Thought I would cross post this - how to use the RA and date circle on mounts that have a polar scope with a calendar dial. There are surprisingly NO examples I could find of simple instructions, or why the thing works at all. This is actually a very hand little feature and amounts to a sort of slide rule for calculating local sidereal time referenced to the position of Polaris.
The key is that the inner dial should stay fixed, and it has to be calibrated just once. There is an index mark on a ring that is fixed on the RA shaft, and so rotates with it. The calendar circle is set so that your distance from the time zone meridian in degrees is indicated on the small scale E 20 10 0 10 20 W. Then you do not touch the calendar circle again! This was the part that was confusing me. BUT - the index mark itself HAS to be calibrated to your location.The only dial you move is the RA dial itself. I'll get to calibration in a minute.
So the general idea is -
1) Start in polar home position - DEC axis straight down, Polaris marker as seen through the polar scope also straight down. That is, the DEC axis and the view through the polar scope coincide. This accounts for the inversion of the image in the polar scope (see below).
2) Now rotate the RA circle *without disturbing the calendar circle* so that the date on the calendar is opposite the local (military) time on the scale that increases right to left. On my mount that is the top scale. If daylight saving time is in effect, subtract an hour to get the local time. Note that you are NOT aligning the RA circle with the index mark for the calendar OR the index mark on the mount casting - you are setting a time opposite a date.
3) Now unlock the RA axis and rotate the mount in RA until 0 comes to the RA index mark on the mount casting.
You are now in the right configuration to proceed to adjustment in azimuth and altitude to bring Polaris into the right place in the polar scope.
This method is really accurate!
Now - to calibrate the calendar circle we have to work backward. We look for a time when Polaris is transiting, directly over the pole. Use a planetarium program to find such a time and date - any pair will do. Let's say it's Jan 15th at 20h 0m.
Set the scope in home position and the RA circle to 0 as indicated by the mount casting index. Now, lock down the RA circle using the set screw, and this time move the CALENDAR circle so that 1/15 is opposite 20h 0m on the RA circle - again, use the scale that increases right to left. Finally, undo the set screw on the calendar index ring and move it so that it points to your distance from the time zone meridian. This index is only used once this way, but provides a reference for resetting the calendar dial should it be disturbed. Tighten the set screw and from then on, do not disturb the calendar circle. ONLY the RA circle needs to be moved.
Ideally the calendar dial should have enough damping grease so that it moves, but not easily. It should resist enough to force the use of both thumbs.
Note that if you change longitude, you must move the calendar circle to account for that. The index mark, once calibrated, should not be moved again. Just move the calendar dial against the index mark to the new displacement within the time zone.
The fixed angle between vertical and the calendar index mark when the scope is in home position, represents the position of Polaris when local sidereal time is 0h 0m. So in a sense, it encodes the right ascension of Polaris itself. Which makes the calibration process even simpler - just convert Polaris' RA into an angle and set the mark at that angle clockwise from the RA index at top center. As of this writing, Polaris' RA is 46.2 degrees.
It is very satisfying to know one can polar align accurately without touching a computer or phone or using constellation matching by just eyeballing it!
-drl
Edited by deSitter, 15 January 2025 - 12:00 PM.
