New CEM70-EC Mount Tracking Challenges

[Michael Griffin]

Hello,

I have a new iOptron CEM70 EC mount where guiding averages about 1" RMS despite a couple weeks of troubleshooting to improve. Stars are oblong in a 5-minute guided exposure. I should be getting closer to 0.5" RMS. I'm relatively new to this hobby so can use some guidance as I've exhausted what I can find online or in the documentation. 

Here's a link to my logs last night:

https://openphdguiding.org/logs/dl/PHD2_logs_3qTV.zip

Log Details (green sections, running at/near default recommendations) 

Section 1-6  - Polar aligning and calibrating. Steps noted below. 

Section 8 - A 10-15 min unguided session

Section 9 - Guiding OK, not great. At end of section, I decided to fiddle with MinMove and Algorithms because I was trying to get DEC not to overshoot as it was bouncing above and below the line. Failed.

Section 10-15 - Fiddling. Tried resist switch to counter DEC overshoot. Failed. 

Section 16 - Reran Guiding Assistant, reset settings to defaults

Section 18 - Guiding with defaults and new MinMov recommendations 

Section 19-22 - Guiding and making small adjustments to Aggression and/or MinMove

Section 23+ - Guiding at default/recommended settings for the rest of evening

Steps:

1. New PHD2 Profile. Changed camera to 16bit. Changed ADU Saturation to 65535. Selected Encoder option. Note I only have an Encoder in the RA axis...it's not a dual encoder.  Changed calibration steps to 40. 
2. Auto Focus
3. Guide Camera Focus adjustment trying to get lowest HFD on guide star
4. Polar aligned with iPolar, then 3 star in Nina.
5. Calibrate (section 1 in log). Clear backlash first. 
6. Drift align - reported back as less than 1 arc-min. 
7. Re-calibration - Move North (up button) for 20 seconds at 16X speed before starting to clear backlash. 
8. Guide Assistant (section 8) - Move North as above. Ran for 10-15 min. 

Questions

1. Should you re-run guiding assistant after changing algorithms, exposure times OR delay times?
2. Am I running the right algorithms despite having an encoder in RA only?
3. I've tried various exposures from 1-4 seconds and delays from 1-5 seconds, with little change to RMS. Should I be making more or less frequent measurements and adjustments? 
4. DEC Backlash compensation was turned off, but also Guiding Assistant registered it as small. I tried activating on prior nights with no real improvement. As long as I clear backlash before calibrating or guiding assistant, it appears I don't need it. 
5. Do you see any evidence for any mechanical issues I should fix? 

Thank you in advance for your help!

Michael

[Bruce Waddington]

You shouldn't have said 'yes' to the high-res absolute encoders question in the profile wizard.  That prompt has already been changed in the dev releases to clarify that we mean encoders on both axes.  However, you're saying you do have such an encoder in RA, but the GA run you made doesn't show the kind of precise tracking we would expect from that:

In particular, look at the 5 arc-sec tracking error that occurred during a 30-sec period as shown by the arrow.  You haven't said anything about your overall setup - for example, can we assume you're using an OAG on the main scope?  What gear is riding on the mount?  Can you be certain that nothing in the payload on the mount is moving around to produce this kind of error?  Were you operating in windy conditions?

Looking at your longer guiding sessions, it looks to me like you had poor seeing conditions.  Remember that the Dec drive is idle except for the very short, small guide commands.  Here's a look at a typical sequence of Dec behavior:

Ignoring the two dithering events, none of these excursions were caused by guiding.  This leaves us with possible contributions from poor seeing or shifts in the payload on the mount.  You mentioned that you are getting elongated stars with 5-minute exposures, yet the guiding statistics - even allowing for the unexpected excursions - suggest you should be getting round stars.  The stars might be larger than desired but they should be round.  This again points us back to what's riding on the mount and where there might be unwanted movement.  If you were using a separate guide scope, it would point to differential flexure.  I think we really need to get a better description of what your setup really looks like, a photo would probably be helpful.

For now, I think you should treat this as a medium-performance mount and guide it accordingly.  Create a new profile with the profile wizard and use the default guiding algorithms and parameters - hysteresis for RA and resist-switch for Dec.  Use 2-second guide exposures to start and forget about using the variable delay feature. When you do a multi-minute GA run, use the exposure time you plan to use for imaging - i.e. 2-sec - and set the minimum moves according to the recommendations. When you do a calibration, always use the Calibration Assistant because, among other things, it will clear the Dec backlash for you. Don't start fiddling around with guiding parameters because those aren't the sources of your issues.  But you also need to be very aware of seeing conditions especially considering the image scale of your guiding setup.  I don't know where you're located but northern hemisphere weather conditions have been very difficult over the past couple of months and seeing has often been terrible.  In my personal experience I've seen my total guiding rms range from under 0.4 arc-sec to well over 1 arc-sec in the space of a couple of hours, with no repeatability from night to night.  Those aren't guiding or tracking problems, they are simply the consequence of poor seeing conditions.  Pay attention to the location of the jet stream and use the various seeing forecasts to set your expectations - I use MeteoBlue for this.  If you can capture data on a night of better than average seeing, you may get a very different picture of how your mount is doing.  The two easiest ways to get a rough measure of relative seeing are to watch the Dec oscillations during a GA run and to start keeping track of the star diameters you get in the main scope for 10-sec exposures.

Good luck,

Bruce

[Michael Griffin]

Hi Bruce. Thank you for the thorough review and advice. 

Here's a link to a video showing the setup: https://share.icloud.com/photos/096gRjcUILlmQHuvZsZ7sOewg

It's a Esprit 120 OTA on a CEM70EC mount with a ZWO filter wheel, ZWO OAG-L, ASI2600MM, Optec SWX30 focuser, DSD Flat Panel and Eagle5Pro. Two cables are hanging off, but don't seem to affect the 3D balance. Everything else is temporarily taped down. I'll see if I can find anything that might be moving. 

The conditions were not windy during the period I sent the log and seeing was reported as average. Is there a way to translate the contribution to RMS from seeing conditions as reported by MeteoBlue? For example, if they are reporting 1 arc-sec, how do I interpret the contribution? Or  can I use the high frequency star motion as a guide? 

My next clear night I'll try out the strategy you recommended and report back. In the meantime, let me know if anything from the video gives you pause. 

Thanks again,

Michael

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[Michael Griffin]

I wanted to post my results and what worked in case anyone else has a similar problem. The team over at Telescopes.net suggested I try to improve polar alignment even better than the 1 arc-min I was getting with drift alignment. They suggested I give SharpCap a shot. Doing this alone, improved my RMS to 0.5-0.6. I can even track unguided and get decent results. I probably could have achieved similar results with drift align, but found this was an easy option. Verifying results with the guiding assistant showed alignment error of 0.0-0.2 arc-min. 

I then implemented the suggestions Bruce made which improved guiding results a bit more. PHD2 ended up suggesting LowPass2 for Dec after running the guiding assistant. 

Additionally, I'd suggest not underestimating the importance of getting the best focus you can on the guide camera. With a filter wheel and filter offsets, it's a bit challenging but sharp stars equal better tracking on this mount under my current Bortle 9 Los Angeles skies. 

Finally, one PHD2 enhancement I'd love that could help: It would be great if the RMS stats could ignore a preset period after dithering so I can get a cleaner read on performance while imaging only.

Thank you Bruce for your help. 

[Bruce Waddington]

Good to hear you're getting better results.  The RMS statistics in both the real-time graph and with the LogViewer already exclude dithering.  The time window in which the guide star movements are ignored is defined by the settling criteria, which is something you control.  At the point there is a "settling complete" event, the RMS calculation resumes.  We think this makes sense - if you've told the imaging app it's ok to start the next exposure after dithering, then the statistical accumulations should resume.

Regards,

Bruce

[Michael Griffin]

Great to hear. I'll need to adjust my settling time then. Does PHD2 register the focuser settle time setting in the Autofocus settings of NINA too? I've discovered that my looping through filters, and subsequent focuser changes, is part of why my RMS is artificially high. 

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[bw_m...@earthlink.net]

PHD2 doesn’t interact with NINA in this way at all and it has no awareness of focuser or filter operations.  The overall architecture is that apps like NINA manage the imaging session and tell PHD2 what to do.  The dither and settling parameters are part of the NINA options and that’s where you specify things like settling time.  For something like focusing, the usual approach is for the imaging app to either pause or stop guiding, then resume after the focusing operations are complete.

 

Bruce

 

In particular, look at the 5 arc-sec tracking error that occurred during a 30-sec period as shown by the arrow.  You haven't said anything about your overall setup - for example, can we assume you're using an OAG on the main scope?  What gear is riding on the mount?  Can you be certain that nothing in the payload on the mount is moving around to produce this kind of error?  Were you operating in windy conditions?

 

Looking at your longer guiding sessions, it looks to me like you had poor seeing conditions.  Remember that the Dec drive is idle except for the very short, small guide commands.  Here's a look at a typical sequence of Dec behavior:

 

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