Planetary
Eyepieces
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 updates as I continue to test and review
“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.
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, various high-quality mirror scopes and catadioptrics
too.
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 Plössls
·
Zeiss Abbe
Orthoscopics
·
TMB Monocentrics
·
Tele Vue Nagler Zooms 3-6mm and 2-4mm
Tele
Vue Plossls are an excellent, underrated choice for planets.
Zeiss’
Abbe Orthoscopics are widely considered among the very best for planets.
Cheaper
and more obtainable than the ZAOs are Takahashi’s classic MC Ortho’s and
Hi-Ortho’s.
After
lots of swapping and comparing I have so far found only two types to be
definitively a little sharper, a little higher of contrast, somehow a little
clearer, than the rest: Zeiss’ Abbe Orthos and TMB Monocentrics.
I
have had stand-out views with Pentax Orthos, Pentax
XOs and Takahashi Hi-Orthos: all seem slightly more contrasty and sharp, with less flare than other types,
but I’ve never proved this to myself consistently.
However,
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.
There
were differences, to be sure:
·
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 only to a very minor extent. BUT, no it’s
consistently not quite as good as the ZAOs and Mono’s.
·
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).
·
I believe 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.
·
A combination of
long focal length TeleVue Plossl
with a 5x Powermate work well: great eye relief, 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.
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 Plössl.
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.
My conclusion: yes, the ZAOs and Monocentric did
slightly improve contrast and sharpness, but not by much.
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. Enjoy
planetary eyepieces (I do), but set your expectations.
For dedicated
planetary use, the finest planetary eyepieces do make a small difference. 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.