barlow, eyepiece, eyepieces, luminos, magnification, plossls, seeing - January 29, 2019

Choosing Eyepieces

When you’re selecting a new eyepiece, the first and most important consideration should be how it fits in with the eyepieces you already have – particularly, your magnification scheme.

To figure out the magnification of your eyepieces, you take the focal length of your telescope (1200mm for an f/6 8-inch dob) and divide it by the focal length of the eyepiece. For example, if you have, say, 25mm and 10mm Plossls that came with your 8-inch dob, they’re giving you 48x and 120x respectively. So, the first analysis is to fill in magnification “gaps” you have – like a high-powered eyepiece, or more of a mid-powered eyepiece in between the two.

Maximum magnification and seeing

In figuring out how high of a magnification to get, you need to acknowledge the limits that the atmosphere places on you. Depending on your scope’s aperture, the scope itself may be capable of giving you 300x, 400x, or higher. The rule of thumb in calculating the maximum magnification that a telescope can deliver is 50 times the aperture in inches or twice the aperture in millimeters. Continuing to use the 8” dob as an example, 50 x 8 inches = 400x, and 203mm x 2 comes out to just about the same amount, 400x.

However, the atmospheric turbulence that is always present, called “seeing”, generally limits you to about 200x maximum, and perhaps less depending on the seeing conditions at your particular location. As an example, I used to observe in Manhattan, and there was a steady flow of air (called a laminar flow) coming in from the west. I was able to get to 200x easily, and sometimes even get up to 250x, the maximum possible with my 5” scope. However, here in the Denver metro area, the wind whips through the saw-toothed Rockies to the west, causing all sorts of turbulence in the atmosphere which limits the seeing to about 150x. On the other hand, if you take your scope down to New Mexico, you should be able to get right back up to, or even past, that 200x level.

If you go above this level that the atmosphere limits you to, you will get what is called “empty magnification”. Whatever you’re looking at, say, a planet, will certainly look larger with greater magnification. However, the seeing conditions will cause the view will break down. The image will deteriorate and look less sharp than it did at a lower magnification. The best way to find out what magnification your local seeing will generally support is to talk to members of your local astronomy club. They’ll know.

To figure out the focal length of the eyepiece that will give you that maximum magnification is, you can take the focal length of the scope and divide it by that maximum magnification. Continuing with our example of the 8” dob, 1200mm divided by 150x = 8mm. If your local seeing will allow you to get to 200x, then 1200mm divided by 200x = 6mm.

On rare nights, the atmosphere will quiet down, and that maximum magnification will increase. To be prepared for this, you should have a good Barlow on hand, like the X-Cel LX 2x Barlow, to increase the maximum magnification your eyepieces are capable of.

Now you have your maximum magnification locked down in terms of knowing what focal length eyepiece to buy. But which one should you buy at that focal length?

Well, there are a few other important attributes that every eyepiece has that you should understand before buying: 1) apparent field of view; 2) eye relief; and 3) better correction. The first two of these features is displayed with the specifications for each eyepiece.

Apparent Field of View

Apparent field of view (AFOV) is the big deal, the “killer app” of eyepieces. This is what you want, this is the most important thing you’re paying for when you spend extra money on an eyepiece. One area where the increased AFOV really matters is if you have a non-motorized scope, like a dob. With a dob, if you use an eyepiece with a wider AFOV, especially at higher magnifications, the object you’re looking at will stay in the field of view longer before you have to nudge the scope to recenter the object as the earth turns and moves the object out of your field of view.

Apparent field of view is different from, but has a great deal of impact on, true field of view – TFOV. AFOV is a feature, a specification of the individual eyepiece; how that eyepiece interacts with your particular telescope will define the TFOV that you will see.

At any given but equal magnification, the larger the AFOV, the larger the TFOV you will see. The easiest way to understand this is to do a simple thought experiment: think of looking at someone through a straw. You’ll see just their face. Then look at the person through a toilet paper roll tube. You’ll see their face, shoulders, and chest. And note that you’re seeing more at the exact same magnification, which in this example is 1x. The toilet paper roll tube has a larger AFOV – you see a wider angle – and that results directly into a larger TFOV at that same magnification.

Getting back to eyepieces, a larger TFOV means a larger-sized patch of the sky that you can see all in one view. Keeping the magnifications the same, a 10mm Plossl will have a 52-degree AFOV; a 10mm 82-degree eyepiece, like the Celestron Luminos 10mm, will give you the same magnification. Because they both have the same magnification, whatever object you’re looking at will be exactly the same size in either eyepiece.

But the increase from 52 degrees AFOV to 82 degrees means that the TFOV you can see is vastly larger. In other words, the 82 will show you a much larger patch of the sky, a much larger area than the 52. And because area is calculated by the formula pi times the radius squared, this means that at the same magnification, the 82 will show you almost a 250% larger patch of sky than the 52 in terms of area in any telescope.

That increased AFOV and TFOV results in a nice extra layer of black sky surrounding a DSO = deep space object = galaxy, nebula, cluster - as you observe it. Most people find that view that the larger TFOV gives to them to be aesthetically pleasing. The extra black at the edge frames the object in the center, and it looks better; you also see the object in the context of where it is in relation to the surrounding sky. And these are all very good reasons to spend more on an eyepiece.

Another reason to get eyepieces with a larger AFOV is that most people prefer the “spacewalk” feeling that you get when you look through them. When you get to an 82-degree or wider AFOV, your peripheral vision can no longer see the edge of the field of view while you’re looking at the center. This gives you the feeling that you’re no longer looking through a telescope, but instead, it’s like you’re outside the spacecraft in your spacesuit, floating in space.

Lower magnification = larger true field of view

You should also consider getting an eyepiece that will give you the widest field of view possible in your scope - a low magnification eyepiece. This makes initially finding the object you’re looking for that much easier, because you’re looking at a larger-sized patch of sky.

This eyepiece, the one that gives you the maximum true field of view, is called the finder eyepiece for just that reason. This is true of both manual and computerized scopes. For example, the when you’re starhopping in a dob, you want that widest field of view to give you the best chance of spotting the object as you move the telescope around, moving from star to star.

The same is true of computerized scopes. The Celestron SE mount that comes with the 4SE, 5SE, 6SE, and 8SE is very accurate, but it’s not absolutely perfect. You want to give it the best chance at finding the object by putting the eyepiece that has the widest field of view in the focuser – that’s generally the one with the largest number on it. These SE mounted scopes come stock with a 25mm eyepiece; in fact, most scopes come with this same focal length eyepiece as your lowest magnification, widest field of view eyepiece.

To get the widest field of view ( = biggest patch of sky) possible to see through the eyepiece with a regular, 1 1/4″ focuser, look at getting a 32mm Plossl, like the Celestron Omni Plossl or the Meade Super Plossl. A 32mm Plossl gives you a lower magnification than a 25mm eyepiece, which maximizes the field of view possible in any telescope in an eyepiece with a regular 1 ¼” eyepiece.

Two-inch eyepieces = an even wider field of view

Some telescopes can use 2” eyepieces. For example, almost all dobs 6” and larger can accept 2” eyepieces directly into their focuser. For the Celestron 6SE and 8SE, as well as the Celestron Evolution and CPC series, you can get a 2” diagonal that will let you use 2” eyepieces. At lower magnifications, these 2” eyepieces can give even wider field of views than 1 ¼” eyepieces can because of their wider barrel size. This goes right back to that straw and toilet paper roll tube example I gave before.

A 32mm Plossl is a 1 ¼” eyepiece that generally has an apparent field of view of about 49 degrees or so. As I mentioned above, it will give you just about the largest field of view in an eyepiece in a 1 ¼” format. The Celestron Luminos 31mm eyepiece is a 2” eyepiece that has an 82-degree AFOV. Doing the math, in any telescope, the 31mm Luminos will give you a 260% larger true field of view at just about the same magnification as the 32mm Plossl.

Because of their wider fields of view, 2” eyepieces are even better at being finder eyepieces than 1 ¼” eyepieces. Depending on your scope, 2” eyepieces will also let you see some of the largest objects in the sky all in one view - large objects like the Pleiades (M45), the Beehive Cluster (M44), the Ptolemy Cluster (M7). They are also ideal for scanning around the Milky Way at relatively low magnifications, just to observe how incredibly gorgeous it is.

Eye relief

Eye relief is the distance that you have to hold the lens in your eyeball from the top lens in the eyepiece so that you can see the entire field of view that the eyepiece offers you. The rule of thumb is that the eye relief of a Plossl is generally 2/3 of the focal length. So, a 10mm Plossl will have about 7mm of eye relief, which is okay. However, note that a 6mm Plossl will have about 4mm of eye relief, while a 4mm Plossl will have less than 3mm.

Eye relief that’s that short is pretty tight. And very uncomfortable. Because of seeing conditions, which I described above, you have to hold your eye and head steadily and very still, right up close to the eyepiece, for 5-10 minutes or more as you observe until the atmosphere quiets down and gets real still. Holding your head that still for that long can lead to neck and back strain in those that are prone to it.

Eye relief is particularly important for folks who need to or prefer to wear glasses while observing. Although normally, you can take your glasses off while observing and let the telescope become your glasses, that isn’t true for people suffering from astigmatism. Obviously, if you’re wearing glasses, you can’t get your eyeballs as close to the eyepiece because your glasses are in the way. People who wear glasses while observing are typically looking for about 16mm of eye relief at a minimum; 20mm is better.

For eyeglass wearers, as well as the rest of us who just prefer to be more comfortable at the eyepiece, most eyepieces beyond the introductory Plossls have eye relief that is significantly greater than the tighter relief offered by high magnification Plossls: 13mm, 16mm, or more. Again, the eye relief for any particular eyepiece is noted under the Specifications tab for that eyepiece.

Better correction

Beyond increased AFOV and better eye relief, the third thing that eyepieces beyond Plossls can give you is better correction. Better correction of what? Well, if you’re using a fast scope, which means a scope with a focal ratio of f/5, f/4, or even faster, your views will start suffering from various visual aberrations, especially at the edge of the field of view. One of these visual aberrations is called coma: the stars at the edge will appear fat and bloated and not as sharp round pinpoints.

Fast scopes include dobs 10” and larger. Many smaller f/5 newtonians (like the Meade Lightbridge 130) and fast, rich-field refractors (like the Meade Infinity 80) are also fast and will have some of these aberrations. Eyepieces beyond Plossls will correct the view to some extent and give you better edge-to-edge performance across the field of view than less expensive Plossls can. By that, I mean that the stars will look more like pinpoints, nice and sharp, across the entire field of view; craters on the moon will be well-defined from edge-to-edge of your field of view.

Eyepiece recommendations for popular telescopes

The Celestron SE series comes with only one eyepiece, a 25mm Plossl. This is fine until you decide you want to see something larger (more magnified) than the 25mm Plossl shows you. Here are our recommendations for additional eyepieces with each of the SE telescopes: