TYPES OF TELESCOPES

There are several types of telescopes that are suitable for astrophotography. The refractor (Fig. 1) has a classic design and is probably the most easily recognized telescope. There are no central obstructions to reduce contrast or scatter light. This produces sharp, crisp images. These are very well suited for lunar/planetary or stellar work (e.g. star clusters, double stars). A refractor can outperform a reflecting telescope of much larger aperture. The main drawback to these scopes is their cost, especially at apertures suited for deep-sky photography. They also tend to be quite heavy. Also, quality refractors must incorporate either fluorite or other extremely low dispersion glass to prevent chromatic aberration from occurring. This refers to the fact that most optical glass tends to bend different wavelengths of light to different degrees, causing a blue halo to appear around bright stars.

One of the most popular designs is the Schmidt-Cassegrain, also known as a catadioptric (see Fig. 2). This design utilizes a lens and mirror combination. As you can see from the figure below, this telescope "folds" the light path. This allows for relatively long focal lengths to be contained in a short (i.e., more portable) tube. This type of scope affords generally good optics although the center obstruction scatters some of the incoming light, reducing the contrast of the image. Also, the multiple light paths and reflective surfaces causes more light to be lost due to poor transmission and internal reflection. On the other hand, their large apertures make them suitable for deep-sky photography. One problem encountered when doing astrophotography with these scopes is known as "mirror flop." Since the telescope is focused by physically moving the mirror back and forth, it is subject to shifts in its position as the scope moves across the meridian. This is one reason why a separate guide scope generally does not work well for long exposures. An off-axis guider would be required.

A Maksutov telescope is very similar to the Schmidt-Cassegrain except for the fact that the corrector plate is thicker and more curved. Also, instead of a secondary mirror, these scopes have an aluminized spot on the corrector plate itself to reflect light back through the optical tube (see Fig. 3). Their quality is equal to or better than the catadioptrics. One problem is that the thick corrector plate requires additional time to reach thermal equilibrium (all scopes need to reach the same temperature as the environment so as to prevent image distortion caused by the gradual shrinking or expanding optical components).

The Newtonian reflector (Fig. 4) is, inch for inch, the most economical telescope to purchase. Large apertures are readily available. Of course, with increased aperture comes increase size and weight. These scopes also tend to suffer from coma in the edges of the field of view. This means that star images tend to appear oddly elongated (like kidney beans). This is especially noticeable in short focal length (f/5 or less) scope of this type. Newtonian reflectors also need to be collimated on a regular basis to retain good image quality. Collimation is the process by which the primary mirror is adjusted to keep the secondary mirror dead center in the field. The spider vane that holds the secondary mirror in place will cause "diffraction spikes" to appear on stellar images. Some people actually like this effect! Overall, these scopes are good for deep sky as well as lunar/planetary photography.

 

There are several other, less common, telescope designs that are suitable for astrophotography. One of these is a hybrid: the Maksutov-Newtonian. It looks like a classical Newtonian except that is has a meniscus lens on the front end. The secondary mirror is attached directly to this lens, eliminating the need for a spider vane (and resulting diffraction spikes). This design also allows for the secondary obstruction to be much smaller than that found in the Newtonian telescope. This produces brighter images with good contrast. Also, the sealed tube prevents dust from building up on the primary mirror.

One important caveat: the best optics in the world are useless unless they are seated on a sturdy mount. An equatorial mount is necessary for all but the brightest of objects. Any long exposures require that we can track the object accurately as it traverses the sky.

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