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Re: DIY Spring Loaded Worms for the G11

 

Nice report Tony.

If you were interested in trying further improvements (vs just autoguide and shoot the stars!)...some thoughts.??

I was not sure if your value of 12-15 arcsec RA PE was RMS or Peak to Peak.

If the 12-15 arc sec PE was an RMS value, that sounds very high for a Losmandy precision brass worm on a G11 RA drive.? At least for an RMS value it is way too high.? ?I think the G11 brass worms run under 3 arcsec RMS in general.? I have found some that are about 1 arcsec RMS. (Would appear double for a GM8 RA drive, as the worm drives a smaller ring gear.)

So your thought to swap the RA and DEC worms is a good idea.??

To convert RMS to Peak to Peak a rule of thumb (if the oscillation were a pure sine wave this is exact) multiply the RMS value by 2.8.??

However other factors can mask the worm itself because they wobble at the same rotation rate as the worm.? They are:

1. An offset or an angle between the input and output of the Oldham coupler.? This is probably the biggest contributor, as it is a challenge to line up those shafts and to get the center plastic coupler part to only rotate, not shift.

2. On a Tucked Motor system:?
... the transfer gears must be centered on their axles.? One is on the gearbox.? One is on a the short shaft driving the Oldham coupler.? They must exhibit no wobble. { The older direct drive systems did not have these 3 parts (2 gears and the short shaft). } You can reposition the transfer gear of the gearbox by moving the gearbox using it's two mounting bolts.?

Note that: The inner transfer gear on the short shaft cannot be moved...it is made at one location in the vertical mounting plate.? There is hardly any room to move that short shaft to optimize the Oldham coupler alignment due to the precise countersink indentations in the vertical plate made for the plate mount bolts.??

You observed that there is a large time lag (hysteresis of about 0.8 sec) in the DEC tracking diagram.? Something is loose...and is moving when you switch direction.? Most likely it is the DEC worm capable of sliding left to right along its axis.? I prefer to prevent that,...add an R4 Belleville spring under the rightmost R4ZZ bearing, and that bearing must be polished down in OD and lubed to allow it to slide under the spring force.? Once you add that the worm will stay pushed toward the Oldham coupler bearing end, and both bearings will stay tight in axial compression.??

The small grub screws that hold the Oldham coupler to it's shafts tend to loosen up too.? Inspect the coupler ends for any looseness and snug those grub screws down.?

Anyway, your autoguiding was nice and fits your needs.? Keep shooting and having fun!

Best,
Michael


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On Sun, May 23, 2021, 6:06 AM GuitsBoy <guitsboy@...> wrote:

Got some initial guide results with the spring loaded worms last night.? My initial reaction is that it may have helped, but certainly didn't hurt anything.

Conditions last night were mediocre at best, so my results should be taken with a grain of salt.?? We had LOTS of passing clouds, and a bit of variable breeze.? I was guiding on M13, near zenith, historically a difficult spot for me because the intentional weight imbalance on the DEC (camera heavy) doesn't work well here.? I was shooting 30 and 60 second subs, dithering every other frame. ?? Seeing was good but not great, but transparency and clouds were below average.? This is using a 60mm guide scope with a 240mm focal length.? I seemed to have the best results from a 1 second exposure with multi star enabled.? I have an OAG that I will eventually move to, but since I'm used to using the guide scope, I wanted to keep things apples to apples.

First, my guiding assistant results (guiding disabled) were lackluster at best.? My peak-to-peak RA numbers seem to be artificially inflated due to some RA tracking drift.? My drift rate is somewhere around 2.4 arc-sec/min, so the RA error peak to peak climbs the longer it runs.? Smoothing it visually, the worm appears to have roughly 12-15 arc-sec native PE.? This is right in line with the results I had with the static mounted worms, so no big surprise here.? I think I should swap the RA and DEC worms to see if it improves at all.? I might also apply some tracking rate adjustment.? I suspect the RA drift is due to a sub optimal RTC in the cheap Arduino board that runs the whole show.? Tracking rate adjustment may help if the drift is consistent from night to night.

Next is the declination backlash.? I really expected this to tighten up, but it did not seem to.? Measured backlash seemed to be between 650? to 900 ms (ran twice), which was higher than the static worms.? I suspect this is from some wiggle room in the pivot bolt.? Perhaps if I can tighten this up, things will improve here.?

Now for the actual meat and potatoes guiding.? Here, it seemed to shine.? I saw .61 and .58 arc-sec RMS overall average.?? While sitting there watching, there were a few times where my number (current screen average) dropped to .48!? And most promising, the breeze did not seem to move the mount nearly as much as usual.? Typically I would set up a wind break to calm it down, but didn't feel like setting it up at 11pm.? These numbers are the best I've seen on any mount without using a wind break.


I think my next order of business is figuring out how to remove any play in the pivot bolt side.? Then maybe swap worms between the DEC and RA to see which has less native PE.? But overall, I think the spring loaded worm mod shows some promise.? I can't wait to test it on a night with better conditions.

Thanks,
-Tony
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