February 12, 2017: Let’s Talk About Eyepieces, Baby

Let’s talk about all the good things, and the bad things, that may be.  Let’s talk about eyepieces!  More to the point, let’s talk about expensive eyepieces, also referred to as premium eyepieces.

Obviously, as a preliminary matter, when you’re selecting a new eyepiece, one of the first considerations should be how it fits in with the eyepieces you already have – particularly, your magnification scheme.  Of course, to figure out your magnification, you simply 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.  But that’s just the first thing you’re looking at.

Many budding astronomers – including myself, up to a point – think that better eyepieces will show them more.  “Oh, if I only save up and buy a $200 eyepiece instead of this $50 one I have, things will pop right out at me.  If I throw some more money at a premium eyepiece, I’ll be able to see fainter stars, more detail.”

Well, it just ain’t so, and this is what many beginners don’t seem to realize. Just about every eyepiece out there is sharp – except for the supercheap pieces of junk, the Ramsdens and Huygenians that come with crappy department store scopes (usually marked H20 and SR4).   Other than those abominations, as long as you’re spending north of about $25-30, just about every eyepiece out there will generally give you a nice pinpoint focus, and that focus will be pretty good almost out to the edge of the field of view, where it may have some problems if you have a fast focal ratio scope – f/5, f/4, f/3.  The cure for this is an eyepiece with good correction, which I discuss towards the end of this post.

People seem to think that if they spend more, $100 an eyepiece, $200, $300, even more, that they will see more; that the view will somehow be better; that the eyepiece will somehow be even sharper and show even more detail than a cheaper eyepiece. Well, as a general matter, it won’t. Something like that may have been true 40 or even 25 years ago, when there was more variability in the quality of eyepieces. Today’s eyepieces are pretty uniformly manufactured to very high quality standards, with excellent anti-reflection coatings, so that that just isn’t true any longer.  There is no magic pixie dust sprinkled on premium eyepieces that all of a sudden reveal to you that which wasn’t previously there.

Now, I’d be lying if I said that you didn’t get anything at all for the extra money.  Premium eyepieces will give you more accurate color rendition.  That means that the colors you see will be less affected by the eyepiece glass – the colors will be more true.  Because premium eyepieces have the best coatings on the lenses, there’ll be very little scattering of the light, which will improve contrast somewhat.  If you are that discriminating of an observer so that squeezing every last detail, every last photon, out of your view is important to you, then yes, a premium eyepiece will get you there.

Premium eyepieces will also be sharper from edge to edge of the field of view.  Many less expensive eyepieces try to push the eyepiece design a little further than it should really go – giving you, say, a 70-degree field of view where the design is only pinpoint sharp out to 65 degrees.  The outer 5 degrees won’t exactly be blurry, but they won’t exactly be sharp, either.  The more money you spend, the sharper those last few degrees will be.

On the other hand, the humble Plossl covers all the bases that an eyepiece should – yup, the cheapo Plossls that come stock with many scopes nowadays.  They provide a decent field of view of 52 degrees, they are sharp from edge-to-edge of that field of view in all but the fastest focal ratio scopes, and are an incredible value for the money.  Plossls can be had for between $30 and $40.  Plossls represent an incredible bargain in eyepieces.  There is just one drawback to Plossls – they have an eye relief problem in shorter focal lengths, which I discuss immediately below.

When you spend more for an eyepiece, say above $50 or so, you’re paying for three things:  1) eye relief; 2) apparent field of view; and 3) better correction.  

Eye relief

Eye relief, as I’ve covered before, 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 9mm Plossl will have 6mm of eye relief, which is okay.  But a 6mm Plossl at 4mm?  A 4mm Plossl at less than 3mm?  That’s pretty tight.  And very uncomfortable.  You have to hold your eye and head steadily and still, very close to the eyepiece, for 5-10 minutes or more as you observe, because of variability in the atmosphere called seeing.  Holding your head that still for that long can lead to neck and back strain in those that are prone to it.

Orthoscopic eyepieces are supposed to be just about the sharpest eyepieces going.  They generally cost between $80 and $100.  However, they are not much better in the eye relief department.  Orthos typically have eye relief that is approximately 3/4 of their focal length.  So an 8mm ortho will have about 6mm of eye relief, which, like the 9mm Plossl, is okay.  Get any lower than 8mm, and you’re asking for trouble in terms of eye relief that is too tight.  This is the reason I bought the ortho I have, a KK Fujiyama, at the focal length I did, 12.5mm.  It has eye relief of 10mm, which is plenty comfortable.  And then I use my Barlow, an excellent Orion Ultrascopic, to double the magnification while maintaining the eye relief.

Eye relief is 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.

Many vendors sell so-called “planetary” eyepieces for about $50-60 each.  No, these aren’t specially designed or tweaked for use just on planets to the exclusion of any other object.  Again, the views through these eyepieces are not going to be any sharper or better than any other eyepiece.  But they are going to be more comfortable to use on planets because they offer additional eye relief, and allow you to have some variability in how you hold your head as you observe.  You don’t have to hold your head as stock still as you would with an eyepiece with shorter eye relief – with the additional eye relief, you can move your head around some and still see through them, and that movement prevents neck and back strain.

Apparent Field of View

The second thing more expensive eyepieces offer you is increased apparent field of view (AFOV). This is the big deal, the “killer app” of premium eyepieces.  This is what you want, this is what you’re paying for when you spend that extra money on an eyepiece. One area where the increased AFOV really matters is if you have a non-motorized scope, like a dob.  Because the AFOV is wider, at higher magnifications, the object you’re looking at will stay in the field of view longer before having to nudge the scope to recenter it as the earth turns and moves the object out of your field of view.

AFOV is different from, but has a great deal of impact on, True Field of View – TFOV.  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 mind 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.  At the same magnification, which is 1x.  The toilet paper roll tube has a larger AFOV – you see a wider angle – and that results directly into a larger TFOV.

A larger TFOV means a larger-sized patch of the sky that you can see all in one view.  So, let’s work some numbers a little.  A 32mm Plossl has a 49-degree AFOV, which will give you just about the same TFOV as a 24mm 68-degree AFOV eyepiece, and the same TFOV as a 20mm 82-degree AFOV eyepiece; but of course, the 24mm and 20mm eyepieces will do so at a significantly higher magnification.

Keeping the magnifications the same, a 10mm Plossl will have a 52-degree AFOV; a 10mm 82-degree eyepiece will give the same magnification, so whatever you’re looking at will be exactly the same size.  But the increase from 52 to 82 means that the AFOV and therefore the TFOV is obviously significantly wider.  That will translate directly into a much larger patch of the sky visible through the 82 versus the 52.

That increased AFOV and TFOV results in a nice extra layer of black sky surrounding a DSO as you observe it.  Most people find that larger TFOV 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 is that most people prefer the “spacewalk” feeling that larger AFOVs offer.  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 being on a spaceship and looking out through a porthole.  Others want to go to higher magnifications without losing much in the way of true field of view.

Another reason for For example, the Pleiades are about 1.3 degrees across.  In my C9.25 with a focal reducer, the ES 68 24mm can just barely squeeze them all into the same field of view.  And they look horrible.  One of the most beautiful objects in the entire sky looks horrible because it’s being squeezed, and there is no room for the object to breathe – no black sky around it.  I can’t go any wider than that in the C9.25 unless I move onto two-inch eyepieces, and even then, there will probably be severe vignetting.  The Pleiades should best be seen through a wider-field scope.

On the other hand, there are people – like me – who do not like having to move their heads around to be able to take in the entire view that some extra-wide-field eyepieces offer – like those that have an AFOV of 82 degrees, or even wider than that.  Al Nagler had something to say about this.

Nagler is the man behind TeleVue eyepieces, which are generally considered to be the finest eyepieces out there – with price tags to match.  Back in the Eighties, he invented one of the first widefield eyepieces, the Panoptic, with an apparent field of view of specifically 68 degrees.  This was because 68 degrees is the widest field of view that the human eye can take in all at once without having to move your head around.  Hey, if it’s good enough for Al, it’s good enough for me.

Better correction

Beyond better eye relief and increased AFOV, the third thing that more expensive eyepieces give you is better correction. Correction of what?  Well, if you’re using a fast scope, like an 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 dots.  With those fast scopes, which include not only big dobs, but also many smaller f/5 newtonians (like the AWB One Sky and its various clones) and fast, rich-field refractors (like my Orion ST-80), premium eyepieces will correct the view to some extent and give you better edge-to-edge performance across the field of view than cheaper ones can.

This is especially true with some 4- and 5-element eyepieces in the 65-70 degree AFOV range, which are basically an older design called an Erfle.  The Erfle invented around World War I for use in reconnaissance planes to take pictures of the battlefield from above.  Here are a couple  of examples.  Erfles work well in slower scopes (f/8, f/10, f/12), but the faster your scope gets (f/5, f/4, f/3), the more edge-of-field deterioration you see.  Whether it’s the outer 10% or 20% of the field of view, or even more, depends on the focal ratio of your scope and how much you yourself can tolerate this deterioration.

Premium eyepieces in that same AFOV range (around 68 degrees) perform much better than Erfles, but of course, cost more.  You can tell that a premium eyepiece in the 65-70 degree range isn’t an Erfle by clicking on the specifications tab – non-Erfles will have more than 5 lens elements. (These are the eyepieces I was discussing at the beginning of the article, where they are pushing the designed 65-degree apparent field of view out to 70 degrees.)

On the other hand, this edge-of-field correction that premium eyepieces bring to the table isn’t very important for someone like me with my Mak. High-focal ratio scopes (like f/10 SCTs and f/12 Maks, as well as long, skinny f/10 refractors) don’t need to have better corrected eyepieces, because there are no (or extremely limited) visual aberrations to correct in the first place. Whatever eyepiece you use in a high-focal ratio scope – even a lowly Kellner, which is even cheaper than a Plossl – will look good from edge-to-edge in a slow scope.  This is the reason why a cheapo, three-element Kellner comes stock with Maks – because they look so good due to that long focal ratio.  So, although slow scopes generally cost more up front, we save money in the long run by not needing to buy premium eyepieces thereafter.

So, these more expensive eyepieces certainly do have their place, but not for the reasons many beginners think.  Indeed, one of the reasons I bought my 24mm Explore Scientific 68 degree eyepiece was to conclusively prove this to myself – that a premium eyepiece is not so amazing or great – or better – in and of itself.  (I bought it for that, and for the most important reason – to absolutely maximize the true field of view possible in my Mak.)  But other than the slightly wider true field of view over my regular cheapo 32mm Plossl, the ES isn’t any sharper, isn’t any better than the 32, which works just fine for me – at less than a quarter of the price of the ES.  There is no magic “Ah ha!” moment with a premium eyepiece.

More important than most of this is the fact that the local seeing conditions will invariably have a much greater impact on the quality of your views than any other factor.  If your local seeing is only average, your views will also be only average.  Whatever eyepiece you use will suffer, so why spend all that money on premium eyepieces?

On the other hand, many well-heeled astronomers buy these premium eyepieces for exactly the opposite reason – so that on those rare nights when the seeing conditions are good enough, the eyepiece will not be the weak link in the optical chain degrading their view.  If you are fortunate enough to have better-than-average seeing where you observe, and to have a better-than-average wallet, this is one further reason to spend more on premium eyepieces – to be ready for these rare nights of above-average seeing.  Of course, those two or three nights of perfect seeing will always come when your mother-in-law is visiting and has tickets to take you all out to the ballet, or worse, the opera.

For this, we frugal astronomers have to give thanks to the better off amongst us.  This is because those developments, those breakthroughs that premium eyepieces achieve trickle down to the rest of us in the form of eyepieces that do more for less, with wider AFOVs and better coatings. Even just 20-odd years ago, 68-degree fully multi-coated eyepieces were prohibitively expensive.  Because of their widespread adoption, we have many more models of these eyepieces to choose from at a variety of lower price points.

At the end of the day, you have to ask yourself one question:  “Do I feel lucky? Well do ya, punk?”  Oops, slipped into Dirty Harry mode there for a second. You have to ask yourself whether these things that premium eyepieces offer are important to you, if they’re worth that premium cost. Certainly, the comfort offered by the extra eye relief that you need in short focal length eyepieces (which are used to achieve high magnification) matters, because such eyepieces are used mainly for lunar/planetary observing, which requires time and patience.  And for non-motorized scope users, the wider field of view in premium eyepieces results in your not having to nudge the scope as often, which is also a nice benefit.  However, the best reason to buy premium eyepieces is to get their wider fields of view so that at the medium magnifications (in and around 100x) that most DSOs show best at, you get better aesthetic framing of them.

But other than those reasons?  Eh.  I say unless you have to go premium to correct the visual aberrations that result because of the fast focal ratio of your scope, or unless you’ve got money that’s just burning a hole in your pocket, instead of splashing out on expensive eyepieces all up and down your entire magnification range, stick with your Plossls.  Buy only one or two premium eyepieces for their nice wide-field views.  Otherwise, save your money for a bigger scope.


One thought on “February 12, 2017: Let’s Talk About Eyepieces, Baby

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s