Are you intimidated by manual polar alignment? To be sure, computer-controlled, GPS-equipped telescopes have largely automated this task. But, like star-hopping, polar alignment is still a good skill to know.
Add-on polar alignment telescopes and illuminated reticle eyepieces can make polar alignment a breeze. Here are a few currently on the market:
- Celestron‘s CG5 CGEM finderscope: https://www.celestron.com/products/cg-5-avx-cgem-polar-finderscope ($61.95) is designed for Celestron’s CG-5, Advanced VX and CGEM line of equatorial telescopes. The scope inserts into the polar finder port in the right ascension axis and shows polar star patterns for both hemispheres.
- Sky-Watcher’s EQ5 polar scope ($95.00) is designed for Sky-Watcher’s AZ/EQ-5 telescope and includes includes a balance bubble for leveling, and an illuminated reticle.
- Svbony‘s illuminated reticle eyepiece ($27.15) has a standard 1.25”-inch barrel for use on any telescope. An essential item for the drift method of fine polar alignment mentioned below, the eyepiece has double red crosshairs and variable illumination.
When alignment is (and isn’t) needed
Fine alignment is crucial to long exposure astrophotography. Even when you’re stacking a series of shorter exposures, star-trailing can show up in short order. But during causal observing or at a public star party, rough alignment is often good enough for the drive motor to track the sky, with an occasional nudge to re-center the target in the eyepiece during the night. When rough alignment is all you need, simply aim the right ascension axis at the celestial pole, turn the drive motor on, and you’re off and running. Just be sure to check the view once in a while.
Many German-equatorial mounts come equipped with a small, secondary polar alignment telescope. Some are even illuminated and etched with guide stars for either hemisphere. Here in the northern hemisphere we have a ready-made bright star very near the north celestial pole: Polaris.
Of course, this is only true in the current epoch. Polaris is two-thirds of a degree from the true north celestial pole (NCP). It’s moving closer to the NCP, getting closest (27′) around 2100, drifting away from it again after that. Precession of the Equinoxes is responsible for the Earth’s rotational axis wobbling, making a complete circle in the sky every 25,800 years.
Polaris is easy to find using the two pointer stars at the end of the bowl of the Big Dipper. The NCP currently lies 39′ in the direction of the 2nd magnitude star Kochab (Beta Ursae Majoris).
Finding the celestial rotational pole in the southern hemisphere is trickier. The only nearby marker star is faint 5.5 magnitude Sigma Octantis, which is over a degree (68′) from the pole. Fortunately, there are several methods to get you in the neighborhood of this faint star:
- Find the Southern Cross, and follow its long axis to the South Celestial Pole (SCP).
- Find the Large and Small Magellanic Clouds and draw an imaginary equilateral triangle, with the SCP at the third apex.
- Trace a line from Sirius to the bright southern star Canopus, then continue on an equal length to the SCP.
Getting fine alignment for astrophotography
When you need precise polar alignment, the drift method is the tried-and-true method. This method works for all equatorial mounted telescopes, and is especially suited for German-equatorial (GEM) mounts where the right ascension and declination axes match the directions in the sky.
Here are the steps for fine polar alignment (NOTE: This is for northern hemisphere observers; reverse the directions in step 6 if you are in the southern hemisphere):
- Rough polar align and level the telescope (see above).
- Aim the telescope at a bright star very near of the celestial equator (within 10 degrees), close to the north/south meridian (as close to due south). Mira, Porrima (Gamma Virginis) and the Belt Stars of Orion are great choices for this.
- Nod the telescope up and down, then left and right, while watching the view. This shows you which direction is north and south, east and west, as some optical configurations invert the view.
- Center the star and watch it for about 10 minutes, noting which direction it drifts. An illuminated reticle eyepiece is great for this.
- If the star drifts north, adjust the mount’s azimuth to the east. If it drifts south, adjust it to the west.
- Find another equatorial star near the east horizon, and center it in the view. Watch it for 10 minutes and note which way it drifts. This time, if it drifts north, lower the pole in altitude (elevation), if it drifts south, raise the pole. (Note: Invert this step for a target on the western horizon).
Repeat the procedure to eliminate any residual errors. It may take a few tries to eliminate drift entirely, but in the end you will have rock-solid tracking.
But how do you polar align in daytime for solar observing and photography, and following bright daytime targets? There are a few options for accurate daytime tracking.
If you have an equatorial mount, level your telescope and use a compass or compass app to point the right ascension axis towards north. Just be sure to take the magnetic declination – the deviation of magnetic versus true north – for your site into account. Another method is to mark off your site beforehand, noting landmarks that are due north and south.
But these methods aren’t as precise as polar alignment using the stars. Thus, the best way to achieve precise alignment during the daytime is to polar align before dawn and leave the drive motor running as night turns to day.
Fine polar alignment can be time-consuming, but the ease of finding and tracking objects with the heavens are properly aligned are well worth the effort.
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