The performance of the most modest telescope can be improved by using a really good eyepiece. Good eyepieces are important to planetary observing, since the observer is often using high magnification trying to see subtle surface markings. Thanks to modern manufacturing, there is a wide selection of very fine eyepieces to choose from today with a broad range of prices (Fig. 7.5). The eyepiece is one piece of equipment that the observer should not be too thrifty about. You should invest as much as you can in a couple of really good eyepieces. They can make all the difference.

To observe Jupiter, you should have a modest number of eyepieces to produce a range of magnifications. A good Barlow lens should also be included. Beginners often attempt to use too much magnification when observing Jupiter, thinking bigger is better. Experienced observers find this not to be true. Although, Jupiter is large and bright, it does not tolerate high magnification well - the image tends to go soft quickly. A good rule of thumb is not to exceed magnification of 40x per inch of aperture. With poor seeing, even that limit may not be attainable. With 8- C

in. telescopes, I rarely use more than 200x when observing Jupiter, even on nights <

of steady seeing. I have had observing sessions in which I achieved 325x usable ^

magnification with my 8 in., but those nights have been rare. With Jupiter, try to go * 5

light on the magnification. You will learn from experience what you can achieve ^

with your own setup.

As with telescope selection, arguments can arise over the choice of eyepiece design. An eyepiece used for observing the planets should be one capable of

Fig. 7.5. An assortment of eyepieces, both orthoscopic and plossl, and a x2 barlow lens. (Credit: John W. McAnally).

producing tack sharp images. This normally dictates a multiple-element design; that is one that uses several lenses mounted in combination inside the eyepiece barrel. Planets are bright objects. Therefore, the eyepiece should have proper coatings to eliminate stray reflections and ghosting. Modern coatings actually increase light transmission through the eyepiece and contribute toward high image contrast. The field end of the eyepiece barrel should be threaded to receive eyepiece filters, to be discussed later.

My all time favorite type of eyepiece for planetary observing is the orthoscopic design. This eyepiece is a 4-element design. Orthoscopic eyepieces perform well at very high magnification, and produce a sharp image over almost the entire field of view. Their shortcoming is that they do not produce a very wide field of view, so they are not well suited for other types of observing. But, this is not a problem in planetary observing. Unfortunately, I do not see many offerings of orthoscopic eyepieces on the market today. A good used one can be a prized possession.

Perhaps the next favored eyepiece design is the plossl eyepiece. Certainly today this seems to be the most popular design, manufactured by many companies. The plossl eyepiece also uses a 4-element lens, but combines the lenses differently than an orthoscopic lens. Being very popular, plossl eyepieces are plentiful and come in a wide range of prices. These eyepieces have a fairly wide field of view and produce sharp images to the edge of the field, both at low power and high power. Plossl eyepieces are very good general-purpose eyepieces and also function well for close double star observing. Most of the eyepieces in my eyepiece box are plossl eyepieces. Here too, you should spend as much as you can afford on a quality eyepiece with good coatings.

Another common eyepiece type is the Kellner. Kellner eyepieces use a 3-element lens design. While Kellners perform well at low power for wide field viewing, they suffer more at high power than the previously mentioned designs. I do not use Kellner eyepieces for planetary viewing or other high power uses.

There are also multi-element designs using six or more elements in the eyepiece. These are usually eyepieces producing ultra-wide fields of view. They are also very expensive and I am not convinced they are any more useful for planetary work than orthoscopic or plossl eyepieces. Generally, this expense can be avoided.

There are other less expensive designs, many of which are 2-element lenses. These are poor for planetary viewing and should be avoided.

Although I prefer orthoscopic and plossl eyepieces for planetary observing, the observer can make up his own mind by trying various designs, especially the multi-element ones if so desired. Whatever eyepiece is used, it should be kept clean and free of smudges, and the lens elements should be held securely in place inside the eyepiece barrel.

7.3 Filters

Planetary observing is one endeavor in which the use of colored filters of specific wavelengths can be quite useful. We have previously discussed the fact that certain features on Jupiter tend to display certain colors, such as the bluish-gray festoons of the southern edge of the North Equatorial Belt, the redness of the Great Red Spot, or the reddish-brown coloration of the equatorial belts themselves. Filters can help us see these features.

Contrast and color differences between features on Jupiter are very subtle and especially difficult to detect by the inexperienced observer. Color filters (Fig. 7.6) can increase this contrast and help with the accurate determination of a feature's color. Filters can also help steady an image, especially when the seeing is poor or when attempting to view at a low elevation, such as near the horizon. An image that is unsteady or "boiling" due to poor seeing can be impossible to observe. Since different wavelengths of light are refracted, or bent differently as they pass through our atmosphere, the use of a filter to restrict the wavelength passing through to our eyes can improve this situation.

I strongly advocate the use of filters in planetary observing. This is a personal choice and some observers believe that filters reduce too much the brightness of the image. However, I believe most serious observers today would agree with me.

So, how do filters do what they do? A colored filter of the proper density will block all frequencies of light except for the one for which it is made to pass through. Simply stated, a red filter will filter out all wavelengths of light except the red wavelength, passing through the red light. Blue filters block all but blue light, and so on. We can take advantage of the transmission properties of these filters. For example, when observing a planet, the effect of a red filter is to make red features appear bright and other wavelengths to appear darker. The wavelength of the filter used determines which colors will appear darker. In general, to increase the contrast of a feature you want to observe, use a filter opposite the color of the feature. For example, to increase the contrast of a blue feature, use a red filter. In this manner, filters also assist with the identification of features. For example, if a feature becomes darker when viewed through a red filter, then the feature probably trends toward blue wavelengths. And, if it appears brighter through a red filter, then the feature

Fig. 7.6. An assortment of color filters, each mounted in a ring made to screw into the base of an eyepiece, shown for comparison. (Credit: John W. McAnally).

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