Side-by-side balancing system

As described above, on InFINNity Deck I have a side-by-side setup (see figure 1). In fact, it is even more complicated: between the Celestron C11 EdgeHD and the SkyWatcher Esprit 150ED, there is another home-made 60mm Galileo Type Telescope (f = 1490mm), while a Lunt LS80THA piggy-back is mounted on the C11. The telescopes are all used both visually and photographically.

The great thing about this set-up is that there are always four telescopes available for observing or imaging. The disadvantage is that when replacing eyepieces (the heaviest eyepiece is 1.2 kg, the lightest a few tens of grams) and/or cameras, balancing is a challenge every time. Until now I did this with an ADM DCW-S-D Series Counterweight that could be slid longitudinally under the C11 on the dovetail plate (see figure 1). However, with this counterweight I could only compensate for longitudinal changes. Transverse changes could only be corrected for by mounting different eyepieces on the telescope(s) that I was not using at the time, or by removing or replacing one of the two guide-scopes.

A good month ago I decided that this had to change: the counterweight also had to be moved easily laterally. Both the Esprit and the C11 were both already on an ADM D-type dovetail plate, with that of the C11 protruding approximately 15cm further forward than that of the Esprit. If I were to make the Esprit dovetail plate longer, I could make a sliding bridge between the two dovetail plates with a second clamp. A sliding weight would then have to be placed on that bridge.

First I made a sketch (see figure 2) and searched the ADM website for stuff that would be suitable and found the following there:

– 21″ long dovetail plate for the Esprit: DUP21-D Series Universal Dovetail Bar, 21″ Long, 2″ Spacing.

– Mini dovetail bar for the bridge: MDS-C9.25- MDS Series Dovetail Bar for Celestron 9.25″ SCT Telescope.

– Mini Dovetail Bar Counterweight Clamp: MDS-CW-S- MDS Series Counterweight with 3″ Threaded Rod.

– Counterweight Clamp for D Series Dovetail Bar: DCW-S- D Series Counterweight with 3″ Threaded Rod.

– 1.75lbs counterweight: EX1.75CW- Extra 1.75 CW for 1/2×13 tpi threaded CW shaft.

Passed this order on to Robtics and I was fortunate enough that they could supply the counterbalanced clamp for the D-series dovetail plate with a 1.75lbs (0.8kg) counterweight and the others without a counterweight. In the end I already had a 3.5lbs (1.6kg) counterweight lying around (with clamp), so there was no need to receive those parts again. This 1.75lbs weight gives the possibility to eventually work with three weights: 1.75lbs, 3.5lbs and 5.25lbs (2.5kg). The 21″ dovetail plate was not available from stock at Robtics and would result in considerable shipping costs due to transport from the US (approximately 50% of the cost of the plate). In the end I was able to do this most economically via First Light Optics (FLO) who at the time (pre-BREXIT) only charged the European shipping costs for this (approximately 15% of the plate).

The mini-dovetail bar that was going to form the bridge needed some rebuilding: on one side I drilled a hole for a countersunk M6 screw (see figure 4), but on the other side a slot was needed to prevent stress between the two telescopes when the bridge was moved (see figure 3). To ensure that the bridge would run smoothly when the clamps were moved, it had to be mounted floating on the clamps. To this end I turned two aluminium bushings that fit into the hollow on the back of the mini dove-tail bar. Of course, a few tenths of a millimetre of play is needed for the bridge to function smoothly. To prevent this from leading to unwanted rattling, I have used Alcomex W61450R wave rings (see figure 4). To be able to slide the bridge as far forward as possible, I drilled new mounting holes in the clamps and provided them with M6 thread. After screwing in and checking for sufficient play, I tightened another M6 set screw against the M6 ​​screws from behind, so that they were fixated.

The 21″ D-series dovetail plate has 8 sets of mounting holes for the Esprit rings, but not at the end. Since a push-pull system was needed to make the ring adjustable in height (this to correct the ortho-error of the Esprit), I added a set of holes. The push-pull system consists of the two screws of the ring and a third screw screwed directly into the ring. The head of this Allen screw fits into a small cavity that I milled into the dovetail plate. A smaller hole in the centre of this cavity allows access to the head of the Allen screw with an Allen wrench. The height adjustment is therefore done by turning the three screws in the same direction (thanks to this construction, the ortho-error is currently only 3″!).

I had deliberately made the bridge slightly wider than the distance between the dovetail plates (see figure 5). However, the knobs of the clamps no longer fit in place, but this was easily remedied by creating two new knobs on my lathe with a longer stem from a piece of aluminium rod.

In the end, it turned out that a small adjustment was required to be able to use the double counterweight safely. In the rearmost position (i.e. closest to the declination axis), there turned out to be just 4mm too little space between the motor housing of the declination axis and the counterweight. An aluminium spacer plate of 10mm thickness provided the solution (see figure 6).

Now that the balancing system has been mounted, balancing has become a piece of cake!

In August 2020 I installed a Baader 8" PlaneWave clamp and ditto side-by-side plate (20mm thick), and removed the 10mm thick aluminium spacer as can be seen in below image.