Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1237
Loc: Estonia
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I'm experiencing a very specific and odd tracking error with my mount. I don't have the faulty exposure with me right now, but I attatched a drawing of what the star trails looked like in a 1000 second exposure. It seems to be a very specific problem, as there's no random 'noise' in the tracking.
Additional information: it's an equatorial mount, I don't know it's name, it was made by LOMO in the 50s/60s, there's no documentation.
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Edited by Hrundi (10/08/09 06:52 PM)
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LLEEGE
Running out of Oxygen
Reged: 03/03/05
Posts: 9075
Loc: Cloud-chester,NY
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Looks like a combination of polar mis-alignment and periodic error.
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"Okay! You draw the straws. I'm-a taking the parachute."
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1237
Loc: Estonia
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Wouldn't a polar mis-alignment produce a constant error though? As far as I can tell, the majority of the shifting happens along the RA axis, with the occasional bump being in declination.
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2270
Loc: Kent, UK
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I'm making a lot of assumptions but 3 humps in 1000 seconds gives 1 in 330 sec aprox, not far off the worm period? Plus some polar error ?
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1237
Loc: Estonia
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The humps may be worm problems indeed. I'll have to go over those.
On the subject of polar error, wouldn't that be constant though? The thing that basically confuses me is that the problems in the RA direction are not constant. I can sometimes get maybe up 50-90s with no error whatsoever (which at a field of 10x7 min, with 7700fl is quite good I think). Wouldn't polar error constantly cause problems, as opposed to kick in every now and then?
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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You can simulate your experiment (exposure) using this simulator.
Seems like you have a significant polar alignment error.
I have put your parameters as far as I know them in this simulation:
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1237
Loc: Estonia
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hm, that could be pretty bad. I hope I can fix that.
Here's an updated version of the trail as I recall it.
Basically my worry is that I don't understand the problem still. There is, as far as I can tell, no drift in declination. There is, however, something of a periodic 'bump' in declination, which is incredibly confusing for me.
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Edited by Hrundi (10/09/09 07:22 AM)
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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Your drawing clearly shows a drift in declination and RA PLUS a wiggle caused by periodic error. Together it may seem to you like jumps in DEC, but that is still the sum of a linear drift in DEC and a sine-like periodic RA error.
Alignment errors can give rise to drift in both RA and DEC, depending on the DEC of the star you image. The DEC of your star must have been greater than 0.
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1237
Loc: Estonia
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My drawing's a bit bad, yeah, The drift in declination is periodic in nature, and seems to be one rapid jump. The drift in RA is periodic back and forth. The thing that confuses me is that I definitely have periods of ~300 seconds with zero declination drift.
With 50 sec exposures, I experience about 40-50% ruined by RA error. Then every now and then (rarely), an exposure is ruined by a very rapid jump in dec. Then after that, zero drift in dec for another approx 300 seconds. So that's what's confusing me. I thought that, as you said, polar alignment creates a linear drift in declination, but there's nothing linear about what I'm seeing.
Added an image with the drift going in the RA direction (which is what it does about 90% of the time). The red axis is RA, which I managed to figure out by finding the stars in DSS (in CdC), overlaying the stretched stars, and then using the Cartes du Ciel declination line. Am I mistaken that during this I should be seeing some sort of twist if it were linear polar error?
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Edited by Hrundi (10/09/09 08:35 AM)
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2270
Loc: Kent, UK
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If you try Alan's How To here it shows how to measure and diagnose errors from trails. You'll need a calibration shot as well for scale, I use the moon.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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rmollise
Post Laureate
Reged: 07/06/07
Posts: 4575
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Normal periodic error...and tighten up the polar alignment...
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Uncle Rod
Rod's New Book:
Choosing and Using a New CAT
Available now!
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