Mount tune-up and new PA method
Posted: Sat Jul 30, 2022 7:00 pm
Somehow, messing with my G11S appeals to me more than the obvious alternative. I added an EqStar controller, replaced it with OnStep, added DIY SLWs (spring loaded worms), added timing belts and still did not get the performance that I wanted. A total RMS of 0.7" was as good as it gets. That is about the accuracy where seeing becomes a factor, but clearly my stars were elongated in RA .
I took the drive trains apart to figure out what was going on. I believe I found one culprit: The inner worm block bearing felt bumpy, occasionally, when turning the worm holding the assembly in my hands. So, I swapped theRA and DEC worm assemblies because guiding in DEC is a minimal issue with good polar alignment. The RA RMS went down from 0.6" to 0.8" to 0.4" to 0.5". The stars are now round, and I got down to 0.5" total RMS at one time. At other times it was 0.7" but that may also depend on atmospheric conditions. BTW while testing this, I got a new appreciation of how nicely the worm assemblies are manufactured and how smooth they run when both bearings are good.
The diagnosis of this defect is actually interesting: One author on the Losmandy file site discovered, using the size of the diameter of the worm axle and the size of the inner and outer races of the worm bearings, that a periodic bump of the balls running over a race defect, results in a 76 second periodic error. The 76 second PE is notorious with Losmandy mounts. It can happen when the worm blocks are misaligned, non-roundness of the worm block holes so they pinch, or when a ball has damage (which is probably what I have in thatRA inner block bearing). I thought that analysis was pretty ingenious, although it is not certain that that causes it. The error cannot be PEC -ed out because it has a different period than the main harmonic of 239 seconds. Before I swapped the worm assemblies, I could see the 76 second harmonic clearly in my open-loop error graph. I presume it has disappeared.
As a side issue, my ASI120MMOAG seems to be a bit noisy. I wonder if I should get a better guide scope. Until I know for sure I play around with binning and imaging intervals. It looks like the faster, the better and 1 Hz is best. One simple check is to use my primary camera for a guide camera, which I can easily do in Ekos. After trying a bit there was not much difference between the two but I need to repeat to convince myself.
Next up, the SLWs. I have posted pictures earlier. On the Losmandy-hackers forum there were voices advocating high spring force. Losmandy itself equips its SLWs with a back-off screw, apparently to engage the spring only at high points to keep the mount from binding up when slewing, and to minimize the effects of SLW-induced extra friction. My SLWs use a fairly low force but when I wiggle theRA counterweight bar, I see the worm making significant radial motion - not good. For DEC it is much less for some reason (from trying to move the saddle, which has a smaller moment). So, I added some rubber bands with a hook in an outer block screw that adds easily adjustable force inner and downward. When moving the worm manually I can feel a shocking amount of extra force needed to turn it. The motors seem to have no problem with it. However, the worm and/or ring gear may wear down sooner. After testing a bit more I think I will apply it to RA only and try not to have to add too much force yet have the worm fully engaged.
I also checked if my single beam couplings add any significant flexure in the transmission from timing belt to worm but I don't think so. They feel pretty stiff so it should be OK. One thing I may consider is an alternative motor, maybe one that has internal decoders to make sure that I don't lose steps inRA . Generally, when I run open loop in the autoguider, I see RA going down slowly, never up. This indicates that I may be losing steps despite the 1:3 timing belts.
Some of you may recall myDSLR -and-clear-tape-with-needle-hole-based polar alignment method. Rotate around RA while taking a 20 sec exposure at high ISO. Mark the center of the star trails on the LCD with the clear-tape-with-needle-hole. Then take 2 sec images while using the Alt/Az controls to center the NCP on the needle hole. It lets me get 1' accuracy, computer-less. The NCP can easily be eyeballed using an asterism that is visible on the subs.
Of course, with my ASI2600MM/MC I can't do exactly the same. As it turns out however, Ekos lets me do essentially the same in a nicer way. Namely, it has a FITS image viewer that allows for multiple tabs. So, I can take my star trail image, save it, load it back up in a 2nd tab. I put my cursor on the center of the trails, use Alt/Tab to switch to tab 1, double-click and a bull's eye is placed exactly where the RA position is. Then I adjust the Alt/Az until the NCP is on the bull's eye while the camera takes 1-second images at 4x4 binning. Very cool! It works super especially for small adjustments when I place my mount in a paver-marked position.
That's it for now. I am getting a good understanding of the whole system, and once I get 0.5" total RMS consistently, I will be happy. The only change I foresee is upgrading the motors. The Losmandy-hackers forum has modifications where the needle bearings of theRA axis are upgraded, or where the motor is placed on the SLW assembly to get zero flex, but that is more drastic, and I doubt if it is necessary. In the end I don't know if all this is better than using the Gemini-2 system that I declined in buying a G11S, but it feels nice to be in full control of the control system from OnStep firmware to timing belts and SLWs.
I took the drive trains apart to figure out what was going on. I believe I found one culprit: The inner worm block bearing felt bumpy, occasionally, when turning the worm holding the assembly in my hands. So, I swapped the
The diagnosis of this defect is actually interesting: One author on the Losmandy file site discovered, using the size of the diameter of the worm axle and the size of the inner and outer races of the worm bearings, that a periodic bump of the balls running over a race defect, results in a 76 second periodic error. The 76 second PE is notorious with Losmandy mounts. It can happen when the worm blocks are misaligned, non-roundness of the worm block holes so they pinch, or when a ball has damage (which is probably what I have in that
As a side issue, my ASI120MM
Next up, the SLWs. I have posted pictures earlier. On the Losmandy-hackers forum there were voices advocating high spring force. Losmandy itself equips its SLWs with a back-off screw, apparently to engage the spring only at high points to keep the mount from binding up when slewing, and to minimize the effects of SLW-induced extra friction. My SLWs use a fairly low force but when I wiggle the
I also checked if my single beam couplings add any significant flexure in the transmission from timing belt to worm but I don't think so. They feel pretty stiff so it should be OK. One thing I may consider is an alternative motor, maybe one that has internal decoders to make sure that I don't lose steps in
Some of you may recall my
Of course, with my ASI2600MM/
That's it for now. I am getting a good understanding of the whole system, and once I get 0.5" total RMS consistently, I will be happy. The only change I foresee is upgrading the motors. The Losmandy-hackers forum has modifications where the needle bearings of the