On Wed, Aug 4, 2021 at 04:26 AM, Michael Ben-Yehuda wrote:
Hi Henk
I'll try and address your questions
Don't the "touch points" as per Peter's drawing, depend on how the scope is balanced? If the scope is balanced towards the counterweight, these touch points would be just opposite.? And if the balance were perfect the touch points would be on the same side (low).Based on a deviation of 0.004" and a 16" distance between the bearings (just guessing), worst case, the angle between the shaft and the axis could be off by plus or minus (2¡Á0.004/16)¡Á(60¡Á180/pi)=1.7'.? ?After a 180 degree rotation the polar alignment would be off by 3.6' if it were 0 at the CWD position.? So much for the arc second accuracy of polar alignment.? Frankly I doubt if it were that much.
The touch points don't change by balancing the scope. The total mass of the DEC axis, telescope and counterweight system is supported by the RA shaft and bearings. Even if there were no gap there would be uneven pressure on the needle bearings. The bottom half of the upper bearing and because it acts like a lever, the upper half of the lower bearing carry the load. The only way to equalize the load on the bearings is to pre-load the system with an opposite force.? The counterweights balance off the force on the worm gear and drive train.?
You are correct, the play in the system is far less than 0.004"? I don't have an easy way of measuring it, but i suspect it's around 0.001"
I see, it's just the DEC axis that has counterweights, my reasoning does not apply to the RA axis.
The number of 2' is normal for my AVX.? If I rotate it manually 15 seconds east then 15 seconds west while taking a 30 second image at 6400 ISO using a DSLR with the scope's RA center of rotation in the FOV, I see 2 star trail circles on the LCD with centers that are about 2' apart.? That's because the manual rotation positions the axis in different directions going one way vs the other because of friction.? I have not tried this with my G11S yet but that would be a simple test to measure the effect of the gaps between the shaft and the needle bearings.If the scope were perfectly balanced when tightening the clutches the touch points would be at the same side and the angle between the shaft and RA axis would be zero but the center would still be offset by 0.004" so the rong gear might still bind up unless using true SLWs or unless adding enough slack
The error from the required clearance of the RA shaft to be able to pass it through the needle bearings does not create the kind of error you describe. Peter has tuned his G11 to achieve sub arc-second RMS guiding accuracy. He posted it in one of the other threads. He was at the step limit of? 0.56 arc-seconds until he installed a higher ration gearbox which brought him down to the 0.14 arc-second guiding range.?
It's easy to lose perspective about the level of precision and accuracy being discussed.
I'm not losing perspective of that.? I started a thread on the OnStep board about this.? I am running TMC2130 steppers for 0.9 degree motors at 64x.? The step angle is 50.625".? ?I want to get to 256x but when I try that, the slewing stops working for some reason.? I will try to find the bug because I know the TMC2130 is capable of that.? At 256x the step would be 12.5", albeit at 4x lower torque.? It's a delicate balance but I don't want to add a belt if I can avoid it.
Having said that, the test I suggested has nothing to do with guiding just mechanical, and I will try to illustrate it once I get back from my road trip.
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