Intro: Adjustment-free Counterweight for AZ-3 Altazimuth Mount
The AZ-3 alt-azimuthal telescope mount and tripod (supplied with many Chinese telescopes) is a popular grab-and-go mount for amateur astronomers who want a light-weight tripod with slow-motion controls that is quick to set up and pack away. An important feature of this mount is that the altitude (i.e. elevation) axis is not coincident with the centre of gravity of the telescope. When using my mount, I found that unless the altitude axis nut is done up very tightly, the telescope can tip of its own accord if at the extremes of its travel, as the system isn't balanced. I also found the altitude slow motion control on my AZ-3 mount functioned poorly at the extremes of travel if the nut was done up enough to prevent tipping. A number of retro-fit counterweight setups have been developed by amateurs and have been published on the internet, but as far as I can tell these all require adjustment if used at the extremes of travel in the altitude axis. So I set about to develop a very simple counterweight system that required no adjustment following initial installation.
In this instructable I will describe the concept that I developed to balance my AZ-3 and remove those aspects of its behaviour that had previously given me trouble. I don't propose to give detailed designs, as those choosing to develop their own version of this concept will have different tools and skills from me. My realisation was based on absolute simplicity and use of readily obtainable materials, but this was only made possible because I have a milling machine and a multi-angle vice, which was needed for one of the holes.
Step 1: Where to Put the Counterweight for Optimum Balance
For a counterweight to be effective at all altitude positions, the moment (force x radial distance from pivot) of the counterweight must exactly match the moment of the telescope. So a heavy telescope whose centre of gravity (COG) is close to the pivot can be balanced by a relatively small counterweight directly opposite the COG at a larger distance: nothing clever here. The difficulty with implementing a perfect counterweight on the AZ-3 is that for angles below about say 45deg the counterweight support rod would hit the top of the tripod.
Step 2: How I Implemented the Counterweight
The essence of my solution of this problem was based on the observation that a) the AZ-3 is normally used for carrying light telescopes such as a 90mm f10 refractor, so doesn't need a large counterweight; and b) the AZ-3 sits on a fairly stiff tripod. I reasoned that I could put a counterweight in exactly the right position with respect to the pivot to balance the telescope in the altitude axis if I moved it out of the plane perpendicular to the altitude axis. This is hard to describe but hopefully the photos make it clear. This positioning means that the counterweight will miss the mount base and tripod over all altitude angles. This approach is probably an abhorrence to any self-respecting mechanical engineer but I found that the tripod and mount is more than sufficiently sturdy to tolerate the small torsional force brought on by the out-of-plane counterweight. I considered that the ease of implementation and the improvement in performance far outweighed any such concerns.
It's worth noting that if this lateral torsional force was cause for concerned, it could be mitigated by adding a duplicate counterweight on the other side of the mount, at the cost of increased complexity and set-up time.
Step 3: The Counterweight
I wanted the counterweight to be as compact and simple as possible, and capable of easy removal to facilitate storage.
Real telescope counterweights seem to be very expensive. I looked for a cheap alternative and settled on the following approach. The counterweight itself was made from a stack of steel roofing washers, threaded onto the end of a piece of M10 studding and held in place with a nut and wingnut. Wingnuts were very helpful when determining the proper balance point as the roofing washers could be added and removed from the studding easily and quickly. At the unweighted end of the studding, an M10-threaded long nut was used to fix the counterweight to the rest of the mount, which carries a second M10 male thread. This makes for very quick removal and assembly when the scope is put into and out of storage.
Step 4: The Support Arm
I wanted the support arm (which holds the counterweight to the AZ-3 mount) to be compact, robust and use as few fixings as possible. I discovered that a piece of 3/4" box section steel could be fixed to the moveable tube ring support with an M6 screw using an existing fixing point just behind the forward tube ring. The loose end of the box section was then pushed back until it struck the body of the tube ring support close to the altitude axis (see first photo). This is good for two reasons:
1. The end of the support arm can be cut to length so that it coincides with the projection of the altitude axis.
2. The support arm is prevented by the body of the tube ring support from rotating under the weight of the counterweight when the telescope is pointing above horizontal, even though only a single M6 bolt is holding it in place.
The loose end of the support arm was fitted with an M10 bolt, passing through a hole which makes a compound angle with the edges of the box section. I marked the box section out by inspection and used an end-mill and angle-vice to cut the hole. I made some wedge-shaped spacers on the lathe to tidy up the parts of the bolt where the thread would have been exposed and to allow me to tighten it up fully. You need these tools to make a nice job of the concept, but using a simple pillar drill and a circular file would probably get you close enough.
Step 5: Conclusions
My offset counterweight has completely transformed my usage of my grab-and-go scope on the AZ-3 mount. It takes a few seconds to fit the counterweight to the support arm. The counterweight functions perfectly over all relevant angles without any adjustment. The altitude slow-motion controls also functions properly at any relevant angle. The telescope can also rotate completely about the azimuth axis without the counterweight striking the tripod.
After an observing session, it is a simple matter to remove the counterweight and fold the tripod so that the whole telescope assembly can be propped into a corner. The short support arm does not add significantly to the overall width of the mount, as shown in the photo.
I hope you found this article useful and may perhaps decide to implement something similar with your own AZ-3 mount (at your own risk, of course!). Thanks for reading...