Updated 31 Aug 2013

Amateur Astronomer's Notebook

Telescope Eyepiece Fundamentals

This page is for those who are just starting out in astronomy and telescopes. It will provide a basic introduction to telescope eyepieces. While you don't need to look into auto refinance rates when purchasing a telescope or eyepiece, you also want to make sure you get the best bang for your buck. The wrong eyepiece can make your telescope useless, while getting the right one can really improve an entry-level telescope. This guide will help you navigate and understand the world of eyepieces as well as give you some tips on purchasing an upgraded eyepiece.

What is the purpose of an eyepiece?

All telescopes use either a lens or a mirror (some use both) to gather incoming light and to form an image from that light. The telescope's eyepiece takes the image formed by the lens (or mirror) and magnifies it to a larger size so that the human eye can see more details in the image.

What do the markings on an eyepiece mean?

If you look at a typical eyepiece you will almost certainly see some numbers and possibly some letters printed on the eyepiece. For example, you might see the marking "25mm" or "7.5mm" (or any other number of values ranging from around 4mm to possibly 60mm). This numeric value is known as the focal length of the eyepiece (measured in millimeters). This is probably the most important characteristic of an eyepiece because it allows you to calculate how much magnification the eyepiece will provide. The actual magnification an eyepiece provides depends on the focal length of the telescope. You may see other letters or markings on the eyepiece (such as "H", "SR", "Pl", etc), these indicate the type of eyepiece (we will discuss these in an upcoming section). For now, the thing to take away from this section that the number printed on the eyepiece is the focal length and understand that it is used to calculate the magnification the eyepiece will provide.

How do I calculate the magnification that my eyepiece will provide?

To determine the magnification that an eyepiece provides, two pieces of information are needed. One is the focal length of the eyepiece and the second is the focal length of the telescope in which the eyepiece will be used. Almost every telescope ever made will have the focal length marked on it, usually near where the eyepiece goes into the scope. Focal lengths for typical beginner telescopes will be in the range from around 500mm to 1200mm. To determine the magnification that a particular eyepiece provides, a simple calculation is done: divide the focal length of the telescope by the focal length of the eyepiece. Let's take an example. Suppose you have a telescope with a focal length of 700mm and an eyepiece of focal length 25mm. The magnification that the eyepiece provides in this telescope will be 700/25 = 28x (often called "28 power"). Now let's take the same scope but use a second eyepiece of focal length 7.5mm. The magnification provided by this eyepiece will be 700/7.5 = 93.3x. Magnifications for other telescope/eyepiece combinations are calculated in the same manner. One thing to note from this: eyepieces with smaller focal lengths produce larger magnifications with any given telescope!

The image below shows the results of using appropriate magnification and excessive magnification. The leftmost two images of Saturn are representative of what you might expect to see at low and high power respectively (in a typical entry level scope). The image at right shows what happens when magnification is pushed to excess. The image is bigger to be sure, however the clarity is terrible, the image will be very shaky and much dimmer. No additional detail can be seen beyond a certain point! The image at right is typical of what you might see in an entry level telescope that claims "675x magnification". The most magnification you will normally use is about 50x per inch of lens (or mirror) diameter. For a 3" scope this would be about 150x. Remember, most of your observing will be done at LOW magnification!

Representative views of Saturn at low, high and excessive magnification!

Do eyepieces come in different sizes?

Yes. Here we are talking about physical size of the eyepiece barrel (the silver part that inserts into the eyepiece holder), not the focal length. There are 3 standard sizes of eyepieces in use for amateur telescopes. They are .965", 1.25" and 2". That said, it should be noted that the .965" size is an older obsolete size that is still being used on some entry level scopes. The 1.25" size is the most commonly used eyepiece size for amateur telescopes. The 2" size is also common on amateur scopes but this size is generally found on larger, more advanced telescopes. Most beginning astronomers will not need to be concerned with 2" eyepieces (a single 2" eyepiece can be as costly as an entire entry level telescope)! The photo below shows the relative sizes of the three standard sizes of eyepieces along with a soda can for reference. Note that the 2" eyepiece shown here is not too much smaller than a soda can!

2", 1.25" and .965" diameter eyepieces with a soda can for reference.

Are the larger eyepieces better quality than the smaller sizes?

All other things being equal, no. The quality of an eyepiece is not related to its size (good quality eyepieces can be made in any of the three sizes). That said, it should be noted that many of the eyepieces that come in the .965" size are not of the best quality. This is not a result of the size but more a result of making things less costly.

Are there different types of eyepieces?

Yes. When we say "type" of eyepiece we refer to the optical design of the eyepiece. There are many optical designs used in various eyepieces, some use more glass elements than others to achieve different characteristics. Plossl eyepieces (an excellent all around performer) often come with the better entry level telescopes. Unfortunately some entry level telescopes come with eyepiece designs that are not as good. These typically include the Huygens (marked with an "H") and Symmetric Ramsden (typically marked "SR"). These are common designs in entry level telescopes as they are inexpensive to make (however their performance is often not so good compared to the better Plossl design).

What is meant by "field of view" of an eyepiece?

There are two specifications regarding field of view when speaking of eyepieces. One is known as "apparent field of view" and the other is "actual field of view". Both are measured in degrees. Apparent field of view is constant for any given eyepiece and telescope combination. Actual field of view refers to how much actual sky you can see at any one time (and this will vary depending on what telescope the eyepiece is used in). So what do these two terms really mean? Apparent field of view can be thought of as "how big a window am I looking through". The wider the apparent field of view the more area of sky you will see. Eyepieces come with apparent fields of view ranging from around 30 degrees (quite narrow) to over 80 degrees (extremely wide), with 40 to 50 degrees being very common (and totally adequate for entry level eyepieces). Eyepieces with apparent fields of view in the 60 degree plus range generally cost a considerable amount (several hundred dollars). Although often excellent performers, they are not included as standard equipment when buying a telescope as they are too costly. Getting back to understanding apparent field of view, here's an example of how to better understand what it means. Picture yourself sitting on a couch in your living room and looking out a standard window (say a 3x5 foot window). You can probably see some trees, maybe a portion of the neighbor's house, etc. Now, sitting in the same spot, imagine that there was a 6x12 foot picture window. Now you can see a LOT more outside! Nothing visible through the window looks any LARGER, we just see MORE of what is outside. For eyepieces with a wider apparent field of view, the results will be similar.

So what is actual field of view? When you use a particular eyepiece in a given telescope, it will "see" a small portion of the sky. In general, the more magnification that is used the LESS sky we will see. For example the Moon is about 1/2 of a degree wide. Most small telescopes using low magnification can see the entire Moon (with a good amount of "breathing room" surrounding it). If we switch to an eyepiece that results in higher magnification, we will more likely only be able to see a portion of the Moon at any one time. So, for any given eyepiece/telescope combination, the eyepiece will allow you to see a particular portion of the sky. For a low power eyepiece, a typical actual field of view (for an entry level telescope) might be in the order of 2 degrees. For the same telescope using a higher magnification eyepiece, the field of view might be (for example) more like 1/4 of a degree.

By use of an eyepiece that has a wide apparent field of view (say 70 degrees or so) and one that also results in a fairly high magnification (for example around 100x), it is possible to obtain some very dramatic views of objects like the Moon (basically you have fairly high magnification AND a wide actual field of view at the same time)! As with most anything good, there is a downside: cost. Eyepieces that have very wide fields of view are often pretty expensive and they will not be "standard equipment" on any entry level telescope.

The main thing to take away from this section is that eyepieces with wider apparent fields of view are generally easier to use (think of the difference between looking out a porthole vs. a picture window). For any given eyepiece, the actual field of view is a function of the magnification the eyepiece provides with a particular telescope (the more magnification the less sky you will see at once).

Are there any eyepiece types I should avoid?

Yes. There are 3 or 4 eyepieces to be avoided that are fairly commonly supplied with entry level telescopes. Ones to avoid include eyepieces with the following markings: H25mm, H20mm, H12.5mm and SR4mm. Especially avoid them if they are of the .965" barrel diameter! As mentioned in a previous section, eyepieces with H markings (Huygens optical configuration) are generally of not very good quality (the image will tend to be blurry around the perimeter of the field of view) and the apparent fields of view are on the smaller side. Huygens eyepieces with smaller focal lengths tend to be worse than those with larger focal lengths. Most users that attempt to use an SR4mm eyepiece will find it of little practical use. Such eyepieces are provided with scopes only to allow the scope to claim a very high maximum magnification (many people just starting out associate "high magnification" with "high quality", a notion that is completely false). In general, the smaller the focal length of the eyepiece the harder it is to physically look through. This is because the opening is very small and you have to very carefully center your eye over it in order to see anything. Couple that with the shakiness that a high magnification will result in using a small scope and you will be lucky to see much of anything. Regarding eyepiece size (barrel diameter), I strongly advise avoiding any telescope that can only accept .965" eyepieces. The reason is this: the availability of quality .965" eyepieces has greatly diminished in recent years and you will have a very hard time finding quality eyepieces to upgrade to. There is a huge variety of quality eyepieces available in the 1.25" size so this is the size eyepiece you want your first telescope to be able to use.

What eyepieces should I look for with a first telescope?

Number 1. Make sure the telescope accepts the 1.25" size (it is OK if it also takes the 2" size as adapters are widely available to allow using 1.25" eyepieces in scopes that take 2" eyepieces). Number 2. If the telescope comes with 2 eyepieces, you want one of them to produce a good low power magnification (in the range of 25x - 50x) and the other one should produce a good higher magnification (in the range of 90x - 120x). These magnifications will be the ones most commonly used in most any telescope (including expensive advanced telescopes)! Also, if the scope comes with 2 eyepieces try to make sure that the focal length of one is NOT simply twice that of the other. For example, I've seen scopes that come with a 20mm eyepiece and a 10mm eyepiece. Why is this not the best choice? If you eventually obtain a Barlow lens (an accessory that typically doubles the power of any eyepiece) then you will effectively have only 3 unique magnifications with those 2 eyepieces. It would be better to have a a scope come with 2 eyepieces that are more like 25mm and 10mm. When used in conjunction with a Barlow lens this would provide you with 4 unique magnifications (instead of only 3 with the other case) when using a 2x Barlow lens. The bottom line is this: The best eyepieces for an entry level scope will typically include one of approximately 25mm focal length and one in the range of 10m - 7mm (depending on the scope's focal length). Always avoid telescopes with the eyepieces mentioned in the previous paragraph!!! As for the design of the eyepiece, Plossl is arguably the best optical design for an entry level scope. Some of the least expensive scopes won't come with Plossl eyepieces (due to cost); just be sure to stay away from H and SR eyepieces and chances are you will be fine.

How many eyepieces do I really need?

For most people, 2 will be fine for starting out, and eventually a third eyepiece could be added to your collection. Ideally a low, medium and high magnification eyepiece set is perfect for most people (with low and high magnifications being the first two to obtain). Alternatively, a decent quality 2x Barlow lens will double the magnification of any given eyepiece. So, if you have 2 eyepieces you can likely end up with 4 unique magnifications by using a Barlow lens (a Barlow lens would cost about the same as a decent quality beginner eyepiece, or around $50- $60). You can always add more eyepieces later if your interest grows!

Is there anything to consider if I must wear eyeglasses at the telescope?

Possibly. If you do wear glasses and have to use them when looking through a telescope, you may find eyepieces that produce larger magnifications (eyepieces with smaller focal lengths) difficult to look through. This is because you cannot get your eye close enough to the eyepiece to comfortably see through it while wearing glasses. There is a specification associated with eyepieces that we have not mentioned yet: eye relief. Eye relief specifies how far away you can hold your eye and still easily see image in the eyepiece. There are eyepieces available with what is known as "long eye relief". Such eyepieces typically have eye relief of around 20mm (this should be adequate for most anyone who wears glasses at the scope). Eye relief is not always specified with eyepieces. If it is not called out, chances are it is not a long eye relief eyepiece. The downside with long eye relief eyepieces is that they tend to be somewhat more costly than other eyepiece designs (maybe $100 per eyepiece). Keep in mind however that most people who wear eyeglasses at the scope will typically only find certain eyepieces (the ones that generate higher magnifications) problematic.

What does it mean when an eyepiece says it has "coated optics"?

Most good quality eyepieces will have at least some of their elements treated with anti reflection optical coatings. Such coatings help to transmit more light and reduce glare and loss of contrast. You can often tell that an eyepiece has coatings as the optics will tend to have a bluish or greenish tint to them. Ideally all air-to-glass interfaces in an eyepiece will have anti reflection coatings, but the more coatings an eyepiece has the more it will cost (and the better the view will be too).

What does it mean when an eyepiece says it is "threaded for filters"?

Filters are used to enhance viewing of certain objects (there are different filters for different subjects and viewing conditions). The filters basically thread into one end of the eyepiece. Virtually all eyepieces available today are threaded to accept filters (all but one brand I have ever encountered use a standardized thread so there is little chance of incompatibility among filters and eyepieces).

What does it mean when an eyepiece says it is "Parafocal"?

In general, when changing eyepieces (to get a different magnification), some refocusing of the telescope will be required. Eyepieces that are parafocal will need only a very minor (if any) refocusing. In general parafocal eyepieces are eyepieces of different focal lengths from the same family of eyepieces. Being parafocal (or not) with each other has no bearing on quality or performance, it is simply something that provides convenience.

Should I clean my eyepieces?

The best thing is to not let them get dirty (keep them covered and in their cases when not in use). If you must clean them, do so carefully. Never use anything like Windex! Use only a soft, CLEAN camel hair brush or use compressed air (from a can, NOT from a garage air compressor), or a cloth that is meant for optics (kits such as Orion Deluxe 6-Piece Optics Cleaning Kit or Orion Optics Cleaning Kit are examples of what should be used if you need to clean your eyepiece optics) . Never disassemble an eyepiece to attempt to clean the interior. No dirt can enter inside the eyepiece, taking it apart almost certainly assures you will not get it back together properly!

Do I have to spend a fortune to get good eyepieces?

No. You *can* spend a lot if you want the very best, however for most people starting out the cost of such eyepieces is not justified (or necessary). Very good eyepieces can be had for around $50 each. As you progress in astronomy you can always move up to more expensive eyepieces. The more costly eyepieces offer very wide fields of view with outstanding image quality throughout the field. Some of these eyepieces cost over $500! However, keep in mind that many of the $50 eyepieces will get you 80% of the view of the very best at 1/10 the cost. If you are just starting out and have $500 to spend, it would be much wiser to buy a better telescope before delving into exotic high end eyepieces.

Some excellent starter eyepieces...

Below are some examples of eyepieces that would be very good choices for starting out. If you purchased a good starter scope chances are you already have eyepieces that are perfectly fine. However if you have an older scope or one with less than great eyepieces, the ones I list below are excellent choices for upgrading. Note that these are all 1.25" diameter eyepieces, your scope must accept this size eyepiece for these to work! The first two are ones I recommend as my top 2 picks for excellent low and high power views in the vast majority of entry level telescopes. These eyepieces are of a quality level that you won't outgrow in a month (these are good all around workhorses that will be useful even if you upgrade to a more sophisticated telescope in the future). These eyepieces have all of the features discussed earlier: a wide 50-deg apparent field of view, and the optics are fully coated with magnesium fluoride on every air-to-glass surface (improves contrast and reduces scattering). Cost is around $55 each including shipping.

Orion 25mm Sirius Plossl eyepiece. This would be an excellent choice for a low power eyepiece for most telescopes.

Orion 25mm Plossl eyepiece, 1.25

Orion 7.5mm Sirius Plossl eyepiece. This would be an excellent choice for a high power eyepiece for most telescopes.

Orion 7.5mm Sirius Plossl 1.25

Orion 12.5mm Sirius Plossl eyepiece. This would be an excellent choice for a medium power eyepiece for most telescopes.

Orion 12.5mm Sirius Plossl 1.25

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