Not Brilliant Guiding

[Stub Mandrel]

I've finally managed to access my PHD2 guide logs from my first time out.

Here's the "best" hour.

I know 1 arc-second RMS isn't brilliant, but is probably reasonable for a first attempt with an EQ3 tripod. My main scope and camera is 450D on 130P-DS with CC, which gives 1.85" per pixel

Surprisingly either I haven't got bad periodic error, or it's being masked by other errors or I can't read the graph! Should I be tweaking PHD2 settings or tweaking the mount (or both) to improve this performance?

To me it looks like most of the RA errors are not generating guide pulses. Does this mean I need greater aggression?

You can see huge backlash issues in DEC, sudden big excursions, then overshoot the other way! Not surprising as DEC was both sticky and a lot of backlash. I have stripped down, teflon greased and adjusted the DEC axis, and that's let me greatly reduce the backlash, I think I will see less stick-slip as well.

 

 

[kens]

You aren't getting many RA pulses because your pixel scale is 4.75 and you set MinMo at 0.18 which equates to 0.87" before a pulse is sent. But the pulses ar ethere - just much smaller than the dec pulses.

Some suggestions:

1. Post the guide log rather than a screen shot

2. Turn off auto exposure - PHD recommends it for when you use AO but otherwise a fixed expsoure is preferred

3. With that level of backlash guide in one direction in dec

4. Run the guide assistant and follow its advice

5. To find your periodic error you need to do an unguided run. For a rough idea you can use PEMPro Log Viewer with its option to subtract the guide pulses 

[Stub Mandrel]

Thanks.

I zoomed in and there definitely aren't any guide pulses until the error is >0.5", taht seems a bit high to me?

Here's the log file:

PHD2_GuideLog_2017-07-16_223641.txt

[SteveA]

As Kens mentioned...you are experiencing quite a high degree of backlash in the Dec axis. You can see this from the nature of the Dec axis graph where Dec is drifting and multiple correction pulses are sent without any noticeable affect on this drift, until finaly the backlash is cleared and the mount responds to these pulses. This is then followed by the same behaviour in the opposite direction. This "saw tooth" profile is very characteristic of Dec backlash. Take note of the guiding assistant...it is a useful tool.

Steve

[mikey2000]

I think you need to look at DEC adjustments too.  You can do this in daylight if you have something distant to focus on (several good EQ3 rebuild/strip down threads on this forum)

 

i try to minimise the need for DEC guiding on my eq3 by aiming for a knockout polar alignment.  It's worth spending the time.

 

are you using St4 or ascom pulse guiding?  I found ascom guiding with the multiplier set at 0.7 for RA and DEC gave me the best results.   I typically get RMS less than 1 arcsecond which is fine for my imaging resolution (but I still get occasional wild excursions in either axis which I put down to owning a "cheap" mount)

 

another avenue to go down is to simply ask the PHD2 chaps.  They have a google groups support forum.  Remember to upload your log files.   If you can post a session that includes a calibration and a lengthy guiding assistant run (at least 20 mins+) you will find them most helpful people.

[Stub Mandrel]

1 hour ago, SteveA said:

you are experiencing quite a high degree of backlash in the Dec axis

Yep, I picked that up from one of the tutorials, I spent  chunk of time on 'tuning' the DEC axis so hopefully this will be much better next time.

[Stub Mandrel]

39 minutes ago, mikey2000 said:

are you using St4 or ascom pulse guiding? 

I am using ST4 - pretty much the only option unless I write an Ascom module for my GOTO box, but that's on my 'only if I have to list'.

I could decrease the guiding speed to 0.5x sidereal, but I'm guessing that won't make a huge difference.

I've noted the comment on not using auto exposure - but an issue for me was that transparency wasn't great.

Also, I suspect my guide star may have been a close binary, it was the brightest one in the FOV but towards the end of the session I noticed it was slightly pear shaped - I guess that may not help...

[Stub Mandrel]

This is one of the calibration runs.

Doesn't look bad to me, the points seem to be evenly spaced. Should I rotate the camera to get them exactly N/S or isn't this critical?

[mikey2000]

AFAIK, the camera doesn't need to be aligned N/S.  As long as PHD sees nicely orthogonal lines, it works it all out for itself.   I'd give 0.5x guiding a go too.  I found 1.0x a little 'overenthusiatic' for my EQ3.  I suppose in RA, at 1.0x it's either on or off for RA.  At 0.5x its always on, just slow or fast.  Maybe this helps.

[RayD]

I've never aligned mine and don't appear to have issues.  I believe this is part of what it "learns" during the calibration process?

[kens]

Correct. During calibration PHD2 works out which way is "up" and also how many pixels a given guide pulse will move the mount on each of the axes. 

I had a good look at Neil's guide log. The calibration on that one was quite poor but the latest one above looks ok. I could also see the periodic error with PemPro log viewer which indicates around 60" p-p - pretty much what one would expect from this mount. 

The polar alignment isn't great either. It looks like about 1 degree. A bit hard to be sure though due to the backlash in dec. 

I'd suggest to sort out the polar alignment then run the Guiding Assistant in PHD2 and follow its recommendations.

[Stub Mandrel]

6 minutes ago, kens said:

The polar alignment isn't great either. It looks like about 1 degree. A bit hard to be sure though due to the backlash in dec. 

That's interesting, I usually get good PA as I can track unguided for 2 minutes. I suspect atht with all teh extra cables and considerations that I knock the mount out of whack somewhere between aligning and guiding. Next time out I will check.

[kens]

Its possible the drift I saw was an artefact of the backlash corrections. PemPro log viewer can remove the guide pulses to show the underlying PE and PA drift bit when dec backlash is bad, especially if in one direction, it shows up as a steady drift that looks like PA error. The only way to really check is with an unguided drift for several worm cycles.

[ollypenrice]

Regarding the Dec backlash, I take the opposite view to Mikey 2000 who recommends an excellent PA. I would recommend a significantly inaccurate PA (but not enough to cause field rotation) and guiding active only in the direction needed to correct that error. This will stop the oscillation across backlash. We've used that method here on worm and wheel mounts.

While it is easy to fight backlash with east-heavy imbalance in RA it is less easy to do so in Dec. You could be camera-end heavy, say, but when pointing at the zenith this would reduce to zero imbalance. It might be possible to obtain some magnetic weights which you could easily move to make one side of Dec heavy in any orientation.

Olly

[Stub Mandrel]

Thanks Olly, I had considered the 'elastic strap' option :-)

For the second run I weas pointing up at about 75-80 degrees and DEC went haywire.

[ollypenrice]

1 hour ago, Stub Mandrel said:

Thanks Olly, I had considered the 'elastic strap' option :-)

 

You wouldn't be the first. I've tried it myself in desperation, before moving entirely away from worms and wheels!

Olly

[mikey2000]

I bow to Olly's greater experience.   I did once plan to try out this kind of PA offset but seemed to have forgotten to try it out (probably in all the excitement of discovering the PA routine built into SharpCap :-). )

 

maybe I should have spent the summer nights practising that kind of thing instead of desperately grabbing LP filled subs on not-quite-dark evenings :-)   

 

So, for a PA offset to encourage one-direction DEC guiding, how much of an offset is necessary/recommended ?   I can see how there might be a practical maximum (if the drift rate is higher than the max guide rate for instance).  

[Stub Mandrel]

I looked at all my log files. Most of my sessions so far have been limited /spoilt by poor DEC guiding. This was my best log though, I've excluded the ten minutes at the end when I started fiddling and spoiled it, 1hour 30 minutes for an RMS of  0.92" in RA and 0.69" in DEC. The challenge now is going to be achieving this repeatably! The guiding after this was U/S because the guide scope misted up.

Actually I think this was a practice session where I didn't take any photos

[mikey2000]

1hr30 on your EQ3 sounds good to me! 

Over the last few weeks, I tried the PA mis-align unidirectional DEC guiding idea.   It only needs a very small misalignment (just a few arcsecs) and then using PHD2 you can see a definite trend in the DEC drift.  Then I set it to only guide to correct in one direction.

 

Then one of two things happens... If the DEC guiding requires lifting the scope against it's own weight, it goes very well with only the occasional DEC jump/glitch.   But, if I get the misalign wrong and the corrections are effectively lowering the scope, I get generally good DEC guiding but some occasional *massive* glitches.  They are most entertaining to see  on the guide log.....  Working out this out in advance is mind-bending as 'slew N' doesn't always mean 'lift the scope' depending which side of the pier it is on.  'Slew N' merely means 'turn the Dec axis clockwise' (or is it anticlockwise....?????) (AAArgh!)  Then working out which way to tweak the PA to force the drift in the right direction is another puzzle, sort of anti-drift alignment

 

.92 in RA and .62 in Dec matches my best efforts on my EQ3.  I can get better RA results if I aim further from the meridian.   I asked skywatcher if there are any adjustments I can make to remove the annoying 19 second RA sawtooth but they said "no".

 

I think they want me to spend some money on a more expensive mount.   However, as I'm only imaging at 2" per pixel, I think my guiding is OK for now :-)

[Stub Mandrel]

18 hours ago, mikey2000 said:

Then one of two things happens... If the DEC guiding requires lifting the scope against it's own weight, it goes very well with only the occasional DEC jump/glitch.   But, if I get the misalign wrong and the corrections are effectively lowering the scope, I get generally good DEC guiding but some occasional *massive* glitches.  They are most entertaining to see  on the guide log.....  Working out this out in advance is mind-bending as 'slew N' doesn't always mean 'lift the scope' depending which side of the pier it is on.  'Slew N' merely means 'turn the Dec axis clockwise' (or is it anticlockwise....?????) (AAArgh!)  Then working out which way to tweak the PA to force the drift in the right direction is another puzzle, sort of anti-drift alignment

That sounds exactly the mind-bending problem I am working with!

[bobro]

This post is interesting as I have been working on the same DEC backlash problem with my recently acquired CG5. With my EQ2 (not much DEC backlash as not a worm drive but a paddle) I made use of dual axis dithering and wanted this to work with the CG5. Initially backlash was very large with the SkyWatcher DEC motor so I changed it for a NEMA14 stepper motor with belt drive. Although there is still significant (though reduced) backlash, I've found a solution that works by having a large MinMov DEC setting and very accurate PA. The large DEC MinMov setting allows for a reasonable DEC settling time after dither - it doesn't matter where it settles, as long as it doesn't move much during an image due to accurate PA. DEC error is typically in the 0.6-0.7 arcsec range.

I wonder if the single direction DEC guiding you guys are trying out will support dual axis dithering? I can't work out how it will - perhaps someone can explain.

As for CG5 RA guiding - let's just say another stepper motor is in the process of being fitted to the RA axis.......

[alacant]

19 minutes ago, bobro said:

support dual axis dithering

Hi. Yes, if your camera control software has the latest interface to PHD2. If not -like APT- you can use the PHD2 dither app.

**dithering RA only produces streaking but of course YMMV.

[bobro]

19 minutes ago, alacant said:

If not -like APT- you can use the PHD2 dither app.

Thanks @alacant. Note - APT supports dithering via PHD2 (this is what I use).

I probably didn't explain my question well - my question about dual axis dithering was more about settling. E.g. if single direction DEC guiding is being used with a PA offset to induce drift, this will require DEC MinMov to be smallish to prevent a large DEC sawtooth. When DEC dithers backlash will come into play, possibly making settling time very long (if at all) as DEC hunts around trying to settle within MinMov and possibly overshooting. Perhaps I'm missing something?

[alacant]

26 minutes ago, bobro said:

APT supports dithering via PHD2

Dithering both axes is then limited to PHD2 being set to DEC auto. Under APT, dither will be RA only if PHD2 is set for unidirectional DEC. HTH.