This article will show you a few methods for polar alignment of an equatorial (EQ) mount, how to use the drift method for alignment without Polaris and how to use setting circles. The article was written using a Skywatcher HEQ5 mount, but the technique should be the same for all equatorial mounts.
If like me your Skywatcher telescope didn't come with instructions, and you don't understand the poorly translated Chinese from the website, then hopefully this guide will help you to set up your telescope mount. It is based on a SkyWatcher HEQ5 with dual axis motor drive, but the principle should be the same for any equatorial mount.
If you watch the stars for long enough, or set-up a camera with a very long exposure, you will notice that there is a point in the sky that does not appear to move, and all the other stars rotate around this point. This point is the Northern Celestial Pole (NCP) and is True North (or Southern Celestial Pole if you are in the Southern Hemisphere).
By aligning the telescope mount with the NCP, we can counteract the effect by moving the telescope with the stars. This enables us to take long exposures on the mount without any star trails in the picture. It also means that when you view an object, such as Jupiter, it stays in your field of view and you do not have to keep moving the telescope.
The NCP is located close to the star Polaris (also called the Pole Star), and Polaris is our starting point for an alignment, but it is not close enough for an accurate alignment.
I find setting that it is best to do a rough polar alignment before you attach the telescope, and make fine adjustments when the telescope is attached.
Before we begin the polar alignment procedure, the mount has to be prepared by aligning the polar scope to the mounts axis. You only need to do this once, or if something comes loose, or just needs recalibrating. You don't need to do it every time you setup the mount.
This bit is a lot easier to do during the day. You need to set-up the mount so you can see a distant object, such as a pylon or telegraph pole.
Remove the dust caps from either end of the RA (Right Ascension) axis, lower the counter shaft bar and rotate the DEC (Declination) axis so that you can see through the polar scope.
When you look through the polar scope you should see an overlay of a cross hair or circles with constellations. My polar scope has constellations and circles as shown below. You can see how Polaris is offset by about 3/4 degree from True North.
Once this adjustment has been made successfully, it should not be necessary to carry out this procedure again.
Don't Panic! There is nothing you have to calculate, it is all done by the mount. An equatorial mount is essentially a circular slide rule, but it must first be calibrated for your location. The index marker represents a 'zero' for the slide rule. If the zero is not set correctly, the calculated hour angle will not be correct.
We need to set a reference 'zero' point where we know the position of Polaris in the sky. You simply need to look up the time and date at which Polaris is at its highest or lowest position in the sky as these are easiest times to calculate. I used the free planetarium software Stellarium to calculate the highest position that Polaris achieves. I did this by playing with the date/time controls to find the highest point in the Month, then going through all the days to find the highest day, then the highest hour and so on.
For my location the highest position Polaris achieves occurs at 00:06 on 17nd of November, and I will be using this date in this document. You should substitute with your calculated date/time.
The alignment procedure requires that you set the Longitude scale to "Zero". Depending on where you live, "Zero" can be any place between the E and the W on longitude scale, so first you need to determine where zero is for your location. Your Zero point is equal to the difference between your actual longitude and the longitude of the central meridian of your time zone. To calculate the longitude of your central meridian, multiply your time zone offset from Greenwich Mean Time (GMT) by 15.
For example, in Waterloo, Ontario, Canada (Eastern Time) the time zone offset is -5 hours. Ignore the sign and simply multiply 5 x 15 = 75.The longitude of the central meridian for the Eastern time zone is 75 degrees west. The actual longitude at the viewing location in Waterloo is 80 degrees 30 minutes West. Ignore the 30 minutes and just use 80 in the equation.
Now it's simple, 80 - 75 = 5. Since 80 is greater than 75 the result is positive 5. That means Waterloo, Ontario is west of its Central Meridian. In this case, the zero point is at the "5" mark on the W side of the scale. If the location was east of its central meridian the equation would yield a negative value. In that case the E side of the scale should be used.
The Polar scope is now calibrated for your longitude and the Index Mark should be on the right hand side of the Polar scope. This procedure should not have to be repeated unless the reticule has been re-aligned or the index ring has come loose.
Before aligning your mount it is important that it is level. Most mounts have an inbuilt spirit level, but if not you can still use an ordinary level. Height adjustment is usually done by adjusting the height of the legs. The surface of the mount should be level in both directions (North-South and East-West). You can optionally skip this step but it will save you a lot of time!
The drift method can be used for the fine tuning of the polar alignment process, polar alignment without Polaris, or if your mount does not have setting circles / polar scope. It is easier to perform with an illuminated reticle eyepiece, but at a push you can use the crosshair on the finder scope for a rough alignment.
You should now have an accurately aligned polar mount.
Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial (RA and Dec) coordinates.
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