Guiding the light

I took my first astrophoto during the 1986 visit of Halley’s Comet. I figured it couldn’t be too difficult, so I bought a few items, put high ASA film in the camera, aimed at my target, and exposed the film for 10 minutes. I had seen all the pictures in the astronomy magazines, and, because I was a professional photographer, I figured this was how they did it.
 
Instead of magazine-worthy images, I ended up with a lot of squiggly lines. My photos lacked “guiding,” a way of compensating for random atmospheric anomalies and other unexpected changes.
 
There are two basic ways to guide, and we will look at both in detail. The first, and arguably the easiest, is to use a guide scope. You obtain an image through the main telescope while guiding — keeping a separate “guide star” lined up — through a different, usually smaller, telescope. The two scopes cannot budge in relation to each other; the term for such movement is differential flexure. You must be careful that both telescopes are securely clamped together and that the focusing tubes are rigid.

Another important consideration when using a guide scope is to pick a guide star from the center of the photographic field. Before I hunt for a good guide star, I always line up the imaging scope and the guide scope by placing the same bright object in the center of both. When looking for a guide star, I try to disturb the guide scope as little as possible. Even if your gear is perfectly polar aligned, not having a guide star centered in the photographic field will result in field rotation (meaning trailed stars) during long exposures.

The other basic way to guide is to use off-axis guiding. As the name implies, this process uses a small prism to gather light from outside the axis of light entering the camera. Because all the light comes from the same source, differential flexure isn’t a problem. This is the guiding method of choice with any large telescope and Cassegrains of all types, whose magnified and sensitive images make it hard for guide scopes to “keep up.” Pick a solidly built off-axis guider that moves the focus point back as little as possible, and check to see if the guider offers connectivity to your imaging setup. You might have to order a custom-made part from Precision Parts, Inc., to get everything to work.

Whatever method you use, once you have the guide star in place, the next step is to calibrate the mount. Your imaging software needs to know how much your mount moves in a given amount of time and in what direction. It helps to line up the movement of the guide star parallel to the right ascension and declination by rotating the guide camera. Set the calibration so that you can move the guide star at least six star diameters in each direction.

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Once calibrated, start tracking and watch the guide star. Every night has different seeing conditions, and you need to adjust each time. If you are lucky enough to have good seeing, leave the aggressiveness of the corrections (how doggedly your camera stays on the guide star) at 100 percent and use a short exposure. You want to take advantage of the still conditions and insistently keep the star “locked in.” But if you see the star moving noticeably, you will need to calm things down by lowering the aggressiveness setting to 50–70 percent and using a longer exposure time.
 
You can’t keep up with “jittery” seeing, and if you use a short exposure setting, you will only be “chasing the seeing”: The mount will continuously try to catch up to the star and in the process actually make the guiding worse! Under conditions like this, you want to give the jittery star room to move around and only correct for the major errors. Your image already will suffer from the jumpy seeing — no need to make things worse.

For those of you just starting out, I recommend an illuminated reticle, which is an eyepiece that has reference lines lit up inside it. Try to guide manually to fully understand the dynamics of your imaging system. If you have guiding problems and all else fails, insert an eyepiece to see what’s going on.
 
Finally, if you have an erratic mount by nature, you might consider buying an adaptive optics device that performs fine corrections via a lens or mirror. The improvement can be dramatic, though less so for a high-end, well-tuned mount.
 
I hope this takes some of the mystery out of guiding.