Reading my PE graph

KC_Astro_Mutt

mikenoname yeah, I plan to. I was curious to see ZWO's answer. As you can see in their response above, they didn't want to tackle it either, lol.

mikenoname

KC_Astro_Mutt You do not seem to have good luck with AM5s.

I am just hoping that with a load yours will perform better than that graph. Only one way to find out. Collect your own unguided data and cross your fingers.

w7ay

ASIMount@ZWO It seems most of the slopes are as same as mikenoname's AM5 and yours got much low numbers of Periodic error.

The amplitude of the Periodic error is not important for auto-guiding (it is only important for non-autoguided visual). The first derivative of the Periodic Error is the limiting number for auto-guiding. Large, smooth periodic errors can be easily guided away (take for example the Avalon mounts), rough slopes are what makes it harder to guide.

Taking your rescaled overlays:

I see a factor of 1.6 between the worse case derivative for Mike's curve compared to the worse case for Mutt's (worse) curve. For auto-guiding, Mutt's mount is worse than Mike's mount, even though Mutt's periodic error has much lower ampitude.

Mutt's curve has very high fifth order harmonics. From the white paper that I am writing, the first derivative of the N-th term of a Fourier Series looks like:

Notice the "N" that is inside the argument of the sinusoid pops outside (using the Chain Rule in calculus) as a direct multiplier of the amplitude of the derivative. A large 5th harmonic therefore contibutes to a much larger error.

Mutt's high derivative regions also occur 5 times more often during a long exposure than Mike's mount.

Chen

KC_Astro_Mutt

w7ay this does not sound promising... this is my 2nd (and last) attempt at the AM5. If this one doesn't work, I won't try another.

tempus

w7ay Mutt's curve has very high fifth order harmonics.

Chen,
Where do you see this in the plot? Did you have to calculate this 5th order harmonic or is it obvious from the plot?

KC_Astro_Mutt

mikenoname I just hope I can collect said data before the return period is up.

w7ay

tempus Where do you see this in the plot?

Count the cycles per period. Use Mike's graph for the period, since Mutt's graph is too irregular.

Chen

KC_Astro_Mutt

So, in the end, I finally got a cloudless couple of nights to work with this mount. Well past the return period. It's WAY worse than my first one, and seemingly unable to achieve an rms below about 1.4". It's a $2300 paperweight.

tempus

ZWO states 0.5 To 0.8RMS in their publicity. O am sure you can make a point to return it.

KC_Astro_Mutt

tempus they keep sending me to view articles, try this, try that... every time I've brought up returning/replacing it, the email conversation ends.

Simon sent me a list of things to try, but with zero success. The harmonics in my current mount are far worse than in my previous mount. When Chen says it's bad, it's bad, lol.

tempus

KC_Astro_Mutt. What about your retailer? They have some leverage with ZWO. Hopefully it wasn't bought direct from ZWO. If not, work with Simon. I can't believe he will not help you somehow. The AM5 is the biggest lottery of the astro world at this point.

KC_Astro_Mutt

tempus Simon did try to help. He sent a long list of things to try. Best I could get is 3 minutes of really great guiding, followed by 3 to 5 minutes of erratic crazy guiding. This time, I can't even get the 3 minutes of great guiding.

I suppose I could chat with the retailer. The first retailer I purchased from couldn't get anyone from ZWO to even acknowledge them. I don't know why a different retailer would do any better. Maybe I can get an exception for a return at best.

Edit to add:
This is what Simon sent:

Don't use long exposures - you need short high frequency guiding ... 0.5 - 1s max
Do not use large amounts of aggression ... 35-40% should see you guide better and never have high DEC agression as RA will just chase it
Have a relatively good balanced mount - while harmonic driver mounts don't need to be super accurately balanced - you do need to be reasonable balanced
Ensure the power supply is robust enough voltage wise / current wise
Don't exceeed the FL guidelines unless you are happy that guiding will be affected
Ensure guide scope to main scope FL ratio does not exceed around 5:1 for best results
Be well polar aligned!

w7ay

tempus The AM5 is the biggest lottery of the astro world at this point.

Nah, the USB hardware in ZWO products beats the AM5 by a mile :-).

The AM5 simply is QC'ing the wrong thing. They are QC'ing the amplitude of the periodic error (and still boasting about it for the AM3), while the parameter to QC is the first derivative of the curve.

They should trash the gears with the large slopes instead of passing it to the customer as a usable product. (Note that large harmonic distortion means large slope by the chain rule of calculus. So, when you see lots of wiggles in the curve, run, don't walk, to return the stuff.)

And if I am not wrong, they have even reduced the period of the AM3 from 430.82 seconds down to 288 seconds. That would instantly bump the already large slopes of the strain wave gears up by another 50% (again, from the chain rule). What are they thinking?

It is not clear, but I saw recent spec for the AM5 to also have a 288 second period. It is either a documentation error, or that mount has been revised too to the shorter period.

Chen

Kevin_A

KC_Astro_Mutt i would try running at 0.5s exposures with 300ms RA and Dec max durations. Any higher than 300ms might cause more erratic guiding as the mount might be trying to correct a over correction if its too large.

tempus

The 2 first point are straight from Chen's playbook. I don't think any of the rest will make that much of a difference with such an erratic guiding unless you are way out on some. Get a hold of your retailer. Hopefully he's a good one. I had an issue with a camera at one point. ZWO was ignoring me. I spoke with the retailer and he contacted them. Things got things moving in a matter of days and I got a resolution. Unfortunately, getting a good retailer is also a matter of luck or careful selection. Keep hope!

KC_Astro_Mutt

Kevin_A thanks Kevin. I started at 0.5 second exposures and 250ms durations. Was told to start here with my last AM5. As Chen described above, the harmonics are just too much, and my best guiding might be had at around 1.1" to 1.2" rms. It might be fine for my Redcat 51, but I won't likely use it for anything else.

Best guiding came at 65% agg (both axes), 500ms durations, 0.5 second exposures. At this, I would get a few minutes of 0.75" rms, and a few minutes at around 1.7" rms. The shaky bits came randomly, and the duration of good guiding to bad was just as random.

tempus

w7ay So without having access to the proper data or any data at all prior to purchase, which harmonic drive would be the safest to buy?

KC_Astro_Mutt

w7ay

w7ay They should trash the gears with the large slopes instead of passing it to the customer as a usable product. (Note that large harmonic distortion means large slope by the chain rule of calculus. So, when you see lots of wiggles in the curve, run, don't walk, to return the stuff.)

This... every bit of THIS!

w7ay

tempus So without having access to the proper data or any data at all prior to purchase, which harmonic drive would be the safest to buy?

I already have two RST-135 that I have been using for a few years now (the first was from RainbowAstro's first production run in 2019), so I am not really in the market for another one.

I was recently tempted to order the Nyx-101 just in case I need more payload ability when a Q3 manufacturing run of the Mewlon 180C is expected (paid for already; just now collecting other accessories I will need -- reducers, FTF, adapters from PrecisionParts, etc). But it looks like (check out their PHD2 screen captures), they too, like ZWO, have no understanding of strain wave gear characteristics before actually selling a product that is based on it. Whatever happened to good old fashioned engineering anyway.

My next strain wave geared mount could be the RST-300 if the Mewlon proves too much for the RST-135. The RST-300 is a little heavy, but I have been leaving the RST-135 outdoors 24x7x365 with no ill effect for 3 years now, so the plan is to leave the RST-300 outdoors too and don't have to carry it in and out.

When I had only one RST-135, back in 2019, I had babied it because I couldn't imagined having to go back to my Tak EM-11 if the RST-135 had failed. But after I bought the second one in 2020, I just left one outdoors all the time. And I still have both (similarly equipped with identical dual saddle mount, USB and power hubs, etc).

Now, if RainbowAstro were to announce something in between the RST-135 and the RST-300, I would buy it in an augenblick. You should see how clean their mount protocol is, a software engineer's dream. It takes after Niklaus Wirth's philosophy -- Pascal was designed with no operation that is not absolutely neccessary.

Chen

Kevin_A

KC_Astro_Mutt that sucks. Zwo should replace it if it does not meet the guiding specs of constantly running below 0.8 as advertized.

w7ay

KC_Astro_Mutt

BTW, keep an eye on the Declination error too. If it is large, you may be able to introduce a small polar alignment error (yes, purposely misalign -- again, I will be called blasphemous) so that there is always a small declination drift. That way, you can use North-only or South-only (depending on the drift) and reduce the declination error. That will get rid of any backlash in the declination axis.

The RA axis will never have a backlash problem (since it is unmöglich when you use a guide rate that is less than 1x sidereal rate), but the declination axis will. However, if you introduce a declination drift, you can use a N-only or S-only declination correction, and thus no backlash either.

A small (0.01 arc second per second) type drift will not cause a field rotation problem as long as you don't do 4 hours long exposures in Bortle 1 sites. A good post processor (not the ZWO live stacker) will rotate the sub-images for you when they stack (heck, the good ones will even let you use Drizzle).

Chen

w7ay

@KC_Astro_Mutt Best guiding came at 65% agg (both axes), 500ms durations, 0.5 second exposures.

If you have no other choice, try (both at the same time) (1) reducing the max pulse durations to 100 milliseconds, and (2) reducing the RA aggressiveness to less than 50%. Try 35% or less.

As bad as the harmonic distortions are for your mount (many large wiggles per primary period - so much so that you can't obviously see the fundamental frequency), you should still be able to restrict the correction pulses to no longer than 70 ms or so.

Allowing the autoguider to issue larger corrections than what is absolutely needed to correct for tracking error will cause problems (notice that the ASIAIR recommends 2000 milliseconds for ZWO's own mount, which it totally bogus). If there is a centroid estimation error (and there will be, even when you use optimal guide scope and cameras) large correction pulses (overshoot) will be issued (and exacerbated by large aggressiveness factor; thus, keep it low) which in turn produces a negative pulse later (undershoots) when the centroid is more correctly measured.

Now, the only problem when you reduce the pulse widths and aggressiveness, dither recovery will also be very slow (since we are only trying to move the mount by 0.1 to 0.5 arcsec/sec). But that is cause by how the current guiding software treats dither (it uses the same pulse width and loop gain). If you wite your own software, you can use different parmeters for dither recovery vs guiding, and the problem goes away. In the meantiome, if you don't write software, is to use as small dither as possible. 2 pixels worth will work with most monochome camera, and 3 will work with any Bayer camera. And if your plate scale is around 2 arcsec per pixel, the dither is limited to 4 to 6 arc seconds, and you can recover in 5 to 10 seconds.

Chen

KC_Astro_Mutt

w7ay If you have no other choice, try (both at the same time) (1) reducing the max pulse durations to 100 milliseconds, and (2) reducing the RA aggressiveness to less than 50%. Try 35% or less.

I reduced the duration to 70ms. You're a genius Chen, but you already knew that.