I own an Ioptron HEM 27 Mount for more than two years now. Ever since the start, or maybe a few months later, I can’t recall, the Dec axis was able to rotate a few degrees to each side even with its clutch locked. For those who are not aware the HEM 27 has a harmonic drive Ra axis and a classical worm gear Dec axis. The rotation wasn’t a problem though, and when not disturbed its was able to guide consistently with an RMS of under 0.7”.
About a month ago, I decided to finally fix this rotation, clearly breaking the “if it didn’t break don’t fix it” motto. So, I opened up the Mount and located the origin of the rotation to a bushing holding a bearing in place being loose. That meant the worm gear itself was able to move along its axis inside of its housing, as shown by the arrow on the image below:
📷 IMG_0418.jpeg
So tightened the bushing and closed up the Mount thinking I had fixed the problem.
However, when I looked at the images of the next night after the reassembly, almost all had streaking stars along the same axis. When I looked at the PHD2 guiding log, I saw some weird spikes present on the Declination graph:
📷 IMG_0656.png
It seems to guide perfectly, and every few minutes produce a spike to one direction, followed soon by a spike of similar duration and magnitude to the opposite direction. This can continue for the whole night consistently, though the distance between the spikes is not always exactly the same. The Ra axis seems completely unaffected. I tried disassembling the HEM 27 quite a few times more, trying different things, loosening the bushing again, tightening it more, playing with the locking clutch, but none seemed to fix the issue. Sometimes the spikes would be longer and more blunt, sometimes sharper, but never completely absent. Pointing at different targets changes the behavior but the spikes are still present. I also rotated my filter wheel in a way that Dec was as balanced as possible, and there the spikes almost completely disappeared. I also tried to turn Dec guiding off, and with that the graph just drifted without spiking, but the next night it produced spikes only in one direction. The HEM 27 is carrying a pretty light payload and I’m guiding through a 50mm guide scope and ASI585MC guide camera.
📷 IMG_0533.jpeg
(Dec off, spikes near the end are just clouds)
📷 IMG_0539.jpeg
(Dec off, second night)
I have uninstalled and reinstalled PHD2 and the problem persists, and as it first occurred right after I disassembled the Mount it’s obvious it’s not a software issue. As it is significantly reduced when the when the rig is balanced, a mechanical issue seems the most likely. I briefly considered it could be residual current turning the motor while not being ordered to, but simply balancing the rig solving it seems to disprove that theory.
Still, the behavior baffles me. What could be causing two similar spikes in opposite directions to appear right one after the other? Since Dec is not rotating constantly like Ra, it can’t be a defect in a gear returning to the same position periodically. When everything is tightened, there’s virtually no backlash in any gear.
I have been scratching my head about this for a month now, testing different solutions, and still I haven’t arrived at the correct one. In retrospect I shouldn’t have disassembled the Mount in the first place, but as it has already happened I’m desperately looking for a way to make up for it. If anybody can provide any insight it would be incredibly helpful, as summer nebula season is rapidly approaching and I have no other rig.
Thank you,
David Kamchis
