Followup: Re: [ap-gto] Mr. Whang's Mach2 with TOA150

Roland Christen

Hello again,

I want to follow up with a bit more information and historical perspective for those interested in how mounts have evolved and why.

Today's mounts are different from previous generation mounts in that today mounts are being used primarily for astrophotography. The precision requirements have increased due to the finer and finer "grain sizes" (pixel sizes) of the detectors compared with the 50 micron grain size of film back in the good ole days.

Originally mounts had their worm gears fixed in place and hard attached to the main axis. This gave the mounts a nice solid feel which was perfect for the visual users that dominated astronomy 20 + years ago. The main problem was slight backlash that was always present because a fixed worm needs a certain amount of clearance to prevent binding. Our legacy mounts, the 400, 600, 800, 900 and 1200 mounts were built this way. Most visual users would simply loosen the clutches a bit and move the scope around manually, so gear backlash was never really noticed. These mounts worked perfectly fine with longish refractors that came in F9 to F12 and beyond focal ratios in sizes from 5 to 10 inches.

Along came CCD cameras and now people want to do astrophotography. At first the 9 micron pixel cameras did not put too much pressure on the precision needed, but eventually smaller pixels come to market and they require more and more precision. So we mount manufacturers develop Periodic Error Compensation to combat worm irregularities. Fixed worms present a problem because of backlash, and that's mainly a problem in Declination where the axis is asked to reverse periodically (RA never reverses so no backlash issue). To combat backlash we introduce spring loaded worms with backstops. The gearbox assemblies now are attached to pivots that gently press the worms into full mesh. This gives the mounts a certain amount of springiness when they are handled in a traditional manner with long heavy refractors, but it produces a vastly superior precise motion in DEC for imaging. The tradeoff then is elimination of backlash and the constant fiddling with worm mesh versus a more springy mount if you push it around manually.

There is also another aspect to the type and geometry of the telescopes that are attached to the mount. In the case of short compound scopes (SCTs, RCs etc) you can weigh the mounts down significantly and not encounter that springiness. There is simply not much moment arm to those short scopes. But lets put a 6" refractor with heavy cameras and field flatteners and auxiliary guide scopes on top of dovetail plates and you may have a 60 - 70 lb setup with a lot of moment arm. How much moment arm a mount can handle depends almost directly on the diameter of the worm wheel. Small diameter worm wheels might be able to carry lots of weight but cannot handle high moment forces. Therefore they will bounce more when disturbed and have longer settling time.

The Mach2 mount has a 6" worm wheel, which means that the teeth are only 3" from the center of the axis shaft. A 60 lb refractor imaging setup with an overall length of 60 inches, weight concentrated at either end, will act as a 10:1 lever on the 6" worm teeth. And it will impart up to 600 lb force on the gear teeth if it is disturbed or suddenly started or stopped. The little spring in the gearbox pivot cannot hold this force and the worm teeth will back out a bit until the backstop is reached. If we use heavier springs to force the worm teeth harder into mesh, then we get more wear during slewing and tracking. We also get more static friction which is the kiss of death for the Dec axis. Static friction causes retrograde motion which causes overshoot and spiking during reversals.

For the RA axis the issue is somewhat worse because the load is doubled at a minimum. So instead of 600lb force on the gear teeth, we have potentially up to 1200 lb with the above setup. That's half a ton that the RA worm teeth are being asked to handle! As long as everything is balanced, there is no problem for the axis to handle the static weight. But we don't want to tighten the clutches and then bang on the end of the scope to see how stable the mount is. Every pound of force is multiplied by 10x.

So, by using spring loaded worms we have much better guiding and less fiddling with worm mesh, but at the cost of mount "feel" or springiness. The Mach2 has mechanical damping internal to the axes, so for normal loads it works quite well. The encoders also help to stiffen the axes when they are active, so under actual use the mount works extremely well for imaging, even when the winds are breezy. The encoders are very effective in keeping the mount pointing to a specific position in the sky when disturbed by an outside force. For example: place a star on a crosshair on your computer screen, video mode, and then hang a 5 lb weight on the end of your scope. The star will scoot across the screen and within a second or so will return right back to the crosshair. Without the encoders the star would have moved off to some other position on the screen and stayed there. Things like cable drag and breezes acting on the telescope are effectively countered by the encoder loop. Without the encoders you only have the guider to bring the star back to the crosshairs, and that is a slow motion thing that is not as responsive as an encoder loop. Encoders sample hundreds of times per second to keep the axis exactly where it is commanded.

I hope the above essay has given some information and perspective and a peek under the hood. I am always happy to answer any questions and clear up any misconceptions you all might have.

Roland Christen

-----Original Message-----
From: Roland Christen via <chris1011@...>
To: <>
Sent: Tue, Oct 12, 2021 10:11 pm
Subject: [ap-gto] Mr. Whang's Mach2 with TOA150

Hi All,

For those of you on CN, you may have seen this entry by Mr. Whang:

He has a 150TOA with fairly heavy camera and feels that the mount is not stable, even though the package weighs just 55lb. I believe that the weight may not take into consideration the added weight of the dovetail assembly. The tube weight itself is listed at 44lb and if you add the camera and field flattener optics plus the refractor on top, plus the dovetail system, the total weight on the Dec axis is probably more like 60+ lb. The moment arm is pretty large for this combo, so we had to change the damping quite a bit.

Yes, it will feel somewhat boingy when pushed or disturbed, but I have put my own large refractors on the Mach2 and as long as you don't disturb it, the guiding is rock solid down in the 0.2 arc sec when the seeing is good. Right now I have a 12" Mak-Cass on the mount and the Mach2 handles it just fine and guides very well. Wind or no wind. I think Mr. Whang feels that it won't guide with some wind, but remember, the heavier the scope the more it will resist moving (force against mass - mass always wins). I had a 160 EDF refractor with my 10" Mak sitting on top, total weight far in excess of 80lb and yet it guided splendidly even with a good breeze blowing. The scope really didn't move unless I pushed on the end and then of course it bounced around a bit, just like the video that Mr. Whang posted.

Sometimes our feelings about something don't translate into actuality. If I had that 150 TOA setup, I would go ahead and take some actual images and see what the results are.


Roland Christen

Roland Christen

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