How do I use the polar axis finderscope for the Advanced GT (CG-5) mount?
November 6, 2008
The polar axis finderscope accessory is an aid for quickly polar-aligning your telescope accurately enough for visual work. It’s also great to speed up a polar alignment using the functionality of the Advanced GT mount’s hand control firmware.
Using it is a three-step process. First, install your finder in the polar axis. Then take off the polar housing cap and turn the declination axis until the hole allowing light through the polar axis is not blocked.
Next, you need to align the optical axis of the finderscope with the mechanical polar axis of the CG-5 mount. This can be done either at night with Polaris or more easily during the daytime on a distant building or landmark. All you need to do this is the mount – remove the tube, counterweight and counterweight shaft to make the process simpler to do. If you do this during the day, use the latitude adjustment screws and the tripod to level the polar axis, again to make it easier to do.
You’ll flip the mount several times and recenter the distant object in the finderscope each time, successively getting closer to alignment of the finderscope and polar axis.
Unclamp the RA and rotate the mount until the dovetail platform holding the telescope is either to the right or left. Ignore all the patterns seen in the finderscope reticle except the crosshairs. Center a distant object in the crosshairs by moving the entire mount. Now move the platform all the way to the opposite side of the mount. Note the shift of the object off the crosshairs. Actually the finderscope and the crosshairs themselves have rotated in a small semicircle around where the polar axis points. You can see where that is by looking through the finderscope as you rotate the mount, watching for the center of motion. Clamp the mount and turn the three setscrews around the finderscope to move the crosshairs over this pivot point. Recenter the distant object in the crosshairs by moving the entire mount.
Repeat the flipping-setscrew-recenter procedure. Each time you go from one side to the other, the off-center distance of the crosshairs from the pivot point will be smaller. After three or four repeats, the crosshairs won’t move when you flip the mount. Your polar finderscope optical axis is now pointing in the same direction as the polar axis.
Reassemble the mount and wait until dark. The final step is done with Polaris and uses the reticle in the finderscope. The reticle has an etched pattern to make quick polar alignment easier. It puts together two patterns – the naked-eye constellations and the magnified, upside down (inverted) finderscope view of Polaris’ offset from the north celestial pole. The magnified view is inset at the center of the reticle.
Start by moving the scope in RA around the polar axis until the pattern you see in the finder matches the real naked-eye orientation on the sky of the Big Dipper – Cassiopeia. (Ignore the pattern labeled Octans, which is for the south polar alignment.) Clamp the scope. Finish by using the small circle and the tiny circle on it to nail down your polar alignment. Do this using your latitude adjustment screws and the azimuth screws on the base of the CG-5 mount to place Polaris inside the tiny circle. Once Polaris is in the tiny circle, your mount is accurately polar aligned for visual work.
The polar finderscope will also work for southern hemisphere observers. Here there is no naked-eye pattern etched on the reticle because there are no bright naked-eye stars near the south celestial pole. Instead there is just a 4-star magnified pattern for you to align on. (Ignore the Big Dipper-Cassiopeia patterns also on the reticle that are used for the north polar alignment.) After setting the latitude, simply point the mount towards the constellation of Octans. (You may need a star chart to do this.) Look in the finderscope and align the 4-star-pattern or asterism in the real sky with the same pattern on the finderscope's reticle. (The pattern consists of the stars Chi, Sigma, Tau and Upsilon Octantis.) Once they are lined up, your scope is aligned to the south celestial pole.