Before you start, make sure you are familiar with your telescope and that you know the parts and vocabulary of using a telescope.
The first thing you need to do is polar align your mount. This means aligning the right-ascension axis parallel to the Earth's axis of rotation. If you do not do this, your photograph will circle slightly around your guidestar, a problem known as field rotation. Wide-field photography with a normal (50-millimeter) camera lens requires minimal alignment. Simply pointing the mount's polar axis at Polaris as best as you can judge is usually enough for normal-lens exposures of a few minutes.
The easiest way to do astrophotography is 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 say that the best 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.
Reticle illuminators today are a large improvement over the incandescent bulbs in the past. Today's standard illuminator is a dim, red LED 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.
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. 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.
It sounds almost too good to be true: a robotic device that watches the guidestar and adjusts your telescope automatically, with a machine's tireless precision, while you relax or even sleep. It's no pipe dream; this is part of the CCD revolution that's sweeping high-end amateur astronomy. A CCD (charge-coupled device) is a chip that records images electronically and sends them to a computer, which in this case is rigged to drive the right ascension and declination motors. An autoguider can use a modest, low-cost CCD, making this capability affordable at the $400 level.
Some CCD cameras themselves (as opposed to autoguiders) have a "track and accumulate" function that allows them to take many short exposures, then stack them by computer, finding the best possible fit, to create one long exposure. This reduces and can even sometimes eliminate the need for guiding. Some advanced CCD cameras now include two CCD chips -- a small one to autoguide the telescope and a bigger, better one to record the image.
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