, particularly if you decide to use a friction drive as I did. Furthermore, I wanted to keep the profile of the platform as low as possible (6 cm additional height to my Dobson).
Of course I was curious how it would work and feel to use the platform ... I chose a star close to the equator, put in my 4 mm Ortho plus a 2x Barlow yielding almost 900x (I normally restrict myself to 450x ). And the star just stayed in the middle of the field ... so simply.
By the way, such a platform is helpful not only for observing planets or the moon. Also in deep sky observing, it may be useful to use 300x to 450x, e.g. if your viewing groups of very close, tiny galaxies or PNs. And it is convenient if you can leave your telescope for a moment to have a look in the star atlas or an observing handbook, without loosing the object.
As the guiding accuracy is less demanding by orders of magnitude for visual observing as compared with astrophotography, the impact of imprecicions of the platform and of its alignment are much less disturbing than one might expect. For setup, I only align the platform roughly to Polaris or use a small compass.
My 14" Dobson on the platform. The platform increases the height of the telescope by only 6 cm. The platform is 9 cm high and replaces the regular 3 cm groundboard of the telescope.
The small rollers on the top table are required for guiding the rocker box of the Dobson (design with open bottom).
Designing the platform
Careful planning is quite important for designing a successful platform.
First, one should really understand, how a platform works in detail. Otherwise, it will be hopeless ... .
It is important to design a platform that fits your telescope. Therefore, I do not post here finished construction plans of my platform, as this one was specifically designed for my 14" truss tube Dobson. A platform for e.g. a commercial 8 to 12 inch solid tube Dobson will necessarily have different dimensions. How to determine the dimensions of a platform specific for your own telescope will be explained below.
First you should specify the geographic latitude for which you would like to design your platform. The latitude determines the inclination of the polar axis to the earth's surface, which is, of course, the same angle alpha between the polar axis and the platform. Second, you determine the height of the center of mass (CM) of the telescope (OTA and Rockerbox). In order to enable a platform operating free of torque, it makes sense to design the platform such that the CM of the telescope coincides with the polar axis (see Figure to the side). You can determine the height of the CM of the entire telescope from the height of the CMs of the tube and the rockerbox: (height of the CM of the tube x mass of the tube + height of the CM of the rockerbox x mass of the rocker box) / total mass.
From the inclination of the polar axis alpha and the height of the CM of the telescope, the distance a between the azimut axis of the rockerbox to the point, where the polar axis intersects the platform table, can be calculated. This distance should for stability reasons be ideally such that the northern bearing is directly under one of the teflon pads of the rockerbox. This may not be possible for low latitudes and for telescopes with a high CM (e.g. smaller solid tube Dobsons with their relatively high rockerbox). You may need to find a compromise, such that you make the platform table stiffer around the southern bearing (as I did) or you shorten the distance a (and abandon the idea of mounting the telescope in its CM). Alternatively, you may design an elevated southern bearing (as here and here), or you use a circular segment bearing as well for this bearing (as here and here).
Some platform builders split the segment of the northern bearing and mount it perpendicular to the platform table. This results in a more direct transmission of the force from the table to the ground (see, for instance, the platforms of Tom Osypowsky and Ulli Vedder). These platforms are more difficult to plan, as you do no longer deal with a simple circle segment. On the other hand, this design is particularly suited for large, heavy Dobsonians.
To achieve maximum stiffness of the whole setup, the bearings of the platform should be placed directly under the teflon pads of the azimut bearing of the rockerbox. As mentioned above, this may not be possible in certain cases for the southern bearing. For the northern bearings, however, the circular segment can always be mounted directly under the two other teflon pads. This defines the dimension b in the Figure above and also the width of the platform table, which should be a bit larger than the distance between two teflon pads (see first figure at the beginning). Now you can determine c'. The radius of the circular segment c, should be chosen such that the circular segment will be as wide as the platform table (see again the first figure at the beginning). Typically, c will be about 20 to 25% larger than c', but this may vary. Again making a scetch to scale of the circle and of the circular segment will be helpful. The angle beta between table and circular segment will be 90° - alpha. The length of the tracks on the circular segment can be calculated from the radius c and the intended guiding time. For one hour guiding time, the table will be rotated by 15°. This requires the tracks to be 2 x pi x c x 15° / 360°. The part between the tracks is not necessary for the function and may be straightened, as I did, which may help to reduce the height of the platform.
With these consideration, you may determine all dimensions necessary for designing the platform.
Generally, I want to encourage everybody to take a look at all the other platforms in the link list below. Every platform features different ideas due to different expectations and capabilities of the person behind it. The way I built my platform represents only one possibility among many others, and I am sure there would be a good number of observers who would not be content with such a platform, and this for right. My platform is easy to build and does not require any fancy electronics. If you are enthusiastic about electronics, you will probably much better like to use a stepper motor, which will be a very useful upgrade.