What is the overall procedure for polar alignment of my CGE mount?
November 11, 2008
While polar alignment can be difficult, the CGE has a number of features to make it easier and accurate.
Accurate CGE 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. It must be within 5 degrees, the limit of subsequent adjustments that can be made using the latitude-azimuth adjustment screws.
Use the latitude adjustment hex bolts to fine-tune the latitude. Likewise, use the azimuth adjustment hex bolt to fine-tune azimuth to properly position the mount for polar alignment.
(The CGE 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. The resulting alignment can be good enough for pointing the scope doing CCD imaging with a small chip.)
Then align the CGE using the two-star alignment plus calibration stars.
Following the two-star alignment, the CGE 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 CGE.
After calibration, select Polar Align > Align Mount from the Utilities menu. The CGE 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 CGE 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 Switch 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.)
Because the CGE is very sensitive to imbalance, 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 CGE to complete the final, slow GoTo when slewing to an object.