Arie's Dobsonian Telescopes

Tunable top

There are basically two strategies to achieve that different users with different interpupillary distances (IPD) can use the same binoscope. One is to use a set of two focusers on both optical systems. One focuser changes the distance between the tertiary and secondary mirror, thereby increasing or reducing the IPD. Since the length of the lightpath changes a bit, a second focuser is needed between the tertiary mirror and the eyepiece for re-focusing. This last aspect may seem tedious, but this set-up has the advantage that the "IPD-focuser" is solidly fixed to the focuserboard/ secondary cage. Furthermore, this construction is simpler than the alternative of a tunable top (see below). Thus I started with this system (picture below).

However, I encountered a few problems. First, the (heavy) "IPD focusers" had a rather long travel. This costed me precious centimeters distance between the secondary mirror and the focal plane of the eyepiece. The secondary mirror provides a 23% obstruction, inducing rather tight restrictions on this spacing distance. In hindrespect a 26-28% secondary mirror obstruction would have made the construction of the secondary/ tertiary mirror set-up much easier.

A second drawback was that the 1.25 inch helical focuser between the tertiary mirror and the eyepiece didn't work out well. There was too much play in those focusers, which operated on two sets of two ball bearings. So after initial trials I decided to construct the alternative option as well: a tunable top.

In this construction, each secondary cage is able to rotate relative to a wooden frame.

This frame is in fact the two secondary cage rings of each optical system, but still joined together for rigidness. This large "8-form" is fixed to the main trusses. Each secondary cage cage is separated from the wooden frame by four sets of teflon pads and connectors. The secondary cages are pushed tightly against the connectors with a gear wheel that runs in a rubber belt. The gear wheels are attached to long bolts with ebony knot for rotation (see picture below). Before I chose the solution with the gear wheels, I found that the secondary cages were fixed to the lower frame either too tight or too loose, thereby moving respectively too difficult or too easy. With the gear wheel solution the secondary cages are fixed rather rigidly to the frame, but with the ebony knots movement is still pretty smooth. And the movement doesn't involve more than a few millimeters for changing de IPD.

With the tunable tops option I replaced helical focusers for 2 inch Jim's Mobile RCF-1 Reverse Crayford Focusers (JMI) (picture above). A 2 inch focuser was not a deliberate choice to observe with 2 inch eyepieces. As shown above, my widest field now comes with 27 mm Panoptics, but I would have been happy with 1.25 inch 24 mm Panoptics instead for my widest field.

Finally, I bought cheap 2 inch diagonals of which I cut corners in order to have them fit between my eyes. As tertiary mirrors I bought 46 mm ULS Quartz mirrors from Protostar. To align these, I created a push-pull system (see picture above) on the plate on which the tertiary mirror rests. How I use this system is subject of the next section.

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