tempus Do they have any secondary harmonics or is this an AM5 "feature"?
They potentially have larger periodic errors than the ZWO mounts (RST-135 are spec'ed as +/- 30"), but they have much lower harmonic distortion (check out a Cloudy Nights posting from about two years ago when someone posted the spectrum, and also that NASA paper that I referenced earlier), and also have 430.82 second periods. Even if the harmonic distortion is the same, the slope error of a 288 second mount with +/- 20" p-p is the same as +/- 30" p-p for a 430 second mount.
But as we know, it is the slope of the periodic error that matters for autoguided imaging. For visual work, you can always buy the Renishaw encoder version (RST-135E) that has a periodic error of +/- 2.5".
With a 2 FPS 12-star guiding, and 0.5x sidereal guide rate and good seeing, I see 0.4" p-p RMS error (total for RA + declination). With my new-founded 0.25x sidereal guiding (based on the sawtooth), I see 0.25" p-p total RMS error with halfway decent seeing, using ASIAIR defaults . You probably missed my report here:
https://bbs.astronomy-imaging-camera.com/d/16029-reading-my-pe-graph/34
(and the post just before that one)
My max pulse durations were set to 80ms for 0.5x sidereal guide rate, and 160 ms for 0.25x sidereal guide rate, so you know what kind of PE slope I was targeting. Don't ever, ever use these numbers for your mount, because it will not work if your mount does not have the same periodic error curve as my mount. Read the above posts to see what happens when the max pusle was tweaked too low.
The error is still seeing dependent, that is why I am now doing experiments on the correct weights to use for averaging the centroids. I think I am now limited by the centroid estimation.
It is not just the guiding that matters with a mount. A modern visual mount must also have decent firmware to be able to model itself (which mounts like my EM-11 cannot do because of lack of compute power), albeit it is of less importance if you can do plate solves when imaging. Especially true if you want to track while in Alt-az mode.
Plate solve does not help with daytime Solar work, though. So I have been using a wide FOV "finder" scope (50m f.l. on 290MM mini) to initially find the Sun (or a Sol-Searcher when I am outdoors). I am on the fence about getting an RST-135E for that reason (but reluctant because I already have two RST-135).
I was also looking at the Pegasus Nyx-101 in anticipation of a heavier Mewlon 180 that is in a 3rd-quarter production run, but also took it off my plate because they seem to also have problems, like ZWO does. Neither comany has any experience building traditional mounts before building their first strain wave geared mount. So, I am right now hoping the RST-135 can continue to handle the extra payload.
I am not exactly a spendthrift, but I do care about bang-for-the-buck for my money; and I especially hate companies that use me as a guinea pig to enrich themselves. And make me buy their upgraded product to have a workable instrument, that I should have received in the first place. Copy cats seldom know the "secret sauce" (witness the tantalum capacitor and lithium battery fiascos) and they too have to learn the hard way and make changes to the design. I just don't want them to learn off of my money.
Chen
