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Transcript
Choosing the Right
Telescope
Discussion guided by:
Thomas C. Smith,
Director of the Dark Ridge Observatory
DRO
Amateur Astronomy Series



This presentation and discussion is part of an
ongoing series of topics that was determined to be
of interest to club members as the result of a poll
conducted by several club members in September
of 2006.
Discussion facilitators, like myself, are not
necessarily experts in the field but through member
discussions it is hoped that we will all get useful
information from these presentations.
Of utmost importance to this discussion is that we
share our collective knowledge and have fun in
doing so.
DRO
Topics of Conversation

Three important questions to keep in mind




Some telescope fundamentals and terms









Refractor, Reflector & Catadioptric
Telescope mounts


Aperture, how does this affect things
Magnification, how to calculate and what is reasonable for a given
telescope
Power isn’t the whole question, how is the “seeing”?
Is bigger always better?
Scopes of all sizes and shapes
The basic principals of the three major telescope types


What do you want to look at?
Where will you be observing from?
How much money do you want to spend?
Tilt and Pan (Alt-Az) or German equatorial
Manual, Driven or Go-To positioning
What’s in a finder scope?
Everything has tradeoffs and a price tag
Conclusions; what is right for you?
DRO
What’s Really Important

Three important questions to keep in mind
 What do you want to look at?
• This one question needs to be answered
above all else and is the foundation needed for
the rest of your decision
 Where will you be observing from?
• Is it from a light-polluted downtown area or a
dark rural site. Do you have a permanent
home for the telescope or do you pack it up
and travel to remote sites?
 How much money do you want to spend?
DRO
Some Telescope
Fundamentals

Aperture, how does that affect things?
 First, the aperture or diameter of the main optical
component can be either a lens or a mirror. The
telescope’s aperture determines it’s light gathering
ability and it’s resolving power (fine detail).
 In real terms; resolution is related to the instrument
diameter so with a 6” telescope you can see a lunar
crater as small as about one mile across or half the
size visible in a 3” telescope used under the same
conditions and same magnification. The same
telescopes turned towards a dim galaxy would,
however, tell a completely different story…
DRO
Telescope Fundamentals Cont.

Aperture cont.


Since the surface area of a 6” telescope is four times
that of a 3” telescope, that galaxy would be four times
brighter in the 6” telescope.
Area = PI x radius2
• so Area (3”) = 28.27 square inches and
• Area (6”) = 113.1 square inches or about 4 times the
area of a 3”
3” versus 6”
telescope
view of M100,
simulated
Not a true representations of what is actually visible in an eyepiece
M-100 CCD Images courtesy of Dark Ridge Observatory 14” LX200GPS with SBIG ST-7XE camera (6X300sec Visual Filter @-15C)
DRO
Telescope Fundamentals Cont.

Power isn’t everything…
 It may surprise you that a telescope’s aperture is not
what determines its magnification or “power”. Most
novice looking to buy or understand a telescope ask
something like “how much does it magnify?” In fact
a telescope can provide just about any
magnification depending on the eyepiece that is
used with it.
 There are two main factors that limit the useful
magnification we can actually get from a given
telescope, those being:
• Aperture (again and used often in discussions) and
• Atmospheric conditions that we observe through
DRO
Telescope Fundamentals Cont.

Power cont…
 There is a finite amount of detail present in an
image produced by the telescope’s main mirror or
objective lens, so what is needed is to find the
optimum magnification to extract that detail without
spreading out the light to such an extent that we
lose detail or sharpness. For this reason most
observers use low power when looking at dim
galaxies and nebula.
 So, how much power is too much?
DRO
Telescope Fundamentals Cont.

There is a simple rule of thumb that indicates the maximum
useful magnification of a telescope:
• 50 times the telescopes aperture in inches or twice its
aperture in millimeters.
Maximum magnification = aperture (inches) x 50
or
Maximum magnification = aperture (mm) x 2
• So, a high quality 4” (100mm) scope should not be
pushed beyond 200x.
• For proper perspective, even a small instrument of good
quality will show you Saturn’s rings or the cloud belts on
Jupiter as these only require about 75x.
• So that 60mm department store “gem” that is quoted as
capable of delivering 300x is all about hype and should be
avoided.
DRO
Telescope Fundamentals Cont.

Power cont…

Calculating magnification
• Since we now know the practical maximum that should be
used for a given instrument, just how do we calculate the
magnification itself?
• Every telescope has a focal length, the distance between
the objective (lens or mirror) and the point where the infocus image is formed of a very distant object. This isn’t
always the same as the length of the tube as many
designs have folded light paths and can mislead you. This
value is normally in the documentation that comes with
the telescope and quite often found printed on the
telescope itself and usually lies between about 400 and
3000mm, depending on the aperture of the telescope.
DRO
Telescope Fundamentals Cont.

Calculating magnification cont…
• Eyepieces have focal lengths too, 25 or 12mm for
example.
• Simply divide the focal length of the telescope by that of
the eyepiece to determine the magnification.
telescope focal length
Magnification = --------------------------eyepiece focal length
• Ex. A 2000mm focal length telescope used with a 25mm
focal length eyepiece will provide 2000/25 = 80 power
(80x).
DRO
Telescope Fundamentals Cont.

Why does the Moon look fuzzy?
 Even with the best telescope you will notice that you can
see finer details on the lunar surface or planets on some
nights as compared to other nights. This is due to the
atmospheric turbulence that exists. It is often
compounded by heat emanating from a sidewalk or
roadway nearby that was heated throughout the daytime
and varies significantly from night to night. Astronomers
refer to this turbulence as bad “seeing”.
 Larger apertures allow observers to pick out faint
objects and fine details on the Moon but regardless of
aperture, the better the “seeing” is the more you can
see. Since steady air is so important, large telescopes,
those in the 10-inch-plus category, are often limited to
250 to 300x on all but the steadiest of nights.
DRO
Telescope Fundamentals
Cont.

“Seeing”, a video demonstration.
Animated GIF image from Adrian Ashford, presented on “Sky Tonight” at http://www.SkyTonight.com
DRO
Is Bigger Always Better?

One question that you might be asking yourself now is why go for
an aperture bigger than 10” if the sky conditions are so limiting?
 Large apertures are most often chosen by observers wanting
to gather as much light as possible for viewing dim galaxies,
nebula and star clusters. These so called “deep sky” objects
are most often viewed using much lower power than when
viewing the Moon or planets so air turbulence and “seeing”
isn’t such an issue. Larger apertures also lead to shorter
exposure times for those interested in astrophotography
especially when combined with short focal lengths.
 Even if you can afford a large instrument you might not want
to haul it around for club star parties and such. Too often
buyers get “aperture-fever” and buy big without thinking
about the “Where” question of where are you going to be
observing from.
DRO
Scopes of All Sizes and
Shape

With all the advertisements and hype in the
astronomical world it isn’t hard to be confused
by the myriad of choices out there. Given the
fundamental knowledge that we have just
discussed you should know that there are
really only three basic types of telescopes to
choose from;



Refractor
Reflector
Catadioptric
DRO
Principals of Three Telescope
Designs: Refractors

The refractor was the first type of instrument to
be turned towards the heavens nearly 400
years ago, then called a “spyglass”. These are
the stereotypical instruments that one thinks of
when the word telescope is first heard. These
telescopes are made with the primary or
objective lens mounted at one end of a long
tube structure and an eyepiece mounted in the
opposite end. Light travels from the distant
object through the objective lens, where it is
refracted to a focus at the opposite end of the
tube where the eyepiece magnifies the image.
It is common to use a 90 degree mirror in the
last part of the light path to project the light to
a more convenient position where the
eyepiece is inserted for viewing. This is called
a diagonal mirror or diagonal for short.
SkyWatch / Gregg Dinderman
DRO
Refractor Cont.



In their current day implementations these instruments are often
sought after by those wishing to observe the Lunar surface or
planetary details. These instruments can offer crisp, high-contrast
views that can take high magnifications. In fact when well made, a
refractor can offer the finest images obtainable by any other
telescope type for the same aperture.
Another advantage to a refractor is that their construction is such
that they are more optically rugged and require little if any optical
alignment adjustments and are a good choice for “pick up and go”
instruments for the field.
This convenience comes at a price, however, as these are far more
expensive to produce and buy than the other types of telescopes
available. Additionally, when the aperture becomes large the
instrument becomes quite long and unwieldy (typically 10 to 15
times the length of the aperture diameter) requiring very large
mounts to support and position the instrument. With such large
mounts; the use of high magnifications can be quite tricky as even
small vibrations get transmitted and seemingly amplified through
the mount.
DRO
Principals of Three Telescope
Designs: Reflector


The second type of telescope is the
reflector. A reflector uses mirrors to
direct the incoming light to a focal
point where an eyepiece is used to
magnify the image.
Light enters the open end of the
telescope tube and is reflected by a
curved mirror (dished out) at the
back of the tube to a small flat mirror
mounted near the tube opening and
then towards the side of the tube
where the eyepiece is conveniently
placed.
Classical Newtonian reflecting telescope.
SkyWatch / Gregg Dinderman
DRO
Reflector Cont.




If you want the largest aperture for your money then the reflector
is the telescope for you.
When well made and maintained a reflector can provide sharp,
clear images of all manner of celestial objects at a fraction of the
cost of an equal-aperture refractor.
The tube of a Newtonian reflector is considerably more
manageable too; its length is seldom more than eight times the
diameter of the primary mirror. So an 8” reflector tube is about 4
feet long and easily fits in the back of a small car.
The reflector’s low center of gravity when placed on a stable
mount will position the eyepiece at a very convenient height for
just about any sky orientation.
DRO
Reflector Cont.


For the best value of all, one should strongly consider a
particular type of reflector called a “Dobsonian”. These reflectors
have their optical tubes placed on a simple, sturdy, low-profile
alt-az mounting and are very easy to transport to the field. There
are Dobsonians in the ranges of 4 to 30 inches in aperture and
are really the ultimate instrument for the casual observer.
Periodic cleaning and infrequent realignment of the optical
components of a reflector type telescope often lessen the appeal
for a lesser mechanically inclined observer. Having an open tube
construction; dust and dirt will accumulate on the optics and
infrequent cleaning will be required. The aluminized surface of
the primary mirror usually needs recoated at about a ten year
interval but this changes based upon the conditions in which the
instrument is used.
DRO
Catadioptric


This is the “best of both worlds”
and the third type of telescope to
consider.
This type of telescope came
about because of the desire to
capture the best features of the
refractor and the reflector in a
combined design. As such these
telescopes use both lens and
mirrors to accomplish their task.
SkyWatch / Gregg Dinderman
DRO
Catadioptric Cont.





The greatest appeal of these instruments is that in their commonly
encountered form (Schmidt-Cassegrain and Maksutov-Cassigrain)
they are very compact. Their tube length is normally only two to
three times their aperture due to a folding of the light path through
the telescope.
The smaller tubes can use smaller and consequently more
manageable mounts and tripods.
With the compound construction of these instruments there comes
the occasional optical alignment maintenance associated with the
mirrors.
The field of view of these telescopes is often very small and the
secondary mirror that sits in the light path slightly degrades the
views of planets and the Moon. Even with these minor drawbacks a
well made catadioptric telescope will deliver very fine images of a
large variety of astronomical objects.
One other advantage of the catadioptric is that it is a sealed tube
instrument and resists dust and dirt deposition on the primary
mirror.
DRO
Catadioptric Cont.



Many people seeking a highly versatile and portable
(for the aperture) telescope that can be used for all
sky subjects and astrophotography will tend to use
some sort of compound instrument.
In short they are excellent general-purpose
telescopes that can be used with a wide variety of
accessories.
On a cost scale, the catadioptric is positioned
between the reflector and the refractor.
DRO
Telescope Mounts


The best telescopes in the world are practically
useless unless they are attached to a stable
mount that allows it to be directed in any
desirable part of the sky and has the ability to
follow a celestial object smoothly and precisely.
A stable mount is one that when lightly tapped
will have its image steady in under a second at
moderate to high magnification.
DRO
Telescope Mounts Cont.



There are two basic types of mounts encountered;
pan & tilt (alt-az) and equatorial.
Alt-az mounts are similar to a photographic
tripod’s pan & tilt head in that movement is up and
down (altitude) and left an right (azimuth)
Equatorial mounts have two axis as well but one
of them is aligned with the rotational axis of the
Earth.
DRO
Telescope Mounts Cont.

Alt-Az:


The previously mentioned
Dobsonian telescope mount is of
this type. It is constructed of
simple materials such as particle
board and Teflon friction pads
resulting in a light-weight, low
center of gravity mount, that
glides smoothly in both axis with
fingertip control.
A Newtonian reflector mounted
in this way is extremely easy to
setup and operate, easily
transported and a great value as
well.
SkyWatch / Gregg Dinderman
DRO
Telescope Mounts Cont.

Equatorial:


For telescopes that are to be
dedicated to astronomical use and
where astrophotography is in the
future, serious consideration must be
given to using an equatorial mount to
counteract the effect of the Earth’s
rotation.
When properly setup the observer
need only turn one axis to keep a
celestial object centered in the
eyepiece. Many mounts come with a
motor to take care of this
automatically.
SkyWatch / Gregg Dinderman
DRO
Telescope Mounts Cont.



Is one mount better than another? Not really as each
has its own strengths and drawbacks.
For the casual observer an alt-az mount and
particularly a Dobsonian is the best choice as it is
highly portable and quickly set up for viewing.
If amateur astrophotography of celestial objects is the
intended use then an equatorial mount in one form or
another is really the only practical solution. Polar
alignment, although not a difficult procedure, does take
some time before observing can begin but becomes
much easier with practice. For this reason, serious
astrophotography is done with an equatorially mounted
telescope on a permanent pier-type structure that has
been previously and rigorously polar aligned.
DRO
Telescope Positioning

Manual, Driven or Go-To positioning:



Manual; as the name implies, the telescope is manually
slewed or positioned to point to the object and has to be
manually repositioned, usually only as the object moves near
the edge of the field of view to compensate for the Earth’s
rotation.
Driven; normally manually positioned and then a motor drive
takes over to compensate for the Earth’s rotation. This
requires a polar type mount to “track” the object.
Go-To; all the rage now and becoming much improved as
technology advances. The latest offerings have global
positioning systems built into them so that they know the
current location, elevation, date and time; that the control
software requires to perform it’s mathematical calculations
needed for Go-To functionality. Earlier models require manual
input of the aforementioned parameters.
DRO
The Finder

When using medium or high powers a
telescope will show you a very small window on
the sky. This can make finding faint objects a
frustrating process without the aid of a finder.
As the name suggests, these are observing
aids that assist you in locating celestial objects,
and all scopes, irrespective of type, should be
equipped with one.
DRO
The Finder Cont.



In their most common occurrence they look like a miniature
telescope. They are normally mounted near the main
eyepiece and have a cross-hair (reticule) on which to place
the desired object.
The front aperture should be at least 25mm (1”) and if
possible, larger as the larger aperture will allow finding
fainter objects less difficult.
There are designs that consist of a laser or LED circular
projection on a non-magnified sky background that many
find convenient and natural but these can’t be used except
to find objects that are visible to the naked eye.
That said they can easily be used to “star hop”
to the deep sky object instead.
DRO
The Finder Cont.

Some individuals prefer to have their finder fitted
with a right angle prism or mirror so that they can
see in the finder and without much head
repositioning, see in the main eyepiece. Some
prefer to use the finder straight-through and sight
“generally” with the eye that is not looking through
the finder to assist in finding those fainter objects.
The choice is of course the individuals and it is
advised that the new telescope purchaser take
advantage of a local club’s star party to determine
which is best suited to them.
DRO
The Local Astronomy Club

I am sure that it is no surprise to anyone that the
local astronomy club can offer a significant amount
of help in finding the right telescope and
accessories that fit your individual needs and
desires. Don’t ever be afraid to ask for advice and
take advantage of the “star parties”, that clubs
routinely hold, in helping you make the right choice
for you. There are more often then not a wide
variety of telescope types available for you to
observe through and talk to the owners to get ideas
about how each instrument operates.
DRO
Everything Has a Price



While it may be tempting financially, resist the temptation to
buy the cheapest telescope available. Many of these
instruments are of poor quality either optically or
mechanically, and frequently both, and will inevitably lead
to disappointment and frustration. If you have a budget of
just $200.00 then you should really consider a good pair of
binoculars instead.
That said, there are a great number of quality instruments
that can be obtained secondhand and an experienced
member of the club may be able to help “weed” through the
offerings and assist you in a sound purchase.
Are you reasonably good with your hands? Then why not
make your own instrument from purchased optics and local
hardware. A Dobsonian can be made in a short time and
offer excellent views as well as the pride associated with
self-construction.
DRO
… Price Cont.


Even if you are a beginner fortunate enough to have a
sizable disposable income, do not buy the largest, most
expensive telescope you can find. If you’re just learning to
identify the constellations, then many of the advanced
features that such an instrument possesses will not likely be
of any use to you.
Remember that there is more to a telescope than a tube,
mounting and steady tripod. Be sure to save some for those
accessories that make the instrument all that it can be;



Eyepieces to give a larger range of magnification.
Filters to aid in light pollution and to bring out those fine details in the
planets.
And what of astrophotography? That is an entire topic in itself that I
leave for future discussions in this series.
DRO
Concluding Thoughts


So is there a perfect telescope out there just waiting for
you? Actually, there is; it’s the one that you’ll use most
often! An optically perfect massive refractor is of no use
if you can’t carry it outside for use, and the largest
Dobsonian will not show you the faintest galaxies if the
only place you can use it is in a light-polluted parking lot
near the center of town.
Consider carefully what you consider to be your primary
observing interest, where you are likely to observe, and
what you consider “portable”. Remember that each
telescope has it’s own strengths and weaknesses; an
ideal instrument for detecting fine detail on the planets
may not be the best at catching faint views of a distant
galaxy.
DRO
Final Thoughts




Get with members of the local club and attend
the star parties, this is really the best way to
decide what is right for you.
A telescope is a big investment to most people,
and the universe is not going away soon, so
take your time over the purchase.
When you buy an instrument that is right for
you, you’ll possess the key to unlock a universe
of wonders.
It’s a clear night, so what are you waiting for?
DRO
Questions, Comments?
Got any?