Vixen ED100sf Refractor

INTRODUCTION:
I hope this review is useful to anyone interested in a 4" refractor.

With 3.5 years in this hobby, I consider myself more of an observer than a gearhead. I do most of my observing from my home in Oakland, CA, which means I do mostly planetary and lunar observing. To get away from light pollution, I take a couple of camping trips a year to semi-dark skies.

I started out with an Orion XT4.5, a very competent little dobsonian reflector. I observed every clear night with the little dob, but I was beguiled by all I read about 4 inch apochromatic refractors. They seemed magical, and I had to have one. Always looking for a value, the Synta 100ED line of telescopes (offered by Orion, Sky-Watcher, and Vixen) immediately became my scope of interest. I had read nothing but good reviews online. In June of 2007, a Vixen ED100sf refractor package with Porta mount was on sale for around $1200 and I went for it.

VITAL SPECS:
Vendor - OPT
Price - $1199 (includes alt-az mount, case, finder and diagonal)
Optics - FPL-53 ED doublet
Aperture - 100mm
Focal length - 900mm
F-ratio - f/9
Weight - OTA 8 lbs. with finder and diagonal, Porta mount ~12 lbs.
Mechanical features - Fixed dewshield (can be unscrewed), non-collimatable lens cell, Single speed, non-rotating crayford focuser, 2" visual back, integrated finder dovetail.
Accessories - padded aluminum case, 9x50 straight through finder with dovetail mount, flip-mirror diagonal, rings and dovetail plate.
Mount - Vixen Porta alt-azimuth mount with slow motion controls and accessory tray.

BUILD QUALITY:
The OTA is typical Synta: cost effective and functional. Only the lens cell is machined aluminum. The dew shield and tube appear to be from tube stock, and the focuser is cast. That is fine by me, because it is a light but solid assembly. As far as I know, this OTA is identical to the Orion 100ED except for the color and focus knobs. The white paint is intact after 3 years of use. The rings are low end, but they serve their purpose. I have taken good care of this scope and used it extensively without signs of wear. The included case is made of thin aluminum panels and stands up fine as long as you don't abuse it (keep away from airline baggage handlers).

SETUP AND USE:
I usually leave the scope and mount assembled in my house. The whole setup weighs 20 lbs. and is easy to carry out the door. All setup entails is extending the tripod legs and screwing on the accessory tray. This is as good as grab and go gets. The dovetail is attached to the mount by a main thumbscrew and a backup thumbscrew. This scope won't fit into carry-on luggage, but I don't aspire to be a jet-set astronomer.

My main initial concern was the mount. It was nowhere near as solid as the dob I first used. To be honest, the 100ED f/9 OTA is a bit too long for the Porta mount. The wobbling after aiming and focusing was undoubtedly irksome. However, I have kept the same setup after 3 years. Why? The convenience of grab and go outweigh the irritating vibrations. I'm an apartment dweller, someone who carries everything he needs to go observing. I have made some modifications and concessions to make this combination work. I have added tennis ball vibration suppressors and replaced the small plastic accessory tray with one made out of a pie tin. The view settles down in around 3 seconds after a hard rap to the diagonal. I also use lightweight 1.25" diagonals and eyepieces to minimize demands on the mount.

Refractors, especially doublets, are quick to reach equilibrium with outside temperatures. The lightweight build of the ED100sf makes it cool even faster. With a 30 degree temperature difference in winter, it takes around 15 minutes for tube currents to settle down.

Out in the field, I expect to spend my time observing, not fiddling with gear. This simple setup excels in that regard. An advantage to using light eyepieces (mostly plossls) is there is no need to re-balance the OTA on the mount. The scope points smoothly and slow motion controls on the Porta mount make it easy to track objects at 300x. The crayford focuser arrived smooth and never needed adjustment. Keep in mind I use light eyepieces and do not hang cameras or binoviewers off this focuser. Dual speed focusers are becoming the norm, but the ED100sf really doesn't need one with its depth of focus at f/9. The straight 9x50 finder gathers light well for star hopping in the city. Near zenith, it is hard on the neck and back, so I may switch to a RACI finder. The bulky and unusual flip mirror, although of good quality, has been replaced by a inexpensive but good prism diagonal from Hands On Optics.

OPTICS:
The first questions with apos, or scopes marketed as such, is: how much false color is there? This is a 100mm, f/9 doublet using fpl-53 extra dispersion glass. I do see a faint purple halo around bright stars like Sirius and Vega, also around Jupiter, Mars and Saturn. Does it detract from observing? Not for me. Colors are rendered true enough; if you spend your time looking for planetary detail instead of purple halos, you will be satisfied with these optics. I would not call this scope a true apo, but I'm not a stickler for such things. This scope delivers as you will read in the observing section.

Star test shows even, well defined, circular rings on one side of focus, and mushy, colorful rings on another side. This is typical for ED doublets. A green filter cleans this effect up somewhat. I have also used an Easytester ronchi eyepiece, which shows straight lines, indicating good optics. A tougher test when used with a barlow, the ronchi eyepiece still shows straight lines with little curvature. This gives me reason to believe my optics are better than the diffraction limited 1/4 wave criterion.

The lens coatings are a deep green and evenly applied. The lens cell is not collimatable, but the f/9 focal ratio is more forgiving of miscollimation than, say, f/5 optics. Using a Cheshire eyepiece, I have tried collimating by shimming the focuser, but I don't think it was really necessary. The tube is painted matte black and has 3 baffles. The focuser is also baffled.

The f/9 focal ratio also means it works well with any eyepiece design, including older ones like orthoscopics. The 900mm focal length is a versatile choice. You get a 1.8 degree FOV at 28x with a 32mm plossl, which is just enough to fit the Pleiades and the double cluster. A 4mm eyepiece gives 225x for high power planetary observing.

A minor beef is objects do not "snap" to focus all the time. Some of this is dependent on seeing, but I have to hunt around a little. This detracts none from my enjoyment of the views.

COMPARISONS:
I have two other refractors, a CR150HD achromat and an ST80. Color correction is no contest, and the ED100sf has the least spherical aberration. It is easily the finest optics I own. I have looked through a TV102 and a Tak 78. The moment you handle one of those refined scopes, you understand why they are more expensive and considered standards of excellence. However, I did not immediately put my ED100sf up for sale, because it is a fine telescope in its own right. Others have said the view in a 100ED is 90% of what you get in a premium scope, and I'll go with that.

OBSERVING:

I. Lunar
The moon shows no false color at its limb in this refractor, and it looks marvelous. When seeing is good, it is possible to get lost in the view and feel like you're on a spaceship orbiting the moon. The moon is a high contrast object, which allows you to push magnification in small apertures. I've detected 4 craterlets in Plato in favorable conditions (lunar perigee) and perhaps a 5th. I have been able to find Triesnecker, Hadley, and Birt rilles using the ED100sf.

II. Planetary
Planetary observing is unencumbered by city lights, and it is what I spend the most time on these days. It took a couple of years before I really started seeing the details, so it would be easy for a beginner to dismiss a small scope for planetary observing. Some experienced observers also feel the same way, citing a small aperture's limits in resolving power. I guess I'm neither beginner nor expert, but all I can say is this 100mm scope reveals enough planetary detail to keep me interested for now. I am aware that 4" of aperture limits magnification due to dimming and loss of contrast. Around opposition, Mars displays its major albedo features, polar caps and orographic clouds. Saturn shows its equatorial bands and the Cassini division with ease. The rings are edge on now, so I hope to detect the C ring at a more favorable orientation.

I have endless amusement using the ED100sf on Jupiter and its moons. Shadow transits are easy to see. I have detected larger moons on the disk of the planet, especially near the limb, giving a neat 3-D effect. The GRS was tough at first, but is now easy. I also observed Oval BA and the Wesley impact scar from July of 2009. Sketching Jupiter is a lot of fun in this refractor, because there is plenty of detail to render. I can't wait for the next opposition of Jupiter.

III. DSO
4" is a competent DSO scope in the absence of light pollution. Sadly, there are few of us who know this pleasure without taking a serious road trip. Between my XT 4.5 and my ED100sf, I was able to observe every object in the Messier list in Orange (Bortle 5) skies. I mostly observe in the white zone, and I still hunt for DSO. Open Clusters are where this refractor excels. Showpieces like the double cluster, M41, M6 sparkle with color and starfire. M37 bursts with stars at high power, even in city skies. I have hunted a number of NCG open clusters at home in Oakland, most recently NGCs 1513, 1545 and 2266 in Perseus and Gemini. On showpiece globs like M13, you can crank up the power and enjoy the stellar fireworks display. I did an urban globular hunt in Ophiuchus and nabbed M10, M12 and M14, with M14 being the toughest. Planetary nebula like the Eskimo, M57, M27, and Blinking planetary are possible in the city, though only M57 and M27 show structure. Compared to the view in my 150mm achromat, the 100mm shows most of what the larger scope does, just less brightly. My 6" refractor wowed me with a nice 3-D effect in the Orion nebula. Looking again in the ED100sf, I realized the same effect was there, just not as obvious. Where this scope struggles the most is with galaxies. You need big aperture to bring out structure in spiral arms. However, I can still do some galaxy hunting in the city, detecting M31, M81/82, the black eye galaxy, the Leo Triplet, and others with bright cores like M94 and the spindle galaxy (NGC 3115).

IV. Double star
The ED100sf is an efficient star splitter. It is less susceptible to seeing than my 115mm reflector. On companions with similar magnitude like the double double, Zeta Cancri, Porrima, Pi Aquilae, and Alpha Piscium, this refractor splits at the Dawes limit. Doubles with unequal companions, like Delta Cygni, Izar, and Propus are more of a challenge, but doable. Colorful doubles like Rasalgethi, Cor Caroli, Albireo, and Almach are simply stunning. One of my favorite things to do is to hop to random doubles on my atlas and see what unknown beauties I will discover. I regularly catch the E star in the trapezium and have only seen F under very steady skies.

V. Solar
I have a Baader Astrosolar white light filter to add another dimension to this hobby. The portable setup allows me to have impromptu sunspot observing sessions. The ED optics do a nice job of revealing detail in sunspots during moments of good seeing.

VI. Terrestrial
I don't do terrestrial observation with this scope, although I reckon one could. I prefer a smaller setup like my ST80 on photo tripod for bird watching and such.

Conclusions:
Pros: Simplicity, reliability, ease of use, versatile focal length, portability, nice optics, smooth slow motion controls, top value.

Cons: No frills OTA, marginal mount. Minor false color on bright objects, doesn't always snap to focus.

Would I recommend this scope to someone else? Yes. Is a 4" apo magic? Not in the sense that it will show more detail than larger scopes. You can't deny the laws of physics, but that is not what my review is about. Can you have fun with a 4" refractor? Yes! I have not run out of things to observe, and I am still learning new things. This scope was a great purchase and I plan on using it for a long time. The ED100sf allows me to escape our terrestrial world and revel in the wonders of our universe with ease and grace. To me, that is magical.

Happy observing,

Ging-Li Wang