Observing the Planets
The close approach of Mars in August 2003 fired up the public's interest in the Red Planet. And some amateur astronomers were also turned on to our Solar System, which is often referred to as the "shallow sky". Of course, there are many who still feel that observing the planets is sort of like dancing with your sister - not unpleasant but not as interesting as all those faint galaxies waiting to be found in deep space.
With deep sky objects, there are so many faint fuzzies that the quest is to see as many different ones as possible, and we tend not to re-visit many of those we've already logged. But except for the most distant members of our Solar System, the planets are easy to spot and we look upon them again and again. Their unique appeal is that most are dynamic; it's not just a matter of picking out details, but also seeing what has changed. Back in 2003 the Mars encounter provided a learning experience about how much change takes place from night to night.
Altitude of the object in the sky is important - the higher the better. Everyone wanted to see Mars near its close approach on August 27, 2003, and all of us were excited to see that bright orange-colored beacon rising in the east. Except that in a telescope it was a quivering out-of-focus blob. If you did acquire an in-focus image, it quickly drifted out of focus. The frustrating thing was that you had to patiently wait for it to climb higher in the sky so as not to be looking through so much of the Earth's atmosphere. Earlier in the summer that meant waiting until the wee hours of the morning.
Experienced observers usually note that until a planet reaches an altitude of at least 25o to 30o up in the sky, it will appear unsteady in the telescope and that there will be some color fringing (dispersion) caused purely by the Earth's atmosphere. This atmospheric chromatic aberration is especially noticeable on Saturn, where one side of the rings will have a blue tinge and the opposite side a reddish hue. It's best to observe a planet near the time when it culminates - reaches its highest point in the sky. An outer planet at opposition (opposite the sun in our sky) is also about as near as it gets to Earth. Around the date of opposition it rises at sunset and sets at dawn, so it culminates at midnight. But contrary to what the public was led to believe by many media reports on Mars, the difference in a planet's apparent size through a telescope isn't very significant for about a month before and after opposition.
The naked-eye planets are bright and easy to see, so they don't need a lot of light gathering ability. A large telescope will indeed provide better resolution (the ability to distinguish fine detail), but high quality optics generally give a more pleasing, high-contrast view regardless of size. The real trick is to see all the low-contrast details that planets have to offer. Design is important too - the smaller the diameter of the central obstruction, the better the image tends to be. The sharpest images were in apochromatic refractors and Newtonians with superior quality mirrors, regardless of size. Biggest isn't always best.
The use of filters depends largely on the planet being observed, the telescope, and the observer's preference. To my way of thinking, Mars is one of the few planets on which colored filters can make a positive contribution. The most popular ones that seem to have emerged in the 2003 Mars apparition are the #21 orange and #80A light blue. Orange provided a more naturally colored view than red filters, accentuated the darker details, and cut the glare on larger scopes. The light blue seemed to diminish the effects of Earth's atmosphere and made the polar cap stand out, while not dimming the planet too much on smaller telescopes. Generally filters have both internal and external threads so you can screw one filter into another ("stack" them) to produce a combination of effects. The one situation where I've found stacking useful is for diminishing the brightness of Venus, which stabilizes the image and sometimes allows a view of shading in its cloud deck.
A less conventional approach to filtering is to use a nebula filter. Yes, that's a nebula filter used on a planet. I heard about this on an Internet discussion group and subsequently tried each of my nebula filters on Mars with my 6-inch APO. A broadband "light pollution" filter didn't really enhance any features and gave an unnatural electric blue color. The OIII line filter colored the planet a deep red without improving the contrast of features. (Perhaps in a larger scope the greater light gathering would overcome the darkening effect of the filter.) But a UHC filter made the polar cap and dark features stand out better, while causing a milder reddening of the image. It'll be interesting to try these filters on other planets.
Some of the latest items on the market are planetary filters for discrete wavelengths. The Baader Moon and Skyglow Filter, available from Astro Physics, is one such product. Sirius Optics has a whole range of "light manipulation" filters, such as the Planetary Contrast (PC1), Variable Filter System (VFS), and the Contrast Enhancement Mars 2003 (CE1) filter. These filters employ peaks of high transmission with the regions in between being heavily rejected, a sort of filtration that is impossible with dyed-glass filters. Reports indicate that they bring out details surprisingly well on the objects for which they are intended, while giving a more natural coloration. Some have also reported that they are useful for cutting light pollution.
Stepping away from the planets for a moment, I've never found any filter that helps in lunar observing. Some feel a neutral density filter makes looking at a bright moon easier. But despite how bright it may appear in an eyepiece, its apparent brightness is only enhanced by the fact that our eyes are conditioned to the darkness of night; even a full moon is no brighter than a daytime landscape. And since a bright moon makes faint objects just about impossible to observe, there's really no dark adaptation to be preserved with a neutral density filter. If anything, "moon filters" adversely affect image detail.
Others will argue that a telescope with high quality optics provides the greatest possible contrast without help from filters, plus it will display all the natural coloration of the planet. I tend to agree. Another issue is that colored filters normally are not optically flat, so even if they're made of "optical" glass, they can still induce a bit of distortion into the image. And a filter adds another piece of glass to the optical system, which diminishes image sharpness, though this may be imperceptible to many observers. After Mars had risen high enough, I found that I always ditched the filters and happily observed its details in their "pure" colors. So the lesson for observing planets in general is to try different filters to see which works well for your own telescope and to see which, if any, you like best.
A unique aspect of the planets is that something is always different, be it the dance of their satellites or the surface features currently facing our direction. Just as you check a reference source when setting out to find deep sky objects, so it should be for planetary observing. Because the planets are always changing, the most useful reference is a computer program that provides a plot of the current position of surface features and/or satellites. This is a great help in knowing where and when to look. I rely heavily on two freeware programs:
- Mars Previewer (covers Mars only)
Many of the feature-rich planetarium programs, such as Guide also include ephemerides for the various planets. For example, LCAS member Martin Willes used the Starry Night software to confirm the position of the Martian satellite Deimos, which he spotted in his 24-inch scope. Another source is the detailed tables printed monthly in Sky & Telescope magazine and info on their web site.
Newspaper articles about an astronomical event are usually written by people who have little understanding of astronomy. So at public star parties we encountered guests who thought that Mars would appear the size of the moon, and that it was only on August 27th that it would be so large. Obviously, before a member of the public looks through our telescopes, we should carefully explain how the object will appear and what to look for.
We ourselves should not approach the eyepiece with a lot of preconceptions. Planets sometimes surprise us with the unexpected - like a dust storm on Mars, a change in Jupiter's Red Spot, or the mysterious white cloud on Saturn. And there are abundant routine events such as seasonal changes in the Martian polar cap, the transit of a Jovian satellite, or Saturn casting a shadow on its rings. This reminds us that despite their distance and inhospitable nature, these are other worlds that just happen to circle the same star as we do.