How to Choose your Eyepiece

Help - How to choose your eyepiece

A telescope eyepiece is the interface through which you see space objects from your telescope. There are many to choose from, many manufacturers, many sizes an qualities. Reviews are a great way to start, but how can you really choose which one and expect a particular view of the stars and planets? Here are a few tips that will help you decide

First things first

Assuming that you already own or have access to a telescope, you must learn two things about it: Aperture and Focal length. These are the two most important things to know about your telescope, from these values you will be able to arrive to other important information as well and better choose an eyepiece.

Focal length
For telescopes with a glass lens (refractors) it is the distance between the lens center point to the focus point of incoming light. Every telescope lens has a particular focal point. If you placed your eye at that focal point you will be able to see an image in focus. If you moved your head back a little, the image will be blurry, the same would happen if you moved your head closer. For telescopes with a curved mirror (reflectors and Dobsonians) a instead of lens, it is the distance between the mirror center point to the focus point of the image reflected off the mirror.

Aperture
This is the diameter of the lens or primary mirror. The bigger, the more light will be focused and the more details you will be able to see, assuming that the lens or mirror are of good quality. A large aperture telescope is of no use if the lens is dirty and scratched.

Choosing an eyepiece

Now that we've defined what focal length and aperture are, we can move on to how to choose an eyepiece.

An eyepiece lens, just like a telescope lens, has a focal length (FL) and a field of view (FOV), these are the important values of an eyepiece. Other values will come into play, but for now we will only be concerned with these two. The focal length of an eyepiece is the distance between the lens's middle point to the point of focus of incoming light. This definition applies to any lens. The field of view is the section (or angle) of space you can see when looking through this eyepiece.

Let's say your telescope is a refractor with a focal length of 600mm and an aperture of 120mm using an eyepiece of 25mm. Using the eyepiece calculator we arrive to these numbers:

- Focal ratio: 5.0 (Telescope focal length / Telescope aperture)

- Magnification: 24x (Telescope focal length / Eyepiece focal length)

- True field of view: 3° (Eyepiece field of view / Magnification)

- Resolution: 1 arc (122* / Telescope aperture)

- Exit pupil: 5° (Telescope aperture / Magnification)

*122 is the wavelength of the electromagnetic radiation)

Eyepiece Calculator
Telescope AP	mm	Magnification	X
Telescope FL	mm	EP true FOV	˚
Eyepiece FL	mm	Resolution	arc
Eyepiece FOV	˚	Exit pupil	˚
Barlow MAG

There are a few important numbers to look at:

The magnification: This is how much enlargement at which you are view the object.

The True field of view which is 3 degrees: This is how much of the sky you are able to see at this particular magnification. If the true field of view was 1 degree, you would have been able to see a smaller portion of the same object at the same magnification. So the true field of view is very important in order to really enjoy looking as space objects, the bigger the better.

Now that we know our telescope and numbers, let's move on to choosing an eyepiece.

Budget

Most eyepiece manufacturers produce all different types of lenses, your budget will determine the quality of the eyepiece. Do not expect a clean and sharp image from an eyepiece worth $25, regardless of how good and expensive your telescope is. Astronomy is a somewhat expensive hobby. But do not let that discourage you, the rewards are great.

Planetary Eyepieces

These eyepieces provide medium to high magnifications in order to see details on planets (Mars, Saturn and Jupiter mainly) and our Moon. All other planets are only viewable with very large telescopes costing hundreds of thousands of dollars, even then, the views are washed out and planets still appear small. Magnification as we saw earlier is a function of the telescope's focal length divided by the eyepiece's focal lengths. This means that the smaller the eyepiece's focal length, the higher the magnification.

If your telescope's focal length is 2032mm and you buy a 13mm focal length eyepiece, the magnification would be 2032/13 = 156.3x This is considered medium to high magnification. If you buy a 7.5mm eyepiece, the magnification would be 2032/7.5 = 270.9, this is considered high. In order to enjoy a good high magnified image, many parameters must be present: No air or atmospheric turbulence, no high clouds, very low humidity and moisture and moderate temperatures.

How high a magnification can you achieve? As high as the eyepiece and telescope will allow. However the image will eventually get blurry and out of focus at a certain point. That point is determined by the aperture and quality of your telescope, as well as seeing conditions, ambient temperature, moisture and other environmental factors. The de-facto rule states that the highest magnification is 50x per inch of aperture. If your telescope is an 8" then the maximum magnification is 8x50=400x. It is *rare* to be able to see clearly at that magnification mainly because of the reasons explained above.

DSO Eyepieces

Deep space object (DSO) eyepieces provide low to medium magnifications, mainly because these objects are usually spread out and can only be viewed as a group at a wide field of view. In order to achieve this large angle, the eyepiece must be of large focal length and a large field of view. A 50mm eyepiece, 72° FOV on a 2032mm telescope for example would yield a magnification of 40.6x and a true field of view of 1.77°. Not bad but not exactly a large field of view. This is due to the telescope's long focal length. The 2032 mm telescope is mainly for planetary viewing. This is where focal reducers come in.

Focal reducers

A focal reducer shortens the focal length of a telescope to allow a wider true field of view. If you own a telescope with a long focal length and would like to enjoy DSO viewing, the cheapest way to accomplish this is to buy a focal reducer.

Eye relief is an important specification in an eyepiece, it determines how close or far from the eyepiece you must place your eye in order to see the image comfortably. If you wear eyeglasses, you will need a long eye relief eyepiece, do not buy short eye relief eyepieces, they will be very un-comfortable (if not impossible) to use. If you don't wear eyeglasses, you can choose either, some people prefer long eye relief others short eye relief. An eye relief of 20mm is adequate for non eyeglass wearers. Some high magnification planetary eyepieces have a very short eye relief (around 10mm), these can be very un-comfortable, choose wisely.

To summarize all this:

Determine your budget
Decide on planetary or DSO eyepiece
Determine the magnification and field of view
Are you an eyeglass wearer?
Your budget will determine which manufacturer to buy the eyepiece from
Read some reviews about this eyepiece to make sure that what you are buying is of good quality
Do not let others determine what type of eyepiece you should buy, only whether the eyepiece you chose is good