Part of the trouble with balancing binoviewers and large eyepieces (like the 13 mm Ethos) in the top eyepiece holder (as opposed to a camera along the axial port) is that the eyepiece weight is off-axis. This means that the counterbalance also needs to be off-axis (underneath) in order to avoid 1) the balance changing with the altitude of the target and 2) potential rotation of the optical tube in its ring mount when viewing targets in the extreme east or west in equatorial mode.
The brass-ringed UV filter and lens hood that I have on the front of my Q allow pretty good on-axis balance at many useful astronomy-related altitudes, but I wanted a universal off-axis detachable, adjustable balance solution that worked at all altitudes, even for vertical zenith and horizontal terrestrial viewing. I don’t have a proper machine shop, so my efforts are restricted to what I can saw, file, and drill by hand, preferably with what I can find laying around in the garage. Here’s what I’ve come up with.
Brass-ringed B+W UV filter for permanent protection of the front Q corrector. 68 grams.
Lens hood. This is a Fittest aluminum tube, 95 mm threads, male at one end, female at the other. 140 grams.
SkyWatcher mounting ring, 101 mm inner diameter. I reduced this down to 98 mm (the outer diameter of the Fittest lens hood) by removing the felt, adding a rubber sheet (cut from a bicycle inner tube and attached with double-sided tape), and re-sticking the felt. 167 grams.
Steel hollow curtain rail, cut to fit, 138 grams. The front end is bolted to the SkyWatcher mounting ring. The ring is threaded so you can adjust the bolt and locknut to get the amount of compression on the rail just right (to remove slop but still allow rotation for flat storage) without coming out of adjustment.
The rear (finder) end of the rail is cut and filed to a special shape that interlocks with the Q’s finder mirror bracket without touching or damaging the finder mirror itself or the finder solar filter. The finder is not useable with this system because it looks through the middle of the curtain rod. But then the finder does not focus with binoviewers or Ethos 13 anyway so no great loss. (I now know what the inside of a curtain rod looks like under high two-eyed magnfication. And what a sight it isn’t.)
The moveable counterweight is a couple of large magnets taped together, along with a steel plate to help contain the magnetic field on the non-rail side of the magnet. I can re-position the magnet anywhere along the rail. The magnets and plate are 440 grams, cannibalised from a broken double-sided window cleaning kit. They are ridiculously, seriously, dangerously overpowered. I have to tilt them onto the rail gently one edge at a time for fear of thwacking the scope out of collimation. I might consider replacing the curtain rod with a proper non-magnetic solid rail and counterweight set, but one would have to figure out how to attach/drill/file/shape the finder end of the solid rail so it fits around the finder mirror and finder solar filter without touching them. Probably something involving swans. (What, no Bleak Expectations fans out there? Over in the Stargazers Lounge that last sentence would’ve been really funny.)
To check the balance at any time, rotate the Dec knob back and forth to see if there is any difference in resistance between the up and down directions. They should be equal.
The system seems to work well for my Ethos 13 mm and WO binoviewers.
Edit: It turns out that my WO binoviewers have a swan logo on the nose cap! How re-happified a bit some more am I? Harry Biscuit would be so proud of... oh, never mind.
Edited by munirocks, 21 May 2019 - 02:29 PM.