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Getting the balance right (part 1)
Modifying a Synta AZ-3 mount to have perfect balance.
By Antony McEwan
Some of you may know of my fondness for small refractors. I find them to be very useful telescopes with quick cool-down times, excellent image sharpness and contrast, and low to moderate weight. For these reasons I usually take a small refractor with me when I go on holiday and, indeed, when I took my Stellarvue Nighthawk with me to Wales in 2004, I had several wonderful nights of exquisite viewing. Now although a good small refractor would seem to be ideal for holidays and quick set ups, the question of what to mount it on requires some thought. Many people use photographic tripods, but unless you get a very heavy duty (and expensive) one the ‘scope can overload the mount and you may have a less than steady viewing experience. There are many lightweight equatorial mounts available, such as the EQ2 and EQ3 from Synta, or the Orion UK GEM mount, but these are bulky to carry around and detract from the ‘grab and go’ holiday ‘scope ethos. A good mainstream alt-azimuth (alt-az hereafter) mount for small to mid-sized telescopes is Synta’s AZ-3 mount. It is quite often supplied with a 3” or 4” refractor atop it, is lightweight, of simple design, and unusually for an alt-az mount it has slow-motion cables attached. These enable you to track an object simply by turning the knobs at the end of the flexible cables rather than actually pushing the telescope itself. This gives a smoother view with less vibration.
I decided to get one of these over a year ago, but found that most of the UK suppliers had sold out. Seems everyone else had the same idea I had! I eventually tracked one down at Pulsar Optical. It was the last one they had in stock and was priced at £89, less than the other prices I had seen quoted. I placed the order and the mount arrived a couple of days later, in sturdy packaging. It was a piece of cake to assemble, but I did immediately notice that once assembled the movements on both axes were somewhat stiff and, yes, squeaky! Of course that’s par for the course with Synta mounts - they don’t use grease as a lubricant but instead some sort of industrial mixture of glue and molasses. It took about an hour to disassemble the head of the mount, scrape off the old gloop, and replace with 25 year vintage Castrol Lithium grease. Result: Smooth quiet motion.
I got many chances to try out the mount with an 80mm f5 refractor for a while, and then with my Stellarvue Nighthawk. Both ‘scopes gave great wide field images and were fairly short and ‘stubby’, making them ideal for carrying around and for quick looks. The views were excellent, but the stability of the mount was variable. When viewing near the horizon or up to an elevation of about 45 degrees the whole thing was very stable and easy to move. When using the AZ-3 mount you move the telescope by turning the slow-motion cables. One cable moves the mount head (and thus the optical tube) left and right, and the other moves it up and down. I noticed very quickly when trying to view objects above 45 degrees the centre of gravity of the telescope would be positioned towards the rear of the tube assembly, quite far behind the actual pivot point on the mount. This meant that the telescope was unbalanced and the altitude slow motion cable was not able to exert enough force (torque?) to move the telescope down (forward) when I wanted it to. I could force it to do so by tightening the single nut about which the altitude bearing rotated, but that meant there was too much tension when viewing at lower elevations and the motion was then sticky and stiff. I realised that if it was a problem with the Stellarvue Nighthawk, it would be a lot worse with the Televue 85 refractor which is a fair bit heavier than the Nighthawk, despite being about the same size. This was a tricky problem - but I found a way of fixing it.
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