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Re: Constant right drift- RA drift vs. Polar align question

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  • Randy Nulman
    Peter, This explanation was very helpful...thanks for verifying and explaining why, emperically, this worked for me. Also adds credence to my logic of
    Message 1 of 34 , Jan 2, 2005
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      Peter,
      This explanation was very helpful...thanks for verifying and
      explaining why, emperically, this worked for me. Also adds credence
      to my logic of "averaging" out altitude adjustments (In various
      pointing positions) to minimize this effect and eliminate the need
      to tweak for each new pointing position.

      Thanks again,
      Randy Nulman


      --- In ccd-newastro@yahoogroups.com, "erdmanpe" <erdmanpe@y...>
      wrote:
      >
      > A polar alignment that is not correct will produce drift in both RA
      > and DEC. The errors in altitude and azimuth are easiest to see in
      > different portions of the sky, so the drift alignment technique
      takes
      > advantage of that by looking in different directions for adjusting
      > each axis. If only the altitude were incorrect, then the RA drift
      > would be the hardest to see near the meridian near 0 DEC, but it
      would
      > then show up in other look directions.
      >
      > The spherical geometry is a little awkward to visualize, but
      imagine
      > some extremes to help out a bit. First, think of a roughly
      properly
      > aligned scope and how much of the sky it sweeps out as RA is
      changed
      > some number of hours. Then think of that same scope pointed at
      high
      > DEC angle and think how little of the sky it sweeps out for the
      same
      > change in hours.
      >
      > Now imagine an Alt-Az mount as a polar mount with the altitude VERY
      > far off. Think of how much of the sky is sweep out near the
      horizon
      > as the Az is changed (really our RA now, but way off), and then
      again
      > near the zenith (or near the meridian, near 0 DEC to be consistent
      > with our first example). Very different from the properly aligned
      > polar mount, but all we have done is shift the altitude of the
      mount.
      >
      > It is only when both the altitude and the azimuth of the mount are
      > properly aligned that drift in both RA and DEC can be eliminated.
      So,
      > yes, an error in altitude can induce RA drift, and it also induces
      > drift in DEC that is maximized near the E or W horizons--so we look
      > there with our drift alignment technique for altitude adjustments.
      >
      > I don't know if I've helped at all <g>.
      >
      > Peter
      >
      > --- In ccd-newastro@yahoogroups.com, "Randy Nulman"
      <rj.nulman@v...>
      > wrote:
      > >
      > > Peter and group,
      > > I also responded previously (assuming this was an RA drift),
      that a
      > > misalignment in "altitude" when polar aligning causes an RA
      drift.
      > >
      > > I have received a few private emails...some agreeing, others
      saying
      > > that there is no way for a mis-adjusted altitude to induce RA
      drift.
      > >
      > > I initially learned of this relationship from Roland at AP, and
      > > emperically found it to be true...when I had some residual RA
      drift
      > > after a crude Polar align, adjusting altitude did, indeed,
      eliminate
      > > this drift. (although, I could never understand the
      relationship
      > > between RA drift near the meridian and polar alignment...could
      not
      > > visualize the scenario in my mind...just knew it worked
      emperically)
      > >
      > > Can someone/anyone explain why a mis-alignment in altitude would
      > > induce a constant RA drift? I'm just curious to verify that
      > > Roland's advice was sound (again, based on my experience, it
      is),
      > > and what the reasons for this are. I know that when something
      > > works, that's often enough reason to use it <g>...but I'm just a
      bit
      > > curious at this point...and would like to respond with some
      facts to
      > > those that emailed me.
      > >
      > > Thanks,
      > > Randy Nulman
      > >
      > >
      > > --- In ccd-newastro@yahoogroups.com, "erdmanpe" <erdmanpe@y...>
      > > wrote:
      > > >
      > > > Drifting to the right doesn't mean anything until you have
      > > identified
      > > > if that is drift in DEC or RA, and where in the sky it is
      > > occurring.
      > > > Your most likely problem is polar alignment. PEC problems are
      not
      > > > consistently in any direction. I wouldn't fiddle with the
      drive
      > > rate
      > > > except as a last resort.
      > > >
      > > > Peter
      > > >
      > > > --- In ccd-newastro@yahoogroups.com, Charles Bell
      > > > <quantumhyperspace@y...> wrote:
      > > > > I can't figure this out.
      > > > >
      > > > > When I try to image at prime focus with the Meade 12
      > > > > inch f/10 equitorial/polar mounted on Meade super
      > > > > wedge, I see a constant drift to the right in my
      > > > > images making all but short exposures useless.
      > > > >
      > > > > Is this due to a polar aignment problem with the
      > > > > altitude adjustment (big knob on back of wedge)? or is
      > > > > it due to an incorrect siderial tracking rate?
      > > > > Is this due to PEC being way off?
      > > > >
      > > > > I have ran PEC several times (and updated).
      > > > >
      > > > > At first I thought I could correct it by adjusting the
      > > > > Guide Rate under the Setup Telescope Autostar menu
      > > > > option. But after experimenting for many hours,
      > > > > thismust only adjust the rate of movement when you
      > > > > manually slew when using the Guide Rate.
      > > > >
      > > > > The error won't let me do a calibration to use the
      > > > > guider. x always shows error.
      > > > >
      > > > > Scope: Meade 12" LX200-GPS
      > > > > CCD: ST-2000XCM with ST-237 guide chip.
      > > > >
      > > > > If I could just get this one problem solved, I could
      > > > > get a decent exposure time.
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > =====
      > > > > Charles Bell
      > > > > Vicksburg, Mississippi USA
      > > > > 32° 15' 56" N 90° 51' 20" W
      > > > > http://www.quantumhyperspace.com/
    • bjohns7764@aol.com
      Hi Peter and Randy, I m a little slow in responding ... as usual ... but here s my follow-up. You guys are absolutely correct! My mental processes, such as
      Message 34 of 34 , Jan 7, 2005
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        Hi Peter and Randy,

        I'm a little slow in responding ... as usual ... but here's my follow-up.
        You guys are absolutely correct! My mental processes, such as they are, was
        thinking in relatively long time periods, such as a BUNCH of one minute exposures
        in a row or such. Over the time and space areas you guys are talking about,
        there just ain't gonna be any significant Dec drift.

        Just write the whole idea off to either very long 'senior moment', or the
        advanced stages of mental deterioration <g> After all, I recently told my wife
        that the second thing to go with age, is the memory. She asked me what was the
        first to go, and I answered ............. I don't remember. <G>


        <<
        I think that the problem in trying to visualize the geometry is that
        the manifestation of the error in a misalignment is different in
        different parts of the sky. Ron has a picture he made a few messages
        down that should help to visualize what I was trying to say in my last
        message.

        While you are quite correct that no matter where we look in the sky,
        in one hour period the polar axis will have rotated 15 degrees, as
        will have the stars--the look direction for the misaligned mount will
        be different than the motion those same stars have undergone.

        Imagine you are at the center of a sphere, the surface of which is
        covered with our stars. Our spherical coordinate system now consists
        only of the two angle variables (RA and DEC) since we have set the
        radius (the third coordinate) to a constant.

        The length of an arc (at constant DEC angle) on the surface of that
        sphere for a 15deg rotation (one hour) about the polar axis is
        different as we change DEC angles. Largest at the equator, zero at the
        poles. This is equivalent to your comment that the lines of RA are
        closer together at the poles--the arc lengths for the same RA angle
        change depends upon the DEC angle.

        Hence, if our mount's polar axis is misaligned in altitude, the length
        of the arc on the sphere's surface is different than the actual star
        motion in that one hour because we had to position the telescope at a
        different DEC angle in order to see those same stars (because of our
        polar axis misalignment).

        This misalignment will also in general result in DEC drift. However,
        at one special location, the meridian, we can get RA drift without DEC
        drift because for a short time the two arcs on the sphere's surface
        are tangent to one another. Elsewhere this is not possible, so in
        general any polar axis misalignment results in both RA and DEC
        drift--but in differing magnitudes depending upon our look direction.

        So, you are quite right that over a long period of time we would have
        to see DEC drift even if our misalignment were only in altitude. But,
        briefly, at the meridian, it would be minimal. That's why we swing
        down to the E or W horizon during our drift alignment proceedure in
        order to maximize the DEC drift due to altitude misalignment.

        Peter

        >>


        <<
        Hi Bruce,
        I don't have the math prowess to explain this, but I can verify,
        emperically, that RA drift _can_ occur without Dec drift...this is
        repeatable...and can be corrected for.

        For example, after a very "anal" drift align, I found that I had no
        drift in RA or DEC at around zero degrees dec..and near the
        meridian. However, when I moved to around 70 degrees north..also
        near the meridian, I still had NO measurable dec drift after 5
        minutes, but very real RA drift. All these measurements were done
        using a CCD and noting the pixel location over 5 minutes of time.

        I was able to eliminate the RA drift in the new position by
        adjusting altitude only. However, when I went back to zero degrees
        dec, I now had some RA drift there (that, as I indicated, was not
        there before). I resolved this by averaging out the Altitude
        between the two points and got to a scenario where, although there
        was VERY minor RA drift in both positions, it was so small that I
        could go unguided easily for 2 plus minutes in any portion of the
        sky. Obviously, guiding was "easy" at this point. I must repeat
        that during the whole process, there was virtually no Dec
        drift..only RA drift...and altitude adjustments did compensate.

        Regards,
        Randy Nulman

        >>


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