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Agena AstroProducts Observing Guide: The Planet Jupiter

By: Brian Ventrudo
May 12, 2017

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Jupiter Observing Guide
Figure 1 - The planet Jupiter (Credit: NASA/JPL).

1. Overview

Jupiter is the largest planet in our solar system and perhaps the most fascinating to observe with a telescope. It's brighter than any star and only outshone by the Moon, the planet Venus and, very rarely, by Mars and Mercury. Jupiter's churning and fast-moving atmosphere reveals many interesting and colorful features in a small telescope, which is why the planet is a favorite target of new and experienced stargazers. This guide will help you understand what to look for on the face of the planet Jupiter through visual observation with a small telescope, and it will help you consider and select some good stargazing tools and accessories to help you get the best view.

2. Jupiter in a Nutshell

Jupiter is the largest planet, by far, and the largest object in the solar system except for the Sun. The planet is 2.5x as massive as all other planets combined. Like Saturn, Jupiter is a "gas giant", a massive planet made up almost entirely of cold hydrogen and helium gas along with traces of other gases like ammonia and methane. (Uranus and Neptune are also sometimes called gas giants, or, because they are much colder, "ice giants"). Gas and ice giants do not have well defined rocky surfaces like the Earth. Their outer layers are made entirely of clouds; farther down the gas turns to compressed liquid, and at the very center of these planets may be a rocky carbon core.

3. What to See on the Face of Jupiter

For backyard astronomers, the most interesting part of Jupiter is the part we can see: the outer layers of the atmosphere. To see any detail on the face of Jupiter, you will need a telescope. But even the smallest scope shows some structure in the atmosphere, usually two dark bands surrounded by lighter regions. The dark bands on the face of Jupiter are called belts and the lighter bands between them are called zones. The two most prominent belts are the north equatorial belt (NEB) and the south equatorial belt (SEB) since they each lie just north and south of the equator, respectively. Many more belts and zones are visible in steady sky with larger telescopes at higher magnification (see Figures 2 and 3).

Jupiter Observing Guide
Figure 2 - The nomenclature of Jupiter's belts and zones (Credit: Wikipedia Commons).
Jupiter Observing Guide
Figure 3 - A spacecraft image of Jupiter. Note the correspondence of this image to the map in the previous image. (Credit: NASA).

The belts and zones are caused by churning layers of gas in the atmosphere. The rotating atmosphere has segmented itself into layers that are confined to certain latitudes on Jupiter, much like the trade winds on Earth blow in the same place and same direction throughout the year. The zones are caused by convection regions where warm gas rises, cools, and sinks repeatedly in the upper atmosphere. The belts are regions of cooler gas that have sunk lower in the atmosphere. The belts are dark and colorful, with tones of red, salmon, tan and brown. The color comes from phosphorus compounds that are ionized by radiation and lightning in the fast-moving winds. The zones, which are regions of warmer rising gas, are usually of a pale-yellow or white color caused by ammonia gas crystallizing and blocking the view of the more colorful gases below.

Jet streams of opposite direction on each edge of the belts and zones confine each structure, and as the gases whirl across the face of the planet at speeds exceeding 300 km/h, turbulence occurs at the boundaries. This turbulence leads to intricate and exceedingly beautiful structures called garlands and festoons between and within each belt and zone. On nights of steady seeing, and at high magnification, you can get glimpses of these structures in a 4" or larger telescope. The dynamics of Jupiter's atmosphere also lead to circulating cells of gas that result in cyclonic storms, the most famous of which is the Great Red Spot (GRS). This spot straddles the south equatorial belt and the lighter south tropical zone, impinging on each. The GRS, which is larger than Earth, has been observed for centuries and may be a permanent feature in the Jovian atmosphere. But it does change over time. The GRS and the SEB both change color at unpredictable intervals. When the SEB turns lighter, the GRS darkens and vice versa. During the past several decades, the GRS was darkest in the periods 1961-66, 1968-75, 1989-90, and 1992-93. No one knows why it changes, or indeed what causes its red color.

Jupiter makes a complete rotation on its axis very quickly, just once every 9.8 hours. The Great Red Spot largely follows the rotational motion of Jupiter. You can get an idea when the GRS will pass across the meridian (the north-south centerline) of Jupiter, in your local time zone, with this online tool:
http://www.acquerra.com.au/astro/software/jupiter.html

4. The Moons of Jupiter

While Jupiter itself has plenty to hold your attention, it also has four large moons that are great fun to watch in a telescope. These moons were discovered by Galileo in the months after he first turned his little telescope to Jupiter in the early 17th century, so they are sometimes called the Galilean moons. They are, in order of increasing distance to the planet, Io, Europa, Ganymede, and Callisto. These moons move around the planet in periods ranging from 1.7 days (for Io) to 16.7 days (for Ganymede). As the moons move around Jupiter, they frequently pass in front and behind the planet, and also fall into the shadow of Jupiter. It all makes for great viewing on nearly every night during which Jupiter is visible.

There are four main types of events to look for when you're observing Jupiter's moons:

  • A transit is the apparent passage of a moon across the face of Jupiter. During transits, the moons appear to move east to west across the face of the planet, as defined in the sense of our sky, not the planet itself.
  • A shadow transit is the passage of a moon's shadow across the face of Jupiter. Such events are governed by the position of the Sun, Jupiter, and the Earth.
  • An occultation results when Jupiter's moons pass behind the planet. Io, the closest moon to Jupiter and the quickest, passes through an occultation in just a couple of hours. Ganymede, the slowest moon, takes much longer.
  • An eclipse of one of Jupiter's moons happens when it passes into Jupiter's shadow. When an eclipse begins and ends, the moon suddenly disappears then appears out of the darkness of space as it enters and emerges from the shadow of the big planet. These are perhaps the most dramatic events involving Jupiter's moons you can see with a small telescope.
Jupiter Observing Guide
Figure 4 - Multiple transits of the shadows of Jupiter's moons (Credit: Efrain Morales Rivera).

With four moons and four phenomena-transits, shadow transits, occultations, and eclipses-one of these events can be observed, on average, about once every 3.5 to 4 hours. To determine which moon undergoes which event, you must turn to a set of tables or a calculator. This online tool predicts, in GMT and your local time zone, upcoming events related to Jupiter's moons:
http://www.shallowsky.com/jupiter/

5. Jupiter Observing Tips

Jupiter is almost always an impressive object. Even at its most distant from Earth, and even with a small telescope, you can see the most prominent belts and zones in the atmosphere of Jupiter, and its moons are easily visible even in a pair of binoculars. The best time of year to observe the planet is at and near opposition, when it's opposite the Sun in the sky and rises as the Sun sets. At opposition, the Earth and Jupiter are on the same side of the Sun and the planet is closest to Earth. Jupiter is large enough to look good and offer plenty to see for a few months on either side of opposition.

Oppositions of Jupiter occur at approximately 13 month intervals. Some dates of recent and upcoming oppositions are: April 7, 2017, May 9, 2018, June 10, 2019, July 14, 2020, and August 19, 2021. At or near opposition, the planet appears largest with an apparent diameter of 44" to 50" (arc-seconds), about the apparent size of the largest craters on the Moon. The planet shines as bright as magnitude -2.9. By comparison, when Jupiter is near conjunction, its farthest point from Earth, it appears about 30" across and shines as faint as -1.6, much fainter than at opposition but still brighter than any star.

In most amateur telescopes, the images of Jupiter and other planets, even at high magnification, will be quite small. Beginners may find this disappointing at first. But even small visual images can present nearly as much detail and color as you see in photographic images of Jupiter, especially if you use the right tools under the right observing conditions.

All planets look better in a telescope on nights of good seeing when the air is steady. You can get a good idea if seeing is good by looking for twinkling of bright stars. If stars exhibit very little twinkling, the air is reasonably steady and you can expect to see sharp images of Jupiter. If the air is very turbulent, even the best telescope will show Jupiter as a boiling featureless blob.

Even with moderate to good seeing, patience is critical. Don't just have a quick look at the planet and move on. You must wait for moments of the steadiest air over the course of many minutes or more. During this time, you will be rewarded with a few moments when tiny details snap into view, including the smaller belts and zones, festoons, the GRS (if visible) and possibly other smaller spots and shadows cast by the moons. But these fleeting moments make the effort worthwhile.

A few more tips...
  • To get the best view, wait until Jupiter is near the meridian, its highest point in the sky for the night. Just after it rises or just before it sets, the planet will lie low in the sky and its light will pass through thick, dusty, and unsteady layers of the atmosphere. As the planet rises, you will have to look through less of the Earth's atmosphere.
  • Make sure you give your telescope at least 15-30 minutes to come to the ambient temperature of your surroundings after bringing it outside.
  • More magnification will always give you a bigger image, but it will not necessarily give you a better image. The steadiness of the atmosphere and the quality of your telescope and eyepiece will influence the optimum magnification you can use on a particular object on a particular evening. You must experiment with magnification each night to get the best view.

6. The Best Telescopes for Observing Jupiter

Aside from good seeing, to get the best view of Jupiter you need a telescope that can deliver contrast, aperture, and has a long focal length. Long focal lengths provide the largest images for a given eyepiece, while contrast makes it easier to discern subtle features on the face of a planet. Larger apertures provide increased resolution of fine detail and higher magnifications.

With a lack of a central obstruction, refracting telescopes generally offer the best contrast for visual planetary observation. Achromatic refractors with long focal ratios of f/12 or f/15 were once the gold standard for amateur planetary observers. But these long-tube refractors have largely been replaced by shorter focal length apochromatic or semi-apochromatic refractors that give sharper views with much less false color from the refractive properties of the lenses. Semi-apochromatic apochromatic refractors with doublet objective lenses are a particularly good value for visual observation of Jupiter and other planets. These scopes are sometimes called "ED" or extra-low dispersion refractors. The f/9 100mm ED refractor and f/7.5 120mm ED refractor from Sky-Watcher are good choices as they offer reasonably large apertures of slightly longer focal ratios than other refractors and come at an affordable price.

Jupiter Observing Guide
Figure 5 - The Sky-Watcher 120mm f/7.5 ED refractor is a good choice for high-contrast views of the planets.

Larger-aperture scopes give higher resolution of fine detail, but smaller high-quality apochromatic refractors can be pushed to provide higher magnifications on nights of good seeing. In smaller scopes, good quality optics is a big help. The 85 mm TV-85 refractor from Tele Vue Optics is renowned for its sharp images. Jupiter is large enough to produce a good image in this telescope, which can provide magnifications of 200x or more, although the relatively small aperture renders the image somewhat dim. Vixen Optics also has an 80mm apochromatic refractor with good optics for viewing Jupiter at moderate magnifications.

Achromatic refractors of 6" aperture or less can also be used for planetary observation, but their inherent chromatic aberration can make for fuzzier images and a distracting purple glow around bright images of Jupiter and other planets.

While not offering the same contrast as a refractor, long-focal length compound reflecting telescopes such as Schmidt-Cassegrains and Maksutov-Cassegrains are a popular choice for many planetary observers and imagers. These scopes pack long focal lengths into a small optical tube, and modern optical anti-reflection coatings help boost image contrast and brightness. The larger apertures of 6", 8", 9.25" or more also offer inherently higher resolution than smaller refractors for revealing fine detail in good seeing conditions. They also give brighter images at high magnification than smaller-aperture scopes.

For visual planetary observers who want the larger aperture of a compound scope, Celestron's NexStar and CPC telescope systems are a good choice. They offer good optics and long focal lengths in compact packages as well ascomputerized alt-azimuth mounts.

Meade's LX-90 systems and the new ETX125 also give good visual views of the planets, as do the more advanced Meade LX200 systems.

Jupiter Observing Guide
Figure 6 - A CPC800 Celestron 8" Schmidt-Cassegrain telescope.

For even longer focal ratios and focal lengths, and therefore larger images, the razor-sharp optics of a good Maksutov-Cassegrain are a good choice for serious planetary and lunar observers. Unlike Schmidt-Cassegrains, these scopes rarely require alignment and have smaller secondary mirrors for higher image contrast. The 127mm, 150mm, and 180mm Mak-Cass scopes from Sky-Watcher are excellent planetary performers.

With whatever telescope you have at your disposal, use as much magnification as it will handle when you look at the planet. This depends on the quality of your optics and on the seeing. Generally, a magnification of 30-50x the aperture of your telescope (in inches) works well on nights of average seeing. For example, if you have a 4-inch telescope, try 120x to 200x. If you have an 8" scope, try 240x to 400x. Again, experiment to get the best view each night.

Jupiter Observing Guide
Figure 7 - A set of Orthoscopic eyepieces from Kokusai Kohki. The Orthoscopic design has long been favored by planetary observers.

7. Eyepieces and Accessories for Observing Jupiter

Eyepieces are another critical component to a good planetary observer's toolkit. Stock eyepieces that come with many telescopes are of modest quality may affect the amount of detail and contrast you can see at high magnification. As you become more serious about seeing maximum detail on the planets and Moon, you may consider acquiring a few good eyepieces for planetary observation.

Any good eyepiece will show you detail on Jupiter, but some eyepieces are designed especially with lunar and planetary observing in mind. A planetary eyepiece does not need to give wide "space-walk" views because the images of planets are small and are usually held near the center of the field of view. To give maximum contrast, planetary eyepieces tend to forgo wide fields to minimize the number of glass elements and glass surfaces. So dedicated planetary eyepieces often have an apparent field of view (AFOV) of less than 60° compared to wide-angle eyepieces with apparent fields of view of 68°, 82°, or larger.

Many manufacturers make a series of eyepieces for planetary observing. Some good choices include:

Plossls. A tried-and-tested design, Plossls are a reliable performer for planetary and lunar observing. They are also a good value for observers on a budget. Many manufacturers make a Plossl design including GSO and Tele Vue Optics. Agena's Starguider eyepieces are also an economical and well-regarded choice.

Brandons. A long-established design that dates back to the 1940s, Brandon eyepieces are favored by planetary observers for their high-contrast and bright views of the planets. They work best with telescope of focal ratio f/7 or greater.

Orthoscopics. Also a long-time favorite of planetary observers,Orthtoscopic eyepieces from Kokusai Kohki, Baader Planetarium, and other manufacturers give excellent contrast in a small form factor.

Tele Vue Delites. A premium eyepiece from Tele Vue Optics, the relatively new Tele VueDelites have replaced the company's Radian eyepiece line which was popular among planetary observers. With a 62o AFOV, Delites are also a good general purpose eyepiece.

Zoom Eyepieces. A high-quality zoom eyepiece is ideal for getting a wide range of magnifications without the bother of actually changing the ocular. The Baader Hyperion Mark IV eyepiece is excellent for planets and for all around use. The 3-6mm Nager zoom eyepiece is intended for use in small refractors to give high-contrast planetary views and relatively high magnifications.

Eyepieces are critical components for serious planetary observers. But if you use a refractor or compound reflecting telescope, don't forget your star diagonal. Lower priced or stock diagonals with many telescopes do not have optimized surfaces or coatings and can somewhat degrade images of the planets and other celestial objects. A higher-quality diagonal is an investment, but it need not be prohibitively expensive. 1.25" diagonals from GSO and William Optics are affordable. And if you want a premium unit, Baader makes an excellent 1.25" diagonal. Larger 2" diagonals are also available from a variety of manufacturers and at various price points.

Finally, a good 2x or 3x Barlow lens comes in handy for extending the focal length of your telescope and increasing image size with your set of eyepieces.

8. Optical Filters for Observing Jupiter

While Jupiter presents much detail in steady skies, you can tease out even more fine features by using color filters. These filters, which are numbered according to the traditional Wratten numbering system used for photographic filters, are mounted in 1.25" cells that thread into the barrel of an eyepiece. Color planetary filters pass some colors and block or darken others which helps improve contrast and visibility of the Great Red Spot, belts, and festoons.

Filters, of course, reduce the amount of light reaching your eyepiece. So for smaller scopes less than 8" in aperture, you should consider lighter-colored filters. Dark-colored filters can be used with apertures of 8" or larger. Also, since filters can be stacked, there are many possible combinations. But again, stacking multiple filters works best with larger apertures. Here are a few good filter choices for observing Jupiter:

Jupiter Observing Guide
Figure 8 - A Wratten #80A blue filter (left) and a Baader Moon and Skyglow filter (right). Both filters enhance the contrast of features in the atmosphere of Jupiter.

You can get a more detailed guide to choosing color filters for observing planets at this page:
https://agenaastro.com/choosing-a-color-planetary-filter.html

In addition to color Wratten filters, the Baader Planetarium Moon & Skyglow filter has proven to be useful for visual observation of the planets, especially Mars, Jupiter, and Saturn. The filter uses a special glass that passes isolated segments of the red, green, and blue portions of the spectrum while blocking light in the yellow and orange spectral regions. The result is a filter that leaves natural colors mostly intact, but which significantly enhances detail. For Jupiter, the filter noticeably enhances views of the Great Red Spot and belts.

8. Summary

Of all the sights in our solar system, Jupiter is perhaps the most fascinating to observe with a telescope. Bright and big, with ever-changing cloud bands, Great Red Spot, and fast-moving moons, the planet presents something new to look at nearly every clear night. This guide has helped you understand what to look for on the face of the planet Jupiter through visual observation with a small telescope, and it suggested some good stargazing tools and accessories to help you get the best view. The rest is up to you! So head out on the next clear night to start seeing this spectacular world.