recommendations for improving my setup with am5n?

[David Osojnik]

My Setup:

• Guide Scope: EvoGuide 50ED with 242mm focal length
• Guide Camera: ASI 174MM 
• Mount: ZWO AM5N

For the last session (logs attached), I did:

• Created a new profile, took darks, and set the exposure time to 1 second since I read that harmonic drive mounts perform better with shorter exposures.
• Ran the calibration assistant, slewed to the default position, and performed calibration (results were good).
• Stayed at the same position and ran the guiding assistant for 17 minutes, accepting all recommendations (except backlash adjustment for now). The assistant suggested increasing the exposure time to 1.5 - 3 seconds, but since I read that harmonic drive mounts should be around 1 second or even 0.5 seconds, I didn’t change it. (I had already tested longer exposure times with no improvement.)
• No dithering for this session, as I wanted to optimize guiding first.
• Ran two 1-hour guiding sessions (19:24 and 20:40) with the guiding assistant settings applied, achieving RMS of 0.70 and 0.76.
  • Exposure = 1s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 70%, Minimum move = 0.065
  • DEC algorithm Resist Switch: Aggression = 100%, Minimum move = 0.100, FastSwitch = enabled

• Ran another 1-hour session, (22:19) this time lowering RA aggression from 70% to 50%, and got 0.68 RMS (best of the night).
  • Exposure = 1s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 50%, Minimum move = 0.065
  • DEC algorithm Resist Switch: Aggression = 100%, Minimum move = 0.100, FastSwitch = enabled

• Ran another 1-hour session, (00:08) this time changing both RA and DEC min_move, and got 0.77 RMS.
  • Exposure = 1s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 50%, Minimum move = 0.100
  • DEC algorithm Resist Switch: Aggression = 100%, Minimum move = 0.150, FastSwitch = enabled

• Ran another 1-hour session, (01:23) this time changing more stuff, and got 0.72 RMS.
  • Exposure = 1s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 40%, Minimum move = 0.070
  • DEC algorithm Resist Switch: Aggression = 70%, Minimum move = 0.100, FastSwitch = enabled

• Ran another 1-hour session, (02:19) this time changing exposure time to 0.5s, and got 0.77 RMS.
  • Exposure = 0.5s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 40%, Minimum move = 0.070
  • DEC algorithm Resist Switch: Aggression = 70%, Minimum move = 0.100, FastSwitch = enabled

• Last run for 2-hour session, (03:16) this time changing more stuff, and got 0.79 RMS.
  • Exposure = 1s
  • RA algorithm Hysteresis: Hysteresis = 0.100, Aggression = 50%, Minimum move = 0.070
  • DEC algorithm Resist Switch: Aggression = 70%, Minimum move = 0.100, FastSwitch = enabled

for me it looked like default settings had some RA oscillations and a lot of corrections that is why i went with lower aggression settings later on

Any recommendations for improving my setup?

My stars look okay (except for some occasional random frames), but since this is a new mount, I want to get the best performance possible.

Thank you so much!

David

[Brian Valente]

Hi David

I don't see any logs attached. Please make sure to use the Log Uploader feature and send the url to the zip file

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Brian Valente

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[David Osojnik]

Of course i forgot... :) here they are:

https://openphdguiding.org/logs/dl/PHD2_logs_KBya.zip

[Brian Valente]

Hi David

In general I think you have the right idea of what you are seeing and testing for. 

It's clear RA is the constraining factor. Dec seems to be relatively well behaved, even though your min moves are higher. 

Strainwave mounts can be finicky, exactly in the way you are experiencing: they need to be aggressively guided but too aggressive introduces oscillations. 

You might try a custom guide exposure between 0.5 and 1 sec (maybe 0.75 sec?) and see if you can strike a balance between those. I would start with a high aggression, watch it, and if it oscillates, drop the aggression down by 5% at a time until the oscillations go away. That may be a reasonable and systematic way to go about tuning it. You can also try dropping your hysteresis value to 0 and see how that improves things.

Brian

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[Jens Scheidtmann]

Hi David, 

You didn't mention your astrophotography setup and total load. Could you please share that info?
Did you take pictures during these sessions? I would love to see FWHM or HFR values from your subs correlated with your guiding results. 

Thanks in advance, Jens  

[David Osojnik]

Hey,

So my setup is:

• telescope: William Optics Cat 91 wifd (Focal Length: 448mm / Diameter: 91 mm / Focal Ratio: f/4.9)
• mount: ZWO AM5N
• Camera: ZWO ASI 6200 MM
• Guide Scope: Skywatcher Guidescope Evoguide 50 ED
• Guide Camera: ZWO ASI 174 MM

Total weight on the mount is 13kg and i have put 5kg counterweight on 25cm length for the above test. (i have as well tried without it but did not see any noticeable difference).

I did take the subs and here are the FWHM and HFR values:

i did 163 subs between different LRGB+SHO filters and the values were:

HFR between: 1.50 and 1.60 and 3 different subs at 2.0, 2.1 and 2.14

FWHM between: 4.6 and 5.1 with one sub at 6.2

Eccentricity between: 0.25 and 0.35 with outliers at 0.40 and 0.48 and 0.57 

please let me know if you need anything else.

Thank you

David

[Bruce Waddington]

Hi David.  I think I can see some unexpected RA behavior that you should talk to ZWO about.  To begin, I see periods of very unusual RA oscillation (RA in red):

This is roughly a minute's worth of oscillation, with each guide correction producing an over-shoot.  This isn't an error with the guide algorithm, each guide command was intentionally reduced by 30% to avoid this sort of problem. It's safe to make this argument because you have a high-quality calibration that, if anything, would tend to create a minor amount of under-correction in RA.  If we look at a typical RA span with a different ZWO mount, it looks normal:

Even though there are changes in guide direction, as expected, there are no instances of oscillation.   Taking this a step further, we can look at the various frequency components of the RA tracking errors in the mount.  This is what your mount looks like:

The worrisome thing here is the 2-3 sizable error components with frequencies in the 60-145 second range.  These kinds of errors are difficult to guide out because of their short periods.  Analysis of the other ZWO mount shows a different, more manageable pattern:

You can see these higher-frequency components are mostly absent, so PHD2 only has to deal with the longer-period tracking errors.  In fairness, the comparison mount was using 0.5 sec exposure times so that's something you should try as well.

With a conventional mount, I would guess that the mesh on your RA drive system might be too tight, but I don't know how well that applies to the strain-wave mounts.  Hence, I think you should probably ask for some help from ZWO.

Good luck,

Bruce

[David Osojnik]

Thank you!

Will do some more testing (this time with 0.5s exposures and calibration) and try to talk to ZWO to see what they say and will report back if i can figure this out.

Could you tell me what was the settings used by the other ZWO mount at 0.5s?

thanks

David

[Bruce Waddington]

These are the settings but I don't think there's anything magic about them.  The min-move values, of course, depend on your image scale so you should set those based on GA recommendations.

Bruce