Poor RA guiding

[chrisund123]

Any ideas on why my RA guiding is so much worse than my Dec?

HEQ5 pro, belt mod.

[michael8554]

Very typical.

Dec usually needs little guiding, if PA is good, and Dec Backlash is minimal.

RA has to cope with Periodic Error, which a belt mod can reduce, but sometimes worsen if the belt tension and worm adjustment are not correct on reassembly.

And less than optimal guide settings.

A screen shot only shows you have a problem, need the GuideLog to see why you have a problem.

Michael

[rnobleeddy]

I don't it's possible to tell based on a graph alone. I don't consider myself that knowledgeable, but I can't see any reason why you should necessarily expect them to be similar, although obviously that's be ideal!

I'd guess you have good PA and so very little correction is needed in DEC. 

For RA there are two options - why is it mechanically worse (imbalance, play in the gears) and then separately, can you make the guiding better without touching the mount.

All of your RA corrections take two or three pulses to correct. I'd probably trust the guiding assistant over me, but Id start by dropping from 3s to 1s exposures and see if things improve.

[Clarkey]

Looking at the graph the system certainly seems to be oscillating in RA. As rnobleeddy has pointed out running the guiding assistant is certainly a good option. Reduce the aggressiveness of the guiding response would probably help.

Also, make sure the FL of the guidescope is correct in the settings.

[The Lazy Astronomer]

What's your imaging scale? And how do your images look? Nice round stars or noticeable trailing?

What I'm getting at is if your images are good, don't worry too much about the graph. 

[david_taurus83]

You've got almost 4 arc seconds peak to peak so it definitely worth trying to smooth it out. How is the balance of everything? What guiding rate are you using? Have you tried the PEC algorithm in PHD?

[ollypenrice]

1 hour ago, The Lazy Astronomer said:

What's your imaging scale? And how do your images look? Nice round stars or noticeable trailing?

What I'm getting at is if your images are good, don't worry too much about the graph. 

Round stars only tell you that the level of error is roughly equivalent in both axes. You can still be losing resolution, even a lot of resolution.

That graph would be consistent with a mount with backlash falling to one side of the mesh and then being pushed to other by the guider, only to fall back again, and so on and on. However, it would be consistent with other problems as well. Have you tried the usual trick of running slightly east-heavy in order to encourage the mount to stay on the driving side of the mesh?

Olly

[michael8554]

Each 2 arcsec error in RA is taking 2 pulses to correct.

Which then sends RA over to the other side.

Where another 2 pulses are required to correct, etc etc.

But notice the second pulse is always larger, which means PHD2 didn't see the expected correction from the first pulse, so then sent a larger one.

Too-high Aggression isn't causing the overshoots, it's probably stiction, or tightness, in the RA axis.

Need to see the GuideLog so that Calibration etc can be evaluated.

Michael

[chrisund123]

Thanks for the replies.

I have run the guiding assistant and I've been using the settings it suggests. I've got the focal length of the guide scope set correctly.

I'm imaging at 1000mm focal length, stars aren't terrible but are noticeably oblong if you look for it.

I have been trying to balance east heavy but it's not seemed to improve things. I'm leaning towards something binding a little being the issue, although things seem pretty much as free as the dec axis, so not sure where the problem may lie.

With that in mind I stripped down the RA axis just now to take a look and didn't find too much wrong with it, but the bottom tapered bearing did seem to have gotten stuck. I think when I last stripped it down maybe I did the bottom nut up too tight and that may have caused the bearing to be too tight? In any case I got the bearing spinning nicely, packed it with fresh grease and did the bottom nut up by hand.

I didn't see any issues with the worm drive, will adjust the backlash and see how things go when the weather permits!

[Pompey Monkey]

Post deleted

Edited July 1, 2021 by Pompey Monkey
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