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A Trealise on Optimizing Planetry Views.

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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)

planetary observation?

1. Resolution

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

primary mirror.

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

increase it.

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

any photography.

•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.

German Equatorial


•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).

7. Summary

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

effectively here:

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

exit pupil.

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?

Try ~4-6mm.

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,

narrow-apparent-field eyepiece.

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

$300 reflector.

•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

junk scope.

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.

Fine focus

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

atmospheric lensing.

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.

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This should answer any and all questions from anyone new to astronomy and should be a mandatory read :-) before any questions are allowed to be asked. (just joking)

that apart it should be considered as a sticky

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This should answer any and all questions from anyone new to astronomy and should be a mandatory read :-) before any questions are allowed to be asked. (just joking)

that apart it should be considered as a sticky

+1 for making this a sticky, loads of information clearly presented.

Thanks for taking the trouble to post.

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Great post. This is especially relevant to me at the moment as I'm thinking of a new scope.

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Very useful indeed as it was this post along with the patient and informative help given by one of the forum members that led me to my recently received Celestron SE8 goto.

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