Takahashi
Mewlon 180C Review
Takahashi s Mewlon
range has been around for many years. The larger sizes 250mm and 300mm (10
and 12 ) are now frighteningly expensive, but this 180mm, the smallest of the
range (there is an in-between 210mm as well), is a bit of a Takahashi bargain,
especially when you consider it includes all the mounting hardware and a nice
finder that would cost extra on a typical Tak
refractor. What s more, they ve recently revamped it, so I thought it was high
time I tried one and reported back.
So
what then is a Mewlon and why might you want one?
The management overview is that the Mewlon is an affordable planetary reflector - lighter,
quicker cooling and possibly sharper than the SCTs and Maks
you get from Celestron, Meade and Skywatcher.
I ll
cover the details in the optics section below.
At
A Glance
|
Telescope |
Dall Kirkham Cassegrain |
|
Aperture |
180mm |
|
Focal Length |
2160mm |
|
Focal Ratio |
F12 |
|
Central Obstruction (incl. holder/baffles) |
33% |
|
Length |
625mm |
|
Weight |
6.2 Kg |
Data from Takahashi.
What s
in the Box?
The Mewlon came in one of Takahashi s nested boxes:
I do like a
good Takahashi unboxing!
Design
and Build
The Mewlon
series used to be a family of four optically near-identical planetary
reflectors, but nowadays Tak have turned the two larger
models (u-250 and u-300) into imaging scopes with sub-aperture correctors.
However, the two smaller models, including this one, remain pure reflectors
aimed at high-power visual use and those looking to image smaller objects.
Similar Takahashi quality complete
with the beautiful castings Tak s specialise in - can
be expected throughout the Mewlon range, but the
u-250 does get some extra refinements in line with its high price, such as an
electric focuser and removable back with special vented cell for fast cooling.
The u-250 and u-300 are very large and
heavy scopes, whereas this 180 is quite small and very portable.
Mewlons
compared. L to R: u-180C, u-210, u-250.
Optics
The Mewlon
range use an optical design known as a Dall-Kirkham. A quick diversion into
some optical theory to explain this is in order first. As usual, if you know
this stuff, just skip it.
Most amateur telescopes with mirrors
fall into two basic camps. There are Newtonian reflectors that mostly turn up
mounted as Dobsonians; these contain just (two)
mirrors. Newtonians are cheap, sharp, quick cooling and generally excellent.
Trouble is, their eyepiece is in an awkward position at the top of a long tube,
making them hard to use when mounted equatorially.
Then there are catadioptrics
Schmidt and Maksutov Cassegrains
from the likes of Meade and Celestron. These have a
combination of mirrors and a glass corrector plate at the front (or in some
variants, just in front of the secondary mirror). They are much more compact
and have the eyepiece in the most useful place, but are slow to cool and can be
compromised optically. Larger Maks especially are
typically expensive and heavy; their thick corrector means they often
just never cool properly.
At one time, most professional
telescopes were a third variant you rarely see today the Classical Cassegrain
which gave super-sharp views and a flat field, albeit at long focal ratios.
The Classical Cassegrain has just two mirrors like a Newtonian, but whilst the
primary is (again, like a Newt ) parabolic, the secondary is hyperbolic
supposedly a difficult curve to grind into a mirror. Consequently, there have
been few Classical Cassegrains made for amateurs (Tak made one of the few), whilst the pro s
have moved on to designs that give shorter focal ratios, like the
Ritchey-Chretien.
The Mewlon is
a relative of the Classical Cassegrain called the Dall-Kirkham. Like the
Classical, it has only mirrors, but the complex parabola/hyperbola is replaced with a simpler (i.e. cheaper) ellipse/sphere.
This gives the DK Mewlon
excellent performance on axis (i.e. for small DSOs and planets), but much worse
off-axis aberrations than the Classical for wide field imaging.
So, what
advantage does the Mewlon have over the usual SCTs
and Maks? Well for a start, no corrector means it s lighter weight. Deleting the corrector scatters less
light and adds fewer optical surfaces to get out of alignment too. Best of all
is that the endless cool-down of larger SCTs (and especially Maksutovs) is cured because this is an open-tube like a
Dob .
In comparison
with most SCTs, the DK has a longer (F3) primary mirror that should (and in my
experience does) make collimation easier, but that leads to a longer tube for
the aperture.
Finally,
how is this new version of the Mewlon 180 different
from the former? Optically, the big difference is in the mirror coatings, which
now have multi-layers on both surfaces for a 7% system improvement in light
throughput. Takahashi refer to these as, HR Multi-Coated .
I should
also point out here that Takahashi finish the mirror polishing in-house and
each pair is a matching set. Bench tests suggest Mewlons
have excellent optics and this one certainly does, as we will see.
The 60mm (30%) central obstruction
is increased slightly to 33% by the baffles, but that is still less than most mass-produced SCTs,
which typically have 35-37%. This is important for contrast and for its planetary and lunar potential,
also for performance in poor seeing.
Baffle tube contains some 10 knife-edge baffles. Secondary is 60mm just 30%
by width, less than most SCTs and Mak s.
Tube
The tube is
seamed rolled steel, but unlike most the join is welded not folded and has been
carefully finished externally to hide the seam. The front of the tube is
elegantly rolled over. Everything is perfectly blacked inside.
Despite the longer-than-SCT
F3 primary, the tube on the u-180C is reasonably short, possibly short enough
to be carry-on portable with the visual back (and maybe focuser knob too)
removed.
The rear is a
superb one-piece Takahashi casting. Whereas on the u-250 this can be removed
for cooling to reveal a vented cell, on this u-180 it is fixed.
The Mewlon 180 has a thin, three-vane spider, better for
contrast than thick vanes. The secondary holder is fronted by a logoed blanking
plate - nicely machined out of metal and threaded on.
Takahashi make
a marketing point of the baffle tube containing 10 separate knife-edge baffles
for contrast and stray light protection. The baffle tube is indeed filled with
carefully blackened ridge-baffles and the secondary is also shrouded by a deep
baffle. I ve seen under-baffled Cassegrains that are horribly
sensitive to stray light, so this is a vital part of the Mewlon s
notable contrast.
All the Mewlons have their finders cleverly mounted into a cast handle
that attaches solidly to the cast back-plate. This makes the OTA easier to
mount, but also makes for a super-stable finder mount that never loses
alignment.
The tube is
covered by a unique vinyl cover with a Velcro tightening strap.
Focuser, 2
visual back and typical Tak 1.25 adapter.
Detail of unique
(to Mewlons) 6x30 finder-cum-handle.
Focuser
Like almost
all Cassegrains, this Mewlon
180-C focuses by moving the main mirror, controlled by a substantial knob that
protrudes from the visual back (which stays fixed). This gives the Mewlon an enormous range of focus enough to accommodate
straight-through viewing, a 2 diagonal or a DSLR without extension tubes. The
mechanism is smooth and accurate too.
The downside
of moving-mirror focus is some image shift when changing focus direction. The
larger Mewlons avoid this with a different focus
mechanism which moves only the secondary via an electric motor and handset.
This
version of the u-180 has a 2 visual back; the old was limited to 1.25 . This makes a substantial difference. In fact, I d
argue that 2 eyepieces are much more necessary here than in a small refractor.
Why? Because otherwise a focal length over 2m means a maximum field of view of
just 0.72 , seriously limiting the scope for deep sky.
But now,
with the largest 2 eyepiece, a 55mm Plossl, the Mewlon has a maximum FOV of 1.2 - just enough to fit in the whole Pleiades open
cluster. In case the improvement from 0.72 to 1.2 doesn t sound like much, check out the difference
on the sky in the simulated image below.
Mounting
The u-180C
is reasonably light weight at just over 6Kg, thanks mainly to not having a
corrector plate. However, that still makes it push the ~7Kg weight limit for
the smallest equatorials and the 180C would work best on a mid-size mount like
an HEQ5 or similar (or, of course, Takahashi s own EM-11 or EM-200).
The Mewlon range all have a dovetail plate attached to a
casting underneath the tube. This avoids the need for tube rings. The dovetail
plate is intended for Tak s own mounts, but has the
same chamfer-angle as the Vixen-compatible ones found on most small mounts. I
have seen a heavy-duty after-market Losmandy D
dovetail attached to a u-250, but that shouldn t be necessary for the u-180C.
Dovetail is Tak s own, but generally Vixen compatible.
Vixen SX2 handles
the Mewlon 180C s 6 Kg weight comfortably.
Mewlon 180C and
refractor of similar aperture (TMB 175).
Accessories
Takahashi
make a version of their fine rack-and-pinion focuser that attaches in place of
the standard visual back. This gets around the image shift, but would only be strictly
necessary for serious imaging.
Also for imaging,
Takahashi make a 0.8x reducer for the Mewlon that
flattens the field and shortens the focal ratio from F12 to F9.6 (1730mm). By Tak reducer standards, the Mewlon
reducer is good value too.
In
Use Astrophotography
The 2 visual
back on this new version means that a full-frame DSLR can image the whole Moon.
And it s not a bad result either, if not as good as a fine small apochromat.
The problem seems to be that things get progressively less sharp toward the
field edge (the Moon takes up most of the field vertically), in line with the
Dall-Kirkham s promised off-axis coma.
The focuser
is smooth and accurate, the optics snappy, but reversing tends to create some
image shift and a slightly different focus point.
One big
advantage of the focuser is huge travel and you can stick a DSLR straight in
the back, with no need for wobbly extensions. Again, I am reminded that the 2
VB on this new version is not a trivial upgrade.
The sub of the
Crab Nebula (M1) with an APS-C sensor shows classic off-axis coma. With a
full-frame DSLR, this sub of the Eskimo Nebula shows just a bit more. Brighter
stars are starting to show attractive spider-vane spikes.
As usual,
these have been reduced in size, but not cropped, stacked or processed in any
way.
M1: Fuji
X-Trans APS-C, 110s ISO 6400.
Eskimo
Nebula: Canon EOS 6D MkII Full Frame, 120s ISO 3200.
Moon: Canon
EOS 6D MkII Full Frame, 1/200 ISO 400. Slightly
increased contrast.
In
Use The Night Sky
Collimation
My first
looks at the Moon with the Mewlon were disappointing
slightly soft and without the clarity and snap I was expecting. Then I reminded
myself, this is a reflector, Roger: you remember, those things that need
collimating . Ah yes ... collimation, my favourite job.
The manual
says that collimation is set at the factory and should survive shipping; well,
not in this case. A quick star test showed it was off.
To collimate
the Mewlon, you unscrew the blanking plate on the
secondary housing to reveal three Allen bolts (much better than the screws on
an SCT). I attached the Allen wrench (supplied by Tak )
to a piece of cord and looped it around my wrist much too easy to fumble and
drop it on the primary mirror otherwise (no SCT or Mak
plate in the way).
A few tweaks
on the bolts - tightening the one(s) on the side where the diffraction rings
are compressed and slightly loosening the opposite to compensate got a nice
round diffraction pattern and tightened the in-focus diffraction pattern too.
After
collimation, the Moon snapped into focus and was sharp and full of detail and
contrast. Ready to start observing (and not as painful as past collimation
experiences)!
Removing the
secondary cover reveals collimating hex-bolts: don t drop it!
General
Observing Notes
No corrector
plate and well-recessed mirrors mean the Mewlon is
resistant to dewing. One dewy morning, the refractor alongside was hazing up
whilst the u-180c remained clear.
One
surprisingly fantastic thing about the Mewlon
(really!) is the finder. Tak finders are always
good, but this one is just superb very sharp with a wide field and loads of
eye relief. But the best thing is that holder. It s super strong and stable
because it s also a handle. But that means you set alignment once and it s
always perfect.
Being able to
rely on the finder putting things bang in the centre of a high-mag eyepiece is
a great feature and makes getting around a doddle with the Mewlon
(so much easier than a small refractor I was using alongside with a dodgy RDF).
Like all
reflectors with larger obstructions, the Mewlon does
suffer in bad seeing compared to a small refractor a theme that keeps
cropping up on this website for those in northerly climes with a crummy thick
and wet atmosphere above them.
Cool
Down
When I m testing small refractors, this
section is just this one line. Not now.
If you are used to refractors, the Mewlon s terrible view and boiling tube currents during
cool-down are off putting. You wait and wait. After 45 minutes from a warm
room, there s still terrible spherical aberration and a huge plume in the star
test, but at least at lower powers the view is good now. Wait some more.
After almost an hour, the Mewlon will finally just split Rigel, but spherical
aberration and plumes are still writ large in the star test. Maybe the Mewlon does cool faster than an SCT or Maksutov,
but it s much slower than, say, a 4 doublet refractor like an FC-100D. And the weather here
doesn t allow leaving a scope cooling for hours unattended (I m worried about showers right now - the Mewlon is out on its own, like an unsupervised child, as
I m writing).
An hour and twenty in and we re still
not there. I get the FC-60 out for a quick look while I m waiting; it gives a
good view in minutes. This is the ground truth of astronomy in cold climes.
After an hour and a half, the Mewlon s spherical aberration has almost gone at last and
the star test finally looks normal.
This larger Mewlon
250 has a vented cell and removable cover to aid cooling. The smaller models
lack this feature.
Star
Test
The star test
shows spherical aberration during cool-down, but thereafter it s just perfect
identical, sharply-defined and evenly illuminated diffraction rings either side
of focus. It s a Takahashi after all, even if, unusually for me, one with
mirrors.
The
Moon
The Mewlon has a better field of view than I was expecting. The
whole Moon fits into the field of a 32mm Plossl at
68x, but it s painfully bright after a run of small refractors and you need a
filter or much higher powers to kill that brilliance.
A gibbous
Moon showed lots of crisp detail, even in seeing that was moderate at best. I
was able to spot three craterlets in Plato and a rille or two in Gassendi without difficulty. The Moon has a wonderful
whiteness if you re used to refractors or SCTs zero false colour with its
all-mirror system.
Using the Mewlon on top of my 7 LZOS refractor and comparing their
view of a 3-day Moon, I notice that the Mewlon
suffers a bit worse in the poor seeing and scatters a bit more light from the
limb, but that it also delivers a noticeably whiter tone to the brilliant Lunar
highlands.
On a
different night of decent seeing, I used a 15mm Panoptic giving 144x to explore
the extended and difficult Sirsalis rille and look
for more rilles and craterlets in Gassendi.
At its best like this, the Mewlon gives a 5
apochromat level of lunar detail. At 240x with a 9mm Nagler,
it s a still great view very detailed and involving and sharp (once it s
cooled).
In good
seeing, the Mewlon will reveal most of the features
in this Moon atlas.
Planets
Once cooled and on nights of better seeing, I had some excellent views
of both Jupiter and an opposition-year Mars (albeit low in the sky).
I found that simple eyepieces worked best for planets with the Mewlon and my favourite views were with Zeiss Abbe Orthos, particularly the 10mm giving 216x.
The huge focus travel meant I could view a low Mars with the eyepiece
straight in the visual back (no diagonal). Mars then showed some significant
albedo detail like Syrtis Major and Mare Acidalium, possibly some limb cloud too. The view was
similar to an apochromat in the 110-130mm range, albeit with more unfocussed
light from mirror scatter and spider diffraction.
In good seeing, I had fine views of Jupiter too multiple cloud belts
and dark embedded storms.
Uranus showed up as its usual dim bluish disk noticeably a bit more
in need of averted vision than through a similar-aperture refractor (my TMB
175).
Deep
Sky
With a 32mm Plossl giving 68x and the maximum 0.72 field of view for a 1.25 eyepiece, the Orion
Nebula (M42) fits nicely in the field. There are knots in the nebulosity and
the extended arms, even a hint of colour: much more nebular structure than you get with a small refractor.
Even with a full Moon in the Hyades nearby, structure is visible in the bright core
around the Trapezium. Those ten baffles certainly deliver good contrast and protection
from stray light is excellent.
The 180C is
particularly suited to smaller DSOs and I had some exceptional views of the
Crab, the Ring and the Eskimo. That superb contrast is really noticeable here.
Trying to
view a larger DSO, like the Pleiades open cluster, you realise the limitations
of a 2m focal length: just a couple of the Seven Sisters fit in that maximum 0.72 FOV of a 32mm Plossl. Then
I remember this isn t an old Mewlon, this one will
take a 2 diagonal! With a 55mm Plossl there s
doubtless some vignetting, but all Seven Sisters now just squeeze into
the field of view, sparkling nicely amid silvery nebulosity that is much more
noticeable than through a small scope.
I am
surprised that stars don t distort that much at the field edge, despite the
DK s reputation for off-axis aberrations. Stars remain points until 70-80%
field width and even after that just become mildly comatic. I believe Tak may use an outsized mirror to help. Stars retain lovely
true colours too (zero false colour remember).
The Mewlon splits doubles well and Rigel s companion showed up
at 144x with a 15mm Panoptic, as clear and bright as I ve ever seen it, even
with the full Moon close by.
Mewlon 180C maximum
FOV with 1.25 and 2 eyepieces compared on the Pleiades cluster.
Summary
The Mewlon is mostly good and its negative points are simply
those suffered by all reflectors.
Optical and
mechanical quality are outstanding as you would expect from Takahashi (this one
has a virtually perfect star test).
For its size
and weight, it offers excellent lunar and planetary views at the level of a
bigger and more expensive apochromat. The field isn t nearly as bad off-axis as
I was expecting either and with the 2 visual back you can enjoy lower powers
and bigger DSOs, or the whole Moon. What s more the complete absence of chromatic
aberration and with no thick lenses to traverse, colours are delivered with
intense purity.
Even for
imaging, the field is surprisingly good for smaller sensors. The Mewlon would make a great tool for imaging smaller DSOs.
The stellar spikes delivered by the spider (that you don t get with an SCT)
look great in images with the brightest stars.
The
mirror-only optics deliver bright views and are much more resistant to dewing
up than an SCT or Mak .
Compared with
a Takahashi refractor of similar capability, the u-180C is something of a
bargain too.
As with all Cassegrains, the Mewlon has three
less positive aspects if you are used to small refractors:
1) In terms of size it could be a quick-look scope, but
the lengthy (hour plus on cold nights) cool-down counts against this in cold
climates
2) In poor seeing it suffers compared with smaller
refractors
3) Like all reflectors, the Mewlon
may need periodic collimation, especially if you travel with it. But
collimation isn t hard to do on a star at high power
One final
niggle is the focuser. It s buttery smooth and has masses of travel, which is
very convenient (no extensions required for DSLR imaging). But there is some
image shift, like most moving mirror focusers.
The Mewlon
180C makes a truly excellent and surprisingly multi-purpose alternative to a
mid-sized apochromat: for the Moon and planets, but for smaller DSOs too. The
brighter optics and 2 visual back make it much better for extended objects
than the old version. Highly recommended.
