EQ6R - Tracking and guiding - any input appreciated

[Chickpea]

Hi,

I've recently purchased an EQ6R, having previously used an EQ5.

I am using a Skywatcher 200P, imaging with a Canon 400D and guiding with a ASI224 on the 9x50 finderscope.

After a few teething problems, I finally managed to get the mount polar aligned using SharpCap on Tuesday night and then ran the Guiding Assistant in PHD2. 

My initial objective is simply to check that there are no fundamental problems with the mount and then I am happy to take the time to fine tune everything.

I will describe what I did, and I would really just appreciate and observations and advice you folks might have.

I initially polar aligned with SharpCap, ending up with the following configuration.

I then ran an initial calibration in PHD2 and then ran the Guiding Assistant for 25 mins, and got the following output (I applied all of the recommendations).

 

As PHD2 was reporting a 4 minute PA error, I tried to fine tune the focus and then ran another PA in SharpCap. SharpCap showed this as the error before I ran the second PA, and this error once it was run.

 

 

I then ran Guiding Assistant again for another 25 minutes, and got the following output. As you can see it reported a much more significant backlash error. I applied the recommendations.

 

I then decided to get some actual data while imaging, so I pointed at the Tulip Nebula and ran about an hours imaging. The link below is a 4 minute video, showing APT and PHD2 during 40 mins of imaging, sped up 10 times.
https://youtu.be/kT1Ou_w6_CY

And finally here is the PHD2 log which covers the whole evening, including the calibration, guiding assistant runs and then the guided imaging run.

PHD2_GuideLog_2023-06-13_235005.txt

The RA rate and DEC rate were both set to x0.5 in the ASCOM PulseGuide settings (a bit difficult to make out in this screenshot which was taking just before the PHD2 calibration).

As I mentioned above, I just really want to check that there are no major problems and would appreciate any advice or observations. I would be especially keen to know if any of the numbers might suggest a problem with the mount and what steps I should take to further analyse and/or calibrate.

Thanks.
 

 

 

 

Edited June 15, 2023 by Chickpea
remove duplicate image

[SamAndrew]

Looks ok to me, but I think 3s is too long given the size of the corrections, I'd try 1.5-2s

[wimvb]

I don't think you can polar align more accurately than a few arc minutes. To move the mount by arcseconds requires  very smal adjustments of the alt and az screws, and virtually no friction between the mount and tripod/pier plate.

What looks like backlash in the GA, can in fact also be stiction. Slew the mount and listen for changes in the sound it makes. If you can measure the current that the mount draws, you can check for any changes while slewing. A binding mount will draw more power than a smooth going mount. Backlash lower than 1 s (1000 ms) isn't much of a problem. If it's much larger, you should adjust the worm gear.

Your RA corrections are small, but random, which is an indication that you are chasing the seeing. Try guiding with a lower aggression next time (30%), and increase if needed. Seeing differs between nights, so what works one night, might be less than optimal another night.

The EQ6-R can do 0.25" rms guiding with an 8" Newtonian, but only under ideal conditions and near zenith. Depending on where you point the scope, rms can be 1" with the same setup. So, don't chase small numbers. As long as you get tight round stars, enjoy your time in the dark.

Edited June 15, 2023 by wimvb

[Chickpea]

Thanks to both of you.

I will drop the exposure and the aggression as you both suggest.

I had clear enough sky last night that I was able to run GA again and also get an extended period of guiding, so I will post up the results of that, but I believe that the reported backlash was ~1500ms.

I am running the mount from a bench supply which does show the current draw so I will certainly trying slewing to see what results I get (both in sound and current).

I think there is certainly plenty that I can do to fine tune the guiding and will start with your suggestions.

The backlash/stiction is the thing that potentially bothers me at the moment, as that is mechanical rather than guiding configuration, so I just want to make sure that it is within acceptable limits for the EQ6R. The numbers I have so far are 949ms, 4649ms and ~1500ms, so I have only gone under 1 sec once and only just under. I will post the results of last night plus the slewing noise/current as soon as I can.

 

[wimvb]

1 hour ago, Chickpea said:

The numbers I have so far are 949ms, 4649ms and ~1500ms,

Because the worm gear isn't perfectly round, backlash and stiction will vary with where you point the scope. When you run GA, try to always do it in the same area, eg on the same side of and near the meridian, and near the celestial equator, but at least 40 degrees over the horizon to minimise atmospheric effects.

[michael8554]

For years the PHD2 guys said Polar Alignment can be no worse than 5arcmins to get good guiding.

Recently after years of improving PHD2 they are quoting no worse than 10arcmins.

So you didn't need to worry about a 4 arcmin error.

Did PHD2 flag it up as a problem in the GA run ?

No.

It would have if it was too much.

It did flag up exposure, focus, and Dec Backlash.

From the GuideLog.

There was about 25arcsecs of Periodic Error in RA.

There are regular spikes in RA, may be dirt or grit at one point on the worm.

 

Apart from those spikes, RA and Dec guiding looked to be under 1 arcsec.

Focus actually got worse as the night went on HFD = 6.72pixels !

Use the PHD2 Star Profile window to reduce that to a minimum.

Michael

Edited June 16, 2023 by michael8554

[Chickpea]

Thanks Wim and Michael.

I managed to do a further run last night including two guiding assistants and ~90 mins of guiding.

I did change the exposure to 1.5 secs before doing the first GA, and also set the RA and DEC aggression to 30 as previously recommended.

I ran GA once, it recommended a recalibration, which I did, but did not refocus before running another GA. The DEC backlash was reported as ~8500 in both runs.

The GAs and the calibration were run in this sky location

The final results of the GA were

Here are some snapshots of the guiding

And here is the log

PHD2_GuideLog_2023-06-15_235602.txt

Michael - the analysis you did on the previous log file is very interesting and I really appreciate it.

I note the RA PE error. Is that something that I would want to change physically (i.e. remove possible grit), or could that be trained out?

I don't believe I have had PHD2 complain about the PA at any point. SharpCap reports a sub-arc-min error, but I am not sure how reliable it is.

Is the HFD value based only upon the selected guide star, or all stars that are in view? I ask this because where guiding last night there was one very bright star in the view (please see screenshot below) and on the night that you analysed the log there were two bright stars in the few, fairly close to the guide star.

When I use the focus assistant in SharpCap I get multi-star FWHM values in range of 3 to 4 (I don't think I have ever gotten below 3).

Is there any possibility that the focus may be impacting the reported backlash values, by messing up PHD2's measurements?

I'm disappointed that there we both approximately ~8500ms. Am I correct in thinking, that value means that when PHD2 sends a guide pulse, it believes that it is taking 8.5 seconds for the mount to respond?

I took some 20 minute exposures, which to my untrained eye don't look too bad, but the stars in the centre of the image are definitely not perfectly round.

I have also taken a video of the mount slewing, which includes the sound and also the current that the mount was drawing, which I hope to post later today.

Thank you again for all the help and advice so far.

 

 

 

 

Edited June 16, 2023 by Chickpea

[alacant]

Hi

If you've ever dismantled a new sw mount, you'll know of the horrors within; performance issues will become less of a mystery. When you've reached the stage where you're still not satisfied with the images...

Suggestions to move on from here:

- clean, re-grease and replace the cone bearing at the base of DEC taking care to pre-load it with just sufficient torque that the axis spin smoothly. That may just do it. Even better...

- dismantle the whole of DEC, clean, inspect, smooth -very fine emery- where necessary and replace the bearings with known performance equivalents. SKF are a little more expensive but more likely to be within specification than those currently in place. 

- Still no good? Repeat for RA.

The dismantling and bearing part number details are covered here. The only gotcha we'd add on newer 6Rs is to place the axis shafts in the freezer for an hour or so before attempting to refit through the new bearings. The worm shafts are not however interference so are easily pushed into -even SKF-  608s at ambient.

Cheers and HTH

Edited June 17, 2023 by alacant

[michael8554]

Many of your questions suggest you haven't read the PHD2 Instructions, or haven't understood them ?

1)  Dec backlash needs at least a worm adjustment, ideally alacant's stripdown and bearing replacement.

2) "I'm disappointed that there we both approximately ~8500ms. Am I correct in thinking, that value means that when PHD2 sends a guide pulse, it believes that it is taking 8.5 seconds for the mount to respond?"

The Dec axis has Backlash, so it could wobble from one end to the other of the backlash range.

Once it's wobbled to one end it may take 8.5secs to move back to the middle.

The PHD2 Backlash Comp can add some extra big pulses to help that along.

But your Dec backlash is too large, so PHD2 won't do that, as there is then a danger of over-shooting into a repeating pattern of back and forth wobbling.

3) The first GA run reported you hadn't cleared that Dec backlash before Calibrating.

PHD2 had to supply 13 pulses before Dec started moving, which marred the Cal.

If you'd cleared backlash before Cal that wouldn't have happened.

4) "I note the RA PE error. Is that something that I would want to change physically (i.e. remove possible grit), or could that be trained out?"

PE can be Trained if the mount has Permanent PE Correction.

Strictly speaking the grit is not a PE error, it's causing a momentary spike that would be present even if the PE was perfect.

5) AFAIK the Star Tool is measuring the primary guidestar when MultiStar guiding is in action.

PHD2 will select the best star(s) to guide on.

Not necessarily the "very bright star", that is probably over-exposed, so has a flat top, instead of a sharp peak.

That's why PHD2 is asking you to improve focus, so that star shape is as "pointy" as possible.

6) in the graphs you inserted, RA error is about twice that of Dec eg RA = 0.61arcsec, Dec = 0.38arcsecs.

So stars may be elongated in the RA direction.

But to an extent will depend on your imaging pixel scale.

If your imaging pixel scale is much bigger than the guide errors, then the guide error is smaller than one pixel, so may not show.

6) You don't need to run the Guide Assistant for 30 mins, most of the time the suggested 2 minutes will give enough info.

You can PA with SharpCap, and check in the GA run that's it good enough.

Bottom line is that despite the large backlash, lucky for you your setup is guiding Dec well.

It's RA that needs looking at, the grit and the PE.

Michael