Design Considerations for Ball Scope Tube Balance

Design Considerations for Tube Balance

I’ve prepared a simple Excel worksheet to calculate how the different dimensional parameters influence the size and weight of a ball scope. Problem is, I don’t know how to post this excel document on Cloudy Nights. If you have a suggestion on how to make this calculation tool available to all, please let me know. I only know how to post JPEG pictures on this forum. Eventually I will post the spreadsheet on my website, which is currently in construction.

In the mean time, here are a few examples I’ve prepared showing different parameters for different telescope sizes:


Please note that in this table all length dimensions are in inches and weight dimensions in pounds.

The way the excel sheet works is you imput information highlited in yellow and the worksheet calculates the instrument’s UTA weight budget.  The worksheet assumes a cylindrical mirror shape (not a conical mirror). You imput the mirror diameter, the hemisphere diameter, the mirror thickness and mirror focal ratio. The diameter and number of trusses is then imputed (I assumed aluminium poles with .050 ” thickness). An other usefull output is the eyepiece height and OTA weight. You can add a counterweight, located under the mirror and add information on the height of the mirror cell, which moves the mirror up, towards the CG of the hemisphere, reducing the efficiency of the moment created by the glass weight to counterbalance the weight of the UTA.

The table shows different examples I thought up for telescopes from 6 to 24 inches in diameter. With the excel file it is, of course, easy to play with these numbers and do some optimization. The 6 and 8 inch examples are using only one truss pole, like my first ball scope, a 6 inch f/4:


The 20 inch f/3.9 is similar to my scope but with a 2 inch thick constant thickness Mirror. It is, in fact, closer to Mike Mconnelley’s 20 inch ball scope which you can read about in this CN thread:http://www.cloudynig…on-mirror-cell/ .  Mike’s ball scope is interesting because it uses a 2 inch thick glass edge supported only on two, fixed posts. As he mentions in the discussion thread, optical quality is not severly affected by this type of support when he observes at different tube angles. Of course, at some point, the image would degrade if the mirror were much thinner or focal ratio much faster (or even worse: both). Each case is different and one must be careful as the mirror increases in size and/or decreases in thickness since, unlike a dobsonian, you don’t have the benefit of the edge of the glass always resting in the same position.

The other scope examples sometimes require a counterweight because the UTA weight budget is too low for the given hemisphere diameter or focal ratio. To get rid of a counterweight you must either choose a shorter focal length, a larger hemisphere, or mix of both.

Conclusion on the OTA

In this first part of my chronicle I tried as best I could to explain how my 20 inch telescope was designed the way it is and how I went about building it. By itself, this large ball scope is quite a pleasure to use. It is finely balanced, ergonomic and can be used sitting down over most of the sky and requires only a one step stool for the rest. It is easy to assemble and dissassemble without any tooling and maintains its collimation very well, requiring only minor tweaks which can be done very easily from a high power eyepiece using the extension rods described previously. Also, changing eyepieces from the permanently mounted 20mm, coma corrected 100 deg AFOV eyepiece (1 full degree of true field) to a higher power, non coma corrected 1¼ inch eyepiece is effortless thanks to the eyepiece turret which holds two eyepieces simultaneously. Thanks to this turret, tube balance is always preserved.