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CHOOSING TELESCOPE EYEPIECES
TOP ROW: 2" Televue Panoptic 41mm, 35mm, 27mm, and 2"/1.25" Televue Ethos 10mm
BOTTOM ROW: 1.25" Televue Radian 6mm, 12mm, Televue Panoptic 24mm, and some .965" Tasco eyepieces
An eyepiece is an assembly of small lenses that collects the light gathered through a lens or mirror and magnifies it. While most binoculars have built in eyepieces that offer a fixed magnification, most modern telescopes do not. The telescope user can select which eyepiece they want to use depending on how much magnification (power) they want. A low power eyepiece is used when you want to observe a larger area of the night sky, such as a galaxy cluster or the Moon . A high power eyepiece is used either when you want to see a close up view of a small area (such as a crater or a mountain range on the Moon), or when viewing smaller objects like another planet. Telescopes for beginners purchased new usually come with one or two eyepieces, while most high end telescopes usually don't. Most of the beginner telescopes I purchased came with two eyepieces which game me a choice of two different magnifications, but as I grew familiar with what was available in the night sky, I found that sometimes I either needed more or less magnification to get the view I wanted.
Allthough the eyepieces that come with an entry level telescope may provide good views to a beginner, there are usually better eyepieces available for purchase. I have often seen posts on astronomy forums by people wishing to upgrade or add to their eyepiece collection asking for advice on which eyepieces to buy. It is not easy to answer this questions for two reasons: not only are there a lot of factors to consider regarding magnification and eyepiece characteristics, but there are also MANY companies that make eyepieces. While I can recommend certain eyepieces which I actually own and know to be of good quality and performance, I am sure there are other eyepieces out there just as good that I have not tried before. I will attempt to explain some of the terms you might hear related to eyepieces so that you have a better understanding of them, which will hopefully assist you if you are considering upgrading or buying more eyepieces.
MAGNIFICATION AND POWER - To magnify something means to make it appear larger than it actually is. For example, if the view through a telescope or a set of binoculars makes the object appear 10 times as large as it actually is, it may expressed in 2 ways: a magnification of 10 times, or 10 power (10x). How much an eyepiece magnifies depends on 2 factors- the focal length of the telescope, and the focal length of the eyepiece. This figure is expressed in millimeters (mm)and is generally printed on the eyepiece itself, or in the case of most telescopes on an information sticker or plate on it.
How to measure a telescope's focal length
The focal length of a telescope is the distance at which the light passing through the main lens (or reflecting off the man mirror) would travel before it shrinks down to a single point if it continued in a straight path. To calculate the magnification effect an eyepiece will have in a telescope, you would divide the focal length of the telescope by the focal length of the eyepiece. For example, imagine using a 25mm eyepiece through a telescope with a focal length of 900mm. The magnification would be 900 divided by 25, which would equal 36X. If you replaced the 25mm eyepiece with a 10mm eyepiece in that same telescope, the magnification would be 900 divided by 10, which would equal 90X.
The amount of magnification through a telescope also affects how bright the image is seen through it. The main lens or mirror in the telescope collects the light from the object you are looking at. The more you magnify the object, the more spread out the light is and the dimmer it gets. If you look at an object at a lower power, it appears much brighter and sometimes sharper than it would at a higher power. Through a low quality telescope, if you use too much magnification the object not only looks dimmer, but can also appear "grainy". There are limits to how much magnification you can use through a telescope and still produce a good image.
(APPARENT) FIELD OF VIEW - You may have seen this term used by companies advertising their eyepieces. Eyepiece field of view (FOV), sometimes called apperent field of view (AFOV) is expressed in degrees. Most entry level eyepieces like those supplied with a telescope have a 50 degree (50°) field of view. You can also purchase eyepieces with larger fields of view. I own Televue Radian eyepieces with a 60° FOV, Televue Panoptic eyepieces with a 68° FOV, and one Televue Ethos eyepiece with a 100° FOV. There are other companies making eyepieces with various fields of view, but what exactly does "field of view" mean?
Apparent field of view cone- feel free to laugh at the head sketch
If you have ever looked through a telescope, you will notice that what you see is not a square or rectangular viewing area- you will see a circle surrounded by darkness. Field of view has nothing to do with magnification, but relates to the size of the circle itself you see in the middle of that field of darkness. Imagine you are looking through a cone as shown in the diagram above. The size of the circle you see depends on the measurement of the angle of the cone itself as seen from the side of it. If that angle is 50°, the FOV of that cone will be 50°. If you used a wider cone with an angle of 90°, you would see a much larger circle of 90°. In this example objects will not look any larger through the cone with the 90° FOV- you will just be able to see a larger circle in the middle of the darkness surrounding it.
Now imagine that you have a 10mm eyepiece and a 20mm eyepiece, both having a 60° FOV. Pretend that you are looking at a tree in the distance through a telescope using the 20mm eyepiece, and the tree just fits in the field of view from top to bottom. If you change to the 10mm eyepiece, the field of view has not changed, but the magnification has doubled. As a result the tree appears bigger, and you can only fit in half of it's height into the field of view now.
EYE RELIEF - The term eye relief refers to how close to the eyepiece itself you must be in order to see the image through it. This can vary greatly depending on the individual eyepiece design. My first telescope with interchangable eyepieces (the year 1973) had eyepieces with such poor eye relief that I almost had to have my eyeball touching the glass to see anything at higher magnification. In general (but not always), eyepieces with a higher magnification will have less eye relief than an eyepiece with a lower magnification through the same telescope. The eye relief can vary between different manufacturers, and even through different types or focal lengths of eyepieces by the same manufacturer. The shorter the eye relief, the closer to the eyepiece itself you must be to see the image. People who wear glasses due to astigmatism may not be able to get close enough to an eyepiece with poor eye relief to see an image. If you are simply far sighted or near sighted, you should be able to view through a telescope without your glasses and adjust the focusser to get a sharp image.
BARLOW LENS - A barlow lens is an accessory that is inserted between a telescope and the eyepiece that magnifies the image. A 2X barlow will double the magnification, a 3X barlow will triple the magnification, etc. A good quality barlow lens can give you multiple magnifications using the same eyepiece. Pretend you have a telescope and 3 eyepieces- a 10mm, a 25mm, and a 40mm eyepiece. If you add a 2x barlow lense to your equipment, it would be like adding a 5mm, a 12.5mm, and a 20mm eyepiece to your collection- giving you 6 choices of magnification. Like any other accessory, barlow lenses can vary in quality. A good barlow lens can be useful, but a poor quality one might not give you very good images through your telescope. Furthermore, not ALL barlow lenses will work in ALL telescopes. In some cases you may not be able to reach focus depending on the barlow lens and the telescope.
.965", 1.25", 2" EYEPIECES - These figures refer to the diameter of the section of the eyepiece barrel that is inserted into the telescope. Telescopes manufacturers today use either 1.25" diameter eyepieces, or have a 2" focusser that can accept either 2" eyepieces or (with the use of an adapter) 1.25" eyepieces. A 2" eyepiece usually has greater eye relief than a 1.25" eyepiece of the same focal length. Furthermore, depending on the telescope type, a 2" eyepiece with a high focal length such as 40mm might produce a brighter image than a 1.25" 40mm eyepiece. Without going into great detail, this is due to the barrel diameter of the eyepiece being smaller than the diameter of the cone of light where the 2 meet. In some cases a 1.25" eyepiece may be too small in diameter, while a 2" diameter eyepiece will be large enough to let all of the light in where it meets the cone. A good 2" eyepiece can be quite expensive compared to a 1.25" eyepiece. I use 1.25" eyepieces for all of my high power and medium power viewing. For low power viewing I use 2" Televue Panoptic eyepieces (35mm and 41mm).
Regarding .965" diameter eyepieces, they were the common size available for entry level telescopes in the 1970's and earlier. Compared to what is available today they generally had small fields of view, and poor eye relief.