Telescope Basics: Focal length, aperture, eyepieces, and mounts

Focal Length – The focal length of a telescope is one of the most fundamental terms that an amateur astronomer should know. The focal length is the distance light travels after it enters telescope’s tube and collects on the objective lens or primary mirror. The objective lens or mirror collects light from a distant object and brings it to a focus. The length over which this happens is called the focal length. For instance the very popular Nexstar 8SE has a focal length of 2000 mm. The focal length is always specified in millimeters for consistency. The longer the focal length of the scope the more potential your scope has for high magnification.

In order to calculate magnification you’ll need to know the focal length of the eyepiece you are going to use as well. The magnifying power of a telescope is calculated by dividing the telescope focal length by the eyepiece focal length. If the eyepiece has a 20 mm focal length and the telescope has a 2000 mm focal length then the magnification produced will be 100x. (2000mm / 20mm = 100).

Aperture – The aperture of a telescope is the diameter of the primary lens (the lens that gathers the light). It dictates the light gathering ability of your telescope. The more light you gather the more detail you can see and the fainter an object you can find. Aperture is the most important factor in determining the objects you will be able to see and the amount of detail you will be able to discern. The largest aperture you can afford is almost always better! Bigger IS better. However, there are limits to this, the scope that you use the most is the best scope to own. So if you try to go out and buy a huge scope you can’t move and never touch, it’s really not worth it… For some reason the typical units of measurement in aperture change from millimeters to inches. Typical aperture sizes are 4”, 6”, 8”, 10”, and so on.

To put the light gathering ability of a typical scope into perspective, I’ll use another example and equation. The typical human pupil dilates to 7 mm in complete darkness so this is the size telescope manufactures use when putting these specs on a product. The area of the round pupil is the area that light can enter and determines how much light the pupil can transmit to the brain. To calculate the area of the pupil in millimeters we first need to know the radius which is 7mm / 2 = 3.5mm. The area of a circle is pi*r^2 so the area of a dilated pupil is 3.14159 * 3.5^2 = 38.48mm^2.

The aperture of our Nexstar 8SE is 8” which is 203.2 mm. 203.2 / 2 =  101.6mm. Then the area is 3.14159 * 101.6^2 = 32,429.28mm. To calculate how much more light the telescope gathers than our own pupil we simply divide 32,429.28 / 38.48 = 842.76. So our 8” telescope lets us gather 843 times more light and/or more detail than the naked eye. This is very important for seeing deep sky objects or the dust bands on Jupiter.

There’s one other piece of food for thought here. If you are considering various apertures of scopes, you need to understand the math. An 8” scope is not just twice the size of a 4” scope giving twice the light gather capability. It’s actually 4 times the aperture providing 4x the light gathering ability of a 4” telescope. Definitely something to understand when choosing a telescope.

Eyepieces – An eyepiece is the optic that further focuses the light that has been collected, reflected, and focused by the primary mirror. The eyepiece focal length determines the actual magnification that the entire telescope assembly provides. In order to find the total magnification provided by your scope, you want to divide the scope’s focal length by the eyepiece focal length. In our 8SE example the scope has a 2000 mm focal length and the eyepiece has a 20 mm focal length which produces a total magnification of 100x (2000mm / 20mm = 100).

A word on magnification, more magnification is not better. This is a common beginner’s misconception. After experience it becomes quickly apparent that wider angle views and less magnification, provide more crisp, detailed, and perhaps even more colorful views.

The second bit of information provided by eyepiece manufacturers is the eyepiece field stop and apparent field of view. The field stop is the metal ring inside the eyepiece barrel that limits the field size. Using this information you can calculate the true field you will be able to see in the sky in degrees. To do so you want to divide the eyepiece field stop diameter by the telescopes focal length and multiply that by 57.3. Sometimes the field stop diameter is not provided with an eyepiece or is hard to find. In that case there is another way to calculate the true field of view seen through an eyepiece. Most eyepieces have an apparent field of view specified in degrees. For instance a Televue 20mm Plossl has an apparent field of view of 50 degrees. In order to calculate the true field of view, divide the apparent field of view by total magnification.

In our example the telescope has a focal length of 2000 mm and our eyepiece has a focal length of 20 mm so 2000 / 20 = 100x magnification. Then our eyepiece specifies it has a 50 degree apparent field of view, so to find the true field of view we divide the apparent field by the magnification 50 / 100 = 0.5 degree true field of view. So we can see one half of one degree of the sky at a time through this eyepiece. This is helpful because many objects you’d like to view have a visible size specified in degrees, minutes, and arc minutes in sky atlases and apps. Knowing the true field of view gives you a reference point for estimating how much of an object you can see or how large or small it will appear in your eyepiece. To put degrees of the sky into perspective, the full moon covers about 31 arc minutes or just about ½ degree of sky.

Mounts – There are various kinds of mounts that most telescopes use depending on its type and purpose.

Alt-azimuth

The altitude – azimuth (alt/az)  mount moves in vertical (altitude) and horizontal (azimuth) directions. It is the simplest way to mount a telescope. This mount is widely used in beginner push to scopes.

There is a second more specialized type of alt/az mount called a fork mount that combines an equatorial mount internally to determine the right ascension and declination of an object then converts that to altitude and azimuth coordinates. The scope then locates and tracks the object using the alt/az motions. The push to alt/az mounts are good for terrestrial viewing, and the computer driven mounts are usually good for viewing but not necessarily great for deep sky photography. Although these mounts are often not optimal for astrophotography, depending on your patience, exposure time etc, the fork type mount on the Nexstar series and others can get the job done nicely. These computer driven fork mounts can be quite enjoyable and easy to use, saving a lot of time locating and tracking objects.

Dobsonian

The Dobsonian mount is a newer, modified version of the Alt-azimuth mount. This mount was invented in the 1970’s by John Dobsonian. Dobsonian mounts are mounted on the ground to a heavy platform, and designed to support massively sized Newtonian Reflectors, while keeping a steady image. It is common for Dobsonian telescopes to have very large apertures making this mount a necessity.

Equatorial

When moving on to the more advanced stages of both astronomical observing and astrophotography, you’ll want to look into the much better (and more expensive) Equatorial & German Equatorial Mounts (GEMs). These mounts compensate for the rotation of the earth by aligning one axis parallel to the Earth’s physical axis of rotation. This is achieved in practice by a method called polar alignment and allows the mount to rotate in one direction instead of two and in doing so compensates for what is called sidereal motion. The axis’ used are right ascension and declination.

In practice you will locate an object using its RA and Dec coordinates which don’t change. Once the object is located, the declination axis of your scope doesn’t change. The right ascension axis rotates with time, which again is called sidereal time, the rotation of the earth. This single axis rotation is far superior for long exposure photography partly because there is very little chance for slight movements or vibrations to occur in the vertical or horizontal axis’. This slight variance is usually due to lower quality (and more affordable) spur driven gearing that can result in slight but obvious star trails and other undesirable visual effects over long exposure times.

German equatorial mounts are differentiated by utilizing a large counterweight that extends out from the bottom of the scope. This weight is vital for providing balance and stability to the mount. These mounts are typically used with Newtonian reflectors and refractors, and are not nearly as easy to setup and use as a altazimuth mount, but are far superior when it comes to how smoothly they track.

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