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Choosing the Right Telescope


Magnification, how to calculate and what is reasonable for a given telescope ... The telescope's aperture determines it's light gathering ability and it's ... – PowerPoint PPT presentation

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Title: Choosing the Right Telescope

Choosing the Right Telescope
  • Discussion guided by
  • Thomas C. Smith,
  • Director of the Dark Ridge Observatory

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.

Topics of Conversation
  • Three important questions to keep in mind
  • What do you want to look at?
  • Where will you be observing from?
  • How much money do you want to spend?
  • Some telescope fundamentals and terms
  • Aperture, how does this affect things
  • Magnification, how to calculate and what is
    reasonable for a given telescope
  • Power isnt the whole question, how is the
  • Is bigger always better?
  • Scopes of all sizes and shapes
  • The basic principals of the three major telescope
  • Refractor, Reflector Catadioptric
  • Telescope mounts
  • Tilt and Pan (Alt-Az) or German equatorial
  • Manual, Driven or Go-To positioning
  • Whats in a finder scope?
  • Everything has tradeoffs and a price tag
  • Conclusions what is right for you?

Whats 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?

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 telescopes aperture determines its
    light gathering ability and its 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

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 M-100,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 _at_-15C)
Telescope Fundamentals Cont.
  • Power isnt everything
  • It may surprise you that a telescopes 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)
  • Atmospheric conditions that we observe through

Telescope Fundamentals Cont.
  • Power cont
  • There is a finite amount of detail present in an
    image produced by the telescopes 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?

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 50orMaximum
    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 Saturns 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.

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 in-focus image is formed of a
    very distant object. This isnt 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.

Telescope Fundamentals Cont.
  • Calculating magnification cont
  • Eyepieces have focal lengths too, 25 or 12mm for
  • 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).

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.

Telescope Fundamentals Cont.
  • Seeing, a video demonstration.

Animated GIF image from Adrian Ashford, presented
on Sky Tonight at http//
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 isnt such an issue.
    Larger apertures also lead to shorter exposure
    times for those interested in astrophotography
    especially when combined with short focal
  • 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.

Scopes of All Sizes and Shape
  • With all the advertisements and hype in the
    astronomical world it isnt 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

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
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
  • 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

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

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 reflectors low center of gravity when placed
    on a stable mount will position the eyepiece at a
    very convenient height for just about any sky

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.

  • 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
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
  • 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.

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
  • 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.

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.

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
    tripods 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.

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
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 Earths 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
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
  • 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.

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 Earths
  • Driven normally manually positioned and then a
    motor drive takes over to compensate for the
    Earths 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 its mathematical
    calculations needed for Go-To functionality.
    Earlier models require manual input of the
    aforementioned parameters.

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.

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
  • 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 cant 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.

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
    clubs star party to determine which is best
    suited to them.

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. Dont 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.

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

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 youre 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
  • 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.

Concluding Thoughts
  • So is there a perfect telescope out there just
    waiting for you? Actually, there is its the one
    that youll use most often! An optically perfect
    massive refractor is of no use if you cant 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 its 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

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,
    youll possess the key to unlock a universe of
  • Its a clear night, so what are you waiting for?

Questions, Comments?
  • Got any?