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.