Three essentials for CCD imaging are finding, focusing, and following: you can't make images if you can't find celestial objects; images have little value if they are not in focus; and your telescope must follow an object for a reasonable length of time for a high-quality image. Equipment that makes these tasks easy is fundamental to long-exposure imaging.
4.4.1 Flip Mirrors, Finder Scopes, and Go-To Mounts
Many observers report some degree of difficulty finding celestial objects when they begin using an astronomical CCD or DSLR camera. The skills needed to find objects for imaging are no different than those needed for visual finding; the difference lies entirely in the accuracy required. While pointing within V20 is adequate for most visual observers, digital imaging is most efficient when you can accurately point the telescope within a few minutes of arc; pointing errors of V20 are frustrating, if not unacceptable.
Broadly speaking, telescope pointing aids fall into three classes: CCD camera finder functions, optical finding accessories, and accurate telescope mountings. The first of these should be built into the CCD camera acquisition software. The software finder mode usually consists of a high-speed CCD readout that puts a fresh short-exposure image on the screen every second or two. The value of a fast finder display cannot be overstated; instead of exposing for 30 seconds and then waiting another 15 to 45 seconds for the screen display to appear, a fast readout mode gives the observer real-time feedback on what the CCD camera sees. If the object is in the field of view of the camera, centering takes just a minute or less.
Given a CCD camera with a fast finder mode, an optical finder and an accurate telescope mounting insure that the object will remain in the field of view.
Big Finders. The finders that come as standard equipment on many telescopes provide too little light grasp and magnification to center deep-sky objects accurately. A large-aperture finder telescope allows the observer to see and center fairly faint objects well enough that a flip-mirror system may not be needed. Many observers own an old 60-mm or 80-mm refractor; these are obvious candidates to become auxiliary high-power finders. Depending on the size of the telescope, a retired 4-inch to 6-inch reflector also makes an excellent high-power finder.
Mount the auxiliary finder in ring brackets on the main telescope. If necessary, replace its original low-quality focuser with a standard 11/4-inch focuser, and add an illuminated cross-hair eyepiece that provides between 20x and 50x magnification. Once the new finder has been aligned, it should be easy to center any visible target within 1 minute of arc. A big finder is usually the most cost-effective way to locate and accurately center deep-sky objects.
Unity-Power Finders. Although Telrad-type finders offer neither magnification nor added light grasp, many observers swear by them. With the aid of a good star atlas, a skilled Telrad user can often point a telescope within 0.2° of the true location of a celestial object. This places the object close enough that the observer can then locate and center it with a big finder or a flip-mirror system.
Flip-Mirror Systems. These systems fit between the telescope and the CCD camera. They contain a swiveling or sliding mirror that directs the image from the telescope to a cross-hair eyepiece, allowing the observer to accurately center any object bright enough to be seen visually. Their great advantage is that they deliver all the light gathered by the telescope to the observer's eye.
Before you order a flip-mirror, check that your telescope has enough back focus to accommodate the extra length of the device. You should be aware that with fast optical systems especially, the flip-mirror housing may block parts of the converging beam, and cause unwanted vignetting.
In addition to its primary function as a finder telescope, when the image is sent to the eyepiece, the flip-mirror should block all light from the CCD camera, either with a separate dark slide or as part of the mechanism that moves the flip mirror. This allows the observer to use the flip-mirror to cap the telescope for taking dark frames, and also allows making dark frames while the observer finds and centers the next target object.
Sky Atlas Software. Bulky books and charts pose a significant obstacle to efficient imaging, especially when the wind is blowing and the temperature has fallen through the dew point. Since any observer using a CCD camera is already operating a computer, sky atlas software such as MegaStar, TheSky, Earth-Centered Universe, and Guide is a godsend. Not only do these programs display thousands of celestial objects, but they also allow the observer to set the magnitude limit and set the on-screen image orientation to match the eyeball view. Combined with a big finder, sky atlas software is an outstanding observing aid.
Digital Setting Circles. Although a quality set of analog setting circles can serve admirably, digital setting circles make locating objects easier with any telescope. These systems consist of digital shaft encoders and a dedicated computer unit to convert signals from the encoders into right ascension and declination. The computer unit can also store a database of hundreds or thousands of popular sky targets, and then signal when the telescope's position matches the coordinates stored in the unit's database.
Go-To Mountings. Computer-driven telescope mountings can make CCD observing extremely efficient. Today's market supports two classes of computer-controlled mountings: those designed for visual observers (typically accurate to about Va°\ and those that attain the 1-minute-of-arc accuracy that is desirable in CCD imaging. Before buying, an observer who is considering the purchase of an automated telescope should consult observers who already own the product for an honest assessment of its performance.
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