University Lowbrow Astronomers

Eyepieces... The Other Half of Your Scope (Part 1).

by Charles Nielsen
Printed in Reflections: January, 2002.

So you got that new or first telescope, and maybe an eyepiece or two came with it.  But what quality are they, and are they the best match for your optics.  Whether you own a reflector, refractor, or catadioptric, consider this.  You have one objective, which is only one element in a reflector and two three or four in a refractor.  You may have a secondary mirror or diagonal, but that only redirects the light.  The objective is the element that concentrates the light to a small point.  The rest of the “work” to produce the image you see is done by the eyepiece.  This eyepiece will have a minimum of three elements, or has as many as eight, and is responsible for magnifying that little point of light produced by the objective.  So shouldn’t we give our eyepiece selection as much consideration as the scope itself?  And back to those eyepieces that came with your scope.  Except for some very high-end scopes, most do not ship with the quality eyepiece that will bring about the best performance the scope is capable of.  Usually you get a Kellner (sometimes called “modified achromat”) or at best a mediocre quality plossl.  As an example, I bought a 90mm short tube refractor (f/5.6) about a year ago, which came with a 26mm Plossl that I was very familiar with and thought was a pretty decent eyepiece.  A few weeks later I bought a Celestron Ultima 18mm.  The improvement was dramatic.  Better edge sharpness, contrast, and resolution.  A Lanthanum Superwide (22 mm) was even more impressive.  And believe this or not, a 7mm Nagler yields a field that has stars as pinpoints right at the field stop.  Yes, this is with a $300, f/5.56, achromatic, Chinese refractor!  One of our club members has a 10-inch DOB that he has always thought had a somewhat inferior mirror.  One night at Peach Mountain I had him try the Ultima and Lanthanum in his scope.  As he put it, “It’s like lifting a film off my optics.”  It was that much sharper and had better contrast.

So what is the best eyepiece for your scope?  The answer is dependent upon what type of scope you have, the scope’s focal ratio, the type of observing that you prefer, whether you have a clock drive, and of course, what you can afford.  Let’s consider the type of scope you have and your focal ratio.  Focal ratio is actually the much more significant factor.  Why?  One obvious reason is magnification.  The focal length of your scope divided by the focal length of your eyepiece yields magnification.  So what does focal ratio have to do with it?  Given equal apertures, a longer focal length (therefore larger focal ratio) will bring the convergent cone of light from the objective to a focus at a shallower angle.  Therefore the eyepiece has less light concentrated at its edges, where aberrations are at their greatest.  This is why less well corrected eyepieces usually work better in longer focal length scopes than they would in a shorter focal length scope, given the same aperture.  Does this imply that shorter focal ratio, or faster scopes, need better eyepieces?  Sorry light bucket owners, but the answer is yes.  Also, a fast reflector will have a correspondingly larger secondary obstruction, which reduces image contrast.  Therefore with these instruments, better contrast on the part of the eyepiece is of great benefit.  Refractors, having no central obstruction at all, tend to have better contrast than reflectors.  Would this be why many serious planetary observers prefer refractors?  Refractors can also use comparatively lower powers than reflectors.  This is because the reflector’s secondary mirror will severely affect the center of the field of view below certain limits.  At this point you can begin to actually see the shadow of the secondary in the field of view!

What type of objects do you like to observe?  Now we need to consider resolution, contrast, and field of view.  For larger, more diffuse objects a wide field of view is important.  This may be to see the whole object or to nicely frame it with background stars.  Good contrast also helps you to see dim objects against the background sky.  Fine resolution is nice, but many deep sky objects are “fuzzy” anyway.  On the other end of the scale, let us consider the Moon and planets.  Here resolution and contrast reign.  Now we are studying small objects and/or looking for very fine and sometimes faint detail.  Since we are concentrating on a relatively small area of the available field of view, a wide field eyepiece is only a luxury.  The one exception to this is in the case of using high magnification (which we probably would be on planets) with a non-clock drive mounting.  A wider field makes it easier to find the object, and gives you more time before it drifts out of the field of view.  I believe this is one of the biggest advantages of the shorter focal length Nagler eyepieces.  Resolution and contrast are also the most important factors for double stars, especially faint or unequal pairs.  Excellent contrast also helps us to see fine and fainter details on a planet’s surface and the lunar terminator.  Looks like contrast is important in either scenario, and indeed it always is something to be desired.

OK, so all we need is good contrast and good resolution for planets and small deep sky objects, and good contrast and wide fields for those medium and larger “faint fuzzies.”  So why not super resolution, excellent contrast, and wide field of view in the same eyepiece?

It can be done, but not easily (or usually inexpensively).  This is where reality can spoil the party.  The problems start with apparent field of view.  The first culprit is that optical aberrations in an eyepiece get harder to control as apparent field of view increases.  To maintain good resolution and a flat field now requires more optical elements.  This means more glass, larger size, more weight, and more cost.  More glass for the light to pass through reduces contrast.  Then to pour salt in the wound, wide apparent fields and long eye relief do not live on the same side of the street.  Yes, the big bummer, eye relief!  Eye relief is the distance between the eye lens of the eyepiece and the point at which the image is concentrated, and the whole field of view of the eyepiece can be viewed comfortably.  This spot of light is referred to as the “exit pupil.”  Normally as apparent field of view increases, eye relief rapidly gets shorter.  The same thing usually happens as focal length of the eyepiece decreases.  This can become so extreme that you almost have to put your eye on the lens to see the whole field of view.  This is very uncomfortable, and for eyeglass wearers, impossible.  So then what good does the wide field do if you cannot see it?  So can you spell compromise?  First, how wide is wide enough?  My peripheral vision is very good and I can just barely detect both sides of a 65-degree field of view without moving my eye around.  So an 82 degree field is only noticably wider if I intensionally look for it.  Not really a big deal.  I would prefer a “comfortable” 65 degress to a “cramped” 82 degrees.  Even a 50 degree field can appear pleasantly wide if eye relief is long enough.  I find a huge difference between 50 and 45 degrees.  There are brands of eyepieces that offer 20 mm of eye relief and 60-65 degree fields.  With these you can see the whole (or most) of the field of view while wearing glasses.  They are not cheap, but very much worth it!  If you really want those 65+ degrees, you will have to accept short eye relief.  As mentioned earlier, eye relief is better with larger focal length eyepieces, so at about 20 to 25 mm and higher wide fields are easier to achieve without a big sacrifice in eye relief.  For this reason many premium quality eyepieces employ a built in Barlow lens assembly.  By doing this the manufacturer can make say a 20 mm eyepiece with eye relief and an eye lens diameter more typical of a 40 mm eyepiece.  The trade off is the addition of glass elements used.  With good design, quality glass, and good coatings, this is a trade off that is definitely acceptable.

Let’s think about optical coatings.  The robber of contrast is light loss due to reflection and dispersion.  All glass suffers to some degree from both.  Lower dispersion, high quality glass of the correct type help, but of course at higher cost.  Optical coatings reduce reflections off the glass elements.  The more surfaces involved the more critical this becomes.  The whole idea is to pass as much of the light as possible through the glass and into your eye.  We don’t want light bouncing off the glass surfaces or scattering around inside the glass.  I believe any quality eyepiece should have at least a coating on every air to glass surface.  This is usually referred to as “fully coated.”  “Multi-coating” usually implies that at least the outer two glass surfaces have at least two layers of coatings.  “Fully multi-coated” means that every air to glass surface, internal and external has received at least two layers of coatings.  I believe the “multi” and “fully multi” models are considerably superior.  Fully coated lenses usually have a bluish or less frequently a reddish tint.  Multi coatings usually display a greenish (preferably a dark green) tint.  Fully multi-coated lenses display a purple tint and all of the colors mentioned above, depending on the angle of your view and amount of light available.

Now for some good news on that pesky eye relief issue.  That involves the sometimes over looked exit pupil situation.  Again, the exit pupil is the circle of light from the eyepiece where the image is concentrated and brightest.  It is also the point at which you can see the whole field of view (or as much as the eye can handle).  The human eye pupil can open as much as 7 mm when we are younger.  At an older age this may drop to 5 mm.  This means that eyepiece that delivered a 7 mm exit pupil can not all be taken in when you are older.  This “extra” light is therefore wasted.  Your pupil now becomes an aperture stop for your telescope!  On the other end of the scale, when you get down to about 1 mm and below, that spot of light is not much larger than some of the “debris” floating around in the fluid of your eye.  At 0.5 mm this really becomes a problem.  The result is you actually start to see it.  No, those were not dark clouds you were seeing on that planet!  So a very small exit pupil can be uncomfortable and dim.  On the other hand, a larger exit pupil will hit more of the edges of the lens in our eye.  Just like optical lenses, our eye’s lens has more aberrations near the edges.  So as you are probably thinking, is there a good point in the middle?  There is a point when a you have the advantages of using mostly the center of your eye, but without losing too much brightness and starting to see “ghosts.”  The magic number is 2 mm.  The eyepiece in your collection that comes the closest to this number when used in your scope will produce the most detailed image that your optics and your eye can deliver.  Below this number you will see a larger image, but dimmer, and with a loss in resolution.  Does this mean you should avoid anything smaller?  No.  Sometimes you will just plain need the extra magnification.  Below 0.5 mm is practically unusable.  Between that 0.5 and 2 mm is where the good news for glasses wearers comes in.  Most faults with vision involve the shape of our eye and its lens.  Just like an eyepiece, these faults can be reduced if we avoid using the edges of the lens in our eye.  You can actually demonstrate this to yourself.  Take a card or piece of paper and poke a small hole in it.  About a pin size or a little more is adequate.  Now take your glasses off and look at objects near and far through the hole.  Pretty cheap prescription or emergency glasses, but not exactly a fashion statement!  The much clearer image you see is because that narrow spot of light coming through the whole “avoids” most of the imperfections with your eye.  So that good news finally... when you get down into those smaller exit pupils, you can start to use your scope without your glasses on.  The magic number will vary per individual.  I personally have a lot of astigmatism and yet when I start approaching about 1 mm exit pupil, I can take my glasses off and see as clear as with them on.  Near sightedness and far sightedness can be compensated for by merely re-focusing the scope.  Remember that as eyepiece focal length gets smaller, so does eye relief.  But as you can see, there will be a point at which you can still deal with short eye relief, even if you normally wear glasses.

Exit pupil, since it relates to image brightness, has a big influence on using nebula or light pollution reduction filters.  All nebula filters work by increasing the contrast between the object and the background sky.  They do not make the object brighter; it just seems that way because the background is now darker.  So there is a specific range where each filter works at it’s optimum.  The size of the exit pupil and the amount of light pollution that we are dealing with determine this range.  In a “dark sky” situation, say limiting magnitude 6 (don’t we wish), a broadband (such as the Orion Skyglow) filter works best between 1 and 4 mm exit pupil.  For a narrow band (like the Orion Ultrablock) the number is 2-6 mm.  For a line filter (such as the Lumicon O3) the numbers are 3-7 mm.  As you can see, the more you filter out, the more brightness of the image is required.  As light pollution increases, so does your necessity to filter it.  As an example, that nebula that looked good with a broadband from the country, might need a narrow band in the city.  Also a different eyepiece may be required to stay in a filter’s optimal exit pupil range.  By the way exit pupil is calculated by first determining the magnification being used, which is the focal length of your scope divided by the focal length of your eyepiece.  Then divide the diameter of your objective lens or mirror by this magnification to determine the diameter of the exit pupil.

OK.  We have looked at the importance of resolution and contrast, and how better coatings can improve contrast.  We considered the significance of eye relief and how it relates to “usable” field of view.  The advantages and disadvantages of using more optical elements or lenses, and lastly, the very important subject of exit pupil and how it relates to the human eye.  But the seemingly easy question was “what are the best eyepieces for my telescope?”  Not so fast; stay tuned for the sequel “Eyepieces... The Other Half Of Your Scope (Part 2).”  Although you probably already have some good hints, in part 2 I will finally come right out with.  Also, I will explain some of the testing and comparisons I have done with various eyepieces, and my opinions of them.  Just remember the opinions expressed are not necessarily those of this Newsletter, the University Lowbrow Astronomers, or anyone else for that matter.  Stay tuned....


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Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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