This guide will help you understand how eyepieces work in conjunction with your telescope, which sizes are best for beginners, and hopefully help you to avoid sinking more money into an eyepiece collection than you need to.
It’s easy to get a little too excited when first expanding your telescope’s accessories, and a side effect of this is that it’s also very easy to over purchase, spending more and buying more than you actually need, and this is particularly true for eyepieces. After all, a telescope’s eyepiece has a large effect on the potential of your telescope’s Optical Assembly. You probably want to view your target with the greatest amount of clarity, resolution and magnification as your telescope and, more importantly, the celestial object in your sights allow. For that reason you might be tempted to purchase an extensive eyepiece collection. In reality, you don’t need to have a giant collection of eyepieces, but rather need to know which eyepiece will suit your specific needs for that moment.
In truth, especially when starting out, you really just need a solid and reliable set of about five eyepieces. Before making any purchase, you first need to make sure that your additional eyepieces will fit your telescope, or will fit the Diagonal attached to your telescope. Typically, there are two base sizes of eyepieces, 1.25” and 2”. A 2” eyepiece will usually be of higher quality, and allow you to accomplish more with your telescope. If you have a telescope that is of decent make, and it is worth upgrading its capabilities rather than upgrading to a new telescope, you are going to want to purchase a Diagonal Adapter.
When planning on buying additional eyepieces, it is a good idea to have all of your telescopes specs handy. While this might be intimidating, there is a good deal of math involved when calculating the type of eyepiece that you will need, so having as many details figured out ahead of time will save you a lot of time and effort.
Don’t forget, kits are available that have pre-assembled many of the popular options.
Focal Length
The first step in understanding eyepieces is understand their Focal Length. The Focal Length is the distance that a path of light and its photons takes to travel through your telescope’s Optical System, from the Objective Lens or Opening to the attachment point of the eyepiece. Eyepieces themselves have their own Focal Length, which is measured from the Inner Objective Lens of the Eyepiece to the point where your eye will rest.
It is important to know what the Focal Length of your telescope is when deciding on which Eyepiece to use for any viewing circumstance. It is also important to know the Focal Length of your Eyepiece, and first time eyepiece buyer’s often want to have more variety than they need when starting out. Your telescope will come with two eyepieces, typically one 10mm and one 25mm.
So, which one of these eyepieces is more powerful?
Magnification
The Magnification, or Power, of your telescope is determined by the Focal Length relationship between your telescope and your eyepieces. Choosing the correct Magnification Power for ideal viewing of your celestial target depends on a number of complex factors related to your target, and honestly requires experience in trying to view it with the specific telescope and eyepiece combinations available to you.
It is recommended that when viewing an object that you start out with lower magnification, and slowly raise the magnification power until you reach the peak level of clarity afforded to you by your gear. The higher Aperture your telescope is the more flexibility you have in terms of eyepiece choices. This will also help you determine when and how to purchase additional eyepieces, and will require some level of experimentation with specific targets.
But, how do you determine the Magnification power?
Firstly, a telescopes Aperture has a high impact on its ability to magnify. A general rule is that each inch of aperture in a telescope’s Optical Lens will allow for fifty time magnification. It is important in choosing an eyepiece that you do not overextend your telescope’s magnification capabilities, so keep its Aperture in mind.
There is a simple formula that amateur astronomers use to determine magnification, and that is by taking the Focal Length of your telescope and dividing it by the Focal Length of your eyepiece. Simply put:
MAGNIFICATION = TELESCOPE FOCAL LENGTH / EYEPIECE FOCAL LENGTH
While this might seem counterintuitive, this means that an eyepiece with a lower Focal Length will provide a higher Magnification.
It is also worth noting that your telescope’s Focal Length is not simple the length of its Optical Tube. This might be the case for Refractor Telescopes, but Reflector and Compound Telescopes will fold the light’s path as it travels through your telescope, so even if these telescopes have a shorter Optical Tube, they very well might have a longer Focal Length.
It is a good idea to keep track of your telescope and eyepiece Focal Lengths and their combined Magnification Powers in your Astronomy Journal for easy reference. This will prevent you from having to perform this formula every time.
When first broadening our your eyepiece collection, it is recommended that in addition to your 10mm and 25mm, you start out by adding a 4mm, an 18mm, and a 40mm to allow yourself a wider range of magnification options. There are also eyepiece kits available that simplify the buying process for you, and as with all things telescope related, it is important to keep in mind that you get what you pay for. It is better to spend a little bit more on a better constructed eyepiece than to try and save a few bucks in the long run. Higher quality eyepieces will better lens coating and more internal elements, allowing for better resolution, clarity, and contrast.
You can also affectively change an eyepiece’s focal length with the use of a Barlow Lens, which attaches between your eyepiece and your telescope. A Barlow Lens allows for more versatility in the eyepieces you own by increasing their Magnification potentials. The two most common types of Barlow Lenses are the 2X and the 3X Barlow, which increase magnification by two and three times respectively. Another way to alter your eyepiece’s magnification potential is with a Zoom Lens. Where a Barlow Lens has a fixed Magnification, a Zoom Lens has a variable Magnification, as it allows you to manually adjust its Focal Length. This does come with a price, as Zoom Lenses are typically much more expensive than Barlow Lenses, and can negatively impact your image quality, which becomes very important when considering their use in Astrophotography.
However, the Focal Length is not the only thing to consider when purchasing additional eyepieces.
Field of View
Field of View is the measure of what is observable to your eye within the full 360˚ circumference around your person. Using both eyes, humans have vertical Field of View about 135˚ and a horizontal Field of View of about 200˚. Because telescopes have a monocular Field of View, what is observable through an eyepiece is more limited in regards to Field of View, though certain eyepieces are able to afford the user a greater Field of Views than others.
There are two types of Field of View that are relevant to telescopes: Apparent Field of View and True Field of View. The Apparent Field of View is the observable field presented when looking through the eyepiece when it is not attached to your telescope. The True Field of View is what is observable when the eyepiece is connected to the telescope and the effects of magnification take place. To calculate the True Field of View, you divide the eyepiece’s Apparent Field of View by the Magnification Power that was determined by the previous formula. Written out, the formula for an eyepiece’s True Field of View appears as follows:
TRUE FIELD OF VIEW = APPARENT FIELD OF VIEW / MAGNIFICATION POWER
Or
TRUE FIELD OF VIEW = APPARENT FIELD OF VIEW / (TELESCOPE FL / EYEPIECE FL)
When viewing celestial objects, you should take into account the size and proximity of the object in relation to the True Field of View of your eyepiece and telescope combination. For instance, to view the entirety of the Moon, your True Field of View will need to be 35˚, but to see the entirety of the Andromeda Galaxy, because of its massive size and distance, you will want to aim for an eyepiece that will allow for a True Field of View of 3˚. It is worth noting that different eyepieces can have the same Focal Length but a different degree of Field of View, which is why it’s important to have experience with the lenses that you already have, as well as to take into account what the object is that you will be viewing. Eyepieces that fall into what is considered a narrow Apparent Field of View are between 25˚-30˚, while eyepieces that are considered to have a wide Apparent Field of View are between about 40˚ and 65˚, while an extra wide Apparent Field of View will be anything wider than 65˚.
The addition of Barlow Lenses and Zoom lenses will negatively impact your Apparent Field of View.
Eye Relief
Your Field of View through your telescope’s attached eye piece can also be affected by the amount of Eye Relief it has.
Eye Relief is the measured distance that your eye can be from the eyepiece to still receive the eyepiece’s full True Field of View. It used to be that an eyepiece’s Eye Relief was relevant to its Focal Length, but as technology advances most modern lenses offer long Eye Relief regardless of Focal Length. Typically, for newer eyepieces, narrow Field of View eyepieces have shorter Eye Relief and require you to remain closer to the telescope’s eyepiece than wider Field of View eyepieces.
Long Eye Relief is particularly important if your wear glasses, as you will have a harder time getting your pupil closer to the eyepiece unless you remove your glasses. Another option for those who wear glasses is to fold the eyepiece’s eye cup, or the rubber ring made to rest your eye in and increase comfort in your viewing, over and onto the frame of the eyepiece, allowing your eye to get closer.
The Exit Pupil
The Exit Pupil is the size of the light that exits the eyepiece and enters your eye. This is important to know, as you want the Exit Pupil of your eyepiece to be smaller than the pupil of your own eye, however if the Exit Pupil of your eyepiece is too small the image it presents will be negatively impacted by too much magnification. Otherwise, your eye will not absorb all of the light coming out of the eyepiece. Your pupil changes thorough the day in relation to the light around you. Using your telescope at night means that your pupil will dilate in order to let more light in through the darkness of the night sky.
There is a formula for calculating the Exit Pupil of your eyepiece, which is done by dividing the Aperture of your telescope in millimeters by the magnification of your telescope. Written out, the formula looks like so:
EXIT PUPIL DIAMATER MM = APERTURE MM / MAGNIFICATION POWER
Or
EXIT PUPIL DIAMATER MM = APERTURE MM / (TELESCOPE FL / EYEPIECE FL)
As we covered earlier, there are a number of different ways to affect your telescope’s magnification, so you need to be aware of the end size of the Exit Pupil. The Exit Pupil of your eyepiece should be between 5mm and .5mm. The ideal exit pupil varies, although there is a lot of argument in support of a Exit Pupil sweet spot of between 1mm and .7mm, which will provide a dramatic increase in clarity and contrast of your image.
