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 wasnít even a proper astroí model) in the first place? The answer is the Moon.† I am collecting phases. Itís 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 5Ē and 7Ē 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 canít afford a Questar, whatís the next best thing?
I wondered if the answer might be the Vixen 95L, a tiny, light-weight Cassegrain with some of the Questarís convenience features like the built in flip-mirror.
I need a small, portable, long-focus scope for imaging the Moon.
Design and Build
Optics Ė ĎVMCí
The OTA is tiny at 36 cm by 10.7 cm (14Ē x 4.2Ē) and weighs just 1.8 kg: a little larger than a Questar, but not much. It 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, itís actually a catadioptric (a combination mirror/lens telescope like an SCT).
Open tube and spider: the VMC 95L looks like a classical Cassegrain, but actually itís a sub-aperture Maksutov.
ĎVMCí means Vixen Modified Cassegrain, but really itís a Maksutov-Cassegrain. To be precise, itís 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 SCTsí. Focusing is by moving the main mirror, again just like an SCT or fast-food small Makí, like a C90.
The VMC has a control box with flip-mirror and straight-through port Ė ideal for imaging.
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 (or viewing) straight through a port on the back. Enough focus travel is provided (conventionally, by moving the main mirror) to allow virtually any eyepiece or camera to come to focus in either position. Whatís more, the focuser is fast, accurate and super-smooth with an oversize-knob for gloved-hand convenience.
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 1.25Ē visual back for the port, but it threads onto a standard T-ring thread so direct DSLR attachment is possible.
Unlike a Questar, there is no built-in finder, but Vixen have provided a standard dovetail shoe and a good RDF to fit.
If you need to adjust collimation, itís via the main mirror only: deeply recessed screws lie behind the oval grommets you can see in the picture. Vixen obviously donít intend regular collimation.
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, 90į 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 ľ-20 thread for a photo tripod (and the Little VMC 95L is easily compact and light enough for a medium sized photo head).
So the VMC 95L is obviously highly portable and easy to mount.
The VMC95 makes for great grab-n-go portability on a GP mount
Like most Vixen gear, the VMC is of high build quality. It has the appearance, fit and finish and mechanical sophistication of a premium product. But of course, appearances can be deceptive.
In Use Ė Visual
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 TV76) 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.
Once cooled the VMC 95L gave decent, but not great images. The star test wasnít perfect, but the collimation looked fine.
The VMC shows a tiny bit of CA Ė 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 Ė 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 isnít quite the same going in reverse, so having focused through the sweet spot, you back up only to find it isnít there. One other thing about the focuser is that it is so smooth and light that itís 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).
The image of Jupiter through the VMC was reasonable at 70x in a 15mm TV Plossl, with multiple cloud belts visible and apparently good congtrast. However, the image became noticeably soft at 100x using the 11mm Plossl.
Like Jupiter, the view of the Moon was good up to about 70x, giving a bright, sharp, high-contrast view similar to a small APO. But again the image went soft at higher magnifications Ė radically different from the Questar whose excellent optics swallow high magnifications with ease.
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 Ė 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. So 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 didnít live up to the fine apparent build quality and premium brand.
In Use - Photography
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 astro-photography. 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 Ė another convenience. The super-accurate long-travel focuser would also be ideal Ö if only the optics were a bit better.
Feature-wise, I found the VMC ideal for imaging.
I tried terrestrial photography first and found the VMC quite reasonable (but only after an extended cooling period):
Branches with an out-of-focus daytime Moon behind.
On the Moon, the VMCís intended target in my hands, I was unable to achieve really sharp images, even with careful live-view focusing:
The Moon, imaged through the VMC, is slightly soft.
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 didnít try this.
So again, the VMC fails to deliver on its promise of premium imaging performance.
In many ways the little Vixen VM95 is a nice scope. Itís tiny, light weight and generally well made. It sports a number of innovative features like the flip-mirror back that are genuinely useful. Whatís 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 Iím not sure. It could be the thick meniscus lens in the secondary taking ages to shed heat, or it could be that theyíve 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 arenít 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?
Itís tempting to see the VMC 95L as a budget Questar field model; it isnít.
So 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 doesnít live up to that promise. It looks like a budget Questar Field Model, but it isnít one.
Not really recommended.