Vixen
VMC 95L Review
I
looked at the listing and dithered. Even at £1700 (a bargain, less-than-half
price) the year-old Questar field model was just what I wanted, but not what I
wanted to pay. I took a deep breath, closed the browser window and forgot about
it.
So
why was I looking at a super-expensive small Maksutov (that wasn’t even a
proper astro’ model) in the first place? The answer is the Moon. I am collecting phases. It’s a long-term
project to get the best image I can of every day in the Lunar month. I already
have many of them, imaged from home with my 5” and 7” refractors. But
unfortunately I live in a valley with no Western horizon. So I never see early
days in a Lunation; to get them I must climb the local fell and image from its
Western slopes. I need a small, light, high-quality long focal length scope to
do the job; hence the Questar.
So if you can’t afford a Questar, what’s the
next best thing?
I
wondered if the answer might be the Vixen VMC95L, a tiny, light-weight
Cassegrain with some of the Questar’s convenience features like the built in
flip-mirror.
At
A Glance
|
Telescope |
Vixen VMC
95L |
|
Aperture |
95mm |
|
Focal
Length |
1050mm |
|
Focal
Ratio |
F11.1 |
|
Central
Obstruction (incl. holder/baffle) |
~36% |
|
Length |
360 mm |
|
Weight |
1.8 Kg |
Data from Vixen.
Design
and Build
The
VMC is made in China and shares its styling and design with other current
Vixens. Apparent build quality is high, with quality finish and good
mechanicals.
Open
tube and spider: the VMC 95L looks like a classical Cassegrain, but actually
it’s a sub-aperture Maksutov.
Optics
The
VMC appears to be an open design like a classical Cassegrain, with a secondary
supported on a curved-vane spider (the curves reduce diffraction spikes around
stars and planets). However, it’s actually a catadioptric (a combination
mirror/lens telescope like an SCT).
‘VMC’
means Vixen Modified Cassegrain, but really it’s a Maksutov-Cassegrain. To be
precise, it’s a type called a sub-aperture Maksutov which has the thick
meniscus lens just in front of the secondary mirror, rather than across the
whole aperture:
The
advantage of the sub-aperture design is that it reduces weight and (in theory)
cool-down time, whilst providing freedom to correct for coma and field
curvature.
Disadvantages
include the need for a spider (which degrades the image a bit) and the need for
the light to go through the corrector twice, potentially increasing some
aberrations.
Sub-aperture
correctors feature in various current scopes, including some by Orion Optics
and the Tal Klevtsov, as well as a number of other Vixens.
If
the basic design is a bit unconventional, the VMC 95L is quite typical in other
ways. The focal length of 1050mm (F11.1) is short for a Maksutov, but slightly
longer than a typical F10 SCT. The central obstruction is quite large at 36%,
but no larger than most SCTs’.
If
you need to adjust collimation, it’s via the main mirror only: deeply recessed
screws lie behind the oval grommets you can see in the picture. Vixen obviously
don’t intend regular collimation.
Tube
The
OTA is tiny at 36 cm by 10.7 cm (14” x 4.2”) and weighs just 1.8 kg: a little
larger than a Questar, but not much.
Like
Meade with the ETX series, Vixen have tried to copy some of the features of the
Questar control box. In this case, the rear of the scope contains a flip-mirror
arrangement: an internal diagonal that can be switched out of the way to allow
imaging straight through a port on the back.
This
flip-mirror arrangement has the potential to be super-convenient for my needs:
use a wide-field eyepiece in the diagonal as a finder, then flip the lever to
the port with camera attached for imaging. As you can see, Vixen provide a
1.25” visual back for the port, but it threads onto a standard T-ring thread so
direct DSLR attachment is possible.
Vixen
VMC95 has built-in flip mirror.
Focuser
The
VMC has moving mirror focus, same as most SCTs and Maks. Enough focus travel is
provided to allow virtually any eyepiece or camera to come to focus in either
position. What’s more, the focuser is fast, accurate and super-smooth with an
oversize-knob for gloved-hand convenience.
Mounting
Like
many small Chinese scopes these days (the VMC is made in China), the little
Vixen has a fixed (obviously Vixen-type) dovetail without tube rings, but you
can attach it in two positions, 90° apart to allow the VMC to be mounted
sideways on a Vixen Porta mount.
The
VMC 95L was very stable and vibration-free on my Vixen GP mount with the
smallest (~1.5 Kg) counterweight; the whole assembly was easily carried as a
unit.
The
dovetail also has a ¼-20 thread for a photo tripod (and the Little VMC 95L is
easily compact and light enough for a medium sized photo head). Tele Vue’s
TelePod head was also an excellent match.
The VMC 95L is obviously highly portable, but easy
and flexible to mount too.
The
VMC95 makes for great grab-n-go portability on a GP mount.
Accessories
Unlike
a Questar, there is no built-in finder, but Vixen have provided a standard
dovetail shoe and a good RDF to fit.
In
Use – Daytime
I
tried terrestrial photography first and found the VMC reasonable (after an
extended cooling period), but slightly un-sharp.
Branches
(with an out-of-focus daytime Moon behind): false colour is minimal, but
sharpness poor.
In
Use – Astrophotography
I
bought the VMC for a specific imaging purpose, do I thought it was worth
briefly discussing use as an astrograph.
The
VMC was clearly designed for astrophotography. The design gives a wide, flat
field and the flip-mirror control-box is very useful and avoids fumbling around
exchanging eyepieces, diagonal and camera; just what I need when imaging from a
steep, windy hillside. Both ports are close to parfocal – another convenience.
The super-accurate long-travel focuser would also be ideal.
Feature-wise, I found the VMC ideal for
imaging.
However,
on the Moon, the VMC’s intended target in my hands, I was unable to achieve
sharp images, even with careful live-view focusing. To put that in context, I
have obtained far better Lunar images with a 50mm achromatic refractor.
For
applications such as imaging of DSOs that require more modest image scale and
where absolute sharpness is less critical, the VMC might work well, but I
didn’t try this.
The
Moon, imaged through the VMC, is soft and un-sharp. This was the best of many
attempts.
In
Use – The Night Sky
General
Observing Notes
Once
cooled the VMC 95L gave decent, but not great images.
The
VMC shows a tiny bit of CA – at a level similar to a short focal-length triplet
APO - but this is not an issue. It is interesting because it makes the point
that catadioptrics are not inherently completely CA-free in the way pure
reflecting designs (Newtonians and Cassegrains) are.
One
advantage of a long focal length is you only need basic eyepiece designs – I
used Tele Vue Plossls almost exclusively for this test.
The
focuser is very light, smooth and precise, but has some image shift and
worryingly isn’t quite the same going in reverse, so having focused through the
sweet spot, you back up only to find it isn’t there. One other thing about the
focuser is that it is so smooth and light that it’s very easy to lose focus by
accidentally brushing against it.
If
slightly soft optics are one problem with the VMC 95L, stray light is another.
Working around a bright Moon gave very bad ghosting and reflections that flood
the view with background light, destroying contrast. A similar effect was
experienced when viewing near a street-lamp. My guess is that the secondary
needs a larger baffle. Perhaps the OTA could do with proper flocking and even a
larger primary baffle tube as well.
An
interesting comparison on one night was with a borrowed Vixen 102SS APO. The
APO is a similar aperture, but size, price and performance are all of a
different order (the APO was pin-sharp on everything, but more than twice the
size and cost of the VMC).
Cool
Down
The
first thing I noticed was how long the VMC 95 took to cool and how poor the
image whilst it was doing so, with massive astigmatism (so much so that it was
impossible to get consistent focus across the field).
In
contrast, the small APO (a TV-76) I was using alongside was usable almost
straight from a warm room. Even a borrowed Questar cooled much more benignly
and was usable before the VMC. I am not sure why this is. Perhaps the VMC has a
plate-glass mirror? Maybe the thick sub-aperture corrector just takes ages to
cool and loses alignment whilst doing so.
Star
Test
The
star test wasn’t perfect, but the collimation at least looked fine.
The
Moon
The
view of the Moon was good up to about 70x, giving a bright, sharp,
high-contrast view similar to a small APO. But the image became soft at higher
magnifications – radically different from the Questar whose excellent optics
swallow high magnifications with ease.
Jupiter
The
image of Jupiter through the VMC was reasonable at 70x in a 15mm TV Plossl,
with multiple cloud belts visible and apparently good contrast. However, the
image again became noticeably soft at 100x using the 11mm Plossl.
Deep
Sky
The
double-double split, but not with ease I would have expected of a 95mm scope.
The
Pleiades looked good, with pin-point stars showing a fairly flat field with
well-controlled off-axis coma, as promised. The whole cluster fits easily into
the FOV of a 32mm Plossl. I noticed that the wider field conferred by the
shorter focal length was useful compared to the narrower field of the Questar.
Compact
DSOs – The Ring, Dumbbell, Orion Nebula etc - generally looked good in the VMC
at low-medium powers, with the large (for its size) aperture and flat field
working in its favour. In particular, M13 looked brighter and resolved more
deeply than with most small refractors.
The VMC 95L might prove a satisfying compact
deep-sky tool.
Overall in visual use I felt that the optics
of the VMC delivered just-acceptable views that didn’t live up to the fine
apparent build quality and premium brand.
I bought the VMC95 to image the Moon, but it wasn’t
up to the job.
Tempting to regard the VMC95 as a budget Questar;
it isn’t.
Summary
In
many ways the little Vixen VM95 is a nice scope. It’s tiny, light weight and
generally well made. It sports a number of innovative features like the
flip-mirror back that are genuinely useful. What’s more, when you finally get
it to cool down it delivers views that are acceptable (but far from excellent).
However, that brings me to the main problems:
1)
Long cool-down
time. Not only does it take ages but the scope is almost unusable meanwhile and
it spoils the VMC as a grab-and-go. Why
the Vixen suffers from this problem I’m not sure. It could be the thick
meniscus lens in the secondary taking ages to shed heat, or it could be that
they’ve cut corners and used a plate-glass (instead of Pyrex) primary; or both.
2)
Stray light and
ghosting. The Vixen works poorly around a bright Moon (or street-lamps) due to
serious reflections, ghosting and general stray light. Again, I am not sure of
the problem. It may be that the tube needs proper flocking, or it may be in
need of a secondary baffle.
3)
Slightly soft optics.
The VMC is OK for casual visual and imaging use, but its optics aren’t up to
high powers or image scales. Is this an intrinsic problem with the sub-aperture
design where the light has to go through the meniscus twice? Or is it just a
problem with mediocre optical quality in a demanding design?
In
conclusion, The VMC has enormous promise as a tiny astrograph with some clever
and convenient features, but some oversights in the design and fabrication
means it doesn’t live up to that promise. It looks like a budget Questar Field
Model, but it isn’t one.
Despite
some excellent features and good design, the VMC95 is not really recommended
due to soft optics, slow cool-down and stray light issues.