Your first telescope - Part 2 (eyepieces)
Published on Dec 29, 2006 at 11:58 pm.
7 Comments.
Filed under amateur astronomy, telescopes.
An important part of a telescope is the eyepiece. Often novices to telescopes don’t realize this. The objective (lens or mirror) focuses the light, but the eyepiece is the lens that you look through to put the light back into a form that your eye can use. Since this is the part that you are actually looking through, it is very important! Frequently novice telescope users will spend a lot of money on a telescope, but skimp on eyepieces. The end result is that nothing looks very good through the telescope! Often serious amateur astronomers spend as much on eyepieces as they did on the telescope itself (and sometimes more!). Most commercially sold telescopes today come with eyepieces, but these are not always the highest quality. Once you get comfortable with your telescope, you’ll likely want to add to these eyepieces. Conveniently, there are only a few standard sizes for eyepieces. The most common telescope eyepieces have a standard diameter of 1.25 inches (3.175cm). This means that these eyepieces are interchangeable among any other telescope taking this size eyepiece. An older standard was 0.965 inch (22.4cm). Few quality telescopes today use this size, and it has been increasingly difficult (though not impossible!) to find high quality eyepieces in this size. Some larger telescopes can take a 2 inch (5.08cm) eyepiece, but most of these telescopes come with an adapter to accept a 1.25 inch eyepiece. I’ve posted a photo here of several of my eyepieces, as well as a Barlow lens.
Not all eyepieces are the same. There are multiple types of eyepieces. “Why so many types?â€, you might ask. Well, that’s a good question. Cost is, obviously, a factor. But, let’s think for a moment what the eyepiece actually does. It is a lens (actually, usually a set of lenses). Parallel light rays passing through the eyepiece is focuses in the same manner as the objective element of the telescope. Here lies the problem. A single lens will focus one color of light entering it just right. But, it is often difficult to focus light that comes from an angle into the eyepiece. Normally this isn’t a problem, but it can become an issue for telescopes having a short focal length. But, more importantly, different colors of light are focuses differently. Any substance has a different index of refraction for different wavelengths of light. This is why a prism breaks white light into many different colors. Each is a different wavelength, so it is bent by a different amount. With a lens, each color is bent slightly differently, so only one color is actually in focus. The rest are out of focus. This means that you can focus an image in one color, but the image will have rings around it of other colors. The term chromatic aberration is used to describe this effect. All refractors have chromatic aberration. But, since eyepieces are lenses, this means that any telescope, refractor or reflector, will have chromatic aberration when used with an eyepiece, and any telescope used visually will need an eyepiece (though you can take photos without an eyepiece, if you do it right). Thus, all telescopes used visually have chromatic aberration. But, chromatic aberration can be partially offset by using a set of lenses, instead of just one. For this reason, modern eyepieces are always complex devices consisting of several lenses, called elements. As you might suspect, the more lenses used, the more you can offset aberrations, but the more expensive the eyepiece will be.
But, chromatic aberration is not the only issue facing eyepieces. You also have to worry about field of view and eye relief, as well as internal scatter. Light passing through the eyepiece tends to reflect and scatter as it passes from element to element. This scattering has the dual problem of making what you are looking at dimmer and reducing contrast, which makes dim objects dimmer and dark sky brighter, making them even harder to see.
Field of view is related to how much of the sky that you can see. As you can imagine, the higher the magnification, the smaller the part of the sky that you can see. But, not all eyepieces show the same amount of the sky, even if they are the same focal length. Some eyepieces, when you look through them, you must look almost exactly along the axis of the eyepiece. So, you can only see things right near the center of the field of view. Such an eyepiece is said to have a small field of view. But, other eyepieces are more forgiving, you can look through them at a larger angle, seeing things a bit farther from the center of the field of view. We say that those eyepieces have a large field of view. The field of view is often given as a measure of apparent degrees from one side to the other. Very small field of view eyepieces give an impression of looking through a peephole at the sky. The larger field of view eyepieces are exciting to look through, because it seems like you are looking through a picture window at the sky. But, off axis focusing is tough, so most of the larger field of view eyepieces only focus clearly near their center, and images farther from the center of the field of view show distortions. Fixing this usually involves more elements, and more cost. Also, the barrel of the eyepiece often gets in the way of very large fields of view, so many of the larger field of view eyepieces that don’t involve a lot of elements are of the larger 2 inch diameter size.
Another major factor to think about with eyepieces is eye relief. You don’t put your eye right up against the eyepiece, but rather you place your eye near the eyepiece and look through it. Put most simply, eye relief is how far you hold your eye from the eyepiece in order to see an image properly. Typically, for a given optical design, the longer the eyepiece’s focal length, the longer its eye relief. Long eye relief is also more forgiving, in that exact placement of the eye is not so critical. That makes using the telescope easier. But, if eye relief is too long, novices often put their eyes too close to the eyepiece and have trouble focusing on an image. Also, if eye relief is too long, then your head is so far back that it doesn’t block much stray background light from hitting and reflecting off of the eyepiece. This is a problem that is particularly apparent in a light polluted urban environment. I wear glasses, and that provides another problem. If the eyepiece eye relief is too short, then my glasses bump the eyepiece (risking scratching my glasses). And, if the eye relief is much too short, then there is no way to get my eye close enough to the eyepiece while wearing my glasses. So, it is no great surprise that when it came time to order eyepieces at the college, I ordered eyepieces with mostly long eye reliefs. But, long eye relief for short focal length eyepieces often comes at the expense of greater number of elements and greater expense.
Different eyepiece designs have different numbers and types of elements. The most common type of eyepiece that you find in good commercial telescopes today is a Plossl eyepiece. Not that many years ago, this was a fairly expensive eyepiece medium to high end eyepiece, but apparently manufacturing has gotten less expensive, because they are now quite common. Plossls have 4 or more elements. There is a significant difference in image quality in the more expensive, well made Plossls, as compared with the inexpensive ones that often skimp on coatings and quality.
An older eyepiece design, called a Kellner, used to be standard for amateur telescopes, but now is often found only on low end telescopes. The Kellner eyepieces are 3 element designs. A disadvantage to Kellner eyepieces is that they often have shorter focal lengths than Plossls, and smaller exit pupils (the circle that the light is coming out of the eyepiece). This tends to make for smaller fields of view, and exact placement of the eye is critical. However, I rather like Kellners, at least for long focal lengths. For focal lengths longer than about 25mm, the eye relief isn’t too bad. For focal lengths longer than 35mm, I feel that the eye relief is every bit as good as any other eyepiece. And one advantage is that the fewer elements, the less light is loss in transmission, so the easier it is to see very dim objects. Unless you are doing very serious work, a Kellner is just fine, as long as it is a good Kellner, and not a cheap knockoff one. Edmund Scientific markets a proprietary design called an RKE eyepiece, which is a modified Kellner. My 28mm RKE is, in fact, one of my favorite eyepieces (that is the one with the red ring around the top in the photo above). Another modification of the Kellner design is the Modified Achromat (MA) design. Like the RKE, it is a three element design, but is supperior to the Kellner. We have several of these at the college, and they are quite good inexpensive eyepieces.
Another eyepiece that used to be a standard eyepiece was the four element Orthoscopic eyepiece. These are very good eyepieces in that they tend to have very good focus all the way to the edge of the field of view (something that most eyepieces don’t have). Some planetary and binary star observers swear by them. I don’t really like them, and their cousins the Ultrascopic eyepieces, because they tend to have shorter eye reliefs than Plossls.
Very rarely, you find Huygens or Ramsden eyepieces anymore. They are simpler, two element designs. They were more common decades ago. About the only place that I have seen them recently is the highest power eyepieces of mass market low end telescopes. As I said in my previous post, though, those eyepieces magnify far more than the effective limit of the telescope, and the manufacturers know this. So, it doesn’t make sense to put high quality eyepieces in with a telescope that can’t realistically use very short focal lengths. Thus, they include cheap Huygens or Ramsden eyepieces that have little use only so that they can claim a “high power†telescope.
There are other eyepieces that have special properties, such as Erfle and Konig eyepieces which are designed specifically to provide extremely wide fields of view. They are great low power eyepieces, particularly the 2 inch ones, because they do provide a picture window view of the sky. There are plenty of other eyepieces, but most novices won’t be interested in them. Serious amateur astronomers, though, generally accumulate many of these specialized eyepieces, some of which often cost more then their first telescope did!
Another type of lens I want to discuss is a Barlow lens. Though not really an eyepiece, it is used with eyepieces. You use a Barlow by removing the eyepiece from its holder, inserting the Barlow, and then inserting an eyepiece into the Barlow. The Barlow is a lens (or set of lenses) that bends and lengthens the light cone from the telescope to the eyepiece. This effectively makes the telescope’s focal length seem to be two or three times longer than it really is. This makes the magnification two or three times more than you’d otherwise have with a given eyepiece. So, a telescope with an 800mm focal length with a 20mm eyepiece would have a magnification of 40 times (see my previous post for how this works). But, with a 2X Barlow, the same 20mm eyepiece would yield a magnification of 80 times, and it would yield a magnification of 120 times when used with a 3X Barlow. So, a good quality Barlow can make it seem like you have twice as many eyepieces! The important point here, though, is high quality Barlow. Often very low quality Barlows are included in budget telescopes, and that tends to tarnish their reputation. High quality Barlows an be quite useful, though. Not only do they effectively double the number of your eyepieces, but they also help with eye relief issues. Remember, shorter focal length eyepieces normally have shorter focal lengths. But, a 2X Barlow lets you use a 30mm eyepiece as if it were a 15mm eyepiece, but still with the eye relief of the 30mm eyepiece. So, for those of use with glasses, Barlows can be really useful. If you don’t have one, consider purchasing a good quality one.
Next time, I’ll talk a bit about what sort of things you might want to look at with your new telescope.
-Astroprof
Astroprof’s Page » Eyepiece Filters on December 31, 2006 at 8:54 pm: 1
[…] Once you get comfortable with your telescope, you might want to consider buying filters for your eyepieces. As I said in my eyepiece post a couple of days ago, telescope eyepieces come in standard diameters. For at least half a century, they have also had standard thread in them, allowing threaded filters to be screwed into them.  But, why would you want filters? […]
robert wachob on July 3, 2007 at 5:03 pm: 2
I am looking for a $25 1.25 eyepiece, wonder if you know of a link online?
Robert
Astroprof on July 4, 2007 at 12:06 pm: 3
Off the top of my head, I don\’t know of any eyepieces that inexpensive. However, Orion Telescopes (http://www.telescope.com) has some eyepieces for only about $30 that aren\’t too bad.
Lucian on August 31, 2007 at 11:49 am: 4
Hi
I find your site useful, but i would need some help from you.
I intend to buy a 6mm eyepiece, and I don’t know which type has better eye relief (Kellner, Ortho, Plossl?)
I know that at such small focal lengths (6 mm) eye relief tends to be a problem, and i’d want the eyepiece at least to accomodate my eyelashes (i’m not wearing glasses).
So what design do you think is better? (of the reasonably priced ones)
Thanks a lot!
Astroprof on August 31, 2007 at 4:23 pm: 5
Lucian,
Of the modestly priced eyepieces, the one with the best eye relief for a 6mm eyepiece would generally be the Plossl. That is really a pretty good all round eyepiece design.
Corbin on June 15, 2008 at 4:43 pm: 6
I have a Celestron Nextstar 60 GT. When I look through with no eye piece I can see things fine, although ‘far away’.
When I add an eyepiece, no matter what size, it will not focus. I am looking for a simple answer as to why. Since this happens with all eyepieces, I’m assuming something with the telescope, but maybe just user error?
Please help!
Astroprof on June 15, 2008 at 11:04 pm: 7
Corbin, Hmm. Sometimes those small refractors don’t have a huge dynamic range of focus. But, adding a diagonal can often add some length to the tube. So, they are made either with a built in tube extender or something that can be put into place for use when not using a diagonal. If that is in place when using the diagonal, you cannot focus. If it is not in place when not using the diagonal, you cannot focus. That would be my guess.