Help! I Can’t See Detail On The Planets!

Magnificent Saturn: Credits NASA, ESA and E. Karkoschka (University of Arizona) — http://hubblesite.org/newscenter/archive/releases/2004/18/image/a/

I often see pleas of help from new telescope owners who purchased a telescope to view features on the prominent planets that grace our evening skies, but are disappointed with the detail (or lack thereof) visible through the eyepiece.

The prospect of seeing Jupiter’s clouds, Saturn’s rings, and Mars’s polar ice caps is tantalizing, but these features often prove to be elusive. If you’ve just gotten a telescope and have struggled to see details on the other members of our solar family, read on for troubleshooting and steps you can take to optimize your chances.

Managing Expectations

The first order of business is to manage expectations. Even under ideal circumstances, a typical entry-level commercial telescope is not going to provide a large view of the planet. You may be expecting to peer through the eyepiece and see this:

But in reality you will see something more like this under good conditions:

Jupiter in a small scope (4–5") under good observing conditions using appropriate magnification.

It should be noted that the surface features on all planets are often far more subtle than they appear to be in images. All image processing adds a certain degree of contrast and saturation, and the nature of viewing the images on a typical monitor in a bright room is quite different from how we perceive the light coming from the telescope when our eyes are dark adapted.

Moreover, the unfortunate reality is that there are often a number of factors working against the observer’s ability to see detail on the planets:

1. Looking at a planet without a lot of detail on it, such as Venus
2. Using inappropriate magnification
3. Poor atmospheric seeing (bad turbulence)
4. Observing the planet when it’s too low in the sky (too close to the horizon)
5. Poor collimation of the optics
6. Observing over heat sources like rooftops and air conditioning vents
7. Observing from indoors through a window
8. Not waiting for the telescope to acclimate to the surrounding air temperature
9. Poor optics
10. Inexperience

The only thing that won’t degrade the view of the planets, is light pollution. Getting out in the dark countryside to observe planets isn’t necessary, since the major planets are bright enough to be seen even in broad daylight (if you know where to look!). Even the most light polluted cities are no match for the bright concentrated light of the major planets.

Let’s go through each of the above factors in more detail.

Factor #1: Viewing the right planets

Not every planet has detail to show. The only planets that have easily observable features or otherwise notable attributes are Jupiter, Saturn, and Mars.

However, Mars only gets close enough to Earth to see meaningful detail for a brief period once every couple of years, and even then, not all Mars oppositions are equal. Close oppositions like we just experienced happen once every 11–13 years or so. Further, Mars sometimes undergoes massive global dust storms which can completely obscure its surface features. One such dust storm occurred this opposition. Even under ideal circumstances, Mars’s features are subtle and require patience to see. Throw a global dust storm on top and Mars will just look like an orange ball.

Venus and Mercury will show you phases like our Moon does, but they will not show any features on their surfaces or clouds. Neptune and Uranus will show you their respective blue and green color as well as some moons, but will also not show any features in their clouds (even in large telescopes). They are also so far away they can be easily mistaken for stars, even in larger telescopes!

The most reliable planets for observing detail on are Jupiter and Saturn. Under good observing conditions, Jupiter’s cloud belts, the Great Red Spot, and shadows of its moons being cast onto its surface are visible, but the larger the telescope, the more detail you’ll be able to see. Jupiter’s four major moons are always easy to spot, even in binoculars. However the moons themselves will not show any detail. Only very large amateur telescopes under the steadiest of skies will show you detail on Jupiter’s moons.

Like Jupiter’s moons, Saturn’s rings are at least detectable in just about any telescope at almost any magnification. Saturn’s cloud belts are also observable, but like Jupiter’s, they can be subtle and fairly low contrast. The Cassini Division in Saturn’s rings should also be visible, as well as Saturn’s shadow being cast onto its rings when Saturn is at quadrature. Saturn’s major moons are also visible, and become increasingly apparent as you go up in aperture.

Factor #2: Magnification

Typically the eyepieces included with telescopes (especially the smaller entry-level telescopes) are not appropriate focal lengths for planetary observation. It’s very likely you will have to buy one or more eyepieces to do planetary observation with your telescope.

As a reminder, this is about what you should expect from a smaller telescope using appropriate magnification:

Figure 1: Optimum magnification in a small 4–5" telescope. The cloud bands are fairly well defined, but also not very high contrast either.

Here is what you might expect when trying to use too much magnification for the telescope or what the atmosphere will allow:

Figure 2: Using too much magnification, the view becomes much dimmer, and the planet looks less distinct. Despite being larger in size, there isn’t any extra detail to be seen in the planet. Atmospheric distortion is also more apparent.

Or this if using too little magnification:

Figure 3: Using too little magnification. The brighter view and lower magnification can make Jupiter just look like a bright white featureless disk to your dark adapted eyes.

Picking the right magnification is a balance between what the atmosphere will support, what the telescope will support, view brightness, and image scale. As you go up in magnification, the view gets dimmer. The larger your telescope, the more magnification you can use before the view gets too dim, but at that point, you are more likely to bump into the magnification limit imposed by the atmosphere for your area.

So what is the right magnification for planetary viewing of a given telescope size?

Setting aside the atmosphere for the moment, and to really over-simplify things, a good starting point is to pick a magnification between 25x and 35x per inch of aperture (or 1x and about 1.4x per mm of aperture).

For a 60mm scope (2.4"), that means between 60x and 83x magnification.

For a 114mm scope (4.5"), 114x — 160x would be good.

For a 203mm scope (8"), 203x — 285x would be good.

For very small scopes (60–90mm), favor the upper end of the magnification range since the limit will typically be the scope, not the atmosphere. For small scopes and above (114mm+), favor whatever the atmosphere will allow. Often times this means going below the lower end of the magnification range.

To determine what eyepiece focal length you need, simply divide the telescope’s focal length in mm, by the magnification. For example if the 114mm telescope has a focal length of 910mm, then to achieve 160x you would need to use a 5.69mm eyepiece (910/160). Since no such eyepiece exists, just find the next closest focal length you can afford. In this case a 6mm would be fine, producing about 152x magnification.

How much magnification you can actually use will depend on the scope’s optics and the atmosphere, but those above rules of thumb will get you in the ballpark magnification range of planetary observation.

My personal opinion after many years of observing, is that about 120x magnification is the minimum for planetary observing. When all other conditions are optimal, you can readily see a wealth of detail on Jupiter at 120x. For this reason, telescopes smaller than 90mm in aperture will struggle to deliver good views of planets unless they are very high quality. If you are currently using anything less than 90mm in aperture, you may want to consider upgrading, but the more aperture you can upgrade to, the better. Aperture is king.

Factor #3: Atmospheric turbulence

The atmosphere is probably the biggest gatekeeper to planetary detail. It refracts, bends, and distorts light the same way water in a pool does, just to a lesser degree. Different parts of the world have different levels of atmospheric turbulence at different times of the year. Mountains, trees, lakes, oceans, latitude, and elevation above sea level can all influence how much atmospheric distortion you may encounter.

The level of turbulence is known as “seeing”, and it is often described by its Pickering number: http://www.damianpeach.com/pickering.htm. The higher the Pickering number, the more fine detail and steadier the view will be, and the more magnification you can use. While the Pickering scale isn’t a complete picture for how small the features you can see through a telescope will be, it’s a good enough approximation for the quality you might be able to expect.

Here is an example of Jupiter in very bad (Pickering 0) seeing:

Figure 4: Jupiter dances around and morphs like an amoeba through an 8" telescope in Pickering 0 seeing.

Here is an example of Jupiter in excellent seeing (Pickering 8 or 9).

Figure 5. Jupiter in Pickering 8–9 seeing.

Seeing conditions can change from second to second, minute to minute, or hour to hour. Some days may be significantly worse than others. Prolonged views of a planet will let you catch those fleeting moments when air stabilizes, and presents a clean, undistorted view of the planet.

In a very small telescope (60–90mm), you will likely run out of telescope capability before bumping into atmospheric limits (unless you have a very high quality small telescope). That is, as long as you stick to the 1x — 1.4x per mm of aperture rule of thumb, you will likely not have to worry about the atmosphere being the limiting factor. But scopes larger than that, regardless of their quality, can reach magnifications that might exceed what the atmosphere will allow, thus it becomes important to pick an appropriate magnification that matches the atmospheric limit.

There is no explicit cliff where the view goes from good to bad with too much magnification. It’s a slippery slope, and it’s all about your personal preference for the combination of steadiness, sharpness, brightness, and size. This is why it can be useful to have a zoom eyepiece, or a tight clustering of focal lengths around the average atmospheric limit for your observing location so that you can pick the right magnification for the seeing conditions.

From my experience, these are the approximate magnification limits for each Pickering level:

• Pickering 8–9: Whatever your scope will allow. Atmosphere is usually not the limit.
• Pickering 7: about 500x
• Pickering 6: about 350x
• Pickering 5: about 180x
• Pickering 4: about 120x
• Pickering 3 and below: 90x or less

Note that these magnifications are entirely arbitrary based on my personal preference for “crispness” of the view, as well as what eyepieces I have available to me. Those are the limits where the view stops looking crisp enough for my tastes. For you it may be different.

So what’s the takeaway here?

There’s nothing you can do about poor atmospheric seeing conditions other than wait for them to get better, or travel somewhere that they are better. The best you can do is pick an appropriate magnification that fits your personal criteria of crispness and size for the given night.

For a rough prediction of seeing quality, check out MeteoBlue and select your location: http://meteoblue.com/en/weather/forecast/seeing/

Factor #4: Planetary altitude above the horizon

This is somewhat related to Factor #3. The lower a planet is on the horizon, the more atmosphere you must look through to see it. If the atmosphere straight above you (90 degrees altitude) contains about 10 miles of turbulent atmosphere, then to look at an object just 25 degrees altitude above the horizon, you’re looking through approximately 24 miles of turbulent atmosphere. For objects just 10 degrees altitude above the horizon, you’re looking through almost 60 miles of turbulent atmosphere (and dust, and moisture, and ice crystals!)

Figure 6: Light from objects lower on the horizon will pass through more atmosphere on the way to your telescope than objects straight above you. This means greater total distortion of light.

In addition to a thicker atmosphere, lower altitude also means atmospheric refraction can become an issue. Our atmosphere acts like a prism that splits white light into different wavelengths. The steeper the angle of light entering the atmosphere, the more pronounced these effects — known as atmospheric dispersion — become.

Figure 7: The effects of atmospheric dispersion for objects at low altitude above the horizon. The atmosphere acts like a prism, refracting light at low angles of incidence, and breaking it up into its constituent colors.

Figure 8: Through the eyepiece, the effects of atmospheric dispersion can be quite pronounced. Blue and red fringes will flank either side of the planet, and the view will always look a bit blurry since not all wavelengths are coming to the same focal point.

How can this be avoided? Unfortunately, the major factor is a combination of your latitude, and where the major planets are in their orbits. Right now (as of summer 2018), the major planets are in the Summer zodiac constellations (Sagittarius, Scorpius, Ophiuchus). These constellations are low in the sky for those in higher northern latitudes (45 degrees or higher). Because the orbital periods of Jupiter and Saturn are very long, they will remain in these summer constellations for a few more years. When they eventually reach the Fall and Winter zodiac constellations, they will be high overhead. Those living in more southerly latitudes will always get to enjoy planets that are relatively high above the horizon, regardless of where they are in their orbits.

To optimize the view, it’s best to wait until the planet crosses the meridian — the time of night when it will be highest in the sky. This changes day to day as the year progresses. I recommend Time and Date’s planetary rise, set, and meridian tools for determining the meridian time and altitude for the different planets for your location.

Figure 9: Waiting for a planet to reach the meridian for the night is the best way to minimize the effects of atmospheric dispersion and low altitude turbulence. Viewing a planet just after it rises is a recipe for poor viewing conditions and zero detail.

How close is too close to the horizon? There is no hard cutoff. Some nights will let you go down to 15 degrees, others won’t. You just have to go out and try it and see if the atmosphere is cooperating. If it’s not, however, then waiting for the planet to rise to its meridian height is the next best option. If that meridian height is still very low (below 15 degrees) due your latitude, then you may have to wait until the planets are in more favorable locations in their orbits.

Factor #5: Poor collimation

For refracting telescopes and Maksutov Cassegrains, collimation is rarely, if ever, an issue. For Schmidt Cassegrains, it’s sometimes an issue (though typically the long focal ratios of these scopes make small collimation errors less noticeable). But for Newtonian reflectors (especially ones with short focal ratios — F/6 and shorter), ensuring good collimation is an important piece to the planetary observation puzzle.

It’s easier for Newtonians to get out of collimation than other designs, and the shorter focal ratios typically found in Newtonians makes collimation errors more pronounced than in the more common longer focal ratio designs of refractors and Cassegrains.

If you are the owner of a Newtonian reflector, then be sure to familiarize yourself with how to accurately test and collimate its optics:

How to Align Your Newtonian Reflector Telescope | Sky & Telescope

A Beginner's Guide to Collimation - Gary Seronik

Astro Babys Guide to Collimation - Newtonian Telescope - Reflector

https://astro.catshill.com/collimation-guide/

Factor #6: Observing over heat sources

Heat rising off of rooftops or from various vents can cause serious degradation to the view — as much or more as the atmosphere itself. Cities and asphalt shingles on roofs act as giant heat traps that absorb heat during the day, and release it at night. As the warm air rises off of these structures, it causes turbulence. It’s likely you’ve seen these effects in the form of mirages over hot asphalt, subtle shadows being created by heat rising from a radiator in the winter time as sunlight passes through the rising warm air, and distortions from a campfire.

Figure 10: The heat and fast moving air coming from a jet exhaust completely obliterates the details of the mountain behind it. Heat rising off of buildings, rooftops, or vents will have a similar effect on the details of planets. It’s best to get away from these heat sources.

Do what you can to get away from these heat sources. The higher the planet is above the heat source, the less pronounced the effects will be. Observing a planet over distant rooftops is not a problem. Observing a planet just a couple of degrees above a nearby rooftop or heat vent of some kind, is a problem.

Factor #7: Observing from indoors through a window

This feels like one of those things that should go without saying, but to get the clearest views of a planet, you cannot view it through a window. A window is not a piece of optical glass. It has substantial imperfections that will badly distort light passing through it. Most windows are multi-layered and are not coated with anti-reflection coatings, so you’re going to be seeing ghost images of the planet when looking through the glass. If the window has a screen, the fine mesh is going to add substantial diffraction effects that will greatly reduce contrast.

Further, typically the indoor temperature is different from the outdoor temperature, meaning the heat escaping through the window is mixing with the colder air outside, creating a thermal gradient and degrading the view in the same manner that was explained in Factor #6.

Further still, most heating units are typically positioned under a window because that is where the greatest heat loss occurs in a room. This means during the winter time if you have a heating unit near the window, warm air is going to be rising and mixing with the colder air near the window, further compounding the effects of thermal distortion.

Just don’t view planets from inside, through a window. You will never see any meaningful details in them if you do. Go outside, and be sure to avoid looking at the planet when it’s just a few degrees above something that could potentially be a heat source.

Factor #8: Thermal acclimation

This is a major factor that is just as important as all of the others. When the surface of a telescope mirror, or the air inside the tube, has a substantially different temperature than the surrounding air, you get thermal distortions (the notion of thermal distortions is a widespread problem, if you haven’t noticed already).

Refractors typically don’t have a major issue with thermal distortions, but the thicker mirrors of reflectors and Cassegrains have high thermal mass, meaning they cool or warm slowly. The temperature differential between the mirror’s surface and ambient air creates a boundary layer on the surface of the mirror that absolutely clobbers light passing through it (which it has to do twice: once on the way in, and then again after being reflected off the surface).

Figure 11: A special test setup that allows a camera to record the effects of the thermal boundary layer of a mirror that is warmer than the surrounding air.

But it isn’t just the boundary layer sitting on the surface of the mirror that’s a problem. Any thermal currents present in the tube can distort the light before it reaches the eyepiece.

Figure 12: A video demonstrating the distortion of effects of any heat sources placed between the mirror and the incoming light.

It’s worth visiting this page for more videos and visualizations of the effect of the boundary layer and tube thermal currents.

So what can be done about this? The easiest thing to do is to bring your telescope outside a couple of hours before you plan to observe to let the scope acclimate to the ambient temperature. Once the temperature of the mirror gets within about 5 degrees Fahrenheit or 2 degrees Celsius of the ambient air temperature, it will perform well.

How long this takes depends on how big the temperature difference was from the start. Keeping the scope in a warm, heated house in the winter means it might take 3 or more hours to reach ambient if it’s particularly cold outside. Conversely, if it’s kept in an air-conditioned house, it might take an hour or two to warm up properly (but you might also exhibit fogging going from a cool to warm environment).

The best thing you can do to speed up the acclimation process of a Newtonian is to add a cooling fan to the back of the mirror to blow air against it. This will dramatically reduce the acclimation time of the mirror. Some telescopes come with cooling fans already installed, but you can make your own easily enough with just an 80mm PC cooling fan and some basic DIY skills:

If you’re curious about how long your telescope will take to acclimate with or without a fan, there is a free program you can use. You will need to know the thickness of your mirror and what type of glass it’s made of in order to accurately compute the time it takes to reach ambient.

Factor #9: Poor optics

Sadly, many entry-level commercial telescopes are either too small or too cheaply made to give you satisfying views of the planets. Telescopes with insufficient optical capability usually come in one of three flavors:

#1: Too little aperture (typical 60mm refractors)

Figure 13: A classic entry-level telescope — the 60mm refractor with erecting image diagonal. In this price class, no single brand is better than another.

Going back to the magnification rule of thumb, the upper limit magnification for a telescope this size is about 84x. This is basically an insufficient magnification for viewing details on planets, and because of how cheaply made this class of telescope is, it won’t be a clean 84x either. It will likely contain a fair amount of chromatic aberration, and the erecting image diagonal it typically comes with (designed for terrestrial use), is of very poor quality and will further degrade the view before reaching the eyepiece.

The short story is that these small refractors are simply *not* designed for planetary viewing. You will see Jupiter’s moons, the shape of Saturn’s rings, and the phases of Venus, and that’s about it.

So what’s the solution? Avoid these “department store” quality refractors. The better quality refractors start at around 4" (102mm) in aperture, but they are expensive compared to an equivalent Newtonian. Refractors, in general, are not cost-effective instruments.

#2: Compact achromatic refractors

Figure 14: Notable offenders. These typical “travel” refactors all have short focal ratios, which means high levels of chromatic aberration when using magnifications needed to observe planets in detail.

As if their small aperture wasn’t enough of an issue, this class of telescope also has a short focal ratio. This means they refract light more aggressively than a refractor with a long focal ratio, and in doing so, they break it up into its constituent colors more severely. When light gets strongly refracted, not all wavelengths come to the same focal point. This is known as chromatic aberration, and all refractors (even camera lenses) suffer from it.

Figure 15: The effects of chromatic aberration. The shorter the focal ratio, the more severe the effect is. The diagram on the left is a lens refracting light at a steep angle (e.g. short focal ratio). The left-most Jupiter image shows significant chromatic aberration and prismatic dispersion from interaction with a badly aligned prism diagonal. The right-most Jupiter image shows less severe aberration, but it still has an unnatural hue to it that reduces the already subtle contrast of its cloud belts. These are common problems in all but the most expensive refractors.

The only way for a refractor to have a short focal ratio and still produce a sharp, true color image, is if it uses more than two lens elements and expensive, extra low dispersion glass. These kinds of refractors are commonly referred to as “apochromatic” refractors, rather than just typical “achromatic” refractors. However, they are often 5–10x the cost of a normal achromatic refractor of the same size and focal ratio.

What’s the solution? If you do want to get a refractor, do not get a short travel refractor with a short focal ratio. Get a long refractor at least F/8 or longer, or be prepared to spend nearly $1,000 or more on a high quality small aperture apochromatic refractor.

#3: Short Newtonian reflectors with spherical mirrors

Figure 16: Small reflecting telescopes with spherical primary mirrors. On the left is a Celestron PowerSeeker that uses a Bird-Jones configuration, and on the right is a very small, short focal ratio Newtonian reflector with a spherical mirror. Neither scope will give you satisfying views of the planets.

Newtonian reflectors gather light from a primary mirror at the back of the telescope. The shape of this mirror is supposed to be parabolic, but to save costs, sometimes it is made to be spherical. Spherical mirrors do not focus light properly, especially if they have short focal ratios. See this diagram for an illustration:

Figure 17: How light comes to a focus in a spherical vs parabolic mirror. A spherical mirror does not bring all light to the same point of focus, but a parabolic mirror does. The shorter the focal ratio, the worse the problem gets for spherical mirrors. This lack of single focal point will dramatically limit the useful magnification of the telescope.

Making a spherical mirror is cheaper than making a parabolic mirror, which is why manufacturers will sometimes include spherical mirrors in their telescopes instead of parabolic mirrors.

Some designs attempt to correct for this problem with a special lens element in the bottom of the focuser tube. This is known as a Bird-Jones (or Jones-Bird) design. The Celestron PowerSeeker 127, Celestron Astromaster 114, and Celestron StarSense Explorer LT 127AZ utilize this design. The problem is that because of the lens element in the focuser tube, it becomes nearly impossible to precisely collimate a Bird-Jones scope (see Factor #5 above for why collimation is important). These kinds of scopes are also made to poor tolerances and the corrector lens often does a poor job of fixing the spherical aberration present in the mirror.

The solution? Avoid any Newtonian reflector that doesn’t have a parabolic primary mirror. Some Newtonians have spherical mirrors but long focal ratios (F/8 or longer). These are passable, but not great. It’s just best to avoid them entirely. Do some research to make sure the model you’re thinking about getting is a true Newtonian reflector with a parabolic primary mirror.

Factor #10: Inexperience

Astronomy has a weird quirk to it whereby you cannot take your vision for granted. You might think that by virtue of the fact that you’re (A) a human and (B) you have working vision, that you’re automatically an expert at seeing things.

The kinds of things we amateur astronomers like to observe are not things the human vision system naturally evolved to be good at perceiving. Just because we have the physiology necessary to see very faint light, or see very tiny features on a tiny planetary disk through an eyepiece, doesn’t mean we have the skill to notice those things easily.

This is where observer experience comes in — like any other skill, it’s something we must work at and develop. The subtle contrast of planetary features is something that takes the brain time to get used to. Our eyes are sending that information to the brain, but the brain may be dismissing it (subconsciously) as being unimportant. Over time, however, the brain will learn to care about the seemingly unimportant signals the eye is sending it.

You can practice your observing skills by taking the time describe what you see in as much detail as possible. Literally write down detailed notes or even talk to yourself while observing. Even better, try to sketch what you see. It forces you to really pay attention to small, subtle details.

Summary

While every factor is almost equally important, the truly most important factor in how much detail you can see (assuming you’re using optimum magnification), is your telescope.

If you already have a small 60–90mm telescope and it’s not a very high quality apochromatic refractor that cost you $1,000 or more, then my recommendation is to start saving up for a larger, better telescope. Aperture is king when it comes to planetary detail, and the more of it you can afford, the better. The absolute minimum aperture I recommend to begin observing meaningful detail on the planets is 114mm (4.5"), but again, the higher the better.

The most affordable telescopes per inch of aperture are Dobsonians (Newtonian reflectors mounted on a Dobsonian base). A decent entry-level Dobsonian from 114mm to 150mm (4.5" to 6") in aperture can be had for $200-$300 brand new. Some examples include the Orion FunScope 114, Meade LightBridge Mini 130, AWB OneSky, Heritage 130p, Orion XT4.5, Orion XT6, and SkyWatcher 6" classic.

Of all those, my personal recommendation for the cheapest possible entry-level scope is a full size 6" F/8 reflector like the Sky-Watcher 6" Classic (also branded the Classic 150p), or Orion XT6, or Apertura DT6.

1. 6" of aperture is still going to be a substantial increase in detail over a 5" or 4.5" scope.
2. The F/8 focal ratio means collimation will be more forgiving, it will have virtually no coma (an aberration of parabolic mirrors), and cheaper eyepieces will still perform well in it. The faster focal ratios of typical “table top” dobsonians are harder on cheaper eyepieces, requires more precise collimation, and the coma is strong enough that only the very center of the field of view is truly sharp.
3. Because it’s a “full size” dobsonian, it does not require a table to be placed on, meaning it will be more stable (though it’s still small, so a chair or stool to sit on is recommended).