Telescope Star Testing
While planetary observing requires primarily great optics, good seeing,
and high magnification, star observing is more varied. Some types of star
observing require the same considerations as planetary work, but other
types of observing require different considerations.
If you are star testing your telescope, for example, you need good
seeing, optics that are cooled down, and high magnification.
What is star testing?
Observing moderately bright stars under good seeing with high magnification
can tell you a lot about the quality and alignment of your telescope. You can,
for example, identify an over corrected or under corrected objective, spherical
aberration, turned down edge (for mirrors), and astigmatism errors just by
examining out of focus star images. In fact, if you don't own a laser
alignment tool, observing a star at high magnification is a good way to tweak
up your instrument's alignment.
Star testing is done by examining a magnified star image with the eyepiece
racked both inside and outside of focus, and comparing the resulting
If you are using a star test to align your optics, you might find it
difficult with a telescope that doesn't have a clock drive. By the time you
go to the business end of your telescope to make an adjustment and return to
the eyepiece, the star may not even still be in your field of view. If you
live in the northern hemisphere, you can resolve this issue by using Polaris
as your alignment star.
A good website that shows the kind of star images produced by different
optics errors is Star Testing Astronomical Telescopes.
Star aligning is a technique I've used a lot. It is sometimes a touchy
procedure, however. Often I either bump my telescope or make too big an
adjustment and cause the star to be moved out of the field of view. If you
want a quicker, easier way, you probably want an alignment tool.
Use this astro-customized search to locate Cheshire eyepieces or laser
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Double Star Observing
Another type of star work that has similar requirements to planetary
work is double star observing.
What are double stars?
Double stars are star pairs (sometimes even more multiples than pairs)
where the component stars are so close together that to the naked eye they
appear as a single star. They may be so close that they even appear to be
a single star with a small telescope.
The stars may be locked together gravitationally, or they may be actually
very far apart and independent and just happen to be along the line of view.
Double star observing is fun for a number of reasons. I use double stars (or
binary stars as they are sometimes called) primarily to compare and evaluate
Observing close doubles under good conditions gives some indication of
optical quality. If a double separated just over the theoretical resolution of
your telescope can be cleanly split, the optics are near perfect. If not, maybe
doing some star testing can help identify the problem.
Doubles are also interesting objects to observe because of the variability.
Often the component stars are different colors, or different magnitudes. These
variations give each double its own identity.
Star Cluster, Galaxy, and Nebulae
For more general star, galaxy, and nebula observing, seeing is not
much of a concern, but transparency is. You'll be straining to see the
faintest details of interesting objects, and nearly every photon matters.
It's with this general star observing that aperture is of great importance.
The bigger the aperture of the telescope you use, the brighter objects will be,
and the more objects you will ultimately be able to see. However, if you get
such a big telescope that it becomes too much of a bother to use, you've gone
You can check out the Telescope Tutorial
for more details on how optical performance relates to aperture. For here I'll
just mention that detail resolution is proportional to the diameter of a
telescope, and limited by atmospheric phenomenon. Light gathering power, all
important for star observing, is proportional to telescope diameter
squared. So for star observing, diameter is especially important.
Star Observing Hints
Most star observing is done at relatively low power. Open clusters, the
Andromeda galaxy, and some nebulae are large enough in apparent size that low
power (less than 100x) is best. For this observing, a clock drive isn't
Planetary nebulae, globular clusters, and some galaxies will generally
work best at moderate power, perhaps 100x to 150x. But even at this
magnification, a clock drive isn't a necessity.
Transparency is usually best when objects are well above the horizon.
While not as critical as with planetary observing, a cooled down telescope
will help you get better views.
If you live in a large city, you may need to find an observing site that has
better transparency. Another constraint caused by cities is all the lights.
While not strictly affecting transparency, stray light definitely restricts the
ability to see dim objects. A different location may provide darker skys.
Let your eyes become fully dark adjusted for observing stars. You'll be
surprised by how much more detail you can see with fully dark-adapted eyes.
If you are using charts between observations, use a red light to observe
your charts. Eyes are less affected by red light, and you'll loose less of
your night vision if you use a red beam. If you use a computer screen for
star charts, see if it has a night vision mode, which will again display
primarily with red light.
If you have a pesky street light or porch light that's giving trouble,
consider using a towel or your coat to drape over your head when at the
eyepiece. This will block out the troublesome stray light. If your
telescope is portable, you may be able to move it to a position where
your house, a garage, or shed can block a pesky streetlight or porch light.
Consider using filters designed to increase the contrast of nebulae. You
can find nebula filters using this astro-customized search engine.
These filters work by screening out wavelengths that interfere with seeing
nebula features. Keep in mind, however, that since these filters screen out
light, they don't work as well on smaller aperture telescopes. I don't think I
would make the investment if my instrument was smaller than a 6 to 8 inch.
For very dim objects, learn to use averted vision to help discern
faint details. Averted vision describes the technique of looking away
from the dim details you want to see, and examining them as best you can
with your peripheral vision. The center of your vision is best for details,
but your peripheral vision is more light sensitive. Experiment to find
which way to avert your vision to find the most light sensitive portion
of your eye.
What Telescopes are Best for Stargazing?
If you are new to amateur astronomy-- start with a decent pair of
binoculars. You can get a good functional pair for under $100. Try for 50mm
diameter or better. And don't get too high a magnification. You'll find you
can't hold them still well enough over about 10x unless you mount them on some
kind of tripod. Check out the Binocular Tutorial
page for more information.
You may find you can get by for some time with a couple of pair of
binoculars, like a 7x50 for spotting objects and a 10x50 or 15x70 for a bit
higher magnification. Or you can spend a bit more and get a zoom binocular.
As to telescope type, there are a couple of considerations. Dobsonians
can certainly give the most aperture for the dollar. An 8 inch Dobsonian
can go for around $500, while an 8 inch Cassegrain will generally cost
$1500 and up.
While more aperture will let you see more stars, aperture isn't the only
thing you want to think about. As a beginner, you may want to consider a
compromise between aperture and ease of use. You can purchase 5 inch or so
Newtonians or 3.5 to 8 inch Cassegrains with computerized clock drives. These
units are more portable, and the computerized clock drives and extensive
star catalogs let the telescopes locate objects for you.
That being said, I point out that there are two kind of popular Cassegrain
telescopes available for the beginner astronomer. See the Cassegrain Tutorial for more details.
Suffice to say, I would argue that for stargazing, a Newtonian or Schmidt
Cassegrain would be better than a Maksutov. The Maksutov has a very large
f ratio (around f/15), which provides too narrow a field of view for some
Purists will argue that you should learn the constellations, and then learn
how to star hop to find objects with your telescope. Others will say it
wastes too much time, and computerized mount telescopes allow you to see many
more objects in an evening.
I think it all depends upon the observer and how he or she enjoys the hobby.
You may find that the ability to see several objects in an evening because of
the telescope's computerized drive is more important than a large aperture.
You may also find that portability plays an important part to you. If you
are a youngster, elderly, or handicapped, you may find that the difficulty of
working with a large telescope out ways its benefits. You may find that for
good views you have to travel to a different site. For these and other
reasons, you may want to temper the urge to get a behemoth telescope. I've
been involved with astronomy for over 40 years, and I currently own nothing
bigger than a 6 inch Newtonian.
You might find the following table useful for determining which telescope
matches up to your anticipated observing. For finding deep space objects such
as galaxies, you'll likely want to concentrate on the wide-field and
general purpose telescopes, the larger aperture the better.
For small objects like globular clusters and double stars, you'd likely
be best served by telescopes to the right side of the table. Again, the
larger instruments will let you see dimmer objects.
Telescope/Observing Preference Table
Use this astro-customized search to browse some excellent telescope
Star photography, as with star observing, is quite varied in technique.
Constellations, rich star fields, and some extensive nebulae can be
photographed with simply a guided camera. You can use either the barn door
mount or the piggyback method. See the Observing
Comets for descriptions of those types of photography.
For photos of clusters, globular clusters, and galaxies you'll need to
photograph through your telescope. This is a most difficult kind of photography.
To do it you need a clock driven telescope and either an auto-guider or
a guide scope mounted along side your main instrument.
Time exposures are a must here, typically several minutes. While this
kind of work can still be done with a standard film camera, if you have the
money you may want to buy a specially designed CCD camera for telescope
Use this astro-customized search engine to find astrophotography cameras