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Tips and Tricks
Finding Difficult Objects Easier With A Telrad
Preventing Telescope Slipping When Adjusting Eye
Piece Position
Scout With Binoculars First
Submit Your Tip and Help Other Beginning Astronomers
NEWTONIAN REFLECTOR
Newtonians (also known as catoptrics) usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece.
Advantages
Lowest cost per inch of aperture compared to refractors
and Catadioptrics since mirrors can be produced at less cost than lenses
in medium to large apertures.
Reasonably compact and portable up to focal lengths
of 1000 mm.
Excellent for faint deep sky objects such as remote
galaxies, nebulae and star clusters due to the generally fast focal ratios
(f/4 to f/8).
Reasonably good for lunar and planetary work.
Good for deep sky astrophotography (but not as convenient
and more difficult to use than Catadioptrics).
Low in optical aberrations and deliver very bright
images.
Disadvantages
Open optical tube design allows image-degrading air
currents and air contaminants, which over a period of time will degrade
the mirror coatings and cause telescope performance to suffer.
More fragile than Refractors or Catadioptrics and
thus require more maintenance (such as collimation).
Suffer from off-axis coma.
Large apertures (over 8") are bulky, heavy and tend
to be expensive.
Generally not suited for terrestrial applications.
Slight light loss due to secondary (diagonal) obstruction
when compared with refractors.
REFRACTOR
Refractors (also known as dioptrics) are what the average person identifies with the word "telescope", a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube.
Advantages
Easy to use and reliable due to the simplicity of design.
Little or no maintenance.
Excellent for lunar, planetary and binary star observing
especially in larger apertures.
Good for distant terrestrial viewing.
High contrast images with no secondary mirror or diagonal
obstruction.
Color correction is good in achromatic designs and
excellent in apochromatic, fluorite, and ED designs.
Sealed optical tube reduces image degrading air currents
and protects optics.
Objective lens is permanently mounted and aligned.
Disadvantages
More expensive per inch of aperture than Newtonians
or Catadioptrics.
Heavier, longer and bulkier than equivalent aperture
Newtonians and catadioptrics.
The cost and bulk factors limit the practical useful
maximum size objective to small apertures
Less suited for viewing small and faint deep sky objects
such as distant galaxies and nebulae because of practical aperture limitations.
Focal ratios are usually long (f/11 or slower) making
photography of deep sky objects more difficult.
Some color aberration in achromatic designs (doublet).
Poor reputation due to low quality imported toy telescopes;
a reputation unjustified when dealing with a quality refractor from a reputable
manufacturer.
CATADIOPTRICS
Catadioptrics use a combination of mirrors and lenses to fold the optics and form an image. There are two popular designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain. In the Schmidt-Cassegrain the light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube and intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3 1/2" and larger apertures.
Schmidt-Cassegrain Advantages
Best all-around, all-purpose telescope design. Combines
the optical advantages of both lenses and mirrors while canceling their
disadvantages.
Excellent optics with razor sharp images over a wide
field.
Excellent for deep sky observing or astrophotography
with fast films or CCD’s.
Very good for lunar, planetary and binary star observing
or photography.
Excellent for terrestrial viewing or photography.
Focal ratio generally around f/10. Useful for all
types of photography. Avoid faster f/ratio telescopes (they yield lower
contrast and increase aberrations). For faster astrophotography, use a
Reducer/Corrector lens.
Closed tube design reduces image degrading air currents.
Most are extremely compact and portable.
Easy to use.
Durable and virtually maintenance free.
Large apertures at reasonable prices and less expensive
than equivalent aperture refractors.
Most versatile type of telescope.
More accessories available than with other types of
telescopes.
Best near focus capability of any type telescope.
Schmidt-Cassegrain Disadvantages
More expensive than Newtonians of equal aperture.
It is not what people expect a telescope to look like.
Slight light loss due to secondary mirror obstruction
compared to refractors.
Maksutov-Cassegrain
The Maksutov design is similar to the Schmidt with basically the same advantages and disadvantages but is not quite as good optically given the same focal ratios. It uses a thick meniscus correcting lens with a heavy curvature and a secondary mirror that is usually an aluminized spot on the corrector.
The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures (over 90 mm).
The Maksutov optical design typically is easier to
make and should be less expensive than the Schmidt-Cassegrain.
ALTAZIMUTH
The simplest type of mount with two motions, altitude (up and down/vertical) and azimuth (side-to-side/horizontal). Good altazimuth mounts will have slow-motion knobs to make precise adjustments, which aid in keeping tracking motion smooth. These type mounts are good for terrestrial observing and for scanning the sky at lower power but are not for deep sky photography.
DOBSONIAN
Dobsonian mounts are a special type of altazimuth mount. Instead of the scope being attached near its center the scope is attached to the base near the its end. Dobsonian mounts are also not intended for deep sky photography. They typically are the cheapest of the different mount types.
EQUATORIAL
Superior for astronomical observing over long periods
of time and absolutely necessary for astrophotography. As the earth rotates
around its axis, the stationary stars appear to move across the sky. If
you are observing them using an altazimuth mount, they will quickly float
out of view in both axes. A telescope on an equatorial mount can be aimed
at a celestial object and easily guided either by manual slow-motion controls
or by an electric clock drive to follow the object easily across the sky
and keep it in the view of the telescope. The equatorial mount is rotated
on one axis (polar/right ascension) adjusted to your latitude and that
axis is aligned to make it parallel to the Earth's axis, so that if that
axis is turned at the same rate of speed as the Earth, but in the opposite
direction, objects will appear to sit still when viewed through the telescope.
Below are the four values that you must know about your telescope and eyepieces in order to determine such things as magnification and field of view.
Telescope Aperture
The clear aperture of a telescope is the diameter
of the objective lens or primary mirror. The majority of the time
this value will be in inches. For the equations below the aperture
must be converted from inches to millimeters.
Telescope Focal Length
This is the distance, in an optical system, from the
lens (or primary mirror) to the point where the telescope is in focus (focal
point). The longer the focal length of the telescope, generally the more
power it has, the larger the image and the smaller the field of view.
This value is typically expressed in millimeters.
Eyepiece Focal Length
Eyepiece Apparent Field of View
Magnification Power = Telescope F. L. / Eyepiece
F. L.
FOV (degrees) = Eyepiece AFOV / Magnification Power
Resolving Power (arcsec) = 4.56 / (Apt / 25.4)
Limiting Magnitude = 7.5 + 5 * Log(Apt / 10)
Light Grasp = (Apt / 7) ^ 2
Exit Pupil (mm) = Apt / Magnification Power
Apt = Telescope Aperture (mm)
F. L. = Focal Length (mm)
FOV = Field of View (degrees)
AFOV = Apparent Field of View (degrees)
Finding Difficult Objects Easier With A Telrad
Using a Telrad can make life a lot easier for beginning astronomers. If you haven't used one get one, compared to other telescope accessories they are very cheap (~$50). You will not regret having it.
If you haven't seen one or never heard of one a Telrad projects 3 concentric circle in space. There is no magnification, the image is not inverted or reversed, and you can keep both eye open when using it. There is one drawback though, the stars you see are what you get. This sometimes makes it difficult to point your telescope when there is not a naked-eye guide star close enough to your target object.
The solution to this is to use binoculars to look through the back of the Telrad. If you have ever look through some binoculars you will notice that you can see more stars then with the naked eye. This still has the advantage that the image is not inverted or reversed and you can use both eyes. It will take some practice learning where and how to look so you can see the circle projected by the Telrad but this will make finding your target much easier.
I have not tried a monocular but that should work also.
Preventing Telescope Slipping When Adjusting Eye Piece Position
If you have a newtonian reflector and an equatorial mount you will notice that sometimes your eye piece gets in a position that may require you to kneel down or stretch over the telescope to look through the eye piece. The eye piece can be adjusted by loosening the scope rings and turning the telescope, however, your scope may slide through the scope rings (this will be more pronounced the higher latitude you are) changing the balance of your telescope.
A simple solution to this is to get some zip-ties and put them around the scope so the zip-ties ride on the scope rings.
Scout With Binoculars First
On of the most frustrating thinks that a beginning astronomer can face is when you find the object that you want to view on your sky map, you point your telescope, "Nothing". So you do it again, "Nothing". From personal experience, this is typically because the star that is next to your object on your sky map is not the one you are really looking at. Use some binoculars to first verify that what you are looking at really matches your sky map. The binoculars will help you see stars that are to dim to see with the naked-eye. The great thing about binoculars is that you get to use both eyes, the field-of-view is larger than your finder scope, the image is correctly oriented, and you can see stars that are to dim to see with the naked-eye. Binoculars also help when star hopping form one object to the next.
Never get in the mind set that "I have a telescope
I don't need to use binoculars".
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