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Guiding error.. ?

LaVey

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LaVey

LaVey

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Hi guys...

What a lovely night last night, set up in the daylight.. then went out as it got dark and was there till 2...

Got about 45 mins of 5 min exposures on M31 and all looked good (apart from the orange glow!).. then kept getting odd 'shifting' in some subs, although not every one after this, but getting more frequent. Below is a couple of shots of PHD tracking, the mashed sub and also my PHD log... Ended up throwing out 10 5 min subs.. and only an hour and 5 mins kept, oh the PHD log is after a forced recalibration..

Is this something to do with the way I setup.. or a limitiation of the CG5 ?

Any help/ideas appreciated...

post-14196-133877398171_thumb.jpg

post-14196-133877398178_thumb.jpg

PHD_log_12Sep09.txt

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LaVey

LaVey

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Cables all good and loose, on grass - so no vibrations, think my balance was ok. Not sure if this helps but here are a couple of graphs produced from the PHD log... looks suspect to me as this is a regular pattern - but what could cause this ?

post-14196-133877398188_thumb.jpg

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David Galvin

David Galvin

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If you decide to strip the mount down for cleaning then you may find this site useful. Before you do this, it may be worth checking that the small grub screw that locks the gear wheel onto the motor shaft is tight. This tightens against a flat face on the shaft and can, over time, work loose. This would show up as regular spike on the P.E..

Worth a quick look first before you get the grease out.

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samtheeagle

samtheeagle

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LaVey, sounds like we had very similar sessions last night :) I was doing the same as you, getting some images of M31, and every so often the DEC axis tracking just went crazy. It just started off in a direction and never came back. Not quite as sudden and abrupt as your graphs, but still plenty to ruin the subs :) Unfortunately I didn't have the logging turned on so I can't look into it any further today.

On the whole the guiding looked good, sticking around the centre line of the PHD graph, and the RA axis seemed to be well behaved, but the DEC, well that was another matter! Does this sound like a purely mechanical thing? That's my gut feeling...

Can anyone elaborate on the different DEC guiding algorithms of PHD? I was using "resist switch" last night.

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LaVey

LaVey

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thanks all.. didnt make any funny noises last night.. but sure I heard a funny grind on the last session.. but it was a quick noise and dissapeared.. thought I imgined it... maybe not :S

thanks for the link david.. the first part of that post reads

' If nuts, bolts, set screws, worm gears, Allen wrenches, and retaining rings make you nervous, you may be wise to find someone else to do the job.'

lol.. that'll be me then! oh well... i'll give it a go, gotta start somewhere - if only it were code to be debugged or a wireless system to break into :)

will keep you updated .. hopefully nothing is broke in there..

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David Galvin

David Galvin

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Tthe red line on your graph shows regular spikes and is for is for the dec drive corrections. If your polar alignment is good then you should not require the dec to be on. So my point is what happens when you turn off the dec motor? Does the tracking cope with R.A only?

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LaVey

LaVey

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hmm.. i dont have a small screwdriver long enuff to get the damn plastic housing off to start with - fail. lol

david - you say i do not need dec on if polar aligned well... can this be turned off then ? didnt know i could do that...

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LaVey

LaVey

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just been having a read around the forums and internet.. the CG5 doesnt have PEC... but from what ive read if i'm autoguiding this shouldnt matter? Is this PE always gonna be there because of the way the gears are machined or can a refurb/grease up sort this out? Is this mount not really suitable for long exopsure photography? Should I take this down to DHinds for a refurb or should I save for a HEQ5/6?

I'm pretty confused :S lol

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samtheeagle

samtheeagle

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David, you're totally right! The thought hadn't even crossed my mind that with good polar alignment I shouldn't even need to guide on the DEC axis... Next time I get a chance to do some imaging I'll guide in RA only and see what happens. Thanks for the idea :)

LaVey, to my knowledge a regrease will improve matters, but you'll always be limited by quality of the mount. Probably the most often quoted comment on these forums is "get the best mount you can afford". You get what you pay for, and the higher value mounts bring higher quality which will really help with imaging. I'm using an EQ3-2 mount atm, and it's adequate for my needs, but of course I realise it has it's limits. It's certainly useable, and I've got some really nice 5 minute subs out of it, but of course an EQ6 would be better :)

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David Galvin

David Galvin

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The CG5 is very similar (clone) to the Vixen SP and GP mounts which I have used over the years. I have auto guided these with AstroArt3 but have never used PHD. I think, it is safe to say, that the general principles are the same.

Your scope may not have PEC (Periodic Error Control) but it will have PE (Periodic Error).

All motor and gear systems have errors that cannot be avoided although they can be reduced in manufacture. This of course adds to the cost of the final product. Generally you will pay for this precision and would appreciate the accuracy of the tracking of the mount.

Basically, the R.A motor drives the shaft which has a gear wheel attached to it, this meshes with a second gear that drives the worm which turns the larger R.A wheel. Each of these will have mechanical error and play. These errors manifest as a back and forth motion in the eyepiece and can be seen under high magnification.

Yes these errors can be improved by re-greasing and adjusting the meshing for a better fit but this would be down to the skill and patience of the person doing the job!

The mount is reasonable for its price range but maybe you would be better doing shorter exposures that may show less error ie 1 or 2 minute subs. Stack enough of these should show good results.

The software PHD is used to monitor and guide on a star whilst you image through another 'scope or camera. If the star drifts to the West or East then a signal is sent to the R.A motor to bring the star back on the cross hair. As far as the imaging system is concerned it has not moved and allows the light to build up on the pixels.

The R.A motor is running continuously through the imaging/ observing session and has to be be monitored constantly by the software.

The Declination motor, if fitted, is used to a much lesser degree. Its main use would be to centre an object in the field of view, once done it would not be needed if the telescope was aligned perfectly on the true celestial pole i.e not just on Polaris.

Poor polar alignment manifests as a star, on the cross hair, drifting North or South. If this is bad then the software will constantly send signals to the Dec motor to bring the star back onto the cross hair.

Maybe somebody on the group could analyse the image and work out if the drift is in Dec or R.A. This would give us a clue to what is happening.

Time to rest and have a think.

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LaVey

LaVey

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wow.. thanks David, appreciate the time you put into awnsering this for me, could be my polar alignment was off then. Will have another bash when the clouds decide to play ball and see what occurs.

Many thanks again to all in helping me climb this learning curve :)

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David Galvin

David Galvin

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I am sure that you will both get there, it does take time. I always think that it best to learn from the mistakes made by others and not to reinvent the wheel. Have a go at using the search facility on the forum and try key words such as PHD, you will be amazed at the useful stuff that you will stumble upon. Never be embarrassed to ask a question or ask for advice as there are many well learned people on this group who will be only to glad to help.

Keep us all posted and show us your results.

Regards from Dave.

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