Using the STV to Obtain Accurate Polar Alignment


AP 900GTO Polar Alignment using an autoguider (STV)

My collection of alignment methods: Dick Locke, Updated 10/2015, v7


With the OTA on the west side of the mount, point the telescope towards SOUTH.  Choose a reasonably bright star +-5 deg near the meridian (the imaginary line running from the zenith to due south) and near the celestial equator (zero degrees declination or 90 degrees down from the pole (60 degrees up from TX). Select "Drive Monitor Slow" using the selector on the STV.  The top graph (x axis) will show dec either drifting up or down. Ignore the bottom graph for now; it shows RA.  Calibrate, track, then set X aggressive to be zero


Pointing SOUTH 60 deg. high, you are on NW side of mount.

1)   UP = Loosen Far

a)     If the line on the top graph is drifting up (S) loosen far, move the scope west

b)    Move the mount CCW by  Loosen far AZ and tighten near from NW side of AP

2)   Down = Loosen Near

a)     If the line on the top graph is drifting down (N) tighten far, move it east. 

b)    Move the mount CW by tighten far az knob

c)     To make the line go up when in south, loosen the near knob and Tighten far – from northwest side of mount looking at mount

3)     Repeat this step until there is little or no drift on the top graph.

4)      If the star drifts north the telescope mount is pointing too far to the west. If the star drifts south the telescope mount is pointing too far to the east.

Point the scope to a star in the EAST that is at about the altitude of Polaris (30 deg. up)

1)   UP = Counter-Clockwise.   

a)     If the line on the top graph drifts up, increase the altitude angle of the mount, rotate against the weight, counter-clockwise when viewing knob from top.

2)   DOWN = Clockwise

a)     If it drifts down, decrease the altitude angle, rotate with the weight, turn knob clockwise.


Repeat this step until the top line shows little or no drift.  Repeat steps two and three until there is no drift in the top graph for at least one complete cycle. At this point, the bottom graph will represent your drive's periodic error and your scope is perfectly polar aligned.   If 90 second drift is less than 1 arc sec you’re done.

Set up your telescope and STV and roughly align on Polaris.  Simply getting Polaris in the finder is sufficient for this step. For the following steps, the camera head was connected to the scope so that the wire lead is facing down. RA changes will be vertical (y axis on the monitor), and Declination changes will be horizontal (x).






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GTO Quick Star Drift Method of Polar Alignment – Using the Meridian Delay Feature


2. Slew to a star that is within 1 hour of the meridian, either east or west. Be sure that your telescope is on the west side of the mount when pointing to a star in the east and vice versa. Center the star on the crosshair N-S-E-W buttons and press the RA/DEC/REV button then select #9 Re-calibrate option (from the RA/DEC/REV Menu).


3. In the RA/DEC/REV menu look at the Meridian Swap selection. If the star is in the west 1W advance the meridian by 1 hour so that the display shows Meridian<1W>. Use the PREV< and NEXT> buttons to advance the hour and also change the direction to either W or E (pressing PREV< multiple times will display the hours in the east). If the star is in the east 1E, enter <1E> (You can verify if the star is in the east by it having a larger RA number than the current meridian or LST value). Press RA/DEC/REV to return to the Objects Menu.


4. Enter the same star again and press GOTO. Now the telescope will swap sides. The star will now appear again in the eyepiece, but may be displaced both in RA and DEC on the crosshairs.


5. Make altitude adjustments.


a) The RA (east/west) error is a function of the orthogonality of the telescope. For polar alignment, this can be ignored for now. Use the mount’s altitude adjustment to bring the star half way toward the center of the reticle in the N-S direction. Center the star the rest of the way using the N-S buttons.


b) Press the Recal button #9 again in the RA/DEC/REV Menu. You will hear a beep letting you know you re-calibrated.


c) Set the Meridian swap display to <0W>.


d) Enter the same star again and press GOTO. The scope will again swap sides and acquire the star on the crosshairs. Repeat the mechanical alignment procedure until the star remains on or close to the crosshair. It is not necessary to do this at high power, nor is it necessary to get the star to fall exactly on the crosshair. 100x should be plenty of power – this is 12 times more sensitive than a typical polar scope.


6. Make azimuth adjustments:


a) Pick two stars that are lined up in the east (or west) at similar right ascension values, but are separated by a large declination value (Differ North & South). In the winter, a good combination would be Beta Auriga and Alpha Orionis (Betelgeuse). In the summer, you can use Eta Ursa Majoris (the end star in the handle of the Big Dipper) and Alpha Virginis (Spica).


b) Slew between these stars. Again, using the GOTO, slew the scope back and forth between these two stars and adjust the azimuth axis of the mount to center each star in turn on the crosshair in the E-W direction (ignore any small N-S displacement). Remember to use the azimuth adjuster to correct half of the error and the E-W buttons to center the star.