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Planetary Eyepieces

Two high-quality, expensive eyepieces on opposite sides of the “gumsight” argument: a Tele Vue Ethos and a Pentax SMC Orthoscopic. In fact, both give great planetary views.

During the Korean War, a group of U.S. fighter pilots, flying the then newly-minted FS86 Sabre, chose to rip out the sophisticated gun-sight Lockheed had provided and replace it with a strip of gum stuck to the windshield. These pilots were known as the “gumsight brigade” and they believed their piece of gum worked better than Lockheed’s lenses and prisms.

A similar mutiny happened in amateur astronomy about a decade back, with many people coming to regard the type of complex, multi-element eyepiece pioneered by Al Nagler as inferior in sharpness and contrast to simpler designs such as the Orthoscopic, Plossl and Kellner. Their rationale was that all those lenses and air-to-glass surfaces scatter and absorb light, preventing the viewer from seeing critical low-contrast planetary detail.

This seems like a compelling argument. For one thing it is an attractive rejection of the consumerist values overtaking this hobby. For another, fine planetary detail is certainly hard to see, so anything that helps should be good. It should have been a money-saver, too, but unfortunately some of the eyepieces now deemed “planet-worthy” – Zeiss Abbe Orthoscopics, TMB Monocentrics and Pentax Ortho’s and others – have become rare and so expensive too.

In this revised version of a ten-year-old Scope Views article I’ll give you my experiences and I’ll gradually add sections on individual “planetary” eyepieces as I test and review them.

My Experiences

Over the years I’ve owned most of the well-known planetary eyepiece types (with the exception of the range Astro Physics briefly offered some fifteen years ago and which I should have bought when I had the chance, along with Bitcoin and shares in Apple).

Planetary viewing is my thing. I have spent countless hours peering at planets through a range of high-end refractors from the likes of Astro-Physics, Takahashi and LZOS/TMB. Add to the refractor roll-call reflectors including, a 1/10th wave 8 inch Newtonian, a 1/10th wave 5.5 inch Maksutov, a Takahashi Mewlon 180 and an 1/8th wave 12 inch Dall-Kirkham and you have pretty much all the types of planetary ‘scope out there.

Perhaps I should also add that old and ugly I may be, but I can still read the bottom line on the eye-chart with ease, so I’m not blind yet...

Over the years I have done side-by side tests between the “standard” premium multi-element eyepieces, such as Tele Vue’s Ethos and Naglers and Pentax XLs and XWs with the following simpler types that are widely regarded as “planetary” eyepieces:

·        Pentax SMC Orthoscopics

·        Pentax XOs

·        Tahashashi 0.965” Orthoscopics (I haven’t tried the more recent Tak’ Abbe Orthoscopics)

·        Takahashi Hi-Orthos

·        Circle-T (UO) Orthoscopics

·        TeleVue Plossls

·        Zeiss Abbe Orthoscopics

·        TMB Monocentrics

Tele Vue Plossls are an excellent, underrated choice for planets.

Zeiss’ Abbe Orthoscopics are widely considered among the very best for planets.

What I found was that I could not, reliably and definitively, prove to myself that I ever saw planetary or Lunar detail with one type that I just couldn’t with another.

Meanwhile I have found only one type to be reliably a little sharper, a little higher of contrast, somehow a little clearer, than the rest; that eyepiece is the TMB Monocentric.

There were differences between the others, to be sure, which I will sum-up as follows:

·        I believe the T6 Naglers may be very slightly dimmer, yellower in tone and less sharp on-axis than some of the others (but sharper off-axis right to the edge).  These would not be my first choice for planets, but paradoxically my best ever views of Jupiter and Mars were with T6 Naglers, due to seeing and circumstance: Naglers are useful for planets in Dob’s, due to their wide flat field.

·        You might expect that the same comments would apply to the Ethos, but in fact I’ve found the Ethos (I’ve tried the 13mm, 8mm and 6mm) to be really excellent : very sharp and contrasty on-axis with little scatter and a very cool tone.

·        As good as any and better than most are a combination of long focal length TeleVue Plossl with a 5x Powermate. This combination has great eye relief and excellent sharpness and contrast with a sensible field of view. I used this combination for a lot of critical planetary viewing with my TMB 175.

·        The Nagler Zoom is an excellent planetary choice, which I use a lot because it’s so convenient. I’ve now owned three 3-6 zooms and one 2-4. Early examples had ghosting problems, but later ones don’t.

·        The best of the bunch in terms of Orthoscopics are (unsurprisingly) Zeiss’ Abbe Ortho’s, with Pentax SMC Ortho’s and Takahashi’s not far behind.

·        The dimmest of the lot were the Circle-T Orthoscopics, probably due to more basic coatings. They were no sharper or more contrasty than the others (but not really less so either and so are a great budget choice).

·        As I said, only one eyepiece has convinced me it’s truly and repeatably a little better than any other – the TMB Mono’.

 

Overall, apart from field of view and comfort, the differences between types are small, much less than moment-to-moment variations in seeing that make planetary detail fade in and out; much less than the difference made by ensuring your eyepieces are properly clean; much less than due to fatigue induced by tiny exit lenses and zero eye-relief; much, much less than the difference between a good telescope and a mediocre one, or between a wobbly mount with poor drives and a rock-solid one which holds the planet in centre field for an hour at a time.

 

So where does this leave those reports of vastly superior planetary detail that we’ve all read? Read on for some theory.

 

The Wobbly Stack

What you see is a pretty subjective thing anyhow, but let’s look at some of the theory behind it.

In Star Testing Astronomical Telescopes, Richard Suiter describes the telescope system as like a wobbly stack of filters which at each level takes away a bit more contrast; tip it too far and the whole thing falls over. Items on the stack include:

1)     Aperture

2)     Seeing (not transparency, but the level of atmospheric disturbance which distorts the image moment to moment)

3)     Quality of the primary optics

4)     Central obstruction size

5)     Alignment of the optics

6)     The diagonal (mirrors scatter much more than lenses)

7)     The ability of the focuser to deliver critical fine focus

8)     The eyepiece

9)     The skill and fatigue level of the observer and their eyes

 

Ignoring aperture and seeing for a moment, the main telescope optics, particularly optical quality, central obstruction and collimation are the most important. Viewing Jupiter with a well collimated, new Meade 8” SCT side-by-side with my 8” 1/10th PV Newtonian, the differences were very considerable, with the Newt’ delivering a lot more contrast, sharpness and detail with the same eyepiece.

 

I do believe that to really assess planetary eyepieces you need complete confidence in the primary optics and would probably choose a long-focal-length refractor or Newtonian of the highest optical quality with a precise micro-focuser.

 

But this is emphatically not the same as saying that other telescope types – pure Cassegrains and catadioptrics – are necessarily worse in absolute terms for seeing planetary detail.

 

In fact, assuming the optics are serviceable and properly aligned and focused, aperture and atmosphere are ultimately the most important items on the stack for the best views of all, regardless of eyepiece. Consider:

 

My best planetary views ever, by a simply enormous margin, were with the 60” Hale Cassegrain at Mount Wilson. Those views were mostly through a 55mm Tele Vue Plossl, but honestly the cheapest functioning eyepiece would have delivered far more detail than the even the very best types in the very best of the rest telescopes. Why? Large aperture and a night of seeing that was exceptional even by Mount Wilson’s world-class standards (likely well below 0.5 arcsecs).

 

The second best planetary views I’ve had to date were with an even more surprising instrument – a Celestron C11. Again, the eyepieces used were basic Plossls. But what made the huge difference in this case was the seeing - that C11 was set up at the summit of Mauna Kea which boasts some of the clearest, steadiest seeing on Earth (Mauna Kea has an average of 0.45 arcsecs).

 

Aperture and seeing win: no eyepiece in your scope will give you views like this monster does with a basic Plossl.

 

Other Factors

So, to get the best views, you’d choose a quality scope with a large aperture and a site with the best seeing (light pollution and transparency are much less important) over any particular eyepiece or telescope type.

 

If seeing is mediocre, my experience is that long focus telescopes with a small (or no) central obstruction perform best and make more difference than specialist eyepieces. This really matters. I used to think that cassegrains and catadioptrics with large obstruction were just useless for planets. They’re not (see above)! I’ve had excellent views with them, but only in exceptional seeing.

 

One way of ensuring you get the best from your eyepieces, of whatever type, is to keep the eye lens scrupulously clean. Even a bit of grease and crud can really degrade their planetary performance.

 

Consider ditching the diagonal and viewing straight through. There has been argument over which type of diagonal is best – single-coated mirror, dielectric mirror (or indeed prism). But in all cases, dusty mirrors and prism surfaces scatter much more light than a couple of extra FMC lenses protected inside an eyepiece.

 

An Example Session Testing Planetary Eyepieces

At the very end of the 2021 Mars opposition, I had an opportunity to do some extended testing of three of the most famous planetary eyepieces under near ideal conditions:

·        Zeiss’ Abbe Orthoscopic 10mm and 6mm with the matching 2x barlow

·        A 5mm TMB Monocentric

·        A 3-6mm Tele Vue Nagler Zoom

 

Mars was small, but just large enough to be able to tease out some low-contrast detail. The telescope was an absolute planetary classic – an Astro-Physics 130 EDT. The EDT was so over-mounted on an AP1200 (a mount capable of hefting almost ten times as much) that vibes were almost non-existent, even when drumming on the tube at 300x! For visual use, tracking was perfect and the conditions in my dome comfortable.

Meanwhile, the conditions were clear and stable but freezing. Mars was just 8.3” apparent diameter, with the dark albedo region of Mare Cimmerium clearly visible as a dark stripe centre planet.

Having tried various powers, I settled on an intermediate magnification, of 217x with 5mm Eyepieces, to run the test.

Over the course of a couple of hours, I swapped repeatedly between a 5mm Nagler Zoom, a 5mm TMB Mono’ and a 10mm Zeiss Abbe in the 2x barlow. I couldn’t convince myself of any major differences between these fine planetary eyepieces, beyond differing FOVs, but I did note that view through the Mono’ seemed consistently a touch sharper and higher contrast.

On another night of slow, steady seeing in late winter 2021, with Mars at 35° altitude and just 6.5” in size, I upped the power trying to see some albedo detail. Swapping between 5mm eyepieces giving 217x, I again noticed the Monocentric gave a slightly crisper image than the Nagler Zoom and perhaps even crisper than the ZAO (but the difference was tiny if so).

Mars at that power was a clean gibbous disk but too small to identify individual albedo markings, so I had to go to a higher power still, with just the 3mm setting on the Nagler zoom and the barlowed 6mm ZAO both giving an extreme 362x. Even at that size, Mars surprised as a fairly well defined, ochre gibbous world with Syrtis Major centre planet and unmistakeable in steady moments.

Overall, I slightly preferred the image with a 6mm ZAO + Zeiss 2x barlow to the Nagler zoom. But again, the differences were very minor and needed repeated swapping to confirm.

Summary

 

All in all, I think many of the “gumsight” claims about simple eyepieces for planetary detail come about due to a combination of wishful thinking, rapid changes in seeing and perhaps dirty eyelenses. All the premium eyepieces – complex and simple - that I have discussed are of high optical quality and work well for planets; contrast differences between them are really very small.  If you expect to replace your Naglers with Orthoscopics and have a transformed view of the planets, I think you will be very disappointed.

 

If you live on a mountaintop in Arizona and have a dedicated observatory with a ten inch, eighth wave APO on a big mount, maybe spending a few thousands more on specialist planetary eyepieces makes sense (you’ll already have spent the price of a house on your rig anyway, so what the heck?). If you observe with a Celestron 8 and you live in Yorkshire, not so much.

 

For dedicated planetary use, you might as well go for a simpler design, like an Orthoscopic from a good make. Just don’t expect a radical upgrade in the view. If you already own Naglers (Ethos, Delos, XLs etc), just use those and keep ‘em clean. Spend the extra on upgrading your ‘scope or mount (or a move to a desert summit!) first.