Through the Telescope

When it comes to deep-sky photography through a telescope, you have two choices: using a separate guidescope or an off-axis guider.

A guidescope attaches to the main telescope via mounting rings that allow it to be aimed independently to some degree. This lets you choose any guidestar up to a couple of degrees from the field being photographed. As a rule of thumb, the guidescope should have about as long a focal length as the telescope you are photographing through. It should also have a reasonably large aperture. Such a guidescope is a substantial instrument in its own right, adding a lot to the whole setup's cost, size, weight, and demands on the mounting.

Guidescopes have another problem: flexure. During an exposure the guidescope must not bend, shift, or otherwise change orientation with respect to the main telescope's optical axis. Nor can anything in the main telescope bend or shift. Otherwise stars will come out elongated, double, or irregular even when you guide perfectly.

For such reasons the guidescope has been largely eclipsed in the last 20 years by the off-axis guider. This device allows you to look through the main telescope at the same time you're photographing through it.

Off-axis guiders generally use a little "pick-off" prism to divert a small part of the image to the guiding eyepiece. The pick-off prism is near or outside the edge of the camera frame, so its shadow has little or no effect on the photograph. You maneuver the prism around to find a good guidestar before starting the exposure. Alternatively, some guiders use a full-aperture window called a pellicle that transmits most of the light to the film while reflecting 10 or 20 percent to the eyepiece.

At long focal lengths, off-axis guiding gives the best results. The starlight you see in the eyepiece goes through the same optical assembly, by and large, as the light going to the film, so tube flexure ceases to be an issue. If it happens you just guide it out.

There are, however, a few inconveniences to consider. Finding a guidestar can be tough, because the area of the field accessible to the pick-off prism is limited. The guiding eyepiece extends out at a 90° angle to the light path of the telescope, and to find a good star you may have to rotate the eyepiece holder around the optical axis to an inconvenient angle. (Some new off-axis guiders allow the eyepiece holder to be rotated independently of the camera.)

Focusing is another consideration. To focus, aim at a bright object, look through the camera's viewfinder, and turn the focus knob until the image seen through the camera is as sharp as you can get it. Leave everything right there. To focus the guiding eyepiece, slide it up or down in its holder; resist the temptation to touch the main focus.

Finally, what you see in an off-axis guider is not exactly what you get. You cannot use a star to guide on a moving comet or asteroid. To track such an object you either have to revert to a guidescope, in which you can track it directly, or calculate the object's expected motion and move your guidestar slowly and steadily at exactly the right speed in the right direction.

Whether you use a guidescope or an off-axis guider, photography through a telescope requires that you work with very high power. The rule of thumb is to use a magnification about five times the telescope's focal length in inches. Thus, with an 8-inch f/10 Schmidt-Cassegrain (focal length 80 inches), try guiding at about 400x. In an off-axis guider, a 9- or 12-mm eyepiece with a 2x Barlow lens will always be just about right, no matter what your telescope.