Good telescope polar alignment is important for long-exposure CCD imaging. When acquiring targets using the small field-of-view typical of many CCD imaging systems, especially CCDs, accurate positioning requires good polar alignment. Inaccurate alignment also leads to drift, which limits the amount of time an exposure can be taken before the stars begin to blur. Even if you are autoguiding, larger drift makes for more corrections and potentially lower guiding accuracy. Finally, with large CCD frames and high declinations, field rotation becomes a significant factor.
Many telescopes include a ”polar alignment scope.” These can work reasonably well if they are aligned properly with the mount. Unfortunately they rarely come from the factory aligned with the polar axis, and some of these devices change alignment if the focus is adjusted! Even if they are well adjusted, the accuracy of the polar alignment is limited to the resolution of the alignment telescope.
Telescope mount modeling software, such as MaxPoint, builds up a model of mechanical errors in the mounting by looking at the difference between the commanded position and the actual telescope position. This naturally includes the polar alignment error, which can simply be read off as a pair of numbers. Achieving accurate results requires measuring the pointing error at a number of points spread around the sky, and the process has to be repeated from scratch every time an adjustment is made to the system. Although this procedure can be a little time-consuming, it does work very well if a sufficient number of measurement points are used.
A technique known as Drift Alignment has been used for many years, and can achieve an extremely accurate polar alignment. Unfortunately it is very time consuming, since the drift of a star over time must be observed. This can be greatly speeded up using a CCD camera, since sub-pixel centroid measurements can easily be made. Here is a suggested procedure:
1. Mount your CCD camera with north at the top.
2. Connect MaxIm DL to the camera, configured as an autoguider.
3. If you have an autoguider connection to the mount, remove or disable it. This can be done by disconnecting the cable, or by going to the Guide tab Settings and turning off Enable X and Enable Y.
4. Point to a star at the meridian, near the equator.
5. Start the guider in Track mode, and watch the error reading in Declination only. It is okay to enable guiding in Right Ascension, if needed (for instance if the star drifts rapidly out of the field of view). Due to the sub-pixel measurement capability of the autoguider function, you will be able to see any error quite quickly. Note which way it is moving and how quickly. (For extremely high precision alignment for a permanent mount, you may wish to wait several minutes to see the drift.)
6. Now stop guiding and switch to Focus mode. Set up a very short exposure, binned for speed, and continuous exposures. Adjust the mount in azimuth, watching the star move. This way you can tell exactly how far you are moving the mount (see below for calculating image scale).
7. If the star drifts North, adjust the azimuth to the East. If the star drifts South, adjust the azimuth to the West.
8. Re-center the star using the telescope slow motion controls and repeat steps 4 through 6 until the drift is small.
9. Point to a star near the East horizon, close to the equator. Make sure the star is at least 20 degrees above the horizon, to avoid excessive refraction.
10. Start the guider in Track mode, and watch the north/south drift as before. Note which way the star is moving, and how quickly.
11. Stop guiding and switch to Focus mode; again, use continuous, short binned exposures. Adjust the mount in elevation, watching the star motion to see how much you are moving it.
12. If the star drifts North, move the polar axis down. If the star drifts South, move the polar axis up.
13. Recenter the star using the telescope slow motion controls, and repeat step 10 through 12 until the drift is small.
14. Go back to step 4 and start again, until the drift is small in both positions. Remember to reconnect or re-enable your autoguider when you are done.
Paul Boltwood has pioneered a very simple and effective technique for polar alignment. Using a planetarium program, create a chart of the area around the pole (north or south). Make sure that the proper pole location is labeled on the chart.
Next, do a rough polar alignment, and then take a 60-second CCD image of the pole. Halfway through the exposure, rotate the telescope about the RA axis as far as it will go. Move it just fast enough to complete the arc in 30 seconds. You will create an image with the stars clearly visible, plus a set of near-circular arcs centered on the position of the telescope’s pole. You can now easily see the difference in position between the telescope’s polar axis and the correct pole location. In fact, you can measure this distance in X and Y using the Information window, and get a very accurate number. Now adjust the mount by that distance (tip: set a rapid binned exposure mode and watch the stars move as you adjust). After one or two iterations you have an extremely accurate polar alignment.
This is a new method from Larry Weber and Steve Brady, the inventors of the Half-Flux Diameter focus measurement technique and the free FocusMax utility. It uses several images taken across a region of the sky. The rotation of the fields is measured using the PinPoint astrometric engine. From there, calculations are performed to determine the location of the telescope’s axis compared to the proper pole location. Their easy-to-use PolarAlignMax utility works with the PinPoint astrometric engine. The PolarAlignMax utility can be obtained from http://focusmax.org.