A Treatise on Optimizing Planetary Views
A very common question posed by astronomy beginners arises the first few times
they aim their shiny new scopes at our nearest celestial neighbors, the Moon
and planets. Many beginners are expecting to see Hubble-like images of the
planets, and even those that are not are often disappointed by what they
describe as "tiny, featureless dots" in the eyepiece.
That's when the questions begin to be posed to more experienced astronomers on this
- Is there something wrong with my telescope?
- If I buy a new eyepiece, will the view get better?
- If I buy a new telescope, will the view get better?
- How do I see views of (insert planet) that look like (insert photo)?
First of all, it's important to note: you're not alone. Virtually every
amateur astronomer has gone through this very process of learning how to get
good views of planets and the Moon.
And the good news is that getting great (or at least optimal) views of planets
is achievable using (for the most part) a straightforward "formula" of sorts
that varies very little from observer to observer and from telescope to
What I hope to achieve with this post is to provide straightforward,
no-nonsense advice to the beginning amateur astronomer, that will provide the
background knowledge and processes required to get the most out of your scope
when observing the planets.
Choosing the right Telescope
If you've already got a telescope, which you most likely do, then you are
welcome to skip this section. Or, just read the following sentence: Any
telescope of reasonable quality can deliver breathtaking planetary views.
Of course, some telescopes may be able to deliver more detail, higher
magnifications, and better contrast than others. But even a modest 90mm
refractor can show you the large equatorial belts on Jupiter, color variations
on Saturn and its rings, the phases of Venus and Mercury, and some texture on
Mars' surface (especially during a favorable opposition).
But surely some scopes are better than others for planetary observation. But
which ones? And why? What features of a telescope can make (or break)
The resolution of a telescope (the smallest feature, measured in seconds of
arc, that the telescope can resolve to your eye) is driven directly by the
aperture of the telescope. This is what we learn from
Dawes' Limit, which tells us that the resolution of a telescope is directly proportional to its
aperture. Thus, a telescope with twice the aperture of another
scope can resolve features half the size as the smaller one.
So, one feature that makes a great planetary scope is a large objective lens or
However, there is a law of diminishing returns with aperture, when applied to
the real world, and namely the real atmosphere. The larger the aperture, the
larger the volume of air that the light rays coming into the scope must pass
through. Thus scopes of larger aperture tend to be more sensitive to turbulent
air ("seeing") than scopes of smaller aperture. But don't let that dissuade
you from buying that larger scope -- if anything, larger scopes can be stopped
down with a mask to make them perform as if they have smaller objectives or
mirrors. You can always cut down the aperture of a scope, but you can't
2. Maximum Magnification
Obviously, when you're observing something just a few arcseconds wide (even
mighty Jupiter never breaks 1 arcminute even at the most favorable of
oppositions) you want to ramp up the magnification.
But magnification isn't free. In order for a telescope to deliver a sharp
image at a high magnification, it needs to have big aperture (we'll discuss why
later on). In practice, a telescope's commonly useful maximum magnification is
equivalent to its aperture in mm. So an 8" scope (200mm) will max out at
around 200x. The atmosphere generally places a limit of around ~300x on the
magnification before aberrations driven by atomspheric turbulence begin to be
distractingly large. However, on nights of superb atmospheric calm, a
telescope can potentially deliver a magnification twice its aperture in mm.
Note that when magnifying the image more than the aperture in mm, the image you
see in the eyepiece will appear larger, but will not reveal any new detail.
However, the extra magnification can sometimes make small or low-contrast
features present at the lower magnification easier to see.
We'll discuss this a bit further later on when discussing eyepiece selection.
But to make sure you don't miss it, the background for these claims can
be described mathematically: Telescope Equations: Maximum Magnification
3. Color correctness
Planets (especially Jupiter, Mars, and Saturn) are colorful things to observe.
As a result, a great planetary telescope should have great color correction.
Reflecting and catadioptric telescopes (Newtonian reflectors, Makustov- and
Schmidt-Cassegrain) focus all wavelengths of light (all colors) to the same
spot, and thus deliver sharp, true color.
"Standard" refracting telescopes (called "achromats" or "achros") focus two
wavelengths of light to the same place, while the others focus nearby, but not
in precisely the same place. This causes an effect known as "color fringing"
where bright subjects (such as a planet) appear to have a violet "haze" around
them. More expensive telescopes with more exotic glasses (called
"apocrhromats" or "apos") focus three wavelengths of light to the same place,
which tends to be sufficient to prevent any visible color-related aberrations.
4. Slow(er) focal ratio
As will be discussed later, slower scopes are able to reach the scope's maximum
magnification with less expensive, longer focal length eyepieces. This can be
considered an advantage because the longer focal length eyepieces tend to have
more comfortable eye relief (though this is not a hard-and-fast rule, as we
will discuss later).
Slower scopes also have a wider area free of visually detectable aberrations
such as coma and astigmatism. This can be important if your scope is undriven;
observing the planet as it zips across the field of view, you will want a sharp
image all the way across, not just in the dead center. Slower scopes deliver a
wider area from the center of the field of view where aberrations are not
5. Optical design
Many say that refractors (specifically apochromatic refractors) are the best
choice for serious planetary observation. The claim is based on many factors:
lack of a central obstruction, color correctness (in apochromatic models) and
(within reason) perfect collimation right out of the box. There are likely
other reasons that people will make this claim.
However, I am going to go against the grain here and put forth the idea that
other optical systems can be just as good (or even better) than an apochromatic
refractor for planetary observation -- assuming similar price points.
Let's go over the pros and cons of each optical design when used for planetary
Refractors - Specifically apochromatic ones. The upsides are that they
tend to stay in perfect collimation right out of the box and onward, they have
no central obstruction, and they tend to have slower focal ratios. But they
are very expensive for the aperture; generally a 4" apo is the largest that a
reasonably well-to-do amateur can afford, though 5" and 6" models are available
for huge premiums. And this, in my opinion, is their first disadvantage.
Resolution is a direct function of aperture, and so is maximum magnification.
So for the money a 4" refractor is going to give you less resolution than
other, larger scopes of the same cost. Big refractors also require massive
mounts, as their long tubes act as a big sail that can transmit vibrations due
to wind and touching.
Schmidt-Cassegrain - These are very popular, as they strike a good
balance between price, optical quality (the Schmidt corrector eliminates most,
if not all, coma from the view), and focal ratio. They're easy to mount on
most motorized mounts available to amateurs (whether fork-mount or GEM), so
most are available with tracking capabilities. They are compact, and thus
don't require as bulky a mount as equivalent-aperture refractors and Newts.
They are available in large apertures (up to 14", then they start getting crazy
expensive). The optical design uses mirrors (aside from the corrector plate),
and thus focuses all frequencies of light at the same place -- no chromatic
aberration. And the convex secondary mirror slows the optical train down to a
very reasonable f/10 or so, which means you don't need super-expensive or
super-short eyepieces to max out the magnification. However, they do have a
central obstruction, and so clarity will suffer compared to an apo of
equivalent aperture. This effect is generally considered to be in proportion
to the diameter of the obstruction. So an 8" SCT with a 3" central obstruction
would perform like a 5" apochromatic refractor. At least that's what the math
says (check out Thierry Legault's website and read his page regarding the
effects of a central obstruction:
Thierry Legault - What are the effects of obstruction ?). Another downside to SCTs is the
fact that they must be collimated. Granted, the collimation doesn't generally
go out that much over time, but the fact remains that there is some adjustment
required over time to maintain an optimal view. Finally, SCTs can take longer
than refractors and Newtonians to cool down, due to the closed, sealed system.
Makustov-Cassegrain - These tend to be less popular, except in
comparatively small apertures. They have all the same pros and cons as the
Schmidt-Cassegrain, with the following exceptions. First, their central
obstruction is smaller as a function of the total aperture (thus you get closer
to that "apo" ideal with smaller apertures). The optical train is even slower
than SCTs -- generally f/13 to f/15. Unlike SCTs, Maks generally don't require
any collimation. A downside is that the corrector in an MCT is much thicker
and thus cool-down times are the longest of all the common telescope types,
especially in larger apertures.
Newtonian - Whether mounted on a GEM or a dobsonian mount, the Newtonian
optical system tends to give you the most bang for the buck when it comes to
aperture. The open tube means reasonably fast cool-down times (especially when
paired with a cooling fan behind the primary). However, there are a lot of
downsides for planetary observation. Newtonians tend to have fairly fast focal
ratios, which means you need short and/or expensive eyepieces to max out the
magnification, which can be tough on eye relief. Newtonians require frequent
fiddling with the collimation to keep them aligned -- often multiple times in a
single session. And their huge bulk makes them limited to the size you can put
on a non-dobsonian mount. A 10" newt is the biggest you'll find on a GEM, and
even then it's an enormous sail that transmits vibrations like crazy. Finally,
the spider vanes in most Newtonian scopes create diffraction spikes that can
be distracting when observing bright targets such as planets. For example,
a dim moon of Saturn could be obscured in the diffraction spike haze.
6. The mount
So what is the best mount for that large (or moderate) aperture scope that
delivers sufficient magnification without chromatic aberration?
Since planetary observation is done at high magnification, it really makes the
most sense to have a tracking mount. There are basically two types of mounts
(four if you count tracking/manual variants); the pros and cons are discussed
Alt-Az (fork or dobsonian)
- Using some form of motorized tracking (typically, fork mounts) will ensure
that not only you're able to position the planet in the dead center "sweet
spot" of the eyepiece, but also that it'll stay there.
- A dobsonian-style mount tends to be able to carry the largest apertures at
the lowest price point; this is what makes them great choices for beginners.
- Alt-az mounts offer lighter weight (no counterweight) and more compact size
than equatorial mounts carrying scopes of similar aperture.
- Undriven mounts (typically dobsonian) have a disadvantage that the planet,
at high magnification, shoots through the field of view in less than a minute.
This requires constant adjustment of the telescope to keep up. And with an
alt-az mount, each adjustment will require tweaks to two axes. So if you step
away from the scope for a few minutes, you'll have to start over locating the
object using your finder and lower magnification eyepiece.
- Over the course of the night, the planets will appear to rotate in the field
as they pass overhead. This can be a disadvantage if you're planning on doing
- Motorized fork mounts have a disadvantage that in order
to track properly, they have to be aligned to at least one (better two or
three) stars, which makes them take a little longer to get set up.
- A German Equatorial mount (or GEM) tracks the motion
of objects in the sky by aligning one of its rotational axes with the
rotational axis of the Earth. Thus they can be quick and easy to set up (just
aim it north, set the latitude properly, and it's ready to track).
- They are also great at tracking, even when used without tracking motors.
Once you get the planet in view, if it zips out of the eyepiece, you just tweak
the RA knob and it comes right back into view, even if it's been a few minutes
since you were at the scope.
- Add tracking motors or a clock drive, and a GEM can track your planet just
as well as anything else, with an advantage that the object will not appear to
rotate in the field as it moves across the sky.
- They tend to be heavy (counterweights are required)
- They carry far less weight (and thus less aperture) than their
similarly priced alt-az cousins
- They tend to be more difficult for a beginner to use at first
- When slewing to different parts of the sky, the eyepiece can move around
into crazy positions (especially with Newtonians).
So what's "the best" scope for planetary observation? Well, that's really up
to you. How much do you want to spend? Do you want your scope to be dedicated
to planetary observation, or will you use it to observe DSOs as well? Do you
plan to do any photography with the scope? I'll try to break it down to what I
see as stand-outs in various categories. This is by no means a complete list,
and is not intended to be. It's also just one opinion from one amateur
astronomer... you'll find many more (likely differing) opinions elsewhere.
When it comes down to it -- go to a star party and observe through lots of
different kinds of scopes, to find out what features you want in your scope.
Note also that these recommendations are geared specifically toward those that
are primarily interested in planetary observation (even if some DSO observation
is desired). Obviously, folks that prefer DSOs as their primary targets have a
whole other set of requirements that will make for different recommendations.
Good: Motorized dobsonian telescope, such as Orion's "g" series. These
scopes will have substantial aperture at a lower price than similar apertures
in other formats such as a fork-mounted Schmidt-Cassegrain. The drive motors
will allow you to keep your planet in the field of view for extended periods,
which makes the fast focal ratios typical with these telescopes less of an
issue (the planet stays in the dead center of the field, so no worries about
off-axis aberrations). However, Newtonian telescopes can be finnicky and
fickle about precise collimation, often requiring recollimation multiple times
throughout an observing session to maintain the optimal view. Their fast focal
ratios also dictate that short(er) focal length eyepieces are required to reach
the maximum magnification, so they can be somewhat uncomfortable to use due to
the short eye relief (or more expensive to get a long-eye-relief eyepiece).
Better: A fork- or arm-mounted SCT, such as Meade's LX200 or
Celestron's SE series. You can still get decent aperture in these scopes, and
the motorized tracking makes it easy to maintain the view of a planet.
However, they cost quite a bit more for the same aperture as a dobsonian mount.
But for that money, you get a scope that is more compact, with optics that tend
to stay in precise collimation over extended periods of time. They also tend
to have slow focal ratios of f/10 or so, which means you can use longer
eyepieces (with correspondingly longer eye relief) at maximum magnification.
You can also use less expensive eyepieces.
I think it's worth a special call-out here to Orion's 180mm Makustov-Cassegrain
scope on a motorized EQ mount. For the money, it's reportedly a fantastic
planetary scope that performs on par with 4" apos many times the price.
Best: I won't even begin to make a recommendation here. Many will say a
4" apo is the best. But I disagree -- aperture rules for resolution, so I'd
think a bigger scope would be better. Keep in mind that a rule of thumb when
comparing a reflecting or catadioptric telescope with an apo, that you can take
the aperture of the primary mirror, subtract the diameter of the secondary
mirror/obstruction, and the resulting value is (more or less) the "equivalent"
apo you'd have. So Orion's 180mm Mak should perform pretty much like a 5" apo
(but there will surely be a lot of hemming and hawing about this -- before
starting that, read this). What's
really and truly "best" is probably something none of us can realistically
afford, like a 10" apochromatic refractor or something.
Choosing the right eyepiece
So far we've talked about the telescope...but that's only half of the optical
system. The other half is the eyepiece. This is generally where most
beginners start when trying to improve their view of the planets; their
brand-new scope came with a couple (or sometimes only one) eyepiece of a
middling focal length. Perhaps a barlow came with it. Reading forums and
doing a few internet searches reveals that there are lots of premium eyepieces
out there -- some possibly exceeding the value of the telescope they just
bought! Surely one of these would make the planets "pop" in ways the cheapo
provided eyepieces would, right? And furthermore, why not buy a 2.3mm eyepiece
to really max out the scope? They sell them, so it must work, right?
Well -- there's a lot to cover before we get to all that.
First, let's talk about maximum magnification. When you look through a
telescope, you are utilizing a set of optics (lenses, mirrors) to enhance the
functioning of your own eyes. At a magnification of 1x (no magnification), the
light rays entering the scope exit the scope in a bundle the exact same size
they came in. This would require an eyepiece with a focal length that matches
the focal length of the telescope (which obviously does not exist).
Furthermore, consider that the bundle of light coming out of the scope would be
huge -- as big as the telescope's aperture in fact. The vast majority of that
light wouldn't enter your pupil; it would spill around the sides and do no
So, we add magnification, by using shorter focal length eyepieces. The ratio
between the scope's focal length and the eyepiece's focal length defines the
magnification provided. The effect of this magnification is that the bundle of
light rays exiting the telescope (the exit pupil) shrinks in size. Since our
pupils obviously have a finite size, we need to make sure that the exit pupil
is at least as small as our dark-adapted pupil. This sets the minimum
Before moving on, it's important to note that the exit pupil can be easily
calculated for any telescope and eyepiece combination by dividing the focal
length of the eyepiece by the focal ratio of the scope. So a 20mm eyepiece in
a f/10 telescope makes a 2mm exit pupil. A 5mm eyepiece in an f/10 telescope
makes a 0.5mm exit pupil. The exit pupil gives us a way of objectively
describing "high power", "low power", and everything in between, no matter what
scope or eyepiece is used. I strongly recommend referring to exit pupil sizes
instead of eyepiece focal lengths when reporting how "good" or "bad" a
particular eyepiece works for a given object.
Focal length - That out of the way, let's move on to maximum
magnification. I won't get into the math, because this has been done more
Telescope Equations: Maximum Magnification
The math tells us that the maximum magnification where you still have a sharp
image, untainted by the effects of diffraction, is with a 1mm exit pupil.
Obviously there's some wiggle room there, as some observers may have slightly
higher cell density in their retina than others. The Airy disc is also
slightly different sizes depending on whether or not you have a central
obstruction (and how large it is). So really consider more like 0.8mm-1.2mm
So choosing the focal length of the eyepiece you want is easy; it should be
about the same as the focal ratio of your telescope. Have an f/10 SCT? look
for a ~9-11mm eyepiece. An f/8 refractor? Look for ~7-9mm. An f/5 Newtonian?
The second part gets more complex, and that's deciding, given that focal
length, which eyepiece to actually buy. There are a few factors to consider:
Apparent field - Eyepieces with larger apparent fields of view (such as
60, 72, 82, or even 100 degrees) have a clear benefit when doing planetary
observation with a manual mount. Since at high magnification, you're having to
observe the planet as it moves across the field, clearly having a larger field
gives you more time to observe between bumps to the scope. A side benefit is
that generally eyepieces with large apparent fields are well corrected out to
the edge, so they work better in fast scopes. Simpler designs such as Plossl
and Orthoscopic have narrow fields of view (55 and 45 degrees, respectively)
and tend to not be well corrected outside the central area of the view; a
tracking mount is essential for doing planetary observation with a simple,
Eye relief - How far you have to hold your eye away from the eye lens in
order to have it positioned properly per the manufacturer's design. Simple
designs such as Plossls and Orthoscopics tend to have eye relief figures that
roughly match their focal length. So a 6mm Plossl will have (roughly) 6mm of
eye relief. 5mm of eye relief is pretty uncomfortable. 10mm is bearable, 15mm
is very comfortable, and 20mm is needed if you need to wear glasses while at
the eyepiece. Getting longer eye relief from short eyepieces generally
requires at least two extra lenses from a traditional four-element Plossl or
five-element Orthoscopic. The extra lenses usually form something like a
Barlow lens at the bottom of the eyepiece that allows a longer focal length
eyepiece to perform like a shorter one, but preserve the eye relief.
Color and contrast - The discs of planets are alive with color and
subtle detail. Optimum contrast is essential to see more detail on a planets'
surface, such as festoons in the equatorial belts on Jupiter. What can
negatively impact color and contrast in the eyepiece? The primary offender is
light scatter. As light encounters the surface of a lens, not all of it
actually enters the glass. Some small percentage of it will either reflect
away or be scattered to an out-of-focus place in the final image. Uncoated
glass is particularly bad about this, which is why eyepiece designs like the
Monocentric, which has only two air-to-glass surfaces, was prized in the past
for its clarity. Modern multi-coatings, however, cut the amount of reflection
and scatter to extremely low values. But not zero. When it comes down to it,
a complex eyepiece with 8-12 elements in it is going to have lower light
transmission and higher amounts of light scatter than an eyepiece with fewer
elements (and the same coatings and quality glasses). Note that modern
coatings are VERY good. It takes some effort and a trained eye to see the
difference between a high quality complex eyepiece and a high quality simple
To barlow or not to barlow? - As noted in the paragraph above, adding
more lens surfaces to the optical train reduces contrast. But a quality
barlow, like a quality eyepiece, will have sophisticated multi-coatings that
keep this effect to a minimum. So it's really up to you. If want to go to the
extreme, minimize the lens surfaces and skip the barlow. There are perfectly
good reasons to use one though -- longer eye relief on an otherwise tight focal
length comes to mind. Judicious use of a barlow can also allow you to purchase
a single, high quality eyepiece at a commonly useful focal length (say, one
that gives you a 1mm exit pupil), and be able to use it on nights of
exceptional atmospheric stability at an even shorter focal length (say, one
that gives you a 0.75mm or 0.5mm exit pupil).
What about filters? - You see sets of "planetary" filters for sale -- a
range of colors from blue to red, yellow and green, that are supposed to make
various features stand out on the planet's surface. Are they worth it?
Possibly...but in general, experienced planetary observers avoid the filters.
There are apparently a few special cases where certain filters can come in
handy, such as enhancing the great red spot, or spotting the ice caps on Mars.
The most useful filter can often be a simple neutral density or "moon" filter.
These can cut down the glare of a bright planet (particularly Venus and
Jupiter) to help you see more contrast than you could before.
Recommendations - Putting the above together, some recommendations can
be made. As with the telescope recommendations above, these are to be taken as
broad and not end-all-be-all. Your mileage may vary. Be sure you consider the
quality of the telescope you're using the eyepiece in; a $600 Zeiss Abbe Ortho
may be worth it in your $12,000 apo refractor, but is probably overkill in a
- For slow scopes (f/8 and slower) - go straight to an Abbe Orthoscopic. The
Zeiss Abbe is the undisputed king of this category, but is stupendously expensive (and
I believe is only availble on the used market). Pentax has their XO series which is
very likely just as good as the Zeiss Abbes. One or two rungs down on the quality scale
is the University Optics "volcano top" Abbe Orthoscopics (not to be confused with their
cheaper and lower quality "Super Abbe" eyepieces). The UOs are generally compared on
equal footing with the Baader Genuine series of Orthoscopic eyepieces. I recommend these
eyepieces on slower scopes because the simple design is time-tested and expert-approved.
The cost for the midrange (but still very good) models from Baader and UO is reasonable,
and the 7+mm models will have comfortable eye relief.
- For fast, manual scopes (f/6 and faster) - Your best bet is going to be a wide
field, well corrected eyepiece. The Explore Scientific 82 degree series really shines
here in a "bang for your buck" sense. Some claim that the Televue Delos is the ideal
planetary eyepiece. For a low price option, the GSO SuperView tends to get good reviews
and has a generous 70 degree apparent field.
- For fast, tracking scopes (f/6 and faster) - either sets of recommendations
above may apply, depending on your personal feelings. Since your scope tracks, the
narrow AFOV of an Orthoscopic won't be a problem. But the eye relief in short focal
lengths (particularly those shorter than 6mm) can be really annoying. A compromise here
could be one of the long eye relief series such as the Celestron XCel-LX or Meade Series
5000 HD-60. These have narrower AFOVs than an ES82 (60 degrees), but also have fewer
elements (6), and a generous 15-20mm of eye relief.
Tuning your instrument
So you've got the scope and the eyepiece. Put them together and you're now
observing the planet with a ~1mm exit pupil. But there's still more you can do
to ensure you're getting the sharpest possible view. And that's to properly
collimate your scope. As demonstrated on Thierry Legualt's website, even minor
miscollimation can lead to noticeable degradation of the planetary image:
Thierry Legault - The collimation. It's critical to get the collimation
If you have a refractor or a mak-cass -- you're in luck -- there's very likely
no collimation necessary (or even recommended) for your scope.
If you have an SCT or Newtonian, collimation will be required (though probably
not as often for SCTs). I won't get into the specifics of how to actually
perform a proper collimation, but be sure you finish your tuning with a star
test. A star test is easy to do:
- Point your telescope at a mag2-4ish star near the planet you're going to be observing.
The position is important, because simply pointing the scope in a different part of the sky
can cause the mirror to sag slightly, or (more commonly) the tube to sag a bit under its own
weight. This can affect the collimation enough to be noticeable in a star test.
- Insert your highest power eyepiece, and combine it with a barlow too if you want. You want
an exit pupil of 0.25-0.5mm if possible. This will enable you to observe the Airy disc -- the
diffraction pattern generated by the aperture stop of the scope. This again is where a tracking
mount is essential -- you want to put the star dead center in the eyepiece.
- Very slightly defocus the star. You should see the star get larger, then break into
a central point and a set of surrounding rings. If you defocus too far the surrounding rings
will merge into a "donut" of sorts that isn't very useful.
- Note the relative position of the central dot and the surrounding rings. If they're perfectly
concentric, you've passed the star test. If the rings are closer together on one side than the
other, then your collimation is off; adjust until you get the rings to be concentric.
For some lower-end scopes with somewhat sloppy rack-and-pinion focusers, a
valid star test may not be possible, since even slight movements of the
eyepiece with respect to the optical axis can make the diffraction rings change
shape. In these situations, do your best with a Cheshire to achieve a tight
collimation, and try not to drive yourself crazy trying to get it perfect
(because you'll never get 100% there, or if you do it won't stay that way for
Another critical factor in tuning your scope is making sure that the optics
have acclimated to the temperature of the surrounding air. When light passes
through a boundary between air of two different temperatures, it refracts, or
bends. This is what causes the shimmering effect you see over a hot roof or
roadway. Your telescope's optics are ground and polished to tolerances smaller
than a single wave of light -- that's only a few atoms thick! Now imagine that
your telescope's mirror is a few degrees warmer than the surrounding air. That
means a sliver of air right above the mirror's surface is slightly warmer than
the air around it. Incoming light encounters this boundary and refracts --
certainly more than the distance of one wavelength -- and then strikes the
mirror. The reflected light encounters this boundary again, and refracts
again. The effect of the warm air boundary is doubled on reflecting
telescopes. And consider a catadioptric telescope -- air boundaries may exist
at the front and back of the corrector plate, *and* in front of the mirror.
Another might exist above the diagonal. All of these effects summed together
can make a fantastically precise instrument perform like a department store
Different scopes take different amounts of time to properly cool down.
Environmental conditions can have an effect too -- did you take the scope from
a cool air conditioned house and move it to a hot Arizona night? If the optics
are cooler than the surrounding air, the same problem occurs. "Cool-down"
should really be called "acclimation" -- but alas it's not. In general, more
glass means more cool down time. Bigger mirrors need more glass, and more cool
down time. Makustov-Cassegrain scopes, with their thick glass corrector, have
some of the longest cool down times of all. You can accelerate the cool-down
process with judicious use of fans; Newtonian telescopes commonly utilize a fan
blowing on the back of the primary mirror to assist with temperature
acclimation. You can also buy a "cat cooler" that circulates filtered air
through an SCT or Mak and accelerates the cooldown process.
At high magnifications, particularly with fast scopes, the region of perfect
focus is tiny -- significantly less than a millimeter of travel on the focuser.
If you want that optimal view of a planet, you'll need to be able to
confidently zero in your focus.
The easiest way to do this is through the use of a fine-focus of some kind.
Many premium focusers come with a 10:1 reducer gear that lets you dial in fine
focus easily (example:
GSO Crayford Focuser for Reflectors - Dual Speed).
Some cassegrain scopes can be upgraded by either replacing the focus knob with
a 10:1 reducer system (example:
Micro-focuser for SCTs), or by locking
the primary mirror and strapping a traditional Crayford 10:1 focuser to the
You don't have to spend hundreds of dollars to get fine focus capability,
though. Adding an oversize knob to the focuser is often sufficient to dial in
a fine focus (example: Fine focus knob for C8's).
The larger diameter knobs mean you have to move your fingers
more to achieve the same focuser travel as with a smaller knob -- tada! fine
Finally, how to identify when you've focused? A Bahtinov mask can show you
directly, by presenting a unique pattern of diffraction spikes that converge
when at perfect focus. But before you go make one -- just aim your scope at a
star, put in your planetary eyepiece and zero in the focus so the star is as
razor-sharp as possible. Then lock the focuser and swing back over to the
planet. Chances are you won't do better than that, even with a Bahtinov mask,
for visual purposes. Remember, your eyes will actually correct small amounts
of out-of-focus, including some of the focus problems associated with
Finding the calmest, clearest air
As discussed above in the section about cooling down your telescope, the effect
of refraction through air of varying temperatures was discussed. As you might
imagine, this effect is not limited to the optical surfaces of your scope; the
entire column of air between your scope and outer space contributes to making
your planetary view worse.
Obviously there's nothing you can do to remove the atmosphere (unless of course
you have access to a spacecraft). So the best we can do is to try and stack
the deck in our favor: choose observing sites and targets that should yield the
least amount of turbulent air between you and your target.
We know that turbulent air can be caused by temperature differentials. So
there's one place to start. Avoid observing locations that have "hot spots"
nearby. These can include hot concrete or asphalt roadways and rooftops.
Aiming your scope over these sorts of things, even well after sunset, virtually
guarantees you're going to have poor seeing in that region of the sky. The
effect is increased as the density of the urban stuff increases. Even at your
favorite dark-sky location, with no hotspots nearby, you may still find that
observing things in the general direction of the nearest city center may have
poorer seeing conditions that those in the opposite direction.
You can also reduce the amount of turbulent air between you and your target by
simply reducing the amount of air. Moving to a higher altitude for your
observing helps to get you up and out of the smog and pollution that tends to
form in valleys. It also puts a little less air between you and your target.
Timing your observations for when your targets are highest in the sky means
you're shooting through much less air.
What about light pollution? - Planets are very bright -- bright enough
to cut through all but the worst light pollution. If you have an idea for an
observing site that will have calm air but lots of light pollution (observing
from a coastline over the ocean or a lake comes to mind) -- give it a try; you
may be pleasantly surprised. Generally, though, the prerequisites to a good
dark-sky location tend to also provide for good luck in atmospheric seeing. So
when all else fails, try a good dark-sky site. But it's certainly possible to
find stable air under very bright skies, and still have a great night of
Learning (or re-learning) to see
I saved this section for last because it's the only one that really takes
practice to master. The other stuff described above is formulaic: Match this
eyepiece to that telescope. Get a clock drive and a Crayford focuser. Cool the
scope. Find stable air to observe through.
But once you've got all that done, you look through the eyepiece and Jupiter
still looks like a tiny, featureless dot. How frustrating!! Folks come
to this forum week after week asking this very question. "Is my scope broken?"
"What am I missing?"
What you're missing -- is practice.
First, let's do a little math to show you what I mean.
Consider the full Moon (when viewed with the naked eye). You can see plenty of
interesting features on the Moon with the naked eye -- dark maria, lighter
plains. If you try really hard you can possibly make out the radiants from the
massive Tycho impact crater. However, you can't actually see any craters. You
don't see any real surface detail, just "colors" of different "regions" on the
Now you observe the full Moon with a 10x50 binocular. At a modest 10x
magnification, now you can see all kinds of stuff! The terminator and limb are
ragged with valleys and peaks. You can see hundreds of craters, and possibly
even some detail within those craters.
Why are you able to see so much more detail with just a modest 10x
magnification? It's because the features that were invisible before, such as
the craters, were less than 1 arcminute in size. Since your retina can't
distinguish between features less than 1 arcminute apart from each other, all
the craters and other features were blended together. Applying just 10x
magnification meant that features that were just at the limit of observability
before, now cover a much more generous 5-10 arcminutes across and are thus now
visible as distinct features.
The full moon is 30 arcminutes in diameter. At 10x magnification it appears to
be 300 arcminutes in diameter. What would it take to get Jupiter to appear 30
arcminutes in diameter? How about 300?
The unmagnified size of Jupiter varies, but for the purposes of this exercise
we'll say it's 40 arcseconds in diameter. Arcseconds and arcminutes are
related the same way regular minutes and seconds are related (there are 60
arcseconds in one arcminutes, and 60 arcminutes in one degree). So to get
Jupiter to appear 30 arcminutes (1800 arcseconds) wide, you need 1800/40 = 45x
magnification. To get it to appear the size of the full Moon in your 10x50's,
you'd need 10x more than that -- 450x magnification.
Let's do the same math on Mars, which is much smaller. Mars varies in size a
lot as it moves across the sky. For this exercise let's say it's 15 arcseconds
in diameter. To get that 15 arcseconds to appear 1800 arceconds in size (naked
eye full Moon size) you need 1800/15 = 120x magnification. To get that "10x
binocular" feel, you'd need 10x more than that: 1200x magnification -- probably
impossible without a research-grade 50+inch scope and adaptive optics to deal
with the atmosphere.
But something about this doesn't feel right -- you look in the eyepiece and
even if you can convince yourself that the projected, apparent size of the
planet is as big (or in many cases much larger) than that of the full Moon, it
doesn't seem like the level of detail you can see is comparable. The truth is,
it's probably not directly comparable. Remember that the object you're
observing is so tiny it appears as a point of light in the sky. Even the
smallest perturbations of the air can affect the light arriving from an entire
hemisphere of the planet. So even though its apparent size has been made
bigger, the atmosphere is having its way with it as well, degrading the view.
You can demonstrate this effect to yourself by observing the Moon. Use a moon
atlas to select a crater or other feature that is the same angular size as the
planet (hint: it's going to be one of the very very small ones). Observe that
crater at the same magnification that you observe the planet, and try to tease
out detail from that crater. Pretty tough, huh? Probably just as tough as
trying ot tease out detail from the planet. So it's not that there's something
especially "hard" or "special" about observing planets -- it's just that there
is a lot working against you.
We now know what the math tells us about what we *should* be able to see at
various magnifications. At the modest magnifications required to make planets
appear the same apparent size as the naked eye full Moon, you should expect to
be able to see surface features on a "macro" scale like you do on the full
Moon. You won't see festoons between the belts on Jupiter, but you will
definitely see the belts themselves. You will probably be able to see the
great red spot. And on Mars, at 100x+ magnification, you should be able to see
the same types of surface features such as dark areas, lighter areas, and even
hints of the polar ice caps. With larger scopes that can push more
magnification, you can start to pick out more detail, particularly on Jupiter,
which grows to an impressive 6.66x the apparent size of the full Moon at 300x
magnification, the usual limit due to atmospheric effects.
The last step in your journey of planetary observation is practice. You have
to train your eye and mind to pick out detail -- it's not just going to jump
out at you like it does in the Hubble photos. Start by trying to locate
specific features, such as the Great Red Spot on Jupiter or the Cassini
Division on Saturn. Once you finally see them, you'll wonder why you couldn't
see them before! The more you learn to see, the more will begin presenting
itself to you. That "tiny" little dot, which you now know is actually being
presented larger than the full Moon, won't seem so tiny anymore (it just looks
tiny because it's swimming in an apparent field that is many many times its
And take your time! If you're really serious about seeing planetary detail,
you're going to need to spend more than 15-20 seconds at a stretch at the
eyepiece. Buy or build an observing chair so you can get your eye carefully
positioned in front of the eyepiece, without it shaking around. Just being
able to hold your head still for a bit will make it seem like your scope has a
couple more inches of aperture. When observing, try to spend at least 60
seconds at a stretch in deep concentration on your target. The air is going to
blur everything, but every once in a while, the air will stop momentarily --
usually for less than a second. During that brief moment, the planet will snap
into focus and you'll see gobs of detail on the surface. Then just as quickly
as it snapped into focus, the air will take over again and blur everything.
But that's OK -- that's just how you deal with the reality of unstable air. If
you're really lucky, you'll have a night of observing where the upper
atmosphere is dead calm. There's no snapping in and out of focus -- it's just
razor sharp, all night. Those are the truly great nights, and the ones where
you can really push your scope to the limit with a 0.5-0.7mm exit pupil.
I hope this document has been helpful for you.
I also hope that it continues to evolve. I don't claim to be any kind of
authority on this topic -- if you find that I misrepresented something, let me
know by commenting on the thread and/or PMing me; I'll work with a moderator to
correct the article. If you have additional useful information, please add it
in the comments. Particularly useful stuff I will try to get incorporated into
the main article with the help of a moderator.
Thanks for taking the time to read all of this. Clear skies!