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Re: [APML]OT-Drift Alignment
Well done Frank. See this post I wrote several years ago:
http://astro.umsystem.edu/apml/ARCHIVES/MAR01/msg00948.html
It also discusses the effect of refraction on drift alignment, which can create a serious error if the drift star is close to the horizon.
Dave Rowe
> I haven't posted in a while but lets see if I can really mess things up...
> I'm going to make a statement that is going to fly in the face of years of
> experience and is going to contradict what we have always been told about
> drift alignment...
>
> But first, a little background...
>
> After a couple of years of being happy with "close enough" polar alignment,
> I installed a perminant pier and I figured it was time to "nail" my polar
> alignment. I had done drift alaignments in the past and was always able to
> figure out which way to correct the mount by working through the geometery
> in my head but it hurt my poor little brain (My brain does a good job with
> geometry - even if it doesn't hardly work in any other way). Anyway, I
> always felt that there had to be a faster way to zero in on perfect polar
> alignment and I got my first idea when I was playing with the Drift Explorer
> feature in K3CCDTools (One of those cool webcam astro imaging programs). I
> decided to write an Excel program to calculate my polar mis-alignment based
> on drift rates. The idea being that if I could plug in the rate of drift
> when the scope was pointed "south" and the rate of drift when the scope was
> pointed "east", I could use Excel to trig-out just how far off and what
> direction my mount was mis-aligned. I worked on this program for a few days
> before I got the math right. I also built (using 3D cad) a model of a
> celestial sphere and a "misalignable" mount to play with. It was during all
> this that I "discovered" something quite surprising...
>
> Please be kind!, I know this is not going to be easy for some to take:-)...
> When doing a drift alignment, you do NOT have to drift on a star close to
> the celestial equator. The rate of drift (asec/min) is the SAME regardless
> of the declination of the drift star!* (* Well this is almost true,
> actually as you start getting near to 90° dec any error in the "other" drift
> measurement direction will start to show-up in the current measurement
> direction - but not enough to hurt it the error isn't huge).
>
> O.K. lets see if this can be explained in a way that can be understood.
>
> First, Imagine that your scope could track a drift star for 24 hours. If you
> could watch the star the whole time, you would see the star drift in one
> direction for 12 hours and then slow, reverse and then drift back to where
> it started. The total misalignment of the mount would equal 1/2 of the total
> drift (in one direction) observed during that 24 hours.
>
> Example (for the Northern Hemisphere): lets say that the scope is misaligned
> exactly 1° due south of "perfect". You point the scope at a star on the
> meridian (due south) at 0° dec and watch the drift. At first you would see
> no drift, but slowly, you would see the star drift "DOWN", after 6 hours the
> star would reach its maximum rate of drift as the scope was pointed due
> west. During the next 6 hours, the rate of drift would slow and finally
> "stop" when the scope was pointed due north. After that - in the reverse
> fashion - the star would drift back "UP" to its starting point. Overall, the
> star would drift "down" 2° in the eyepiece for the first 12 hours before
> reversing...
> Now you point the scope at a star at 80° dec (without adjusting the polar
> aliagnment) And repeat the observation. Just like the star at 0° dec, the
> star at 80° dec would have the same 2° drift over 12 hours. The rate of the
> drift would be the same!
>
> So what does this mean... Well, It means that for a drift alignment, you
> could pick a star at zenith on the meridian (counter weight shaft
> horizontal) and then much closer to the pole (say 60° dec) when using a star
> in the east (counterweight shaft "vertical"). The eastern star may still
> suffer from atmospheric refraction, but not as bad as if were right on the
> horizon!
>
> BTW - If anybody wants to try/test my little Excel program, Send me an email
> privately... If you own a Losmandy G-11, it will even try to tell you how
> far and what direction to turn the ALT-AZ knobs.
>
> Frank Holub
> http://www.my-spot.com/astronomy.htm
>
>
> ----- Original Message -----
> From: StarManGMH@aol.com
> To: astro-photo@seds.org
> Sent: Monday, November 24, 2003 1:31 PM
> Subject: [APML]OT-Drift Alignment
>
>
> Hello All,
> I received my new GM-8 Sat. and saw 1st. light with it last night. My
> only previous experience is with a LXD55 mount which I never tried to drift
> align. I have Jerry's CD book and am trying to learn how to drift align. I
> am semi-familiar with celestial coordinates. I know the meridian goes from
> Polaris through the zenith. My question is about the celestial equator. If
> my latitude is 37.21 N, will the celestial equator be 90 degrees from that
> (about 52.79 degrees from the horizon)? If I read the article right it said
> about 20 degrees above the celestial equator which would put me about 73
> degrees from the horizon, which is approaching the zenith. Is this right
> or am I way off for the meridian adjustment? I have no view of the western
> horizon so I will need to use the eastern for the second alignment. If I am
> right for 73 degrees from the horizon for the meridian, can I just rotate
> the mount in RA eastward until it's about 20 degrees from the eastern
> horizon for the second drift adjustment? I'm using a refractor with
> diagonal. Should I have the diagonal oriented so I am looking straight down
> into the reticle? Is the North or South drift true north/south or as it
> appears in the eyepiece? I apologize for my ignorance on this but I want to
> learn this technique so I can take better pictures without as much guiding
> input as I have been having to make with the Meade. Any
> guidance and/or
> suggestions would be GREATLY appreciated. Thanks!!
>
> Gary
> Thaxton, VA
>
>
>
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