[Tom Krajci Capt] I use it to keep the mirror box shorter so the parts will fit through the car door. (I also have a thinner mirror and want to use a 20mm Nagler (2 1/2 lbs. of eyepiece!). . .so without springs or counterweights the mirror box gets kinda long to balance things out.)

The geometry is surprisingly simple: A single post/attachment point is on the elevation bearing (actually one on each side of the mirror box), but not in the center of the bearing, so a force on it will generate torque.

Call the distance from center of elevation bearing to post R. There is also an eyelet/pulley on the rocker box. When the telescope tube is pointed straight up (and doesn't need help from the springs for counterbalancing) the center of the elevation bearing, the post, and eyelet/pulley all lie on a straight line. (It doesn't matter if this line is vertical or not. ..on my scope it's not - so that there's enough room for the spring/cable).

Lets call the distance from post to pulley/eyelet P (when the post and pulley/eyelet are closest to each other. . .when the scope tube is vertical).

A cable is attached to the post and runs through/around the eyelet/pulley, and is connected to a spring. This spring is attached to some place on the rocker box. It doesn't matter where you attach the end of the spring since the pulley/eyelet redirects the direction of force from the cable and spring. I ran the spring/cable diagonally across the rocker box to have enough room.

When the scope points vertical, the spring is at its least stretched position, as you lower the scope the spring stretches, forces increase, and the nose-heaviness of the scope tube is countered. BUT the spring is NOT at a zero stretch/relaxed state when the scope points vertical. When the scope is vertical, the spring is stretched from its relaxed state by the length of P (the pulley/eyelet to post distance I mentioned above.)

With this setup you exactly counterbalance the scope tube at all elevations.

If you want to design your own system you must know: The amount of imbalance of your scope (I do it in inch-pounds). If you have a figure like "my scope is two pounds nose heavy (when the scope tube is horizontal)" then tell me the distance from the OTA end to the center of the elevation bearing (such as 80 inches). In this case your scope is top heavy by about 160 inch-pounds. The spring coefficient (pounds per inch) of the spring you will use. (That's not hard to measure - I used a coat rack and some barbell weights that I had measured accurately on a friendly postman's scale.) Measure how much the string stretches at various weights, and plot the results on your favorite spreadsheet or graph paper. If you have good data the points will lie on a line. . .the slope of the line is your spring coefficient. How far you can stretch your spring before it goes too far and then permanently deforms. . .and then in your design don't come closer than about 75% of that max stretch distance. (I bought several of the same kind at Home Depot, tested all (they all had very similar spring coefficients) and tested one to destruction to learn max limits.)

Draft/lay out your design to see if the size of your elevation bearings, rocker box, and springs (and max expected spring travel/extension) will fit on your existing telescope parts. My setup just barely fits on the rocker box. . .but if I had gotten stiffer springs I could have done it in a more compact manner. Make sure your center of gravity doesn't shift too close to your elevation bearings when the scope is at low elevations or it will not move well. (A normally balanced scope has the tube C/G between the elevation bearings. . .the C/G shifts forward as you use more and more spring counterweighting. . .too much and the C/G is beyond the bearing, and the tube won't stay in the rocker box!

It helps to have bigger elevation bearings, especially if you're going to use the springs for a big effect.

I've set up a spreadsheet to help "what if" the design of the springs, or another one for balance analysis (assuming you know the weights of individual components, and distances too). I can e-mail copies of the spreadsheets, or run numbers for you for tentative designs - takes only a minute or two.

In my particular case the scope was heavy by about 260 in-lbs. Two springs with coefficients of 1.13 lbs./inch were used. R was about 9.25 inches, and P was about 3.5 inches. The springs stretch about 13 inches, so you need enough room on the rocker box for the travel length and also the unstretched length of the spring. . .comes out to about 20 inches in my case. My springs were not very stiff, I could have probably doubled the amount of top-heaviness I could have effectively countered with this setup. I used small pulley wheels (about 1 1/4" diameter) on ball bearings. ..the kind for rolling glass sliding porch doors. . .about $4 for a pair at Home Depot. These aren't precision bearing assemblies, but you don't need that for this application.