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RA Drift??

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#1 cvedeler

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Posted 08 July 2008 - 04:14 PM

I have an AP900 GTO2. I was working with it last night to try and get a good sample of it's periodic error to try out PEMpro. For whatever reason I was only seeing a peak to peak tracking error of about 3 arc seconds. Good news, I know. However in previous tests using K3CCDtools I was seeing peak to peak error of around 9 arc seconds.

Anyway I decided 3 arc seconds was probably too good to be true, so I swapped programs and watched the same star in K3CCDtools for 1000 seconds. The PE was indeed around 3 arc seconds, but I saw about 20 arc seconds of RA drift! This wasn't the typical sine wave PE type of curve, but a "my polar alignment is off declination linear movement drift" type thing.

So what can cause RA to drift in a linear way? Motors not tracking at sidereal rate. Checked that. Atmospheric refraction. I didn't think it would cause 20 arc seconds of drift in 1000 seconds, but I may be wrong. Tube flexure, mirror slop, guidescope differential flexure. None apply to my test as was testing with the primary scope, which is an AP160 refractor.

Any thoughts?

Thanks!
Chris

#2 cvedeler

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Posted 08 July 2008 - 05:25 PM

Ok, here is where I think I'm confused. I was always under the impression that Dec drift is always due to polar alignment error and RA drift was always due to tracking accuracy. After some investigation I'm thinking that RA drift can be due to polar alignment error too. Roland stated this in an email recently but I just couldn't get my head around it or past my previous assumptions. He said that RA drift was due to altitude error.

So it could be my polar alignment that is off. With some clear skies I may be able to test this theory. Perhaps sometime in October. :confused:

The good news is that my AP900 mount was then showing dramatically good tracking! 3 arc seconds peak to peak is remarkable without PEC on.

#3 rmollise

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Posted 08 July 2008 - 05:34 PM

Ok, here is where I think I'm confused. I was always under the impression that Dec drift is always due to polar alignment error and RA drift was always due to tracking accuracy. After some investigation I'm thinking that RA drift can be due to polar alignment error too. Roland stated this in an email recently but I just couldn't get my head around it or past my previous assumptions. He said that RA drift was due to altitude error.

So it could be my polar alignment that is off. With some clear skies I may be able to test this theory. Perhaps sometime in October. :confused:

The good news is that my AP900 mount was then showing dramatically good tracking! 3 arc seconds peak to peak is remarkable without PEC on.


Yep...listen to your Uncle Roland. Poor polar alignment can definitely be a cause of RA drift. We had an INDEPTH (excruciatingly so) go-round about this on SCT User a few years back.

:lol:

#4 Dean

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Posted 08 July 2008 - 05:53 PM

Yep, polar alignment can cause Ra drift (over 24hrs the tracking error will describe a circle)

It could also be the clutch slipping or a loose spur gear. I had a loose set screw on one of the spur gears on my AP1200 once that caused a similar problem, so it may be worth checking.

#5 rsarmiento

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Posted 08 July 2008 - 09:49 PM

also include flexure. As the scope moves towards the horizon, a small amount of flexure between the scope and the saddle is present specially on heavy setup translating to a small amount of RA drifting.

#6 Joe Cipriano

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Posted 09 July 2008 - 01:26 AM

Chris:

Peter Kennett's ("ClownFish") Drift alignment tutorial

Graphically displays drift alignment, and how stars drift depending on which axis (alt or AZ) is off.

HTH

#7 robjohn

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Posted 16 July 2008 - 04:56 PM

Yes. When the polar axis of the mount and the star the scope is viewing differ by 0h or 12h in RA, the drift from misalignment is all in RA. When the polar axis of the mount and the star the scope is viewing differ by 6h or 18h in RA, the drift from misalignment is all in Dec.

#8 amateur

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Posted 09 September 2009 - 03:58 AM

simulate yourself out of this question:

PE and PA simulator

#9 amateur

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Posted 10 September 2009 - 01:38 PM

For DEC=0 and a small polar alignment errors, the stars will describe a vertical line over 24h:

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#10 amateur

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Posted 10 September 2009 - 01:40 PM

For DEC>0 and a small polar alignment errors, the stars will describe an ellipse, over 24h. The ellipse will be wider for larger DEC:

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#11 amateur

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Posted 10 September 2009 - 01:44 PM

The periodic error will always add a wiggle in RA to the curve:

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