PART VII: HOW TO USE YOUR TELESCOPE
MAXIMIZING WHAT YOU SEE
Magnification is the basic advantage
a telescope has over the eye. The telescope forms an image on its final
focal plane and the eyepiece acts as a simple magnifier to increase the
size of that image. Since the telescope gathers more light than the eye,
the image thus formed is also brighter than we see with the naked eye.
Brighter and larger equals more visible.
Objects in the sky come in all sizes and shapes-from large to tiny and
from very bright to exceedingly faint. This is the primary reason we want
to have more than one magnification for our telescope (accomplished with
multiple eyepieces or Barlow lenses).
The sky's natural brightness seems to diminish as we magnify-its light
gets spread out over a larger area, reducing the brightness per unit area.
As a result, fainter objects are often more visible at higher powers,
but there is always a compromise between "large enough to see" and "too
spread out to be visible". It's desirable to experiment with magnifications
to get the best view possible.
Low powers in a telescope are considered to be from 3X per inch of aperture
to 10X per inch (e.g. on a 3" telescope, 9X to 30X). These powers are
used for large objects and the widest true fields possible.
Medium powers are from 10X per inch to 29X per inch. These powers are
used for inspection of larger objects, or for viewing most objects. These
are typically the most comfortable powers to use, and combine the right
combination of magnification with object visibility. (e.g. 30X to 87X
on a 3" telescope)
High powers are from 30X to 50X per inch of aperture. These powers are
used to split double stars, examine the planets and Moon closely, or to
see details in a small, bright, object. To use these powers, the Seeing
has to be good (we'll go into that later). This is 90X to 150X on that
Extra high power (60X per inch and higher) are rarely used except on
double stars or the Moon, and only under conditions of exceptional stillness
in the air (Superb Seeing). The number of nights where these powers can
be used, over a decade, would number on the fingers of one hand, typically.
Averted Vision is a technique that amateur astronomers use to see very
faint details in an object, or even to see a very faint object. The eye's
rods (the part of the retina we use in night vision) don't occupy the
center of our retinas, yet they are incredibly sensitive to light. Accordingly,
our peripheral visions are more sensitive to light than our central vision.
Rods are most heavily concentrated in our retinas just slightly outside
the central area (known as the fovea centralis), so a slightly "averted"
vision will often allow us to see a LOT more than we can see by looking
straight at the object. Fainter stars become visible, and the full size
of a faint object may become apparent when we look slightly to the side
of the object. The typically most sensitive area of the retina can be
employed, for the person who is using the right eye, by looking slightly
to the right of the object, or, for a left-eyed observer, slightly to
the left. Normally, this puts the object's image directly on the part
of the retina that is most sensitive to light.
Phase of the Moon: This influences what we can see.
The Moon acts just like a big city nearby, and its light makes the sky
quite a bit brighter. When the Moon is out, we simply cannot see objects
or stars as faint as we can see when the Moon is down. So always try to
observe when the Moon has set or has not yet risen--unless you're observing
the Moon, of course. The best time of the month to observe all night is
when the Moon is new.
Dark adaptation: When we first go from a lighted room
or car into the darkness, our eyes have not adjusted to the dark. The
eyes' sensitivity to light will gradually increase to their maxima for
about 45 minutes after stepping into the darkness. What's happening is
that the body is naturally producing a chemical in the retina that is
incredibly sensitive to light. This chemical is destroyed by light, so
we have to be in the darkness before it begins to build up. It goes without
saying that if we want to see objects as well as possible through our
telescopes, we should wait this period until our eyes have fully dark
adapted. One easy "rule of thumb": if the sky appears black, you are not
dark adapted. When fully dark adapted, the night sky is seen as silvery-gray.
The eye is said to go from photopic vision, in which the cones in the
eye are turned on and we see color, to scotopic vision, in which the cones
are turned off so we lose color vision but the huge increase in sensitivity
of the rods (over 90,000 times!) gives us the ability to see fairly well
in the darkness.
To stay dark-adapted, never use a white light to read by, or to see with.
Only use a deep red light (our eyes are not very sensitive to deep red),
like a red LED flashlight, or a regular flashlight with several layers
of red cellophane taped in place over the lens. Red flashlights can be
obtained at telescope stores or many sporting goods merchants.
Local Light Pollution: This is the bane of the urban
astronomer. Certainly, when you view the sky, you'll extinguish all exterior
lights on your house and pull the shades so interior lights don't bother
you, but there's little we can do about street lights or neighbor's yard
lights. To maximize our observing experience, try to always set up your
telescope where you are not bothered by such lights. And when you are
at a dark site, remember to turn off the interior lights in your car.
A 1-second blast from a white light may require 15 minutes for your dark
adaptation to recover.
Where to Observe in the Sky: When we look straight up,
we are looking through about 10 miles of air, with ten miles of water
vapor, dust, and light scatter. When we look at an object on the horizon,
that column of air is extended to almost 100 miles, with a huge increase
in atmospheric aerosols. Everything at the horizon is a lot fainter and
harder to see than when the objects are higher in the sky. The highest
above the horizon that any object achieves is when it crosses the meridian
(astronomers say the object Culminates, or reaches its highest elevation),
the imaginary line running from due north to due south, passing over our
heads. To see the most of each object in the sky, try to observe each
object when it passes the meridian, and try to avoid looking at objects
close to the horizon-especially the 30 degrees or so from the horizon
up, where the air is thickest. The best place to observe any object, of
course, is the zenith. We get the best view possible of objects that cross
at or near there.
When to Observe during the night: The atmosphere quickly
loses heat into space when it first gets dark. This causes turbulence
in the atmosphere and blurs the images in our telescopes. But later, the
atmosphere settles down and heat loss becomes quite uniform. It is this
time the sky is steadiest, and images are the best. Additionally, the
moisture in the atmosphere will settle out as dew, causing the air above
us to dry out and become clearer. Plus, many outdoor lights go out after
midnight, making the sky darker. At most locations, these improved viewing
conditions occur after midnight and until dawn. These are the best hours
to observe at most sites. It may not always be possible to view during
these hours, but some steadfast observers will take a nap earlier and
get up to observe during the morning hours.
Seeing and Transparency: or HOW ASTRONOMERS RATE THE
QUALITY OF THE ATMOSPHERE (understanding how the air influences the view
through a telescope)
The atmosphere interferes with the telescope's ability to see. Every
beginner learns that axiom very quickly. First, we blame the scope: "I
just didn't spend enough money to get a good one." But soon we talk with
other amateurs or telescope shop employees and we quickly learn we're
all in the same boat. Young or old, big telescope or small, we are all
limited by this thick soup of an atmosphere we look through.
Though there are many factors at play in our abilities to penetrate that
thick soup, there are two definitions that stand out in nearly every beginner's
text on the use of a telescope:
Seeing and Transparency.
Seeing refers to the steadiness of the
atmosphere. It's the "Great Equalizer" that makes the big, expensive scopes
perform like smaller scopes, and limits even the biggest observatory scopes'
resolution to quantities similar to an amateur's small scope. It's as
if we were viewing the pebbles on the bottom of the shallows in a small
lake-the quieter and steadier the water, the sharper and easier the view
of those pebbles. But let a small amount of turbulence enter into the
water above the pebbles and it becomes hard to tell what's even there.
So it is with the atmosphere-just a little turbulence in the air between
your telescope and space and the star images dance around and send out
spikes in every direction until they become large fuzzy balls, denying
us the clear view of the skies we paid our money for when we purchased
our telescopes in the first place.
Seeing limits the magnification we can use; sometimes to quite low powers
of 50X to 60X. Don't worry-it's probably not the telescope. Just be persistent
in your observing and those moments when you can use higher powers will
The Seeing differences have been scaled by various observers
in the past. A good example of a Seeing Scale (of horrors) can be seen
in the Pickering scale of Seeing, a ten point scale that allows you to
estimate how good your Seeing is. It can be found on the web at: http://uk.geocities.com/dpeach_78/pickering.htm
Unfortunately, Pickering 1 is all-too-common, and Pickering 10 is quite
Probably, more than anything else, this is what limits our abilities
to see detail on the planets and the Moon. The good news is that out of
every minute of horrible Seeing, there will be a few seconds of relative
calm and the images will sharpen up, revealing to us that we did indeed
buy decent scopes and that it's just the crumby atmosphere preventing
those scopes from seeing more.
Patience! Keep at it, and sooner or later you will have one of those
mystical moments when the Seeing steadies and allows us to see the Universe
through a veritable Hole in the Atmosphere. Those of us out West and those
of us in Florida see more of those nights than the people who live east
of the Mississippi, but all of us are eventually treated to one of those
nights of especially good Seeing.
One clue: the Earth's atmosphere steadies, in most places, more completely
between the hours of midnight and dawn. If your interests in planets,
Moon, or double stars makes you seek the higher powers, these are your
best hours to observe.
Transparency is the other atmospheric issue for us amateur astronomers,
according to the books. It is usually defined as the darkness of the night
sky. The farther you are from civilization, the more Transparent is the
sky, or so it is said.
Let's deviate from the book definitions, here, and refer to Transparency
as being actually a combination of two atmospheric characteristics, which
can be defined as Darkness and Clarity. We could use the same term, transparency,
for the second characteristic, but this might be confusing.
Darkness is what is often referred to as transparency. But it doesn't
take very many trips to a dark site to notice that the stars don't always
appear equally bright-especially toward the horizon. The stars overhead
may appear just as impressive, but the fine details in the Milky Way seem
to be less obvious and the sky slightly less impressive on some nights
compared to others.
Many have used a sky brightness meter to actually measure the brightness
of the night sky, and these nights of seemingly lower clarity can measure
just as dark as the nights when the Milky Way looks like big, cumulus,
clouds in the sky. An obvious case in point is that if you are at a site
well away from civilization and it's overcast, the sky is quite dark,
yet the atmospheric Clarity is terrible.
So, while Darkness is an important thing to seek out-it determines how
"deep" our telescopes reach, both in magnitude and into the Universe-it
becomes obvious to the sky observer that it has to accompany atmospheric
Clarity to really be called Transparent.
Clarity is probably best explained as the absence of aerosols and dust
in the atmosphere. When water vapor or wind-blown dust is present between
your telescope and space, you will just not see the faint details in deep-sky
objects that a clear atmosphere allows. There will be more Extinction
(the reduction in your ability to see faint stars with lower altitudes)
in the air, and more light scatter. At its worst, the sky seems to take
on a silver sheen and the Milky Way's details are lost.
These two characteristics can be exclusive. Here in the LA area, we often
get strong winds that blow all water vapor and dust and smog out to sea,
and the visibility of distant mountains and buildings is superb-as if
they were just next door instead of many miles away. We have wonderful
Clarity. Yet, we still have the worst light pollution in the nation, and
the skies are not dark at all. It is true during these times that somewhat
fainter stars can be seen, but the dimmest are several magnitudes brighter
than what can be seen in a dark site, and the Milky Way is never visible.
So the stargazer has to accept that conditions will not always be perfect
when he/she heads for the dark place to observe the sky. But is there
a way we can help ourselves experience better Seeing, Darkness, or Clarity?
Yes there is.
Seeing is often better in those places where the wind, if it blows at
all, blows in a smooth manner without turbulence. You can look for sites
near a body of water, or in a saddle-shaped mountain valley. There's a
site in LA famous for its wonderful Seeing-Mount Wilson. You may recall
they built a big observatory there over a hundred years ago.
Darkness is usually better the farther from cities you are. State or
National Parks and Monuments excel in this regard. Look at the information
for your local areas on the website, www.cleardarksky.com to find the
nearest dark site. Be prepared to drive a bit if you live in a big city.
Clarity is best at high altitudes (one of the advantages Westerners have),
so if you live at low altitudes, the days just after a front passes may
be the best days to observe the fainter objects. Seeing can be turbulent
at these times, but there is usually a day or two after the front comes
through when the air has not yet "steamed up" again, yet the air is also
not strongly turbulent. That's the day to go out.
This simple explanation will hopefully help you understand what you are
seeing when you take your telescope out under the stars. This is a hobby
that rewards perseverance-observe often and the magical combination of
good Seeing, Darkness, AND Clarity will be encountered a lot more often.
Cooling of the Optics: The surface of good optics are
accurately figured to just a few nanometers-if a telescope mirror were
stretched out to the size of the United States, there would be no bump
higher than an inch-we're talking SMOOTH. So, it probably comes as no
surprise that when warm optics meet cold air and start cooling, that surface
accuracy is distorted by the process of cooling. The optics flex and warp
enough to reduce the quality of images from our telescopes until the optics
come to thermal equilibrium with the surrounding air. This can take quite
a while, hours, in fact, for larger optics, and even an hour for smaller
scopes. So, if the quality of the image is important (and, on planets
or the Moon, it is especially), it is important to remember to let the
telescope cool down before viewing. This is ultra-important for high-power
viewing in particular.
Fortunately, for those of us at the usual latitudes of the US, there
is about 90 minutes between sunset and complete darkness (astronomical
twilight falls when the sun is 18 degrees below the horizon) that we can
use to let the telescope cool down. Set the telescope outside during this
period, and you can begin your observing with a telescope that is already
Collimation of Optics: This will be explored in detail
later on, but suffice it to say that no telescope will deliver its best
possible images without having its optics aligned. This process, called
collimation, is simple and requires a fairly simple tool or tools.
Cleanliness of Optics: Though a small amount of dust
does not hurt the image quality from our scopes, grease, oils, and fingerprints
do, as well as possibly damaging the fine coatings on lenses and mirrors.
We'll go into this more, later. Clean optics, though, are essential for
quality images through the scope. At the very least, eyepieces should
be inspected before every outing, and eyelash oils or mascara carefully
cleaned from the lenses. The rule, here, is to NEVER CLEAN OPTICS THAT
Observing Site Optimization: There are a couple of don'ts
that should be remembered:
- Don't look at objects directly above a roof. Heat rising from the
roof will distort the images.
- Don't set the telescope up on concrete, if possible. Concrete holds
its heat and releases it all night, causing heat waves visible in the
scope. Grass or dirt is the ideal material to have underfoot.
- Don't store the telescope where it can get very hot. This might damage
the optics. At the least, it will slow the cooling of the optics after