Mohawk Valley Astronomical Society

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Planets in a New Light

by Perry Pezzolanella, MVAS

Did you ever get frustrated with your telescope because seeing any detail on the planets seemed impossible? Your first thoughts might be to buy expensive specialized planetary eyepieces, upgrade the telescope, or perhaps sink your life savings into a new, huge telescope. Before resorting to such extremes, there is a simple and less expensive solution: invest in color filters. A single 1¼” glass filter costs roughly $15 and simply screws onto the bottom of any standard 1¼” eyepiece. Each planet has a specific filter that enhances the contrast of subtle details and each filter has a specific Wratten filter number corresponding to the color.

➢ Mercury: A #21 orange filter helps steady the trembling atmosphere enough in most cases to see the tiny phases and under very steady seeing it might be possible to see dusky surface detail.

➢ Venus: A #38 deep blue filter cuts the dazzling glare and increases the chance of seeing cloud detail. It also softens the terminator, which gives the added feeling of 3-D.

➢ Mars: A #25 red filter is best as it brings out the darker surface features and the icy white polar ice caps. Since this filter is rather dark, it is best used on larger telescopes or when Mars is near opposition. An excellent alternate filter is #21 orange as it is lighter and still brings out surface details.

➢ Jupiter: Nothing beats #58 green filter for enhancing the cloud belts and especially the Great Red Spot. This filter is great for seeing finer detail such as festoons and also makes it possible to see a moon in front of a darker cloud belt.

➢ Saturn: A #15 yellow filter increases the contrast of the cloud belts and polar hoods and will make the rings contrast nicely with the globe. Saturn may seem bland, but a yellow filter will prove otherwise.

➢ Uranus & Neptune: Use a #11 yellow-green filter with a telescope of at least 16” or larger to help see any possible cloud detail. It will be tough because their discs are so small, but both planets have had a dramatic increase in white spots and darker belts the past few years.

The best all-around filter for any planet is #21 orange. It will give steady views of Mercury, cut the glare from Venus, enhance detail on Mars and Saturn, and will still reveal plenty of fine detail on Jupiter. Try combining two filters and see what happens. Combining #21 orange and #58 green filters when Mars is biggest and brightest closest to Earth brings out wonderful detail while preserving its natural color for fantastic viewing. An orange filter is also good for color-correcting the Sun when using a Mylar solar filter. The #47 violet filter is very dark for all but very large telescopes, but might reveal cloud details on Venus.

The Moon also looks nice with the orange and green filter combination as it cuts a lot of glare and gives it a warm hue. A variable polarizing filter, which contains two polarizing filters mounted together, is also helpful. Turning them varies the amount of light passing through it. Simply screwing it onto an eyepiece and then rotating the eyepiece in the telescope will adjust brightness without altering the color.

Filters are useful astronomical tools, but it is important to realize that, if the weather is too turbulent, or if a feature is not visible without a filter, it will not magically appear with one. These little pieces of inexpensive glass enable us to see the planets in a whole new light, which reveals them to be unique, detailed, colorful worlds.