Eyepiece cheat codes: Observing Jupiter and Saturn

Jupiter and Saturn, and sometimes Mars, are the planets that will yield the most detail to backyard astronomers. Not only are they bright, but they are large enough for even the smallest telescopes to see them as balls with shading and details. And of course, there are Saturn's rings! Mars generally needs to be at a favorable opposition to see surface details well. 

Jupiter has its four Galilean moons and Saturn has between two and seven moons accessible to typical backyard telescopes. The moons of Mars are generally too close to the planet to spot except when Mars is close to opposition and you have a steady atmosphere with good equipment. 

A night with a steady atmosphere—good "seeing"—will allow you to have much better views than a night where the seeing is soft, turbulent, or mushy. This is probably the single most important factor in how sharp the view will be. Try to observe when the planet is highest above the horizon. Viewing through a lot of "soup" at low altitude will also make for disappointing views, even on a night of good seeing. Heat rising from rooftops, asphalt, and concrete also wreaks havoc with seeing.

If you are observing with a Newtonian reflector, the image will be rotated 180 degrees (generally south is up). In a refractor or Cassegrain with a mirror diagonal the view will be mirror reversed (north up, but mirror reversed). See this explanation of directions in the telescope.

Jupiter

A complete novice can expect to see two main cloud bands on Jupiter and its four Galilean moons. With more practice, not only the South and North Equatorial Belts (SEB and NEB), but temperate belts in each hemisphere may also sometimes come into view, as well as darkened polar areas. 

In addition, features such as festoons, barges, and other spots that represent the turbulent swirls and storms in Jupiter's upper atmosphere become visible with practice and good seeing. 

The Great Red Spot is also sometimes visible when it is rotated towards us, although in recent years it has become rather wimpy in its size and color compared to previous decades. Look at some Jupiter images to see the types of features to look for.

Above: The moon Io and its shadow visible against the cloud tops of Jupiter. Image by Steve Hill, CC by 2.0, via Flickr

Below: The four Galilean moons are aligned on one side of Jupiter in this image by Ivana Peranic, CC by 2.0, via Jeremy Keith/Flickr.

Jupiter's Galilean moons—those that Galileo was able to see in his tiny refractor: Ganymede, Callisto, Io, and Europa—are the only moons, out of the currently identified 95 Jovian moons, that are visible to amateur observers, and can even be spotted in binoculars. Because their orbits are well known, predictions as to transits across the face of the planet and the corresponding shadows, disappearances and reappearances behind the planet or its shadow, and even occasional occultations and eclipses of one moon by another are available. You can plan an observing session to add these to the interesting details you can see in your telescope. 

The easiest are the shadow transits, which show up as dark black dots on the face of Jupiter. The moons themselves are more difficult to see when they pass in front of the planet, and much depends on the level of contrast with the cloud deck below them. I have seen them many times in my 4.5-inch reflector, but have been unable to see them just as many times. 

Averted vision is unnecessary for Jupiter and its moons. In fact, you'll see the most by looking directly at any feature. Bore your vision into the feature, almost as if you are looking through it, to get the most detail to register. Relax your eye and just let the detail burn into your retina. Really stare into it!

Sketching the cloud belts and swirls that you see can really help you focus on the details. You don't always have to sketch what you see, but try it a few times and you'll be surprised at the amount of detail that is actually visible. You may not see it all at the same time, the same with deep sky observing, but you will build up a complete picture with fragmented glimpses. This teaches you to place a detail within the greater context and you'll also see how the features slowly traverse the globe of the planet in an (astronomical) westward direction as Jupiter completes a full rotation in less than 10 hours—the fastest rotating planet in the solar system. For more on observing Jupiter, I recommend How to Observe Jupiter Through a Telescope by BBC Sky At Night Magazine.

Saturn

Of all the sights a beginner can see in the telescope, Saturn is probably the most striking. When I show it to people at public outreach events, most people are thrilled and some even question whether what they are seeing is real.

While Saturn doesn't show nearly the same amount of detail as Jupiter, and it's remarkably smaller in the eyepiece, the sheer beauty and uniqueness of the planet will keep you coming back whenever you can. Something about the rings is precious. Really.

Above: Saturn by John Spade, CC by 2.0, via Flickr

The rings change their tilt over the years, and with Saturn now in the evening sky, the rings are nearly edge-on. This makes it difficult to see the major feature in the rings, the Cassini Division. This thin dark lane is sometimes visible on nights of excellent seeing with the rings tilted towards or away from us at a significant angle. Look for it at the outward ends of the rings, where they become more visible because they begin to curve the other direction and the gap is seen at its fullest width. This gap that appears so tiny to us is actually almost 3,000 miles wide! The next ring plane crossing is in March 2025, when the rings, being an average of only about 30 feet thick, become invisible in our telescopes. The Cassini Division may have to wait.

Above: Saturn's varying ring tilt, image by NASA and the Hubble Heritage Team (STScI/AURA), CC by 2.0, via Flickr. Cassini Division label added.

If you look carefully you will usually see a slightly darker band around Saturn and perhaps some subtle shading elsewhere, especially at the poles. Saturn is much smoother than Jupiter, but it does have very infrequent storms visible in our telescopes, such as the great white spot of 2011.

For Saturn's moons, you'll have to use averted vision for all except the largest, Titan, and Iapetus when it is furthest out on the western side of Saturn and its bright icy side is turned toward Earth. Iapetus strays pretty far from Saturn in its wide orbit and can easily be confused with background stars. The inner moons are dimmer, but with good seeing, patience, and a telescope of around 4 inches or more, you should be able to pick out Rhea, Tethys, Dione, and possibly Enceladus. Mimas is quite difficult, Hyperion requires a larger telescope of 10 inches or so, and you won't have a chance at any of the other moons of Saturn, which currently number 146 and counting.

Jupiter and Saturn observing resources:

Cloudy Nights Planet Gallery (more recent images at top)

Cloudy Nights Major and Minor Planetary Imaging thread (latest images)

Online interactive observing tool for Jupiter's Moons (Sky & Telescope)

Great Red Spot transit times (Sky & Telescope) (when it crosses the planet's central meridian)

Online interactive observing tool for Saturn's Moons (Sky & Telescope)

Apps:

Moons of Jupiter and Saturn (Android)

JupitersMoons (iOS)

SaturnsMoons (iOS)

Eyepiece cheat codes: Averted vision

Getting the most from your visual observations involves using a few "tricks of the trade," or cheat codes, as gamers might view them, at the eyepiece. I've picked up several over the 30 or so years I've been visually observing, but the most useful and well-known is the use of averted vision.

Averted Vision

One of the first things you hear about when you start observing is "averted vision," looking slightly away from a dim object to see it better. This uses the rods in your eyes to best effect. I would posit that there are several variations of averted vision (AV), what I call close, medium, and extreme. 

In all three variations, which are really just part of a continuum, you are usually not keeping your eye fixed on one spot, but moving it around slightly to find the sweet spot where you can get a fleeting glimpse every now and then of the object or detail. When logging my observations, I'll sometimes refer to the percentage of time I can actually get glimpses of an object to indicate how dim it was. For example, I may see a brighter or larger galaxy 50% of the time, but a tiny dim one only 10%, sometimes even less. 

Consciously trying to determine this number is a fun way of determining how difficult the observation is for you and how it compares to others. You might be surprised how infrequently you are getting glimpses of an object, despite being absolutely certain you're seeing it. This gives more meaning to terms such as "bright," "difficult," or "extremely dim."

Medium AV

Medium-sized objects such as some galaxies, globular clusters, more compact open star clusters, and some double stars respond well to what I would call medium averted vision. This is the most common variation I use as an observing "generalist." In medium AV, with the object centered, you are usually looking about 1/5 to 1/3 of the way from the object to the edge of the field of view (FOV) to see it best. Depending on your own eyesight and equipment, you may find looking in one particular location consistently yields the best results, but more often it just pops out randomly as you move your eye around the periphery of the object. Be aware that your eye has a blind spot, but by moving your eye around, you will likely not notice it.

Above: Image of M22 by StudentAstronomyGroupUoC, CC BY 4.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

A good example of using medium AV is observing the line-of-sight "double" star 55 Cygni. This one has a 4.9 magnitude primary component and an 11.1 magnitude secondary, with a separation of 22.7 arc seconds at position angle 174. Close doubles with a big difference in magnitudes are a fun challenge. At first glance it's not easy to spot the secondary in my 6-inch reflector from my light-polluted Redneck Observatory. But it's not particularly hard if I use medium averted vision at around 150x. It pops into view if I look about 1/5 of the way to the edge of the FOV with the pair centered, but blinks out when I look directly at the primary. Give it a try or a similar one that is a good fit for your telescope.

Close AV

Close averted vision is useful for observations of tiny objects like Saturn's dimmer moons or detail in the core of a galaxy. When viewing Saturn, I notice that the brighter moons require a bit more distance (medium AV) and the dimmer moons often require I look right next to them (close AV), sometimes seeing them if I look directly at Saturn itself, which also helps keep my orientation fixed. I recommend a free app called Moons of Jupiter and Saturn. There is also a paid app for iOS called SaturnsMoons, although I have no experience with that. Apps like Sky Safari also will show the positions of planet moons. Just zoom in on the chart.

Above: Image of Saturn and its moons by Kevin M. Gill, CC BY 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Close AV is also useful for seeing more detail in larger or brighter galaxies. For example, the other night I was observing NGC 5907, a striking 10th magnitude edge-on galaxy in Draco about 11 x 2 arcminutes in size. I can pretty easily see the ghostly slash-shaped galaxy in medium powers in my 10-inch in a Bortle 4.5 sky, with a hint that the center is slightly brighter. This core area pops out a little better if I use close AV, looking right next to it. 

Above: Image of NGC 5907 by By Kết Nối, Việt Nam, Public Domain, via Flickr. Edited to simulate eyepiece view with annotations.

M82 in Ursa Major is another good galaxy to practice AV on. It will likely yield more detail in close AV. You might even be able to pull out a brighter core or some mottling in dimmer and smaller galaxies with this technique.

Left: Image of M82 by David Warrington from England, CC BY 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Extreme AV

I call it extreme, because in this case you're really trying not to look directly at anything in the eyepiece. Instead you're trying to take in the whole FOV at once rather than focusing on a particular spot. Just think of relaxing your vision and letting the entire view wash into your brain. Your eye is moving around trying to soak up every photon to make some sense out of the scene. It's almost like you're trying to pull your eye back into your head a bit to get the widest field possible to register.

This is useful for extended objects or dimmer objects such as a dim but rich star cluster. A good example is NGC 6645 in Sagittarius. It's a beautiful and interesting cluster for a darker sky, but often ignored because of all the other flashy stuff nearby (M8, M20, M17, etc.). 

Left: Image of NGC 6645 by Mike Durkin from Forest Hills, NY, derivative work: Winiar, CC BY-SA 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Extreme AV is pretty much essential for most nebulae other than planetaries. Coupled with a nebula filter (I use an NPB narrowband filter), this can often yield great results where you can see the shape of the brighter portions of nebulae. 

Left: Image of NGC 2174, the "Monkey Head Nebula" by Nigel Hoult, CC BY 2.0, via Flickr. Edited to simulate eyepiece view with annotations.

Sometimes even a small object that's extremely faint can benefit from extreme AV when you're trying to get any hint at all that something is slightly brighter than the surrounding sky. You just have to experiment, as everyone's situation can be different.

Eyepiece cheat codes: Determining directions in the telescope

In the Space Walk Among the Stars audio guides, I frequently refer to compass directions or position angle. Sometimes I say "right" or "left" from a path we're following in the scope, but that really only applies to Newtonian reflectors, and for that I apologize.

Directions in the telescope can be confusing for beginners and even long time observers. It all depends on how many mirrors your telescope has. Generally, an odd number, and your telescope will mirror-reverse the view. An even number and your telescope will rotate the view 180 degrees.

A Newtonian reflector has two mirrors, the primary at the bottom of the tube and the secondary, the smaller one which directs the light to the eyepiece. In this case, an even number, therefore the view is rotated 180 degrees. (The view is rotated additionally because your focuser is usually located off to the side and your eye is positioned differently throughout the sky, so don’t assume south is always “up.”)

Most people with refractors and Cassegrain telescopes use a 90-degree mirror star diagonal before the eyepiece to give a more comfortable viewing position. That counts as one mirror in a refractor and three in a Cassegrain (which has a primary and secondary mirror plus the diagonal). An odd number, therefore these telescopes will keep the image correct side up, but mirror-reversed. (How your diagonal is rotated will affect what direction is actually "up" in your view.)

If you use an “erect image prism diagonal,” such as an Amici prism or pentaprism, in your refractor or Cassegrain, then you get a “correct image” that is neither rotated nor mirror-reversed (but there may be disadvantages that I won’t get into here).

One thing that stays the same regardless of your telescope type is that without any tracking motor engaged, the stars will always drift to the west (or, if you like, enter the field of view from the east). That’s because the Earth is rotating toward the east, and your telescope is fixed to the Earth. So you can always start with an easy reference point by noting the direction toward which the stars are drifting- that’s west. From there, you apply the correct diagram below and you are good to go!

If you’re interested in more information on how your equipment affects image orientation, see this article from the British Astronomical Association.

Choose the diagram that applies to your telescope

In all telescopes, stars and other objects will always drift to the WEST if the scope does not have a tracking motor operating. Know your telescope and directions in the eyepiece. 

The diagrams below show an example of how position angle (PA) is used to indicate the direction from a primary star to its secondary companion (PA 225 in this example) in reflectors and refractors/Cassegrains (with diagonal). You can also give any directions in the sky using PA or compass direction (270 or west, for example), as in many of these Space Walks. This view would be facing south.

This is for a NEWTONIAN REFLECTOR, such as a Dobsonian, and also for a straight-through finderscope. These show the image rotated 180 degrees from what you would see just looking up or in binoculars. North is COUNTER-CLOCKWISE from West:

 

Tip: In Sky Safari Pro, tap the field of view measurement in the upper right and select "Flip: Both" so the chart will match your view in the eyepiece. Note that it might still be rotated somewhat because of your eye's orientation to the eyepiece.

REFRACTORS and CASSEGRAIN telescopes, typically used WITH A MIRROR DIAGONAL, will show the image correct side up but mirror reversed from what you would see just looking up or in binoculars. North is CLOCKWISE from West:

Tip: In Sky Safari Pro, tap the field of view measurement in the upper right and select "Flip: Horz" so the chart will match your view in the eyepiece.