Planetary Observing

Dan Boyar - November 5, 1997

In an effort to make the upcoming evening planetary observing opportunities more fruitful and enjoyable, editor Kye Ewing recently interviewed veteran planetary observer and ASPB past-president, Dan Boyar, on the subject. Dan has been an avid visual observer of Solar System objects for many years, having his sketches of Jupiter and Saturn displayed in such distinguished publications as Sky & Telescope Magazine, as well as our own Stars & Scopes Newsletter.

With Venus and Mars in our early evening skies, and Jupiter and Saturn appearing in easily observed areas of the evening sky for the next several months, this is an ideal time to have Dan share some of his knowledge on the subject with us.

Questions by Kye Ewing
Answers by Dan Boyar


Q: What are the most important things to consider when planning a planetary observing program?
A: There are two important considerations for planetary observing: seeing conditions and equipment.


Q: What is "SEEING"?
A: As most Stars & Scopes readers know, seeing is the term used to refer to the steadiness of the atmosphere. "Good seeing" is experienced when the atmosphere is tranquil, rather than turbulent. An example of "bad seeing" can be demonstrated by looking at a celestial object slightly above a warm road or rooftop. The object appears to boil and can't be brought into sharp focus. When observing through bad seeing conditions stars appear to twinkle frantically and planets shimmer. Stars shining steadily or twinkling slowly are an indication of a steady atmosphere/good seeing.

Q: Please explain how we can best deal with the seeing conditions that we experience here in South Florida.
A: In summer you can find the best seeing conditions in the late evening (after 10:30PM) through early dawn. In the early evening the atmosphere is generally quite turbulent due to terrestrial heat being released back into the atmosphere. Also, remnants of thunderstorms are often breaking up for several hours after sunset.

During the rest of the year, conditions are usually turbulent right after a front passes through. When the wind direction begins to clock around to an easterly or southeasterly direction (usually a few days later), you will find that seeing conditions improve and observing the planets is much more rewarding. During breezy east and southeast winds, however, avoid viewing near clouds as they drift across the sky from the ocean. They create turbulence; be patient until they move across the sky away from the planet you are observing. Also, atmospheric conditions tend to be steady around dusk and dawn when there is thermal equilibrium.

Local seeing is also important. Setting up your telescope in a grassy area rather than on one emitting heat (like asphalt of concrete) will make a big difference in what you can see. Also, observing over trees or shrubs rather than over rooftops gives a steadier view. Avoid setting up near or viewing over vehicles. In our warm climate it is seldom necessary to put the telescope out ahead of time to acclimate to cooler temperatures. You will find clearer details on the planets if you keep people (including yourself!) away from the front of the telescope, preventing close-up distortions from body heat.

Q: You mentioned another important point to consider when observing planets -- equipment. What should a person look for when choosing a telescope, eyepieces, etc.?
A: The main goal of planetary observing is to see the most details and get the clearest view. Those details don't need to be BIG to see them, but you must have enough magnification and resolution to be able to discern details. The aperture of the telescope determines how much resolution can theoretically be achieved at 13X per inch. About 20X per inch is a better working value because it increases the image scale, making planetary details larger (although slightly less sharp). With 2" to 4" scopes, try using about 30 power per inch of aperture (60X for a 2" scope, 120X for a 4"). With 6" to 10" equipment you may need to drop the ratio back to about 25 per inch. 12" and larger apertures usually require even less magnification per inch, being more affected by seeing conditions. You begin to see fine detail on Mars, Jupiter and Saturn is with a 6" aperture. In good conditions with this size instrument, you should be able to get 3/4 arc second resolution.

"F ratio" is another important concern, although not a critical one. Generally longer f ratios are better for bringing out details, but larger aperture and quality optics will produce real results. The biggest advantage of longer f ratios is that you are more likely to have better optics for the same price, since they have more generous tolerances. Also, longer f ratios give you more "depth of field", which translates to a more forgiving focus range. Small disturbances in the seeing will not degrade the image as easily.

Collimation is another important consideration. How well your optics are aligned makes all the difference when you are trying to pick out details. This is one factor that you have some control over, since adjusting collimation in most scopes is a relatively easy.

Q: Do you have some pointers for enjoying the four most easily observed planets?
A: Let's start with Venus. As the planet emerges from the glare of the sunset this winter (1998/99), try observing Venus as early in the evening as you can. Block the glare of sunset with a tree or structure to spot the planet early while it's still light for the best view. Venus will be southeast of the Sun (upper left of the Sun when facing west). Look for hints of delicate "shadings" on the planet's disk. This is a sign of irregular cloud cover.

Mars will be in the best position to observe in the pre-dawn hours this winter (1998/99), high in the southern sky. Mars' disc is very small, due to its current distance from us. You will need excellent seeing conditions and at least 200X magnification to glimpse any features at all.

Jupiter is high in the southeastern sky at sunset this winter, making an excellent observing target for many months. This planet is also good to check out as soon as twilight permits. As with Venus, its brightness after dark is glaring! For larger instruments, an aperture mask might help. 3" to 4" scopes do well on this one at about 90 to 120X. Watch for developing festoons (loops in the equatorial regions), the ever-evolving Red Spot and interesting motions of the Galilean moons. Also of interest for those with 8" scopes and up - the moon Gandymede can be viewed with 200 to 300X (in good conditions) to distinguish its disk and some markings.

Saturn, following Jupiter into the southeastern sky after sundown, is another fine target. Its features are not as bright as Jupiter's. 25 to 35X per inch of aperture may show the best details. Saturn is now closer to us than it has been in many years. This coupled with the fact that the ring system is much more tilted to our angle of view than in past years makes Saturn an especially interesting target. Look for white areas and other markings on this planet and report them to ALPO or our club President. If you observe regularly your eye will pick up changes. Sketching is a good way to improve your observing skills and is a record of details for future reference.More information on observing the planets and a Novice Observer's Handbook may be found at the ALPO Training Program web page.