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Telescope Accessories Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 19, 2011

Focal Reducer/corrector

Most hobby SCTs have f -ratios of around 10. This is all right for viewing planets, lunar surface features, and some star clusters. However, when looking at nebulae or galaxies, often you will want to reduce the magnification as much as possible. By so doing, you can concentrate the light so that dim, diffuse features show up more clearly against the background of the sky. Reducing the magnification with a given eyepiece also increases the absolute field of view.

A focal reducer/corrector is a convex lens that shortens the effective focal length of the SCT objective by a certain amount, usually 37 percent. This means that the effective focal length and the f -ratio are both cut to 63 percent of their values without the device installed. You might think of it as the opposite of a Barlow lens. With a 37 percent focal reducer/corrector, an f /10 telescope becomes an f /6.3 instrument. A focal reducer/corrector is larger in diameter than a Barlow lens and is equipped with a threaded mount that can be screwed into the opening in the objective mirror that passes light into the eyepiece holder.

Suppose that your SCT has an objective with a focal length of 2,000 mm. If you have a 40-mm eyepiece, a focal reducer/corrector shortens the effective focal length to 1,260 mm. This reduces the magnification from 50× to a little more than 30×. It also increases the absolute field of view by a factor of about 1.6. The corrector feature helps to ensure proper focus throughout the apparent field of view.

Solar Filter

You can use a telescope to look at the Sun, but there are some precautions you must take to avoid damage to your telescope, your eyesight, or both. Before you point a telescope toward the Sun, get a solar filter that fits over the entire skyward opening of the telescope. The filter must be as large in diameter as the objective and is called a full-aperture solar filter . With such a filter, direct sunlight does not fall on any of the telescope optics. Only certain types of filters are acceptable; these block ultraviolet (UV) rays that otherwise could damage your eyes even if the image is not uncomfortably bright. The brand-name telescope manufacturers such as Celestron supply excellent solar filters. They’re not cheap, but neither are your telescope or your eyesight.

If you have a finder that uses lenses, such as a Keplerian refractor with cross hairs, cover it before aiming the telescope at the Sun. Otherwise, you risk damage to the finder’s eyepiece and cross hairs.

Never use a “sun filter” that screws into telescope eyepieces! Such a device is at the prime focus of the telescope objective, so it will heat up. Such filters have been known to melt or crack. If one of these “filters” fails while you’re looking at the Sun, you will remember the experience for the rest of your life. You’ll be lucky if your retina is not injured permanently.

Have you heard that you can aim a telescope at the Sun without a solar filter, with the eyepiece installed, and let the brilliant light shine onto a white piece of paper or a screen to see details of the Sun’s surface? In theory, this scheme works, and you can in fact get a decent image without risking damage to your eyes. Several people can view the image at the same time. But this is a bad idea. It subjects the eyepiece to direct focused sunlight, which can permanently damage the eyepiece. Besides this, as the Sun moves in the sky or as you move the telescope around while locating the Sun, the focused spot will strike and heat up interior components of the telescope.

Think of the focused, unfiltered rays of the Sun as the business end of a blowtorch. Would you turn a hot flame on anything you value? Of course not. So follow the universal rule: Always filter sunlight before it gets into a telescope (Fig. 20-9). Treat your telescope as kindly as you treat your own eyes.

Your Home Observatory Telescope
Accessories Moon (lunar) Filter

Figure 20-9. A solar filter should keep direct sunlight out of the interior of a telescope.

Moon (lunar) Filter

If you have a telescope whose objective lens or mirror is larger than about 10 cm (4 in), the Moon will appear extremely bright at low magnification when it is near the full phase. In fact, at the lowest obtainable magnifications with SCTs using focal reducer/correctors, the full Moon can appear so brilliant that it hurts your eyes to look at it. A Moon filter , attached to the eyepiece, renders the Moon’s image tolerable under these conditions.

A Moon filter, also called a lunar filter , is tinted grayish, gray-green, or brownish, like the lenses in a high-quality pair of sunglasses. It is mounted in a threaded ring that screws into the objective side of the eyepiece. This placement is all right; the Moon’s light is not intense enough to cause damage to a telescope’s interior components or to an eyepiece. You’ll know when you need a lunar filter and when you don’t. At high magnification levels or when the Moon is a thin crescent, you won’t want one. At low magnification, after sundown, and when the Moon is gibbous or full, you will.

Other Filters

It is not easy for most people to find places where the nighttime sky is not polluted by airglow. Airglow doesn’t interfere very much with viewing of the Moon or the planets, although dust and particulate pollution, along with convection currents rising from the day-heated land that roil the evening air, can blur even these images and reduce their contrast. If you want to see nebulae, globular clusters, and galaxies, you will have trouble with airglow unless you take some measures to reduce it.

A light-pollution-reduction (LPR) filter reduces the effects of airglow at night. Most big outdoor lamps are sodium-vapor devices that emit most of their radiation at well-defined wavelengths in the yellow part of the visible spectrum. Mercury-vapor lamps are less common, but they too emit most of their light at certain discrete wavelengths. A line-type LPR filter is designed to transmit light at all visible wavelengths except specific ones. In this way, the airglow from sodium-vapor and mercury-vapor lamps can be attenuated, whereas light at other wavelengths passes through the filter unaffected. Other LPR filters include narrowband and broadband types. The particular filter that will work best in a given situation must be found by trial and error. The folks in your local astronomy club can give you advice based on their own experiences. All LPR filters, like Moon filters, are designed to be screwed into the objective side of an eyepiece.

Planetary filters are simple color filters that are screwed into eyepieces in the same manner as are Moon filters and LPR filters. They are available in almost any tint you can imagine. You can use an orange filter to look at Mars, a yellow-green filter to look at Jupiter, or a red filter to look at Venus. You can even use these filters ( in addition to a full-aperture solar filter—never all by itself! ) to look at the Sun. Experimentation is the key. Try all the filters you can find. Look at anything you want with them. See if you can borrow some from friends, so that you don’t spend a lot of money unnecessarily. You’re bound to see some interesting things.

Practice problems of this concept can be found at: Your Home Observatory Practice Problems

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