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Vega

EQ3-2 motor drive strange behaviour

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ok here's the deal....

When drift aligning using webcam and K3CCD tools (using the reticule and drift explorer tool). I find the azm adjustments can be fine tuned until the star doesn't move up or down' for a good 5 minutes, great. I then swing to a star in the East and fine tune the alt so it doesn't move up or down. However, I find in the meantime the star is drifting left quite dramatically! I'm presuming a drift left means the azm is out again (Should't this already be aligned and not really moving left or right too much?).

Anyawy the real strange thing is, I moved back to a star in the South on the meridian and re-checked my azm and it was almost bang on as expected. Even worse was I found the star moving downwards for a shortwhile then back up! Drift explorer shows a u turn of about 5" ! The movement up and down is 'waving'. Now logic tells me that this is my R.A motor not keeping a consistant speed. :D

I've checked tube balancing and also that the motor drive bracket and sleeve is secured tight to the flat side of slow motion control thingy. Obviously the movement is too small to affect visual observation but it's going to make my stars somewhat 'fatter' in my images. Do I just have to live with it ?

{desperate sounding voice} Auntie FLO... any news on that GOTO upgrade for EQ3-2 ? :D

Matt

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I find in the meantime the star is drifting left quite dramatically!

I think you have to tell us what "left" means. Is it up/down, east/west?

I found the star moving downwards for a shortwhile then back up! Drift explorer shows a u turn of about 5" ! The movement up and down is 'waving'. Now logic tells me that this is my R.A motor not keeping a consistant speed. :D

You have just discovered Periodic Error, I believe. If that's 5 arc-seconds, that's pretty damn good.

See the EQ6 wobble 30 arc-seconds here (in french) and an EQ3

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I think you have to tell us what "left" means. Is it up/down, east/west?

I believe its West. (diagonal almost confused me then!) While using a star in the East at approx 20deg.

See the EQ6 wobble 30 arc-seconds here (in french) and an EQ3

Thanks for that, I feel a million times better. I can't be sure on how many arc-secs the wobble was as my initial alignment was out anyway. From the start it went down to -20" then back up but cant honestly remember what it went back up to (definatly below -8" so a wobble of no more than 12").

Matt

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A drift left, using the K3CCD Tools method is a drift in RA. This is due to a lack of tracking accuracy, or more likely the adjustment has made the star move. After completing the adjustments it should stay where it was for ages, but any adjustment of the 'scope position using the Alt. Az. or RA / Dec. motions will move the star off the cross hairs. It's just how it works.

The up down waving is periodic error and it's caused by all sorts of things. The usual things are that the worm is not absolutely centred on the shaft (usually by 1/100th mm ish so no sweat), a cog hasn't got the shaft hole in the centre etc.

If you time the wave, you can investigate the cause more thoroughly. A wave which takes the same time as the worm turns through 1 turn would point at the worm, other stuff turns at different rates so you can measure stuff and point your finger at the culprit.

5 arc seconds of PE is something a lot of people aim for, not something to tear your hair out over.

Captain Chaos

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The wobble, "waving" or periodic error should be affecting the East-West direction, around the RA axis. It's whatever is making the scope rotate in the RA axis not being able to push the scope around evenly = sometimes it pushes too much sometimes not enough. If there's wobble in Declination, I would guess that something is loose or bearings are worn.

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Just to clarify what I posted above. By "up down waving" I meant the graph that K3CCD will draw. The PE will, as Themos points out, show up in the RA plot only, unless you have lumpy bearings in the RA axis.

Captain Chaos

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Thanks all.. I'm eager to set her up again and see just how big that wobble was.

Astronomy and directions... such a confusing subject.... then we go and throw mirrors in the works! :D

Matt

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