Reading my PE graph

KC_Astro_Mutt

With a graph that looks like this, how do I find my hypothetical rms error?

Kevin_A

Very good low numbers but what the heck is going on with that partial? Lots of harmonics or crazy dance goin on! I would like to know how it guides! Maybe need 0.5s exposures to tame the small jittery dance steps! Haha

KC_Astro_Mutt

Kevin_A yeah, this will be my 2nd attempt. The first mount had pretty bad secondary harmonics, but this one looks nuts!

Kevin_A

KC_Astro_Mutt i would try it out and use short 300ms durations at 1s and 0.5 and see. Remember that the corrections won’t be huge as the errors are small… but 0.5s might be the sweet spot for this mount.

mikenoname

And may want to take your own data plots with your rig on the mount to see if that plot that ZWO sent you is even accurate. These mounts can behave quite differently depending on load and location in the sky.

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.

ASIMount@ZWO

Hi KC_Astro_Mutt,
Here is the Partial Zoom pic from https://bbs.astronomy-imaging-camera.com/d/15989-getting-the-best-performance-from-my-am5/18

Let's put your Partial Zoom pic and mikenoname's to the same scalar, it will be like:

It seems most of the slopes are as same as mikenoname's AM5 and yours got much low numbers of Periodic error.
We zoom out the Y-axis for a better look.

BTW, we are considering a better way to describe the specific behavior of SW gears.

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

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.

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