How to Use a Telescope Filter

Once you've mastered telescopic object magnification, it's time to increase the brightness and detail of what you're viewing. Use a telescope filter to increase contrast anytime or reduce glare during daytime viewing. Examine the polar ice caps of Mars or try to count the infamous rings of Saturn. Colored telescope filters alter the image you see by refracting all other colors in the visible light spectrum. They're effective and easy to use.

Instructions

Difficulty: Easy

Things You’ll Need:

• Telescope
• Astronomical filters

Select a Viewing Objective

1. Decide which specific result you wish to achieve, such as enhancing detail on the Earth's moon or increasing the contrast of Venus when viewed during the day.
2. Determine the numerical visible light transmission (VLT) you require to achieve your viewing result.

Select and Use Telescope Filters

3. Choose a light-green or moon filter of 53 percent VLT for enhanced moon views. You can also use this filter for viewing frost, fog or ice on Mars or encircling Saturn or Jupiter.
4. Choose a yellow filter of 74 percent VLT to brighten desert areas on Mars or darken the blue belt at Jupiter's equator.
5. Choose a light-red filter of 25 percent VLT for daytime viewing of Mercury and Venus against our blue sky.
6. Choose a light-blue filter of 73 percent in order to bring out detail of clouds on Mars and Jupiter. It also fine-tunes the details of galaxies and the binary star Antares.
7. Insert the proper color filter in front of the telescope eyepiece. Use the thumbscrew to loosen or tighten the eyepiece, slide out the filter and replace, taking care to follow your specific manufacturer's directions.

Tips & Warnings

• Astronomical filters fit the standard 1.25-inch telescope eyepiece mount.
• VLT is the total amount of light that can pass through a given filter and is indicated by a number. Lower numbers will be dimmer and higher numbers will be brighter.
• Use filters with a VLT of 40 percent or higher or your target may be too dim to see.
• You'll find several strengths or shades of primary and secondary colors of filters.
• Your filter results will depend on a changing set of circumstances, including true object color and environmental fluctuations. You may not get the same results with the same filter every time.
• Colored astronomical filters are for view enhancement, not safety. To be safe, make sure you always use a solar filter in front of the objective as well.
• Don't clean your filters in the field, where they can easily be scratched by airborne dust. Instead, clean your filters at home and transport your cleaned equipment in a secure, dust-free case.

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