Astronomy Terminology

How Telescopes Work:

Here is my very layperson’s explanation for how telescopes work.  Telescopes serve two primary functions:

  1. Enable seeing of objects that are too dim to see with the naked eye
  2. Provide magnified, or zoomed in, views of the night sky

It may not be intuitive, but this is actually the order of importance of function when it comes to amateur astronomy.  The way to think about it is that you can already see stars with the naked eye, and they don’t necessarily become more detailed when you zoom in on them (planets are a different case).  The real magic of a telescope though, is that it can reveal stars and deep space objects (DSOs – such as nebulae and galaxies) that don’t emit enough light for your eye to recognize them.  In this case, the telescope acts like a giant photon funnel.  It captures more light than is possible with just your eye and condenses it together for you to see.

The above functions are controlled by the following:

  1. Aperture, or the size of the opening in your telescope (measured in inches or millimeters).  A bigger aperture allows you to see dimmer objects.
  2. Magnification, or the ability to zoom in, is controlled by the formula Telescope Focal Length / Eyepiece Focal Length.  The Telescope Focal Length is a specification of the telescope (usually printed on the box or instruction manual).  The Eyepiece Focal Length will be labeled on the eyepiece.  As an example, a telescope with a 450mm focal length using a 10mm eyepiece will have a magnification of 450mm/10mm = 45x.  Keep in mind though, that as you increase magnification, your field of view gets smaller.  This can make it difficult to navigate in unfamiliar skies.  Also, there is a limit to the useful magnification of a telescope.  This limit is approximately 2 times the aperture in millimeters or 50-60 times the aperture in inches.

What does all this mean?  If you’re just getting into astronomy, and especially if you want to do astrophotography, your goal should be to get the most aperture possible for the money you want to spend.  Even a modest telescope can show you details of the planets.  However, the bigger aperture you have, the more Deep Space Objects you’ll be able to find!

Types of Telescopes

  1. Refractor Telescope:  This is probably what first comes to mind when you think of a telescope.  They were the first type of telescope and work by using lenses to focus and magnify an image.  You can find good, high-end refractor telescopes, but beware of cheaper models!  They may advertise impressive magnification, but unless you really shell out some cash, the aperture won’t be enough to give you good views.  Check out this wiki article for more information and this link for some different refractor telescope models.
  2. Newtonian or Reflector Telescope:  These are likely the second most popular telescopes after refractors.  Rather than relying on a lens to focus the image, these telescopes use a large, curved mirror. This mirror collects a large amount of light, then reflects it using a secondary mirror so that it can be focused at your eye.  Newtonian telescopes generally offer more aperture for the money, so they can be a great starting option for someone looking to get into astronomy.  Check out this wiki article for more information on reflecting telescopes and this link for some different reflecting telescope models.
  3. GoTo Telescope:  Welcome to the 21st century!  GoTo telescopes are computer controlled, motorized telescopes, and they’re pretty amazing.  GoTo telescopes are preprogrammed with coordinates of literally thousands of stars and objects in the sky.  After some basic alignment steps, you simply tell the telescope what you want to look at, and it will adjust the direction of the scope accordingly.  The technology is really impressive, but as a beginner I’ve actually enjoyed the challenge of having to manually control my telescope.  In a sense, I’ve found that it’s helped me understand astronomical principles.  However, GoTo telescopes are excellent for tracking and astrophotography.  Check out this wiki article for more information on GoTo telescopes and this link for some different GoTo telescope models.
  4. Dobsonian Telescope:  A Dobsonian telescope is a special class of reflector telescope first designed in the 1960’s.  They are designed to optimize the aperture size, while still being easy to operate and easy to transport.  The distinctive feature is the Dobsonian mount which uses a swiveling platform close to the ground.  Dobsonian telescopes tend to be very easy to operate and great for Deep Sky Object (DSO) observation.  However, they have certain limitations that make them less ideal for astrophotography, particularly with an iPhone.  Check out this wiki article for more information on Dobsonian telescopes and this link for some different Dobsonian telescope models.
  5. Catadioptric Telescope:  Catadioptric combine elements of both refractor and reflector telescopes.  You’ll notice they have large apertures but come in relatively compact sizes.  They tend to be a bit more advanced and expensive than the aforementioned scopes.  However, they can be excellent for more advanced astrophotography, specifically with DSLR cameras.  Check out this wiki article for more information on Catadioptric telescopes and this link for some different Catadioptric telescope models.

Parts of a Telescope

  1. Tripod:  Pretty obviously, this is the thing with three legs that holds your telescope up off the ground.
  2. Mount:  The mount is essentially the connection between the telescope and the tripod.  The tripod consists of the three legs, but the mount contains the actual feature(s) that hold onto and control your telescope.  Mounts come in two types:
    1. Altazimuth: This is a fancy term for the typical tripod you’re probably familiar with.  In fact I had no clue there was a name for this type of mount other than “tripod” until I started getting into astronomy.  Altazimuth mounts are relatively simple to use, with two axes.  One vertical and one horizontal.  They tend to be easier for beginners to control and get used to.
    2. Equatorial:  These mounts are specifically designed for astronomy.  Equatorial mounts also have two axes.  However, these axes are shifted to be aligned with the tilt of the Earth.  If you’ve ever seen a long exposure image of star trails such as the one below, you may have some idea that the stars appear to move very slowly through the night sky.  This is caused by the rotation of the Earth about its axis.  Through an alignment process called polar alignment, equatorial mounts can be used to effectively track objects through the sky and eliminate this trailing effect.  These mounts are especially important for astrophotography.

      Example of star trails

      Example of star trails

  3. Optical Tube:  This is the main telescope tube.  Remember, the magnification achieved with a telescope is equal to the focal length of the optical tube divided by the focal length of the eyepiece being used.
  4. FinderScope:  This is a very low magnification fixed scope that is attached to the main optical tube.  The FinderScope is aligned to the main telescope, much like the scope of a rifle, so that both scopes will have the same object in the center of the field of view simultaneously.  This will allow you to find objects in the night sky easily with low magnification and know that they will be properly located in your main optical tube.
  5. Focuser:  The focuser is a precise mechanism that brings the eyepiece closer to, or farther away from, the main optical tube.  This enables precise focusing.
  6. Eyepiece:  An eyepiece is obviously the part that you look through!  They come in various lengths (commonly 25mm, 20mm, 10mm, etc.) and diameters (most commonly 1.25″ or 2″).  The different lengths will correspond to different magnification based on the magnification formula.  They will also have different fields of view, with smaller eyepieces generally having smaller fields of view.
  7. Filters:  These can be attached to individual eyepieces to polarize and/or filter the incoming light.  They are particularly useful for lunar viewing when the moon is overwhelmingly bright.  They can also help bring out specific planetary filters and enhance astrophotography.  They are rather essential for astrophotography with an iPhone.
  8. Counterweight:  A counterweight is a big heavy weight that can be attached and adjusted to an equatorial mount.  Based on the mechanics of equatorial mounts, these counterweights are required to prevent your telescope from tipping over when it is put in certain alignments.
  9. Slow Motion Cable:  Slow motion cables enable very precise fine tuning of telescope mounts.  This is critical when operating at a high magnification where even slight deviations can cause you to lose an object from view.
  10. Motor Drive:  Motor drives can be used with non-GoTo equatorial mounts to enable tracking of celestial objects.  If the telescope is properly polar aligned and the object centered in the view, the motor drive will rotate the telescope in unison with the rotation of the Earth to negate any movement.  Motor drives are very helpful for iAstrophotography with non-GoTo equatorial mounts.

General Terminology

  1. Latitude: Latitude is a coordinate on Earth’s surface that specifies a North/South position. Latitude is measured in degrees, with 0 degrees being the equator and 90 degrees being at the North and South poles. The counterpart to Latitude is Longitude, which specifies an East/West position. You can check the Latitude/Longitude of your home city here!
  2. Tilt: As you hopefully recall from middle school science class, the Earth is rotating on an axis that is tilted with respect to its orbit.  Specifically, this tilt is 23.5 degrees.  The Earth’s tilt is the cause of the seasons and is also very important to how we position our telescopes for astrophotography.  For more on the tilt of the Earth, check out this great article at Universe Today.
  3. Declination: Celestial objects in the sky have been assigned to a coordinate system, similar to Latitude/Longitude.  This helps us easily locate and track objects in the cosmos.  However, this coordinate plane is centered about the tilt of the Earth, so that we can track objects through the night sky as the Earth rotates.  On this coordinate system, Declination is analogous to Latitude on the Earth.  Likewise, it is measured from 0 to 90 degrees.
  4. Right Ascension:  The other measurement on this celestial coordinate system is Right Ascension.  Right Ascension is the celestial equivalent to Longitude.  One complete rotation of the Earth takes 24 hours, so Right Ascension goes from 0 to 24 hours and is broken up into hours, minutes, and seconds!
  5. North and South Celestial Poles:  As mentioned before, the Earth is rotating about an axis which is on a tilt with respect to its orbit of the Sun.  If you were to draw a line through this axis and extend it up and down into the sky, that line would mark off the North and South Celestial Poles.  These are the only points in the sky that do not appear to move as the Earth rotates.  The North Celestial Pole happens to be very near a special star called Polaris (or as you may know it, The North Star).  Though often mistakenly thought of as the brightest star in the sky, The North Star is actually special, because it helps us find a point in the northern sky that is very close to the North Celestial Pole.
  6. Planet: A planet is a celestial object that orbits a star.  It cannot be a star itself, but it must have enough gravity to create a rounded shape.  There are officially 8 planets in our solar system:  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

    amateur astronomy planets solar system

    The planets of our solar system (plus the Moon). Can you name them all?

  7. Star: Stars are gigantic spheres of ultra-hot plasma.  They give off immense energy by way of the thermonuclear fusion of hydrogen into helium which occurs in their cores.  There are various types of stars in the galaxy, and the different types follow very different evolutionary patterns.  The nearest star to Earth is obviously the Sun!
  8. Galaxy: A Galaxy is an enormous group of stars and other celestial objects which are mutually bound gravitationally.  Our own Solar System is part of the Milky Way Galaxy, but there are literally hundreds of billions of Galaxies in the universe.
  9. Nebula: A Nebula is simply a region of space containing an abundance of gas and dust not necessarily bound into planets or stars.  They are often the birthing grounds of new stars, which then illuminate the cloud in spectacular fashion.  These are some of the most impressive objects to look for and photograph with a telescope in the night sky.
  10. Supernova: When some stars run out of hydrogen fuel in their cores to continue thermonuclear fusion, they will literally collapse under their own mass.  This immense pressurization and gravitational collapse will reignite a runaway fusion process leading to a tremendous explosion.  The explosion, known as a Supernova, will propel the star’s mass out into the universe.  This is a very important process for the creation of the heavier elements found throughout the universe.
  11. Open Cluster:  An Open Cluster is a group of stars located relatively nearby to each other that share a loose gravitational collection.  They were formed from the same giant molecular cloud.  Several Open Clusters are visible with the naked eye in dark skies.


3 comments on “Astronomy Terminology
  1. albionses4u says:

    I am completely new to the astronomy scene, but got really interested after taking the Earth and Space Science course and am really motivated to get into astronomy and astrophotography. I’ve snapped a few pics of Mars and Aldebaran (and unconfirmed but I think it might be betelgeuse) with my iPhone, and really want to get some nicer pics and better views with a telescope. I’m more interested in seeing galaxies and nebulae in contrast to individual stars (etc the stars in the Orion Belt / the Pleiades). What would be a great starter telescope you recommand?

    thanks! =D

  2. Matt says:

    Hey there, thanks for stopping by! Glad to hear you’ve gotten interested in astronomy! It’s a great hobby, and there’s a whole universe out there to explore. So telescope recommendations….there’s obviously a ton out there on the subject. Now that I’ve got some more experience shooting with an iPhone under my belt, I’m going to write an updated section on the site. Stay tuned, and I’ll post it here when it goes up!


  3. Matt says:

    Alright, just made some updates! Check em out here:

    Let me know if you have any questions!