University Lowbrow Astronomers

Eyepieces... The Other Half of Your Scope (Part 2).

by Charles Nielsen
Printed in Reflections: March, 2002.

In part one of this article (see “Eyepieces... The Other Half of Your Scope (Part 1).”) many characteristics of a good quality eyepiece were described.

These were resolution, contrast, eye relief and field of view.  It was also pointed out that it is difficult to produce an ideal eyepiece that is good in all of these aspects.  Compromises are therefore involved.  These are the considerations.  As eyepiece focal length decreases (more magnification), our image scale increases but becomes dimmer, and eye relief usually becomes shorter.  Also, as field of view increases, eye relief decreases, unless more optical elements or lenses are used.  This increases weight, size and cost.  It also means more light loss and reduction of contrast.  This light loss can be reduced significantly when low dispersion glass and excellent optical coatings are employed.  That of course also increases cost.  It was also stated that most aberrations in the eyepiece and the human eye as well, tend be worse around the edges of the field of view.  That is why acuity (or sharpness and detail) actually improve as exit pupil decreases.  This reaches its maximum at around 2 mm.  Reducing the exit pupil further may be of benefit to most eye glass wearers because the smaller cone of light will “avoid” many imperfections of the human lens, allowing that person to observe a well defined image without using their glasses.  Without those glasses, the inherent short eye relief is not as much of a problem.  As we approach the 1 to 0.5-mm exit pupil range, reduction in image brightness starts taking a heavy toll.  At the lower end you will also start to see “floaters”.  Debris in the eye (floaters) tend be become worse with age.  Age reduces the maximum size that the pupil in our eye can expand as well.  Exit pupil also has a great bearing on what light pollution filter is the most effective.

Since the center area of most eyepieces is the best corrected, and longer focal length telescopes will concentrate more light through this center area, longer focal length will let you use cheaper eyepieces more satisfactorily.  Longer focal length will also produce higher magnification and smaller exit pupils than shorter focal length, given the same eyepiece.  One side bar to magnification is this:  As higher magnification reduces image brightness, it also reduces the brightness of the background sky.  In a sense this has an effect similar to a light pollution filter in that the darker background may help you see a dim object easier, even if the object’s small size was not the reason for using more power.

So what types of eyepieces are good for your scope?  First let’s go over the types of eyepieces commonly available and then we can really answer this question.  The first type worthy of even mentioning is the Kellner.  Kellners employ three optical elements and so produce bright images, even with lesser coatings.  In longer focal lengths (20-25 mm and up) field of view and eye relief are not bad.  Contrast is average at best.  Kellners are often referred to as “modified achromats”.  Stepping up in quality, we encounter the Orthoscopic.  Orthos are usually not expensive and are in many respects the best eyepiece ever designed.  They are almost unbeatable when it comes to resolution and contrast, due to sound design and use of only four lenses.  This design is probably also the best at accurate color transmission.  Orthos also have very flat fields with excellent edge correction.  Where is the down side?  Eye relief can become an issue in shorter focal lengths, and field of view is typically only in the 42-45 degree range.  If you have a clock drive and are observing the moon and planets, this smaller field may not bother you, making this probably the best planetary eyepiece of any design.  The desire for wider fields is the main reason for the popularity of the Plossl eyepiece.  Plossls also employ four optical elements, but in a different arrangement then the Ortho.  I believe there is a wide range in quality with Plossls.  A good Plossl demonstrates good resolution, contrast, and have fields of view around 50 degrees.  I believe a good Ortho still beats a good Plossl in all areas except field of view.  There also exists a modified version of the Plossl, which uses five elements.  The most common of these would be the Orion Ultrascopic and Celestron Ultima family.  The same factory in Japan produces both of these brands, as well as a Canadian version called Antares Ultimas, so I am quite sure they are optically the same.  I suspect the Takahashi LE and some of Park’s eyepieces are also these.  They produce very good resolution and contrast, rivaling the Orthoscopic.  Field of view is typically 50-52 degrees.  These eyepieces also use top-notch coatings, probably the best available.  Eye relief does get sacrificed somewhat.  A 20 mm in this category will probably have less eye relief then a 20 mm “pure” Plossl or Orthoscopic.

Now we get to the big boys on the block, the wide fields.  The first wide fields were the Erfle and the Konig.  The Erfle contains five or six elements.  They produce bright images across fields from 60-70 degrees.  Edge of field sharpness falls off rapidly in medium to shorter focal lengths.  A better quality Erfle in focal lengths of 25-30 mm and up will provide a pleasing view with good eye relief.  Konigs are usually a four element design with field widths in the 60-65 degree area.  They are a strange eyepiece in my opinion; more about this in my review section.  Beyond this is the land of the giants.  Combining very wide fields with very good optical correction (especially at the edges) requires large glass elements and plenty of them.  One of the earliest, and still in production, is the Televue Nagler.  Naglers all have 82 degree apparent fields of view.  They are very well corrected and very well coated.  Resolution is in the Ortho league, but not contrast.  Since they typically employ eight glass elements, they are large and heavy.  Eye relief sadly, is quite short, even in some of the medium focal lengths.  This and their high cost are the only real downsides of this design in my opinion.  Otherwise they are one of the finest types of eyepieces money can buy.  Meade has a line of eyepieces named Ultrawides.  They have field widths almost identical to Naglers, and I have heard somewhat better eye relief.  In the 65 degree field area we have several players.  The Televue Panoptic series sports 68 degree fields with very good optical correction and good coatings.  They are smaller and less expensive than Naglers.  Eye relief is generally on the short side.  Meade also has a 68 degree entry called the Superwide.  I have not tested this one, but reviews generally indicate they are somewhat below Panoptic quality.  Our next two entries are real winners.  Each has a 65 degree field, and 20 mm of eye relief!  The Pentax XL series feature retractable eye guards, excellent coatings and quality glass.  They produce excellent, comfortable views that are hard to beat at any price.  They are unfortunately pretty expensive themselves.  For slightly less money there are Vixen Lanthanum Superwides, which also have that magic 65 degree field with 20 mm eye relief.  They are fully multicoated, and use eight elements of high quality glass.  Excellent correction, contrast, and edge sharpness complete the resume.  More comments about this one later.  Recently Televue introduced their Radian line up.  These eyepieces show 60 degree fields with 20 mm eye relief.  Contrast and resolution are top-notch, right up there with the Orthoscopic.  Coatings are excellent, and so is edge of field resolution.

Although I have missed a few, this was a brief on most of the popular eyepieces available today.  What should you have in your scope?  First of all consider that faster focal ratios (about f/6 and faster) benefit more by very well corrected eyepieces.  An eyepiece that has average edge correction in a f/4.5 scope may show marked improvement in a f/10 scope.  Better coatings translate to better contrast, and contrast is always important.  Good coatings also help reduce light lost by passage through many glass elements.  Avid planetary and lunar observers need resolution and contrast most of all.  Spend your money on those qualities, and sacrifice field of view if you must.  A good planetary eyepiece with a wide field of view (50+) will be expensive.  Not to pick on Dobsonian owners again, but that wide field is very handy when you must hand slew and track.  Also if deep sky is more your thing then wide fields are desirable.  If the 65 degree class is good enough for your tastes, then you can get good eye relief, and very nice images for less money then stepping up to anything wider.  Also consider size and weight.  Be extra careful with heavy eyepieces in a diagonal that could rotate and drop it on the ground.  Will that hand grenade eyepiece cause you to re-balance your scope every time you use it?  I believe if given two eyepieces of exactly the same qualities, but much different size and weight, most people will feel more comfortable reaching for the smaller one.  I have found that eyepieces are like many other things; you pretty much get what you pay for.  So would you be happier with that one hundred dollar Plossl versus the fifty dollar one?  You would see enough difference in image quality that I believe the answer would be yes.  Spend what you can, but wisely.

In the remainder of this writing I would like to comment on some eyepieces that I own, have used enough to form an opinion, or actually compared to others.  Many of these experiences are the primary foundation for many of the opinions I have expressed.  My first really good eyepiece was actually the first one I ever bought!  It is an Orion 9 mm Ortho in the .965 barrel size.  I thought it might improve the view in my 60 mm department store refractor that so many of us started out with.  Of course the scope did not do it justice, but it certainly did improve the image, big time.  I retired that scope, but kept the eyepiece and later adapted it to my 8 inch reflector.  This eyepiece has about a 45 degree apparent field that is very flat and sharp to the edge.  Resolution and contrast are excellent.  Eye relief is pretty short.  I also own a 12.5 mm University Optics Ortho, and have tested vintage Edmund Scientific 18 and 6 mm Orthos.  All had flat but small fields, sharp right to the edge, and excellent resolution.  The University showed better contrast, I suspect due to significantly better coatings.  I compared the Edmund 18 to my Celestron Ultima 18 and found that resolution was almost too close to call.  The Ultima has a much wider field and maybe slightly better contrast, but edge definition was a bit better in the Ortho.  In another Ortho comparison I put my University 12.5 against a Celestron Ultima 12.5, in a 10 inch DOB with an excellent mirror.  Several other observers and myself agreed that the Ultima had a much wider field of view, and resolution was very comparable.  I think the Ortho may have had a slight edge.  Contrast was considerably better in the Ortho, and the Ortho seemed to have an edge on eye relief.

My least expensive eyepieces are Rini’s.  These are named after the optician that builds them from surplus lenses.  They are built inexpensively, but then they are very inexpensive eyepieces.  I have an 11.4 mm that I do not like and will not mention further, as well as a 21 mm (RKE design I think), and a 35 mm Erfle.  The 21 is quite sharp in the middle of the field but falls off rapidly near the edge.  The edge is hard to see because it is so mushy, but it is wide.  Was it worth $17.50?  Sure.  The Erfle was a real bargain.  It has a bright wide field of around 60 degrees, and pretty good edge definition.  It does suffer from stray light reflecting off the very large eye lens.  I believe this eyepiece was around $25, and it is worth at least twice this in my humble opinion.  I have the Orion Sirius Plossls in 40, 26, and 10 mm sizes.  All are pretty good concerning resolution, and have decent contrast.  They show a fair amount of edge of field aberations, however the 40 mm is much better here, but is restricted to a 43 degree field of view.  The 40 mm is actually my favorite.  The 10 mm has very short eye relief, but a good image if you can get close enough.

Moving up the scale, I own two Celestron Ultimas, the 18 mm and the 30 mm.  Both are very nice, having resolution and contrast very close to Ortho standards.  The 18 mm has a 51 degree field that actually looks a little wider.  Eye relief is slightly short with glasses, but just barely.  This eyepiece seems very sensitive to sky conditions and some nights seems to perform better than others.  This seems especially true at the edge of field.  Sometimes I think this eyepiece is playing games with me.  The 30 mm was purchased fairly recently and has already become one of my favorites.  It throws up a beautiful 50 degree field that you would swear is wider than that.  This may be because of the perfect eye relief and good edge of field resolution.  This is an excellent deep sky eyepiece and also works incredibly well with my Ultima Barlow lens.

I recently borrowed several Vixen Lanthanum eyepieces from a club member for testing.  These eyepieces feature 20 mm eye relief in all focal lengths, and are fully multi-coated.  The field lens is made from lanthanum crown glass to reduce optical aberations.  They are six to eight element designs.  I compared a 25 mm to my 26 mm Plossl, and a 10 mm to my 10 mm Plossl.  Both Lanthanums showed better resolution and contrast, but were a little dimmer.  Edge of field resolution was much better in the Lanthanums.  Due mostly to the Lanthanum’s generous eye relief, these are perhaps the most comfortable eyepieces I have experienced.  I also put the 7 mm Lanthanum against my 7 mm Nagler.  This was a little disturbing.  The much less expensive Lanthanum was right there in resolution and even better in the contrast department.  The Lanthanum has only a 45 degree field compared to the Nagler 82, but you could see all of that 45 degrees with comfort.  Would I have bought the Lanthanum instead?  Probably not, but I don’t want to think about this one too much!  I also compared the 15 mm Lanthanum to my 30 mm Ultima with an Ultima Barlow lens.  This was a very close call, but I finally decided I liked the Ultima combo slightly more.

There is also a series of Lanthanum eyepieces called “Superwides”.  They are much larger and heavier, but achieve a 65 degree field with eight fully multi coated elements, and still retain that 20 mm eye relief.  I own the 22 mm model and it has become probably my favorite eyepiece overall.  It has very good resolution, excellent contrast, and is sharp right to the edge of its beautiful field of view.  I had a chance last summer to compare this eyepiece to a 22 mm Televue Panoptic in a 14 inch reflector under very dark skies.  I had to spend quite some time deciding this one, and enjoyed every minute of it.  The Panoptic showed a very slightly wider field, but the Lanthanum may have been slightly sharper at the edge.  I finally preferred the Lanthanum mostly because it had the more comfortable eye relief.  Also consider these are both expensive eyepieces, but the Lanthanum is at least 80 dollars less.  Obviously I would have to very highly recommend these Superwides.

At that same dark sky sight last summer I also had the pleasure of using the Televue 35 mm Panaoptic, as well as the 12 and 16 mm Naglers.  All are big, heavy, and expensive, but oh wow!  They all have the typical Televue sharpness and very wide fields.  The 35 Panoptic is larger than most finder scopes, but if you have a 2 inch focuser and can handle the weight this eyepiece will impress you, garanteed.  The two Naglers were also optically excellent, but I found the eye relief to be rather short, so it was hard to really take advantage of their 82 degree fields with glasses on.  I also compared the 16 mm to my 16 mm Konig.  This seemed strange switching between equal focal lengths, but the Nagler is about five times bigger.  The Konig was just as sharp right near the middle of the field, and has comparable or better contrast.  But wander even slightly from the middle of the field and the Nagler destroyed it.  This would have been a closer comparison in a longer focal length scope, since the Konig definitely prefers that.  The Konig also has very short eye relief, so you have to almost jump inside it to see it’s 65 degrees.  I have also tried the 32 mm Konig is several scopes and am quite impressed with it.  I once viewed a star cluster with a 2 inch 32 mm Konig in a f/9 refractor and was very impressed.  I sometimes wonder if the 16 and 32 Konigs are really from the same family.

And so ends my rather long commentary on eyepieces.  For those that enjoyed it; it was a pleasure.  For those that couldn’t wait for the merciful end; sorry.  I enjoy using and testing eyepieces and will continue to do so.  So maybe I will reveal further findings and opinions in a future article.  For readers in the before mentioned group A; when I get around to it.  For people in the other group; I warned you...

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