Various methods of achieving telescope polar alignment exist. They vary from the quick and imprecise to the tedious and highly accurate. The method here will be in-between these extremes of setup time and accuracy. It is basically the method used by auxiliary polar alignment telescopes and can be used to check the accuracy of such.
The Local Hour Angle Method
The circle that Polaris traces in a 24-hour sidereal hour period will cover every possible LHA. In this worksheet the LHA of Polaris is visualized on charts that simulate the view through a finder or telescopic wide-field. Telescope alignment is obtained by aiming the telescope in altitude and azimuth to mimic the view on the charts.
Accuracy of this alignment will depend on: 1) how true the optical axis is with the polar axis and 2) the degree that you can accurately place Polaris in the field-of-view. The later is best done with a reticle eyepiece having an index point or circle whose distance from the field center equals the apparent separation of Polaris from the pole. Lacking that you can make a position guestimate based on the fact that Polaris is about ¾-degree from the pole. This would be ¼ of the way from the center of a 6-degree finder field. Wide-angle 2" eyepieces or a low-power eyepiece coupled with a focal reducer can achieve a sufficient field (at least 1.5-degrees) through the main scope of many systems.
Align your finder or eyepiece cross marks with the declination and right ascension telescope axes. Do this by moving slightly in RA or DEC while sighting an object through the eyepiece. Rotate eyepiece so cross mark lines up with object movement. This will make it easier to judge the location of Polaris in the field-of-view.
For the charts to be of use you will need to input accurate data. All required data is input through blue data fields in the vicinity of the chart.
CHART #1 Calculations
CHART #1 Data Input Instructions
Adjust Chart View
Note that a typical finder and a Schmidt-Cassegrain without star diagonal will yield a mirrored and upside down view so both the last two values would be 1. A star diagonal used on a refractor or Schmidt-Cassegrain will yield mirror views so mirror should be 1 and upside-down 0. You can tweak the view using the built-in graph formatting to change the number of grid lines so that the outer concentric circle represents the finder view and the inner circle the eyepiece view.
Remember that converting your watch time to UT can cause the date to rollover. This is true in North America at summer start where in UT the sun doesn't set until the next day: 8:30 PM EDT is 4-hours behind UT; so: 20:30 + 4 = 00:30 UT the next day. In this example the date entered would be that next day.
Chart #1: Position Angle of Polaris to North Celestial Pole
Chart #1 Output Data
Polaris Coordinates and Epoch
Polaris location is taken from the STARMAT table and processed to the date of observation. The observation date is then converted into year format and displayed as the epoch year.
Chart #2 Explained
This chart is meant to give a quick indication of when Polaris will be at one of eight special points around the celestial pole. These points are at 45-degree increments -- or every 3 sidereal hours and are the easiest to align on for users of simple cross hair eyepieces. Immediately following the chart is the Polaris Position Table that numerates the date and time that these points are reached. The first of the eight points is selected as the next one in the future of the time and date entered for Chart #1.
Since it is based on data from Chart #1, no new data need be entered for this chart and the accompanying table. Orientation of the chart is taken from Chart #1 and is displayed at the top of the chart. Field of view is fixed. No correction is made for precession or nutation so the times listed in the table may be off by a second or two.
CHART #2 Calculations
Chart #2: Polaris in 45-Degree Increments Around the Pole
Note: Because of rounding the "Sec" column may display 60. Treat this as zero and increment the minute.
Astro Utilities Electronic Book Copyright © 1999 Pietro Carboni. All rights reserved.
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