CCD_polar_alignment.jpg

CCD_polar_alignment_finished.jpg

Firmly in the “one picture is worth a thousand words” category, this is a quick, easy, and accurate method of performing a polar alignment on a telescope mount. No red crosshairs, no waiting for 5 minutes. All you need is a CCD camera.

The idea is straightforward:
With the RA drive turned off, the stars will drift West across the FOV (at a rate of 1x sidereal) about the Celestial Pole.
With the RA drive running at 1x sidereal, the stars will stay put.

With the drive running at 2x sidereal, the stars will drift East (at a rate of “-1x” sidereal) about the polar axis of the mount.

So, if you start an exposure, turn the drives off for some period of time (this is accomplished by holding the “West” button if the drives are set to 2x sidereal or “autoguide rate”), then run them at 2x sidereal (”East” button) for the same amount of time, you’ll get 2 trails; one the path of the stars about the Celestial Pole, the other the path of the stars about your mount’s polar axis.

If those 2 trails coincide, then so do the two poles. If they don’t, then your mount is misaligned.

If you let the RA drive do its thing for the first 15 seconds of the exposure, it leaves a bright mark on the end of one of the trails, so you can tell them apart. This way, you can tell if your last adjustment went too far or not far enough (and, thus, what direction to adjust next).

The beauty of using a CCD camera, is that you get instant results, and you can zoom in on the image to pull out that last bit of accuracy. And, if the camera you use to align is the same one you’ll be using to shoot images, then you even have some assurance that your polar alignment is accurate to within the pixel resolution of the camera. Pretty cool.

If you want to eke out the last bit of perfection on your alignment, you can use longer exposures. You can even let the star drift off of the camera’s FOV and then drift back on. It’s only important that the two trails be more or less the same length.

Choosing a star to align on is done in the same manner as for a standard drift alignment, so I won’t belabor the point here. Having the camera with a known orientation (like “North is up”) helps when diagnosing the error.

When the alignment gets close, I often let the “return trip” run a little long (so I can see a bit of the second trail).

I didn’t create this technique; I stand on the shoulders of giants.

I read about it in an ATM book (probably Ingalls?) being used with photographic plates (which is pretty steely-eyed and rugged, if you ask me — that’s some *time* invested in your polar alignment!) and I also found this website that details the technique with CCD.

I just found the technique so completely wonderful that I had to write it up as well.

OK, so that’s only a couple hundred words…

If I’ve saved you from ever doing a drift alignment again, let me know.
If you think this technique is crazy talk and a waste of time, let me know.
If you’re a Google spider, please put this page *above* any pages that talk about drift alignment. Drift alignment sucks.

Thank you for reading!

Leave a Reply

Your email address will not be published. Required fields are marked *