Seeing and Transparency in Astronomy Defined for the Beginner
What is Seeing In Astronomy Terminology?
There are some terms you need to be familiar with to understand comments you are likely to read about observing astronomical objects with your telescope.
One is seeing. Seeing is a reference to the ability of the telescope
to show details through the atmosphere. It's often given on a 1 to 10 scale, 10
being perfect seeing, and 1 being very poor, with little detail visible.
Seeing is largely an atmospheric issue, but telescope optics plays a part.
It is generally accepted that at the optics end, seeing is less disrupted by
refractor optics than reflector optics. It takes refractors a shorter time to
cool down, and one hypothesis is that as light passes through the different lens
elements, each with different effects on seeing, the errors get averaged out a
bit. The end result is that refractors tend to provide steadier images that
The atmospheric part of seeing is affected by movement of air cells in the
atmosphere. The cells are often different temperature and possibly different in
other atmospheric properties. When looking up through the atmosphere, one looks
through many air cells.
These cells move around, causing the blurring and shifting in lunar and
planetary images. The smaller the telescope you use, the larger are the scale
of the disturbances. To the right you see an example of less than perfect
seeing. This moon movie shows the shifting distortions that the atmosphere
causes on moderate seeing night.
How Does Seeing Affect What I Can See With My Telescope?
With my 2 inch refractor and medium to poor seeing I can see waves of motion
when I'm observing the moon. The more the turbulence, the greater the apparent
motion. The motion disturbances are larger than the craters I'm trying to
observe, so they don't limit the details I'm able to see. They are only a
distraction, not a limit to the details I'm trying to perceive.
When I look at the moon with my 6 inch telescope, the waving phenomena are
smaller in apparent size, being on the order of the smaller objects I want to
see. In fact, the atmospheric motion forms a limit to the smallest details I'm
able to resolve.
This explains why with medium to small telescopes, there are more evenings
of acceptable seeing. The motion effects must be virtually gone for larger (8
inch or better) telescopes to be able to function up to their potential. An
excellent source of additional information about astronomical instruments and
observing issues is the book: The Backyard Astronomer's Guide.
You can minimize the effects of bad seeing by looking at your targets
when they appear near their highest altitude. In this way you are looking
through the least amount of atmosphere, which can reduce the turbulence.
Also, trying moving your telescope, if it is portable, so that you aren't
looking directly over a nearby roof. A low object viewed just over a rooftop
will definitely be badly affected by turbulence.
Even if you are seeing a lot of turbulence, don't necessarily give up.
The best lunar and planetary views often come to patient observers who keep
looking. The atmosphere is dynamic, and sometimes it can steady while you
are looking at your target, if even for just a few minutes.
Walther Crater Video
Check out the video above of the Lunar Walther Crater region. It was taken
through a NexStar
5SE, using a Celestron NexImage
Camera. You can see a lot of motion involved as the video plays. Pick out
an area with tiny features and watch how they can even disappear momentarily.
The NexImage camera allows me to record the movie, and I use a Linux utility to
separate the movie into individual frames, then use a Yorick program to
select, align, and average frames to get an optimized single image. With that
technique, I can obtain some pretty good results. A section of the averaged
version of the above video is shown below.
Walther Crater w/NexStar 5SE
What is Transparency?
Another term you'll see when reading about observing is transparency.
Transparency has to do with the ability to see dim objects through the
atmosphere. There are many atmospheric phenomenon that can adversely affect
If you live in or near a city, pollution can reduce transparency. Thin, high
altitude clouds can affect transparency. Forest fires can put up enough
material to affect seeing over a very wide area. City or neighborhood lighting
can brighten the background of the night sky, also limiting the faintness of
objects you'll be able to see.
Transparency is also often indicated with a 1 to 10 rating, 10 being the best transparency.
If transparency is low, then your ability to detect dim objects is greatly
I've found, at least in my area, that transparency is better in the
early morning than in the evening. I think it's related to traffic. In the
evening there is still a lot of traffic, and that causes dust to be constantly
churned up, some of it getting up into the air. But in the early morning,
things have settled down, the air is usually less turbulent, and transparency
If you have the time and you have a portable telescope, you can resort
to moving your apparatus to a better observing area, with less light
interference and clearer skies. If you can see the Milky Way with naked eye,
then you've found a good spot.
Incidentally if you have such a site not far away, you may find you can step
down to a bit smaller, more portable telescope. You can easily gain a magnitude
of star depth just by observing from a darker, more transparent site.
If you find you enjoy observing, you might consider keeping a journal of
your observations. If you decide to do that, make the observations more useful
by annotating (with a number from 1 to 10) the seeing and transparency
conditions, as well as the telescope and other equipment you used. Consider
posting your observations on your website, or join an e-group and share your
experiences there. I can assure you that other amateur astronomers will
enjoy reading about your observing adventures.
First, you'll enjoy reviewing your notes when you view the same or similar
objects again later, perhaps through different conditions or a different
Plus, we all get clouded in now and again, and reading about someone else's
observations helps make those days less terrible. Reading the observations of
others, if they are well documented, is also helpful when you are shopping
for another telescope. It helps you visualize what you might see with different
sized and types of instruments.
Of course it's not only atmospheric conditions that affect what you will see
in the night sky. The views of different object types is also a function of the
type and size of telescope used. For observing those dim objects, you need
good transparency and a large aperture, small f ratio instrument.
To view high resolution objects such as planets, transparency is less an
issue, but you need good seeing and a telescope designed to deliver
high resolution. That type of telescope is usually a long focus or high
f ratio telescope.