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

Chris White

On Wed, Oct 13, 2021 at 12:54 PM, Roland Christen wrote:
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.

I have a legacy mount, 900GTO and perhaps I've never pushed the equipment enough but I'm not sure how much more precision I could get that would even translate into better images.  I purchased the mount second hand last winter, and I don't think I've thrown out a single sub due to mount issues.  I'm guiding at +/- 0.3" Total RMS using APCC Pro modeling and PEC.  My load is pretty light for the mount, with a GTX and full imaging train so maybe I don't notice any of these issues discussed here.  I'm using tiny pixels 3.76um and generally take 10 to 20 minute subs.  My stars are tight and performance is seeing limited as far as I can tell. 

Roland, are you saying that I might notice some limitations with the mount if I wanted to use for example a 12.5" iDK? 

The only thing maintenance-wise that I have had to do is re-mesh my DEC worm.  I suspect that is something I may need to do seasonally as I live in a climate with vast temperature swings through the seasons.  Otherwise it seems like this mount's performance is simply astounding, and I don't know what getting a newer design with encoders would do for me in my final images.  If I do decide to upgrade mounts in the future it would be to acquire a second mount not an "upgrade." 

I do appreciate the history lesson, and it makes total sense why the design has evolved to respond to more demanding usage situations.  And I know you aren't saying that the old mount designs are bad, just that the newer designs are the way they are for the reasons stated.  Thanks again!

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