Polar Alignment of an Equatorial Telescope Mount
- Observational Astronomy
- Getting used to the Dark - Eye Adaption
- Binocular Astronomy
- Light Pollution
- Why do Stars Twinkle?
- How do I take Long Exposures with my Canon DSLR?
- How to Photograph the Moon with a DSLR
- Buying Your First Telescope
- Your First Night With Your First Telescope
- Sky Orientation through a Telescope
- Polar Alignment of an Equatorial Telescope Mount
- Astronomy Filters
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.
Why align the mount?
From our point of view, the planets, Sun, Moon and stars all move across the sky from East to West making a complete circle once every 24 hours. In fact it is not the stars that move, but the Earth rotating that causes the appearance of the stars moving.
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.
Preparing the Mount
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.
Aligning the Polar Scope Reticule
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.
WARNING: Do NOT use any telescope device for looking at the sun without appropriate solar filters. Doing so may cause permanent blindness. Do not use the sun for calibration. Do not use objects close to the Sun, or where the Sun could move into the field of view.
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.
I should point out that you cannot actually see the Big Dipper or Cassiopeia through the polar scope whilst looking for Polaris. I spent ages trying to work out why I could not see them until I realised that they are there only as a guide. Looking through this example, the big dipper should be above me and to my left.
There are three adjustment screws set 120° apart. This photo shows two of them, the other is underneath. Only make small adjustments to the screws, and only adjust two at a time. Do not over tighten or you can crack the reticule.
The Polar Scope Alignment Process:
- Locate a distant object and centre it in the crosshair using the Altitude and Azimuth adjusters.
WARNING: Do NOT use any telescope device for looking at the sun without appropriate solar filters. Doing so may cause permanent blindness. Do not use the sun for calibration.
- Turn the RA axis through 180°
- If the object has wandered out of the crosshair, the reticule needs to be aligned.
- Use the three alignment screws (see below) to move the reticule half way back to the object
- Re-centre the object back in the crosshair
- Turn the RA axis through 180° in the opposite direction
- If the object has not moved the reticule is aligned, if it has moved repeat until you can rotate the axis in either direction with no movement.
Once this adjustment has been made successfully, it should not be necessary to carry out this procedure again.
Calibrating the Index Marker Ring
Polaris, like all the other stars, appear to rotate around the NCP. Since the NCP is invisible we use Polaris as a visual guide because it is the closest star to the NCP. Since Polaris is moving we must calculate the position of the NCP in relation to Polaris.
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.
Polaris not only rotates around the NCP once every 24 hours, but it rotates in a bigger circle every 356 days with the Earth's tilt, so it appears to 'wobble' through the year.
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.
- Unlock the RA axis, and while looking through the polar scope, rotate the mount until Polaris is pointing at the 6 o'clock position.
- Lock the RA axis. In this position Polaris is at its highest point in the sky. Remember that the view is up side down in the polar scope.
- Unlock the RA setting circle by loosening the set screw. Rotate the RA setting circle so the pointer indicates 'zero' and lock the RA setting circle.
- Now Unlock the RA axis and rotate it so the RA setting circle indicates 0h 06m (use your calculated time!) on the top set of numbers. The bottom numbers are for use in the southern hemisphere. Lock the RA axis.
- Rotate the Date circle so that 17th November (use your calculated date!) is lined up with the Index Marker.
- Unlock the RA axis again and rotate it so that the RA setting circle reads Zero. You should find that Polaris is back where it started - at the bottom.
- Loosen the set screw on the Index Marker Ring and adjust it so that it reads your Longitude offset, East or West of the reference meridian for your time zone. (see below)
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!
- Rotate the Date Circle until the Index Mark is lined up with zero or your longitude offset (obtained from the section above, or to how you set the Index Marker at the calibration stage above.
- Unlock and set the RA Setting Circle to 0 hours and lock the setting circle again.
- Unlock and rotate the RA axis so that today's date on the Date Scale is lined up with the current time on the RA Setting Circle. Lock the RA axis.
- The reticule is now in the correct position. You can now use the Altitude and Azimuth adjusters to align Polaris in the small circle on the perimeter of the larger circle.
Polar Alignment using the Drift Method
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.
- Start by performing a rough polar alignment. This can be done using a compass to point the RA axis North. You can set the declination axis to your longitude.
- Find a bright star close to the meridian, just north of the celestial equator.
- Point the telescope towards this star and centralise it in the cross-hair.
- As the star drifts, align the cross-hair with the direction of motion of the star.
- Once the star is moving along the cross-hair, re-centre the star.
- Watch the movement of the star and adjust the altitude adjustment knob so that the star is half way back to centre. Use the hand controller to re-centre the star and repeat this step until no movement is observed along that axis.
- Repeat the above, this time for the azimuth axis.
- Repeat the procedure for a second star on the opposite horizon (e.g. if your first star was in the West, repeat for a star in the East).
You should now have an accurately aligned polar mount.
Using the Setting Circles
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.
- Point the telescope towards a known star near the object you wish to locate
- Using a star atlas or software, set the RA and DEC setting circles to the location of your known object.
- Without moving the setting circles (!IMPORTANT!) move the telescope until the objects coordinates line up with the setting circles.
- The object should be visible in a WIDE (low power) eyepiece field of view.