The Polar Alignment Demon

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Why take pains to point the scope's RA axis precisely at the celestial pole? Poor polar alignment causes objects to drift in declination, resulting in trailed stars. The only cure for that is to improve polar alignment accuracy. Just aligning on Polaris isn't good enough. Even the fairly accurate alignment produced by polar finders is usually not precise enough for exposures more than a minute or two long. What may be needed is a drift alignment.

I used to recommend that astrophotographers always drift align their scopes. Today, that's probably less necessary. The combination autoguiding, short focal lengths, and short exposures make dead-on alignment less importantl. Something better than pointing the polar axis at Polaris is still required, but that "something" may be quickly and easily achieved with the polar alignment routines featured by some go-to scopes' hand controllers.

What is drift alignment? It is the process of accurately adjusting a telescope's mounting by watching the north/south and drift of selected stars. This procedure seems complicated at first but will soon become second nature. Begin a drift alignment by doing a good "normal" alignment. Point the RA axis at the pole as accurately as possible using a polar scope, the hand controller's polar align procedure, or any other means that's better than "point at Polaris." The closer the mount is to the pole initially, the quicker the drift alignment will be. Next, locate a star close to the Local Meridian (the imaginary line that runs through the north and south celestial poles and the zenith and nadir) and around approximately 15 to 20 degrees north of the celestial equator (i.e., at a declination of about +20). This is not hyper-critical. Just find a nice medium-bright star in the general area. When the star is in the crosshairs of an illuminated reticle eyepiece at 250x or higher (using a Barlow if necessary to achieve this magnification), rotate the eyepiece and diagonal until the crosshairs are aligned so up-down is declination. Ensure the star moves precisely along this vertical crosshair when the scope is moved in declination and along the horizontal one when it's moved in RA

Now, let's drift. Put the star in the center of the crosshairs using the hand paddle and watch for movement up or down—for drift along the N/S crosshair. Don't worry about E/W drift. It's OK to guide in RA with the hand controller to keep the star near the center of the crosshairs, but all we want to know at this time is whether the star is drifting up or down in the field. If the star drifts up in the field, adjust the azimuth (left-right) of the wedgeor a GEM's polar axis to make the star move right in the field (Figure 9). How much? About the same distance the star drifted. If the star drifts down, move the star left in the field. After these azimuth adjustments, use the hand controller to re-center the star in the crosshairs. Keep watching for up/ down drift and moving the wedge or polar axis in azimuth until the star doesn't move up or down for at least five minutes.

An accurate drift alignment usually requires the mount also be adjusted in altitude. Locate another medium-bright star, this time right on the celestial equator and roughly 15 to 20 degrees above the eastern horizon. Set the eyepiece up as above (by rotating the diagonal in the visual back as required), with the up-down crosshair again defining declination movement and the left right crosshair defining RA. Center the chosen star in the crosshairs and watch for drift. We're still watching for up/down drift, but the wedge/polar axis adjustment differs. If the star drifts up, adjust elevation to move the star down in the field. If the star drifts down, move it up. Keep doing this (re-centering the star between adjustments) until there's no visible up/down drift for five minutes. With the "elevation" star steady, carefully tighten down the wedge's bolts to lock it in altitude and azimuth. A careful "five minute"

Telescope Drift Alignment
Figure 9. (Drift Alignment) A drift-type polar alignment is easy to do and offers the precision necessary for long exposure astro-imaging. Credit: Author.

drift alignment should allow exposures of at least an hour, with no trailing due to declination drift.

The Focus Demon

The only thing that makes an astrophoto look worse than trailed stars is bloated and out-of-focus stars. Getting good focus is important but not difficult. Make focusing easy, first of all, by not trying to focus using the dim viewfinder of a DSLR. If at all possible take a laptop into the field. It's hard to overstate how much easier it is to get good focus by looking at a PC display than squinting through a viewfinder. Many DSLR and CCD programs have additional focus aids, too, like a quickly updating focus mode, a zoomed-in magnified focus frame, and an "intensity" indicator—the higher the number, the better the focus.

What's a good way to focus? Start out with a bright star, usually your last alignment/sync star. Set the camera to expose for 1 second or less (so the star is not insanely overexposed), and adjust focus until the star is as "small" as possible. You can observe the changing numbers of your program's focus indicator, but mostly rely on the visual appearance of the star. When it's as good as you can get it, ramp up the exposure to two, three, or more seconds, until dim stars begin to appear in the frame. Continue to modify focus until these are hard little pinpoints. How long should you spend focusing? As long as it takes. Continue to check focus over the course of the evening and refocus when needed; aluminum scope tubes can and will expand and contract with temperature changes, altering focus. If you are having difficulties achieving focus on a particular object, try slewing to a Messier globular cluster, if one is available. As a friend of mine once said, "Globs are God's gift to astrophotographers." What that means is that bright globulars, with their scads of tiny stars, provide a perfect target for achieving exact focus.

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