Good Guiding

Guiding long photographic exposures brought CCD cameras to the world of amateur astronomy. When the Santa Barbara Instrument Group (SBIG) introduced their ST-4 camera, it was intended as a guider that would automatically sense tracking errors and restore a guide star to its correct location. It didn't take long before observers realized that a tiny CCD (such as the 165x192-pixel TC-211 in the SpectraSource Lynxx) was capable of imaging faint stars and galaxies. CCD imaging took off, and the rest is history.

There are three basic methods of guiding:

• Off-Axis Guiding

• Auxiliary Telescope Guiding

• Software Virtual Guiding

The first two methods are employed by autoguiders and by observers who guide manually. The third method is unique to astronomical CCD cameras.

In the old days, astrophotographers stared into an eyepiece for hours on end to guide long exposures on film. Today many observers use an autoguider to do the work; but for others, even the simplest autoguider is a budget-buster. For these observers, manual guiding is still an option, and because CCDs and digital cameras do not require hour-long exposure times, this option is relatively easy compared to the old days.

Either guiding process exacts a significant toll in observing time, because after acquiring the object, it is necessary to locate a guide star and to designate it in the guiding camera or center it in the crosshair eyepiece. Whether you guide manually or automatically, you will need to develop efficient techniques for the task.

5.2.1 Off-Axis Guiding

CCDs were introduced to amateur astronomy as autoguiders to relieve the tedium of guiding long exposures. With a second CCD to detect the motion of a guide star and automatically command the slow-motion motors on the telescope, an observer can make long exposures without suffering from a stiff neck for days afterward.

Many self-guiding CCD cameras are off-axis guiders; that is, a second CCD in the camera head picks up a star image that is off the optical axis. This guiding method eliminates potential problems with shifting optics and telescope tube flexure.

The operating technique in off-axis guiding is to locate the target object in the imaging CCD, and then find a guide star. Because the guider CCD is more sensitive than the eye, often there are several stars bright enough to serve as the guide star. After selecting one and starting the autoguider, the observer can make one or more exposures with the imaging camera.

Manual guiding works much the same way—except instead of a CCD watching the guide star, an eyepiece equipped with illuminated crosshairs receives the off-axis image. The observer's job is to monitor the position of the star and when it drifts, to press control buttons that recenter the star. Manual guiding is tedious—there is no other word for it. However, it is a skill that can be satisfying to master, especially when the result is small, perfectly round star images.

5.2.2 Auxiliary Telescope Guiding

Using an auxiliary telescope to guide is the principal "real-time" alternative to the off-axis method, and the only practical technique for use with digital cameras. The principal requirement is a second telescope solidly mounted on the imaging telescope. In practice, you place the autoguider head at the focus of the auxiliary telescope and focus the image. After you center your target object in the imaging telescope, locate a suitable guide star with the auxiliary telescope, designate the star to the autoguider software, allow the guider to self-calibrate, and then relax while the autoguider keeps the star centered within a fraction of a pixel.

For guiding manually, first acquire the target object in the imaging telescope, place a high-power eyepiece with an illuminated cross-hair reticle at the focus of the guide telescope, align the latter on a star bright enough to see easily, and then, after starting the exposure, use the telescope's slow-motion controls to keep the star centered precisely in the reticle. It is easy to take five-minute exposures providing you have a telescope that can track more than 30 seconds without trailing.

Although it looks a bit odd, the auxiliary telescope technique works especially well when the guiding instrument is larger than the imaging one. The number of potential guide stars is much larger, and of course, using the small telescope for imaging gives you a system that is more forgiving of guiding errors.

5.2.3 Software Virtual Guiding

Few telescope mounts satisfy a "less-than-one-pixel" tracking standard for exposure times of five to ten minutes, but many mounts track adequately during exposures of one or two minutes. With a sinusoidal periodic error, the drive rate is nearly correct for about one-fourth of each rotation of the worm. With a standard 360-tooth drive gear, the worm rotates once in four minutes. Without guiding at all, you will get two slightly trailed 60-second images and two reasonably well-tracked 60-second images during each rotation of the worm. Furthermore, in a 60-second exposure, less-than-perfect polar alignment tracking errors are usually not large enough to matter.

CCD cameras can exploit short exposures using the track-and-stack tech-

Exact Focus

Exact Focus

Geometric Disk Geometric Disk

Inside Focus Outside Focus

Figure 5.2 At best focus, star images are maximally concentrated and compact, limited in size by diffraction. On either side of focus, the image expands. Depth of focus is defined as the distance over which a star image remains sufficiently small that it is effectively still in perfect focus.

Geometric Disk Geometric Disk

Inside Focus Outside Focus

Figure 5.2 At best focus, star images are maximally concentrated and compact, limited in size by diffraction. On either side of focus, the image expands. Depth of focus is defined as the distance over which a star image remains sufficiently small that it is effectively still in perfect focus.

niques; instead of one ten-minute exposure, program the camera software to make twenty one-minute exposures. Typically, somewhere between half and two-thirds of the exposures will have acceptable tracking, and those that don't can be discarded. The good images are registered in software and stacked to form an image with the signal-to-noise as good as a single long exposure.

Although the track-and-stack method requires lots of space on the imaging computer's hard disk, if you cannot afford a high-quality mounting and an autogu-ider, it is a practical and cost-effective way to accumulate enough photons to make an outstanding image.

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