Drift alignment with an LX90 using a diagonal
(for the Northern Hemisphere)
Iterative method for precise alignment


Drift alignment In Detail
(Quick and Dirty here)

1) Your tripod must be level and wedge in close polar alignment. Align the scope doing a one star alignment with the Autostar, or Kochab's clock and an easy or two star alignment with the Autostar. Your reticle eyepiece should be perpendicular to the centerline of the OTA (diagonal facing up), and the eyepiece crosshairs aligned to RA and declination axes. A star placed on the crosshairs should stay on them when slewing slowly in RA (left/right) or declination (up/down) from one side of the FOV to the other.


2) Slew the scope to a star in the south near the meridian and celestial equator, and bisect the star with the horizontal crosshair (Fig 1), and observe the declination drift. (Fig 2). Ignore any RA (East-West) drift, or you may guide in RA if you wish.

Note that a diagonal prism or mirror shows correct North / South orientation, but East / West are still reversed.

Wedge azimuth adjustment for #2 drift (Fig 2)
Indicator: If the star near the southern meridian drifts DOWN (south) in the eyepiece of the telescope, the northern direction of your telescope axis is too far East of the NCP.
Adjustment: Move wedge CCW as viewed from the top (azimuth adjuster) This should move the star to the left in the EP
recenter and check drift, adjust until 3-5 minutes accuracy

(or) Wedge azimuth adjustment for #1 drift (Fig 2)
Indicator: If the star near the southern meridian drifts UP (north) in the eyepiece of the telescope, the northern direction of your telescope axis is too far West of the NCP.
Adjustment: Move wedge CW as viewed from the top (azimuth ajuster) This should move the star to the right in the EP
recenter and check drift, adjust until 3-5 minutes accuracy

3) Adjust azimuth adjuster until drift is minimal (5 minutes or more without drift)
4) Slew scope to a star in the East on the celestial equator and observe the declination drift (Fig 2)

Wedge latitude adjustment for #2 drift (fig 2)
Indicator: If the star near the East horizon drifts UPWARD (North) in the eyepiece of the telescope, the northern direction of your telescope axis is too far above the NCP.
Adjustment: Move the wedge tilt plate
DOWN (latitude adjuster counter-clockwise) This should move the star down in the EP
recenter and check drift, adjust until 5-7 minutes accuracy. Reverse adjustment for a Western star

(or) Wedge latitude adjustment for #1 drift (fig 2)
Indicator: If the star near the East horizon drifts DOWN (South) in the eyepiece of the telescope, the northern direction of your telescope axis is too far below the NCP.
Adjustment: Move the wedge tilt plate UP (latitude adjuster clockwise) This should move the star up in the EP
recenter and check drift, adjust until 5-7 minutes accuracy. Reverse adjustment for a Western star.

5) For higher precision (i.e., astrophotography), go back to #2 and verify 7 minutes tracking or more without declination drift, Readjust as needed.

6) Check the RA (East-West) drift at the southern meridian, and adjust the custom tracking speed until RA speed drift is minimized (Fig 3). After longer than 3 minutes, periodic error will probably start to become evident. Custom tracking speeds will not remove PE, so don't try to remove it by changing the speed. You'll have to manually, or autoguide, out the periodic error (see update below). A good worm/gear set will show a few arc-seconds of smooth drift. Newer, less worn in gears, or poorly made gearsets may show as much as a few arc-minutes with a possible "bump" at the period of the worm. The LX90's worm period is ~ 9 minutes.

Some mounts, like the LX200, use periodic error correction (PEC). PEC is accomplished by marking the rotational period with a sensor, and storing the error in memory. A user must guide out the error while recording. The recorded guide movements are utilized when PEC is turned on.

Updated September 2004: Meade has updated the firmware for the #497 Autostar that includes periodic error correction. It is not permanent PEC as found in the LX200 series, but it *is* a workable method of PEC for the LX90. See the TOC and click on the firmware update to read the text file included with the download.

7) If the star drifts off to the right of the vertical crosshair, increase the tracking speed. If it drifts to left of the vertical crosshair, reduce the speed. Remember that a diagonal reverses left and right. Custom tracking is under Setup/Telescope/Tracking, then scroll down to Custom, and hit Enter. I've found that speeds from 8 to 12 are normal, so high numbers don't neccesarily mean something's wrong. After a couple years of use, it's settled down to a +3 on my scope.


Quick and Dirty

( The steps below are available to download for Field Reference or to open as a Text File )

Remember to reverse altitude adjustments (#9) if using a star in the west for the second star.

1.  Level the mount
2.  Roughly polar align mount as best possible, Do an Easy align.
3.  Pick a star on meridian, just north of celestial equator
4.  Adjust reticle eyepiece crosshairs with north-south and east-west axes of mount
5.  Place the star on the horizontal crosshair with the Autostar

If the star drifts up, adjust the wedge in azimuth to move star
right
If the star drifts down, adjust the wedge in azimuth to move star left
6.  Place the star back on the crosshair using the Autostar and check drift
7.  Repeat adjustments as necessary until no drift is seen for 5 minutes
8.  Pick a star in the east, about 20 above horizon

9
.  Place the star on the horizontal crosshair
If the star drifts up, adjust the wedge in altitude to move star
down
If the star drifts down, adjust the wedge plate in altitude to move star up
10.  Place the star back on the crosshair using hand-controller and check drift
11.  Repeat adjustments as necessary until no drift is seen for 5 minutes

Southern Meridian Problem Adjust Azimuth Adjust Azimuth - -
Drifts North (Up) Az too far West

clockwise

- - -
Drifts South (Down) Az too far East - counterclockwise - -
Eastern Celestial Equator Problem - - Adjust Latitude Adjust Latitude
Drifts North (Up) Alt too Low - - tilt plate up -
Drifts South (Down) Alt too High - - - tilt plate down

 

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