Filters

Color Filters

Color filters are used for planetary and lunar viewing, and some are known to enhance images when doing black and white astrophotography. Each filter is listed by its color and its Wratten number, a number designated by Kodak for photography filters. These filters are fairly inexpensive, ranging from $15 to $20. A good basic starting set would include a yellow filter (either #12 yellow or #8 light yellow), a red filter (#23A light red, #21 orange, or #25 deep red), a green (#56), and a blue (#80A). In general, the yellow and red filters increase contrast between light and dark surface areas on Mars. The yellow filter also works well for improving detail on the moon, for blocking UV light when doing black and white photography, and for enhancing comet dust tails and heads. The orange filter brings out cloud bands, festoons, and the Great Red Spot on Jupiter. The blue and green filters bring out the Martian haze and cloud layers. The green accentuates details around the polar ice caps as well. For Jupiter, a blue or green filter enhances the Great Red Spot and brings out the darker cloud bands. A blue filter brings out the detail on Saturn and increases the contrast of comet tails. Finally, a #58 green filter will block street light while passing the wavelengths needed to view emission nebulae. If you have a small-aperture telescope (under 6") try out the lighter colored filters since they block less light than their darker cousins. Remember that the difference filters make in your viewing is subtle most of the time. Something that is not there at all in your eyepiece without a filter will not jump out screaming at you when you add one. They are, however, an important accessory if you enjoy planetary viewing (and who doesn't like that?), it is just to your benefit to realize that your best asset is a trained eye, made so from lots of viewing hours tucked under your belt... with and without filters.

Neutral-Density Filter

The first time you look at the moon, you eyes will be dazzled by the bright light. A neutral density filter will block a good portion of the light passing through your telescope without adding a color tint, and all of a sudden, details in craters and maria pop into view. A neutral density filter also works well on bright planets like Venus, allowing the viewer to see the phases more clearly.

Polarizing Filter

Polarizing filters are two filters that are mounted together in a specially machined cell, and when one is rotated, light transmission can be varied between five and twenty-five percent. This allows the viewer to vary the brightness of the moon with regard to what phase it is in and what magnification is being used to view the surface.

Light pollution/deep sky filters:

Unfortunately, light pollution is a problem in most areas of the United States. Planetary and lunar viewing are not affected by mild light pollution because of the brightness level of the objects being observed. However, as soon as the viewer ventures out of the solar system and into the wonders of deep sky, the view is diminished.

Light pollution filters help to block out the wavelengths of light most commonly used to light our city streets, and depending on the filter's transmission characteristics, allow wavelengths to pass that are most commonly found in nebulae and galaxies. These cost substantially more than color or ND filters, but can mean the difference between seeing an object and not seeing it. You can buy these filters in several sizes. They are available in .965", 1.25", 2.00", and Cassegrain rear cell. The rear cell filter allow you to screw it directly onto the back of your telescope before adding the diagonal and eyepiece. This means that you don't have to switch the filter from eyepiece to eyepiece... a convenience if you need to use the filter all or most of the time.

There are lots of deep-sky filters available on the market. The most common are "Broadband" and "Narrowband".

The Broadband is the best choice if you have moderate light pollution and want help on the largest variety of objects. The Narrowband filter works best if you are in an area of high light pollution and have a larger-apertured telescope, or if you want to enhance certain nebulas (the Veil Nebula, for instance, tends to magically pop into view with this filter, especially used in conjunction with dark skies).

The Narrowband filter, however, blocks out too much to be very useful in astrophotography. Use the Broadband Filter when you want the benefits of a light pollution filter in connection with your astrophotography.

Solar filters

It is possible to observe the sun with a telescope, but certain precautions must be taken. The only safe solar filters on the market today consist of either a full-apertured glass filter coated with a nickel-chromium alloy called Inconel, or a metal-coated Mylar material. Both of these are mounted in a cell that then fits over the front end of the telescope. The Mylar filters give a blue image of the sun, which can be corrected with the use of a #23A filter, while glass filters show a truer orange/yellow view of the sun. These filters allow the viewer to watch sunspots and eclipses safely. Do not go out and buy yourself a sheet of Mylar that you can get at a local hardware or auto-supply store and make yourself a solar filter! It is not safe. Use only solar filters that have been manufactured for the express purpose of viewing the sun through a telescope! Supervise children at all times when viewing the Sun, or when viewing in the daytime. A quick mistaken pass by the Sun can cause damage.

Cover any finderscope that you have on your telescope before doing solar viewing. Some department store telescopes still come with a dark green filter marked "Sun" or "Solar". These screw into your eyepiece like any normal colored filter, and are VERY DANGEROUS. Throw them away immediately. They take the full brunt of the Sun's heat and light, and can crack or parts can melt, allowing the filter to fall away and let the full force of the Sun's light into your eye.

Inspect your solar filter before each use for pinholes (especially if you buy one that is not glass) and defects. Remember not to stare at the Sun when you are aligning the telescope for a solar viewing session. Watch the shadow of the telescope on the ground. When the shadow gets the most narrow, and you can see the sharpest, most distinct image of your scope on the ground, the Sun will be very close to your eyepiece field. Then you can look around from there.

There is another filter, called an H-Alpha filter, which allows the viewer to see solar prominences and flares that are usually only seen by the eye during a total eclipse of the sun. The H-Alpha filter works by blocking all wavelengths but the one emitted by hydrogen atoms. This accessory is not cheap, costing between $500 and $6000, depending on the bandwidth of the filter.

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