The first thing you'll need to know is where your observing site is in relation to the 'standard longitude' for your time zone. You need this to accurately set the date/time circle on the polar alignment scope.
Multiply your offset to UTC by 15. So, if you are at GMT -5 (Eastern standard time) this comes out to '75' (absolute value).
Subtract this from the longitude of your observing site. For my observing site at longitude 82W this means that I am 7 degrees WEST of the standard. When setting up the date/time circle I'll set it's pointer at 7W instead of 0. If your observing site is east of the standard longitude offset the longitude scale the correct number of degrees EAST.
Now follow these steps in sequence and you're off to the races.
- Level the tripod and set to north using a compass (don't forget your magnetic deviation).
- Set the head on the tripod and get your telescope ready.
- Set the pointer on the polar alignment scope at your offset from standard longitude on the
longitude scale of the date/time circle.
- Rotate the scope so that the RA circle pointer points at the current date.
- Rotate the RA circle so that the RA circle pointer points at the current time. NOTE you
want the STANDARD time not the daylight savings time. Adjust accordingly.
- Rotate the RA axis so that the RA circle pointer points at 0h0m.
- Lock the RA axis and wait for Polaris (if it's not dark yet).
- SLIGHTLY loosen the centre post so you can ajust the azimuth easily.
- When you can see Polaris through the polar alignment scope use the altitude and azimuth
adjustments to put Polaris in the little circle.
- Tighten down (not TOO tight) the altitude and azimuth adjustments.
- Tighten the centre post.
If you've done your job right, you should be within 4-6 arc minutes of true alignment and ready to get going. Below is a chart of just how much declination drift you can expect for 1-30 arc minutes of polar alignment error.
Another artifact of a bad polar alignment is field rotation. This means that as the scope and the earth don't have the same axis the field will rotate around the optical axis of the scope during an exposure. The math is too complex to do here but a simple explanation of the artifact will suffice. The field rotation will be 0 either directly east or west, and will become larger towards the zenith. While the guide star will be stationary in the image, the field will rotate at a constant angular rate which means that at the outside of the field you'll see larger trails than at the centre. Another reason to get your polar alignment close. Within 5 arcsec should be enough for mid-range exposures in the 15-30 minute range; any longer and you'll want to get even closer.
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