What is the overall procedure for polar alignment of my Advanced GT mount?
November 11, 2008
Accurate Advanced GT polar alignment consists of 5 parts: rough alignment, Two-Star Align plus calibration stars, Polar Align, repeating Two-Star Align plus calibration stars and finally mount calibration.
Start by setting up the telescope and roughly position the mount in azimuth and altitude towards Polaris. Get it close enough so that it’s within the limit of subsequent adjustments that can be made using the latitude-azimuth adjustment screws.
Use the latitude adjustment screws to fine-tune the latitude. Likewise, use the azimuth adjustment screws to fine-tune azimuth to properly position the mount for polar alignment.
(The Advanced GT polar axis finder is an aid for accurate polar alignment to replace rough polar alignment. It has a reticle of the circumpolar sky that’s oriented to agree with the real sky by moving the mount in latitude and azimuth.)
Then align the Advanced GT using the two-star alignment plus calibration stars.
Following the two-star alignment, the Advanced GT will ask, "Add Calibration Star?" Adding calibration stars is recommended to help compensate for small optical-mechanical misalignments in the scope-mount system. (These misalignments are called cone errors and are usually one-half degree or less. The error is greatest for stars near the equator for an equatorial mount.) Press Enter to add calibration stars. The hand control will list ones on the opposite side of the meridian than those originally used for alignment. Using these stars and those near the equator is best to offset any cone errors in your mount. Adding 2 calibration stars is good, and adding at least 3 stars is recommended for the best results with the Advanced GT.
After calibration, select Polar Align > Align Mount from the Utilities menu. The Advanced GT will then slew to where it thinks Polaris is located, based on the positioning of the mount and the two-star alignment performed earlier. Now use only the latitude-azimuth adjustment screws to center Polaris in the eyepiece. (Don’t use the hand control!) Always move the scope up when centering on Polaris to take up any backlash in the adjustment screws. Once it’s centered, press Enter. Now the polar axis is pointing towards the north celestial pole. (The function takes into account the offset of Polaris from the true pole.)
Repeat the Two-Star Align with calibration stars so the Advanced GT is again aligned to the sky.
Finally, calibrate the mount. This process is the final step in optimizing the performance and pointing accuracy; it compensates for individual mechanical variations in the mount. It is automatic and applies the cone error correction on opposite sides of the sky and applies this constant to alignments. Go to Utilities > Calibrate Mount > RA Index to do the calibration. (Note: Though both are related to the cone error, this is different from the use of calibration stars after Two-Star Align.)
A last, very useful adjustment is GoTo calibration (Utilities > Calibrate Mount > GoTo) when using heavy accessories with your telescope. It calculates and compensates for balance changes to the amount of time and distance it takes for the Advanced GT to complete the final, slow GoTo when slewing to an object.