Help with PHD2 Guiding Graph Please

[Barry]

Hi

Can anyone help me with the cause of the obvious problem with the second of the guiding graphs below?  They were two days apart but with the same equipment and the first outings with this guider. The stars in some subs in the second one were slightly worse than the first (when other causes such as no flattener are taken into account) but not terribly worse by my standards. I think that the balance was OK and PA was with the polarscope followed with Sharpcap. Everything was as tight as possible with a finder guider, with the guide camera and adapter screwed on to the finder body. The exposure is 0.1 sec in the first but 0.5 sec in the second, in an attempt to even out the guiding. The mount is a Celestron Advanced GT and the guiding with a finderguider/QHY5LII.

The graphs are more squashed in widthways than displayed by PHD.

Thanks

Barry

 

[alacant]

Hi. My interpretation: DEC has backlash. In the first session you had the telescope balanced better. In the second graph, when it does finally reverse the direction, it overshoots -as if the gears are sticking and then released when the torque of the guide pulses build up to such an extent so as to overcome the friction.

To be able to use 0.1s frames you'd need very good seeing indeed. Try starting at say 2s and running the guiding assistant for at least 5 minutes. Then take the time and all the min-move settings it suggests.

But remember to look at the photos, not the graphs. Just my €0.02. HTH.

[wimvb]

I don't use phd (I use lin_guider), but read this guide for trouble shooting.

http://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

Maybe it can be of some help. A graph similar to yours is in the examples.

Hope this helps

[libraryman]

Dec backlash

Ray

[iapa]

ignore - thought was a reply to a similar post I'd made

[Tommohawk]

I agree with dec backlash - but could probably be minimised by ensuring balance is definitely over to one side. If too finely balanced, it will slop across, the creep back then repeat.

[Barry]

Thanks all, that makes sense as the calibration started with quite a number of backlash corrections.  Looks like a stripdown and lubricate/adjust is needed.

 

[Tommohawk]

Just now, Barry said:

Thanks all, that makes sense as the calibration started with quite a number of backlash corrections.  Looks like a stripdown and lubricate/adjust is needed.

 

Not necessarily - some backlash is inevitable unless you have a perfect mount. I'd suggest try leaving it deliberatley not too finely balanced and see what happens before you get your hands dirty.

[Barry]

The mount has a backlash compensation routine in the menus; the mount is secondhand so I don't know if anything has been set so I will look at this first. 

The first session did not show this pattern so it could be that a slight imbalance might help.  I will give it a try when we next get a clear night.

Barry

[alacant]

13 hours ago, Barry said:

a slight imbalance might help

Let's say you go north heavy. phd2 issues south pulses. It responds to the pulses and then swings back north due to the backlash and imbalance. If you're balanced, it will respond to the pulse and stay there. Your best bet would be to polar align say 2º from the pole and guide north -or-south- only. Just my €0.02. HTH.

[cfpendock]

43 minutes ago, alacant said:

Your best bet would be to polar align say 2º from the pole

My suggestion too.  Maybe 2 degrees is a bit high?  

Chris

[iapa]

1 hour ago, alacant said:

Let's say you go north heavy. phd2 issues south pulses. It responds to the pulses and then swings back north due to the backlash and imbalance. If you're balanced, it will respond to the pulse and stay there. Your best bet would be to polar align say 2º from the pole and guide north -or-south- only. Just my €0.02. HTH.

Do you not then run the risk of some trailing with longer exposures the stars and your mount ar no longer rotating around the same point?

[cfpendock]

No.  The idea is that a small amount of mis-alignment can be dealt with by PHD.  But the corrections will only be in one direction, thereby eliminating any backlash that direction.  But you will have to establish which direction, by guiding north or south only.

Chris

[alacant]

3 hours ago, iapa said:

run the risk of some trailing with longer exposures

Unless you are snapping -very- close to the pole, I think you're gonna be OK. HTH.

[alacant]

4 hours ago, cfpendock said:

Maybe 2 degrees is a bit high?

Polar align using a polarscope. That should put you far enough away;)

[Tommohawk]

At the risk of being pedantic, can I just elaborate a couple of points. Happy to be corrected though if anyone knows better!

There are three separate but related problems with most mounts.

1. The gears don't mesh perfectly so there is a bit of "slop" in the drive - this is backlash.

2. The gear train doesn't like to be driven with the weight following - it likes to be driven against a resistance and is much smoother being driven this way. If you dismantle the gear train and try rotating the mechanism by hand this can be felt quite obviously. I cant in all honesty explain quite why this is ... but it is. I dont know what this error is called, maybe someone has a name for it. Call it "error 2".

3. The drive speed may vary as the gears rotate due to eccentricity of the gear axes - periodic error.

Backlash can be eliminated by ensuring you are weighted to one side and not too finely balanced. Because of "error 2", RA is usually biased to be East Heavy. This means that although the command speed will vary according to the guiding pulses, it will always be driving in the same direction, and against some resistance.

DEC is a bit more tricky - in theory with good polar alignment, DEC guiding could be switched off - in practice I think most folk leave it on. With normal polar alignment, DEC guiding pulses could come from either direction, north or south, so it's not possible to ensure you are only driving one way.

By deliberately misaligning N/S, corrective pulses will only occur in one direction - BUT to avoid "error 2" you will still need to ensure there is DEC imbalance in the correct direction. Now TBH I dont know which way that would be - if you misalign to the north, presumably correction will always be to the south, so being north heavy should do it.

So far as I can see, periodic error should be controlled by guiding, so long as is it isn't too severe, and isn't so great that it completely reverses the drive direction.

Take home message is, go East heavy, consider misaligning NS deliberately as advised above in previous post, but be aware that direction of DEC imbalance is significant.

Hopefully that sounds about right?