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Vega

Drift align using 1 star only ?

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Yes it's Vega waffling on about drift alignment again.

Having spent quite some time before imaging sessions drift aligning using webcam & K3CCD tools. I thought to myself ...there must be a way of adjusting both axis with the one star. After all, the main aim is to keep that star stationery in ALL directions for as long as possible. So it's just a case of finding what direction to counter any movement. You could start off by adjusting the azm on the mount as normal on a star on the meridian (if it moves up or down adjust so it moves left right etc). Then using the same star, work out which direction u need to adjust the alt to keep it still. Has anyone tried this and already knows what direction the star would move for alt adjustments ?

Crazy idea ?

Matt

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Havent a clue to your question sorry :D ...but Ive got another.. Does it matter what or more correctly where the star is ??? I cannot see the horizon in any direction from my back garden....pretty much straight up :D and a little to the East. Does this make it impossible ???

Chub

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It seems to me that stars on the meridian will not drift due to altitude error. They'll have to go a fair bit to the west before the drift becomes appreciable.

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It seems to me that stars on the meridian will not drift due to altitude error. They'll have to go a fair bit to the west before the drift becomes appreciable.

A strange suggestion as it does indeed drift if my setup is not polar aligned correctly. Besides dirft alignment instructions all over the web clearly state for best results use a star near the meridian and just above the celestial equator.When I say drift, I do mean after a period of a few minutes at high magnification (webcam) with motor drives running. Where's CC... hellp

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Are you sure that the drift of the meridian star is not due to azimuth error?

When you look at the sky, each star follows a "parallel". If your scope is polar aligned in azimuth but not in altitude, it will track along a "parallel" that is, at south, actually parallel to the star "parallel". To visualise it, reach out and grab one of these parallels, at south, and nudge it up or down (the northern end will get nudged the opposite way).

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It won't work using one star, here's why.

The drift will always be Dec. drift as the mount should track the stars at the correct rate. The movement in Dec. will be most pronounced for a mount that is out of alignment in Alt. when the star is low down east or west. The movement will be most pronounced for a mount that is out in Az. when the star is in the south at 90 degrees to the pole. In between and the movement could be caused by one or the other.

The reason for two stars is that if the mount is out in Az., there will be next to no drift whan pointed at a star low down in the east or west, so you can correct for Alt. errors without worrying about the Az. adjustment. Similarly, the drift observed when pointed at a high up southerly star is all due to Az. errors.

I'm sure that you could use a trial and error method to use one star, but I'm not at all sure that the iterations would converge (I think you would get it worse and worse until you gave up). It would take a lot longer though using a single star.

A simpler way would be to go from one star to another using only the setting circles, then calculate the mount errors using the difference between the actual position of the second star in RA and Dec. and the measured position. It might be simpler, but it still involves lots of hard sums, so I'd stick to the old fashioned way as it works really well.

HTH

Captain Chaos

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Thanks CC as always a very useful response. I'll stick to old faithful. I must admit, that night after all my worries (and wobbles). My photographs all came out very well with zero star trails (however they were only 60 secs long). In future, I think I'll just ignor any sideways movement's and just do as it says in the book on each axis.

Matt

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Matt, the sideways movement is due to periodic error and has to be ignored

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Havent a clue to your question sorry :D ...but Ive got another.. Does it matter what or more correctly where the star is ??? I cannot see the horizon in any direction from my back garden....pretty much straight up :D and a little to the East. Does this make it impossible ???

Chub

It does make it harder. See if you can perfect the Kochab clock method instead. Google "Kochab clock" or see here

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It does make it harder. See if you can perfect the Kochab clock method instead. Google "Kochab clock" or see here

....behave :D thought drift aligning was difficult.... :D...Just another reason to move.

But thanks anyway :p

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Astroman, your web site says

If you picture a star placed on the larger circle being tracked by your misaligned scope, it will appear to move southward in the eyepiece as the Earth rotates. Remember that the larger arc represents the actual arc a star will take across the sky. This is an important concept to visualize, because after the star passes the meridian, it appears to move back in the opposite direction! That's why you need to be consistent in which side of the meridian you chose a star to use.

I think the star will appear to rise and furthermore it won't change direction when crossing the meridian. In your diagram, the apparent motion of the star relative to the mount direction can be calculated as the difference in height between the thick curved lines as you sweep from left (east) to right (west). We are subtracting two curves looking like 1-(x*x)/2, to second order, the second one being horizontally displaced by a small amount d.

(1-(x*x)/2) - (1 -((x+d)*(x+d))/2) = d*x + (d*d)/2

The difference is first order in x so nothing special will happen at the meridian. The sign of d determines the slope. This means that for positive d (southern end of polar axis displaced to the east) the star will appear to rise and for negative d it will appear to set.

I also think that your third diagram is wrong. The two tracks should cross at West and East. Your diagram suggests that a misaligned mount will somehow shrink the 180 degree difference from East to West to something smaller. I think a better diagram for the Eastern horizon would be an inclined shallow V shape with the apex at East and both legs going up-and-right ( or in the West and both legs going up-and-left).

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The arcs in my diagrams represent the path a star takes across the sky, (larger), and the path a telescope traces in an an effort to track said star, (smaller). The first illustration represents a telescope that is misaligned both in azimuth and altitude. The azimuth error causes the star to appear to drift in one direction, north or south, depending on which side of the meridian it is observed upon. Through personal experience, I can tell you that an azimuth error causes the star to appear to drift in the opposite direction once across the meridian, your math notwithstanding. I at least once began drift aligning on a specific star, where my error was large. While adjusting the mount in azimuth, bit by bit, the star crossed the meridian, and I found it drifted in the ep in the opposite direction. I made several adjustments in the wrong direction before I realized what had happened. It doesn't take much.

The third illustration demonstrates the error in altitude. If misaligned this way, the arc of the curve the telescope follows is too shallow and the star appears to drift according to the text. Even though the vertical axis of the arc traced by the telescope is low, it still traces a semicircle from horizon to horizon. The beginning of that horizon is more toward the meridian, due to the smaller circle traced. I stand by my illustrations.

That said, I drew the illustrations as an example, and very much exaggerated. If it helps someone grasp the concept, it has worked, regardless of whether I've put the arc on the wrong side of the meridian or not.

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Many thanks for those illustrations Astroman,they make the reams of text I have read fall into place,I can now actually 'picture' what it is I am doing! instead of just following blindly D.A. instructions,

Cheers Frank

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I found it drifted in the ep in the opposite direction.

If I remember correctly, once across the meridian, equatorial mounts need to do the "meridian flip". Could it be that the apparent motion in the ep changed due to this flip but the real motion remained the same? It is clear to me that if a star appears to rise before the meridian (due to azimuth misalignment) it will continue to rise after crossing the meridian. That's the real relative motion. How it appears on the ep may well change after a meridian flip.

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I found it drifted in the ep in the opposite direction.

If I remember correctly, once across the meridian, equatorial mounts need to do the "meridian flip". Could it be that the apparent motion in the ep changed due to this flip but the real motion remained the same? It is clear to me that if a star appears to rise before the meridian (due to azimuth misalignment) it will continue to rise after crossing the meridian. That's the real relative motion. How it appears on the ep may well change after a meridian flip.

No, since my mount is a fork mount. If you look at the illustration, you see that the path the scope makes continues in the downward direction while the distance between the track of the scope and the path of the star increases. This I believe is what causes the effect.

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I made a video last night, demonstrating the azimuth/altitude adjustments with a toy equatorial mount, a torch, marker pens and a sky-board. I need to to do some editing and then I'll put it up somewhere for comments.

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