Tips on Observing Jupiter

Dawn Jenkins, Cleveland

For planetary observing with an amateur instrument, use the highest power you find acceptable. Don't underestimate the power of a Barlow. I find sometimes a lower power eyepiece used with a Barlow is better than using a higher power. I use a pale yellow filter on the planet under "normal" circumstances. I also like neutral density filters Magnification is the key to observing.

Set up early, before sunset. The best observations of Jupiter will come as soon as you can see it. If there is a large difference in the temperature of your mirror and the temperature of its surroundings, you may not get a good view until the optics have equalized. (Set up early, the session will end soon enough)

When looking at Jupiter, imagine a line down the center of the disk. The satellites help to determine North and South on the disk because they orbit Jupiter at it's equator (roughly). This is the central meridian of Jupiter. Watch the spots as they hit the center of the disk. The leading edge of a feature is called the preceding edge and the later edge is called the following edge.

The technique I use to see features is to focus my eye on some spot on the SEB (Southern Equatorial Belt). But I concentrate on seeing the details in the south polar area. This is averted vision for planetary observers.

One more thing. Sometimes good results can be obtained by making an aperture stop for your telescope. With a 121/2" mirror on a telescope with a conventional spider, a clear aperture of nearly 4" can be obtained. A direct benefit from this arrangement is that much of the light from the planet is "cut down". Secondly, this will be a clear aperture, with none of the support structure in the way. Some mirrors that suffer from bad edges can get improved results; making a 12" aperture stop for a 121/2" can make a difference.

What is considered optimum magnification for planetary observing is 30 power per inch of aperture. Because cutting down the aperture of the mirror doesn't change the focal length of the instrument, the same magnification results. On a 10" instrument 100 power is 10 power per inch of aperture. If the aperture is cut down to 2", 100 power is 50 power per inch of aperture. I find the more power per aperture the better. As always, atmosphere and local conditions are limiting factors.

(Note: While a yellow filter helps bring out details on Jupiter, I find that a light blue -- #80A -- filter does the best job in enhancing the Great Red Spot, which has been very pale in the last couple of years. J.K. )

Published in the September 1996 issue of the NightTimes