Last updated: 31 December 2001

Counterweight Design

Sent: Friday, December 28, 2001 10:18:47
From: (Victor van Wulfen)

For a long time balancing has been a problem with the ETX. We can all see it for ourselves; put your ETX on a flat surface, unlock de declination (DEC) and watch the tube swing down. It has been suggested that the ETX was designed with this unbalance in mind. However, the DEC axis is driven by exactly the same type of DC motor that drives the R.A. axis. The DEC motor and gears are therefor under higher stress than the R.A.'s. This will surely, over time wear down the gears faster. So why not balance it?


The first time it came to my attention was after I had degreased the gears. They make a lot more noise without all that green Meade grease on it. Judging by the sound it is very clear that the DEC motors are under much higher stress than the R.A.'s. The R.A. motors don't have to work against gravity as much as the DEC motors (in polar mode) and not at all when in Alt/Az mode. The effect of gravity is brought on the DEC motors by the front heavy optical tube. It became more evident after I mounted my newly acquired dewshield, dew clip and piggyback adapter. The DEC gears strained to do their work when slewing up. Adding a camera would even make the gears slip. Definitely not what I wanted to see and hear. Since observing without dew protection is virtually impossible in the Netherlands so I needed to find a solution. Dr. Sherrod advised me to 'do whatever it takes' and I did.


How can you calculate how much counterweight you approximately need to balance your scope? Pretty easy. It's no astronomical math and probably only a close estimate, but it works fine. To have the scope in balance you need to make sure that the sum of the moments M (M = F×d = (m×g) ×d = m×9.81×d) left and right of the pivot point (read: the declination axis) is 0 (zero). This is not the case on a standard ETX. The tube drops down when you unlock the DEC lock because the moment in front of the scope is bigger than that of the rear of the tube. So that's where you need to add weight. For use without any accessories it may not be necessary, but if you have any attached to the front side of the tube you sure can use a counterweight.


So how much and where? Well, first determine where you can place the counterweight. Then use the formula mentioned above to calculate the sum of the moments of the accessories mounted on the front side of the tube. Then determine where on the rear of the tube you can place the counterweights, measure the arm [d] and calculate the weight needed. Make sure you use only the metric system on your calculations and don't confuse it with pounds and inches. If you don't your ETX may burn up in the martian atmosphere... or was that something else? Anyway...








Created by AccuSoft Corp.


The picture above shows my ETX with counterweights and accessories attached. The moment of 1 has to be equal the moments of  2+3+4 to have the tube balanced as if there were no accessories attached. Any extra counterweights on the rear of the scope will only help to further balance the tube.


I thought it would be easiest to use the space next to the mirror-flip control knobs. I wanted to make the counterweights out of lead for ease of work and maximum weight. To make sure the weights would have the right shape to adapt to the scope I used clay (not Dr. Sherrod!) I bought in a toy store. It takes some effort to get it into the right shape. Once finished place the clay shapes in a little tray and add plaster to it. Let it dry for a day and –carefully- take the clay out. You now have a mold to pour your weights.

 Created by AccuSoft Corp.


Melt the lead but use the necessary precautions. Lead is toxic so do not melt it inside but in a well ventilated area. Be careful while pouring it, it may splatter. Eye protection is advised. After pouring the lead will solidify very quickly. Let it sit for a while and then shake your counterweight out. Once cooled off use a file and hammer to make sure it fits to the scope as it's supposed to.


Created by AccuSoft Corp.


If you want to you can paint it black. Repeat the process for another counterweight. I spraypainted both black and attached velcro to the flat side and the back.

Created by AccuSoft Corp.


Next stick velcro the rear of your scope as well, attach the weights and you're finished.


Created by AccuSoft Corp.


Created by AccuSoft Corp.


By pouring smaller or larger weights you can adjust you balance easily. The picture below shows my ETX-125, in Alt/Az with my accessories attached and the DEC lock unlocked.


Created by AccuSoft Corp.



What you need to do all of this precisely is a ruler, a scale, a calculator, clay, plaster, lead and velcro.


Good luck balancing your scope, it's worth the effort!!!

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