…continuedGuiding a Telescope for Imaging
The easiest way to enter the realm of guided deep-sky astrophotography is by the piggyback method. Simply attach a camera to the side of the telescope, point skyward, and open the shutter. You guide the camera by tracking on a star seen in the telescope itself.
Piggyback photography puts the least demands on your guiding ability. Most piggyback photographers start with a normal or a wide-angle lens (such as 28 mm) that will capture whole constellations at once. Almost any camera lens has a much shorter focal length than the telescope, so the image scale is much smaller. This means you can make numerous small guiding mistakes without affecting the photograph. Piggybacking provides just the type of practice you need to gain experience for other, more difficult forms of astrophotography.
What kind of eyepiece do you need for watching the guidestar? For piggyback guiding you might try a plain, extremely-high-power eyepiece with no cross hairs or reticle. Simply keep a bright guidestar centered in it as best you can judge.
For photography at longer focal lengths, you need an eyepiece with an illuminated reticle or cross hairs. Dozens of guiding eyepieces are on the market, and many perform other functions as well. What do you really need?
Some people feel that the best design is still the old-fashioned plain cross hairs. Any motion is revealed when the star emerges from behind their intersection. Some astrophotographers like to keep the star in view, tucked in a corner adjacent to the intersection. If you use a double or dual cross hair, the guidestar can be placed at any of the four intersection points or defocused to nearly fill the small central square.
Another approach is to use an eyepiece reticle with concentric circles, each denoting a different guiding tolerance. If you can keep the star inside the appropriate circle, you know that all is well. For this approach to work, however, you have to know the guiding tolerance for your particular photographic setup and which circle on the reticle this corresponds to. Guiding tolerances are usually very tight, so most astrophotographers simply prefer to guide as accurately as they possibly can and hope it's good enough.
One often overlooked approach is the projection reticle. This device superposes a reticle's image onto the view in an ordinary eyepiece. Some designs have a 3x Barlow lens built in to allow the use of medium-power eyepieces that have comfortable eye relief. Another advantage is that the reticle's image can be moved around the field to align on a guidestar; you don't have to move the whole telescope to the star. This allows more flexibility in aiming and composing your photographs.
Reticle illuminators today are a vast improvement over the incandescent bulbs with their tangles of wires that were the rule in the past. Today's standard illuminator is a dim, red LED (light-emitting diode) that draws only a tiny current from a small battery that's right inside the eyepiece unit itself. The brightness should be adjustable so you can set it to the best level for any guidestar. One of the latest and greatest improvements is the blinking LED, which gives you alternate views of the guidestar with and without the cross hairs. This allows much fainter stars to be used for guiding.