Re: Understanding a GM8 for Possible Purchase
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Hi! The only motor that is clearly shown from side view is the RA motor, visible from your rear view photo.? That motor shows no "cooling fins" so that is an oldest version servo motor.? The latest HiTorque motor has about 6 cooling fins around the barrel, and I don't see any in that photo.? Photos of the various motor versions are at this website, so you can clearly tell the motor from counting the cooling fins: But if the old motors work, there is nothing wrong with them.? You can increase motor torque by supplying say 17 to 18 volts to the Gemini DC power input instead of 12 volts to get proportionally higher motor torque.? If a motor does wear out, a replacement motor is about $225.? All the motors are interchangeable.? They all use the same stock gearbox from the company.? ?? Now...as you requested understanding.... here we go: Autoguiding is essential and the Gemini-2 gives you that, but it is better if the autoguider needs as little corrections to do as possible.? So if you are looking to do deep sky imaging, then low PE is the factor of interest in any mount.? ? It is pretty easy to tune this earlier style mount to low PE, as the RA drive has only got a few components (motor into gearbox into coupler into worm in worm bearings).? The OPW on the RA generally improves PE by holding the worm bearings in line, getting the best worm bearing behavior, and the OPW makes worm to ring gear space easier to adjust because it helps hold the worm bearings tight on the worm.?? The latest version "tucked" motor has its own OPW, and adds in two large transfer gears and another shaft to hold the inner transfer gear, and these add a few more elements to control for low PE.? ?The "spring loaded" version adds a spring and a back off bolt.?? Owners can improve these mounts by adding a Belleville spring washer to eliminate worm axial shift, and putting in higher quality RA worm bearings.? These are easy to do.? Perhaps this mount already has these elements.?? The GM8 has a 180 tooth ring gear on both RA and DEC, and a 480 second worm period.? The larger heavier G11 has 360 teeth in the ring gears and a 240 second worm period.? The smaller diameter GM8 ring gear would amplify the GM8 PE of the worm by 2X.? But the worm is only a part of the PE contributors.? The lineup of the Oldham coupler, perfection of the RA gearbox and worm bearings are important factors as well.? My own GM8 looks just like this one, and has about 1.6 arcsec RMS PE (about 4 arcsec peak to peak). I imaged with it using 5 inch or 6 inch refractor, and the GM8 was designed for up to 8 inch SCTs.? The GM8 has 3 inch OD clutch disks, so if you are planning to image, you may want to install high friction disks, but these are inexpensive and easy to install.? The bigger G11 had 4.25 inch OD disks.?? This unit has also got a nice polar scope and latest illuminator.? That will help you get a good polar alignment.? It has a lightweight foldable tripod.? I have only heavier tripods, if you need a more beefy base there are other tripod options if you want one.?? The Gemini-2 is a good go-to controller.? This system shows some nice holders in the legs for the G-2 handset.? I notice this particular GM8 has what looks like my DEC extension under the dovetail clamp.? That DEC extension allows the dovetail to rotate past the RA motor end, and gives the mount an ability to do a meridian flip.?? ( Have I left anything out?! ) Have fun, Michael On Fri, Jan 22, 2021, 7:59 PM Brian Valente <bvalente@...> wrote:
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Michael Herman