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Review of the Orion by Vixen 102 Telescope OTA

Posted by Ed Moreno   06/30/2004 07:00AM

Orion by Vixen 102mm Achromat (Discontinued)
By: Ed Moreno ([email protected])

This is a review of the Orion by Vixen 102mm f/10 Achromatic Refracting Telescope. The subject telescope in this review was acquired used. The OTA as received was in essentially new condition, and included the finder and stalk, original rings, lens cap, and CG5 style dovetail.

4” refractors are generally thought of as great all-around small scopes. In general terms, bigger scopes are to my thinking, better scopes. Still, there is a certain aesthetic to the view provided through 4” refractors that gives them a special allure, in spite of their generally low overall light grasp as compared to the majority of scopes on the market today.

I think that there are several factors that contribute to this allure. First is elemental simplicity. While a high quality 4” refractor is probably far more demanding to manufacture than even an 8” dob due to the number of optical surfaces that must be ground and polished to an exacting standard, the IMPRESSION is that they are imbued of elegant simplicity, and this is evident in their purist appeal and the current zest for APOs costing multiple thousands of dollars. Many I have talked to over the years say that they just think a small refractor seems simpler to use than other scopes, and while I don’t totally agree, I would imagine that because the design itself is small and simple, people come to think that they are maybe less complicated to use. Also, a well designed 4” refractor can usually go years without adjustment, and there are no aluminum mirror coatings to loose reflectivity or tarnish. With reasonable handling, a refractor should work as well 50 years from now as it works today. Care and feeding of small refractors is about as low effort as can be expended.

The second reason that many find 4” refractor to be appealing is because they are VERY efficient. In modern multi-coated objective lens sets, a very high percentage of the light entering the aperture continues its journey through the optical train, and the lack of central obstruction means that a very high percentage of that light is focused into the airy disk, resulting in high contrast performance on planetary targets and perfect airy disk patterns when observing stellar targets. I find that the 4” refractors I have owned SEEMED to be as efficient as 6” compound scopes, even though I know that a typical 6” reflector will ultimately win in absolute light gathering. But to me, the bigger difference between the two lies more in the physical size and weight between a 4” refractor and a 6” reflector. 6” reflectors start to make more demands on mounts (assuming that one is at all interested in equatorially mounting and driving their telescopes). To significantly offset the light efficiency in a 4” refractor and equal it in contrast, you will probably have to push up to a physically larger and heavier instrument and may have to push up to a slightly heavier mount as a result. Again, simplicity enters the picture, because a bigger mount might have to be broken down to move, so the trade-off for the small amount of extra light gathering might be an important factor for some. I can carry the Vixen 102 on the CG4 mount, with counterweights and battery pack, fully assembled, from my house to my patio in one trip.

The third reason, touched on above, is contrast. It is hard to improve on the contrast in a 4” refractor with most reflectors up to about 6 inches in aperture, without going to long focal length reflectors with optimized secondary mirrors, or getting up to 6” aperture in a compound system (MN51/61 being the exception – because of their small secondary obstruction, they should match/outperform the 4” refractor in contrast, and my own experience with a MN/61 easily confirms that). While I have heard numerous accounts of people saying that their 4” refractor equaled or beat this or that 8” telescope on planetary contrast, my own experience with the 4” refractors (APO and non-APO) I have owned is that an optically good 8” scope of any design currently manufactured, that is properly aligned and collimated, will deliver slightly better planetary contrast than a 4” APO or Achromat. It may, however, take a lot of owner devotion to keep a reflector properly collimated and aligned. If you aren’t willing to do this alignment and collimation and it gets off by very much, on even up to an 8” reflector, I can promise you that the Modulation Transfer Frequency Curve will dip fast enough that a good 4” Achromat refractor will eat your lunch on planetary performance. Really, it will. It is quite remarkable how good the planetary contrast and resolution provided by a 4” Achromat can be. When I look at Jupiter or Saturn through 4” refractors, I am always surprised by the rich saturation of the banding, and subtle detail available at such a small aperture. Yes, here is where refractors really shine. Contrast on planetary targets doesn’t improve that much until you get into either 5”-6” refractors, or 7” to 8” reflectors, and only then if you are pretty fastidious about keeping them aligned and collimated.

Finally, 4” refractors are as about immune to less than perfect seeing conditions as you can get. Even the 5” compound scopes I have owned (C5, Orion Apex 127) would start to bleed light into the space between the airy disk an first diffraction ring of in-focus star on nights of moderate seeing, while the 4” refractors would still be showing clean first diffraction ring. Seeing has to get pretty bad for a 4” refractor to start to muddy stars. While this relative immunity to the effects of seeing will show on the moon and planets to some degree, where you REALLY see the difference is on stellar targets, and especially on doubles. On nights of typical seeing for me, something like a magnitude 1 primary star will be clearly distinct from close companions in the 4” refractor, but will appear smudged in my larger reflectors so that the split just isn’t clean at all. A beautiful double is only beautiful to me if it is CLEANLY split, and refractors do this better than anything else in almost all conditions except when perfect seeing is present.

So, what this means is that even with the potpourri of choices in the market today, I for one still really “Get” the endurance of the 4” refractor design.

Now, on to the Orion by Vixen 102. This basic OTA used to be sold in the United States under the Celestron brand as the SPC 102, Orion by Vixen 102, and recently by the manufacturer, Vixen, as the 102M. It is easily distinguishable from other 4” Achromat scopes buy the use of a larger 114mm tube assembly. The over-sized tube allows superior baffling. It is the most effective baffling I have seen in any small scope. Looking down the light path with even a bright flashlight is like looking into a deep well. The Celestron and Orion versions use a 1.25” focuser with no provision for a 2”. I believe that the latest Vixen 102M is apparently available with a 2” focuser, and at its price point as sold new, I would expect that to be the case.

The OTA with dew shield in place is about 40 inches in length with the focuser racked in and no diagonal. This is a bit long when compared to some of the better APO scopes, but only a little bit. As you will see, I am going to do numerous comparisons to APOs. The OTA is nicely finished, but not up to typical APO standards, but then at this price point, who would expect that? Still, fit and finish is better than the new generation of Chinese made achromats by a fair margin.

The dew shield is a pressure fit, and tolerances are exceptional here. It takes a firm press to seat it against the lens cell, but once in place, it stays in place. The lens cell is a lightweight casting that keeps the nose of this scope much lighter than on the Chinese imports. This is important in getting a good balance, and allowing the OTA to sit forward enough in the mounting cradle to keep the eyepiece from scraping the ground when viewing at zenith. There is no provision for collimation in the model I own (some Celestron versions apparently did have this), but I am at least the second owner, and perhaps there have been more, and when I checked collimation with a Cheshire when the scope arrived, collimation was SPOT on. This implies that if it is well designed, and if good quality control ensured that things were right at the factory, then things will apparently stay right through normal shipping and handling, hence, no need for collimation adjustment screws. Again, the collimation on this scope was PERFECT!

The stock focuser was very nice. It is a rack and Pinion design, and movement was smooth, though a tad heavy for a 1.25” drawtube, even with the tension screws at their lightest setting. If I loosened the small Allen screws used to adjust tension on the top guide, the draw-tube would get a little loose in its bore, so I put them back to the original tension. Still, the movement was very smooth, and fine adjustment was easily achieved. I say “Was” so many times because I like the little scope so much that I thought that it deserved a 2” focuser so I replaced the focuser with a Burgess 2” Crayford focuser, so the original focuser sits unused. More on the Burgess Focuser later…

Optically, this scope is practically textbook perfect. It is medium focal length Achromat, and consequently by design, it is not color free. That being said, within the bounds of optical manufacturing capability, I just don’t think you could make meaningful improvements on the quality of the lens in this scope, and if you could, I doubt that those improvements would yield a difference detectable at the eyepiece.

Star testing presents excellent patters for correction and smoothness. In-focus airy disk patters are flawless. Presenting clean in-focus airy disks is one of the GREAT strengths of a refractor, and anything less than a perfect in-focus airy disk just makes a refractor unacceptable to me. To me, if the Airy disk is not perfect, the whole value proposition for refractors is negated and the higher cost of manufacturing a refractor would not at all be justified. The Vixen certainly delivers. In fact, I have never heard of a Vixen telescope with poor optics. Credit that to the systemically excellent Japanese manufacturing quality control. I don’t know if you can be safer when buying on the new or used market than when buying Vixen made products. Note that you routinely see used Chinese made 6” refractors selling for just a few dollars more as used Vixen-made 4” Achromats do. I think this price equality is rooted in the comfort that potential buyers have that if they buy a Vixen, it will be good optically, while they may be less comfortable with the non-Japanese imports. I see the same phenomena in the used eyepiece market, where “Made in Japan” stamped on the barrel of a Plossl will allow it to command a much higher price than one without this assurance of quality.

The Vixen 102 is fairly light. I have it mounted on a CG4 class mount, which I consider to be barely adequate for this application. This scope is SO good optically, that you WILL want to use it a higher powers, and to do so WILL push a CG4 class mount to the limit. It would not be over-mounting to put this scope on a CG5 class mount if you do regular high-power planetary or double-star observing.

The Orion/Vixen came with a 6 X 30 finder of excellent quality. I am not a big fan of 6 X 30 finders as a general rule just because they are severely aperture challenged, and I can often star-hop off of a bright star using a zero-power finder just as effectively when the scope can get to about 30x. If you use optical finders though, this is probably as good as any 6 X 30 finder you will probably see.

Observing with the Vixen 102 is quite enjoyable. A 32mm Plossl eyepiece will yield a coma-free 1.6 degree field of view, so larger clusters will easily fit into the field. I recently visited M44 with this scope and the view was beautiful. Stars were crisp, and sky background was as dark as my viewing site allows. Brighter galaxies are easy, and globulars like M3 and M5 are identifiable as such, but true resolution would require a bit darker sky than available to me in my back yard... err, observatory… The Vixen 102 easily matches the 4” APO I used to own in terms of quality of view for open clusters and globulars, but all of the 4” APOs currently on the market are shorter focal length, and all come with 2” focusers, so out of the box, the Vixen can’t achieve the same wide field views. Still, I replaced the stock focuser with a 2” Burgess Optical focuser and now I can use my 35mm Panoptic (28x and 2.4 degree FOV) or even my 55mm Televue Plossl (18x and almost 2.75 degree FOV) and this has made all the difference. In this configuration I find the Vixen to be almost equal in the practical achievement of field of view to even the fastest 4” APOs because even the best fast APOs I have looked through start to show significant coma near the edges of the field when generating fields of view wider than about 3 degrees. This happens even with superbly corrected eyepieces like the 35mm Panoptic.

As an aside to the telescope review, here are some thoughts on the Burgess Optical Crayford pattern 2” focuser. The Burgess focuser itself is very nicely made, and has exceptionally good movement characteristics. I especially like the nicely sized, knurled focusing knobs. Anyway, it practically glides with light eyepieces and with heavy ones, it allows for quite precise movement. Here are some thoughts of Crayford focusers: It probably costs $30 more to make a Craford focuser because of the 6 ball bearing assemblies needed, but some of this is recouped by eliminating the machining required for the rack and pinion. The reportedly sublime feel of the focusers on the mega-dollar APOs is probably achieved by maintaining exceptionally high tolerances during manufacture, and probably by doing lapping of the pinion and rack gears, and perhaps by doing hand fitting and adjustment. This surely drives up the price. I don’t understand why manufacturers don’t abandon rack and pinion at both ends of the spectrum. This is the second Crayford focuser I have owned, and they have spoiled me so that nothing else feels as good to use. Crawford focusers should be standard equipment on all new telescopes. Who wouldn’t pay $25 or even $50 more for a perfect focuser? Hello, marketing people, are you reading this???

I actually like the 1000mm focal length of the 102. This is a sweet spot of the 4” f/10 design. The medium focal ratio makes for coma-free medium-wide field viewing, while the 1000mm focal length means that you can achieve reasonably high magnifications without having to resort to looking through pin-hole eyepieces or introducing Barlows into the optical path. Many hold the opinion that modern Barlow lenses do not affect the quality of the view, but my recent experience comparing complex eyepiece designs to simpler ones makes me feel that on very bright, low contrast objects (like planets, where Barlows are most likely to be used) you will suffer a tiny bit of contrast loss due to reflections and light scatter when having to go through more glass. The difference might be very, very slight, but I have come to believe that it is enough to affect the final view to the point that I am trying to avoid Barlow lenses when viewing planets or very bright objects. Engaging Barlow lenses for double star work is completely acceptable to me, as I have made numerous comparisons with Barlow lenses against high power lenses, and found the difference on these kinds of objects to be extremely difficult, maybe even impossible, to see when viewing double stars.

Doubles are splendid in the Orion/Vixen. Local light pollution limits me to the brighter pairs, with magnitude 9 secondary stars being about the lowest I can go in a close pair, but splits are always as good as or better than anything short of my bigger refractors, and often seeing conditions limit the larger scopes from really doing any better. The big shortcoming here is just raw light gathering. Except on the darkest nights, my local conditions just don’t support detection of many fainter secondary stars. The excellent optics allow for 200x on doubles with superb results, though this kind of magnification can only be used on somewhat brighter pairs.

On the moon at medium high powers (120x to 140x or so), the Vixen displays a narrow, faint yellow band just inside the limb, and an even fainter violet or deep blue fringe just outside limb. Still, the color is less than with ANY other Achromat I have ever owned. I cannot detect any color around crater or mountain shadows. Frankly, the color here just isn’t a factor. I sincerely doubt that it has any noticeable impact on what detail you can actually see as compared to an APO. The level of color is so low that it is in no way distracting. Still, this is the one area that the one 4” APO I owned was clearly superior though. The APO just didn’t show any color at all. That being said, I can’t really say that I could see more detail in the APO. I find best power on the moon (and planets) with this to be about 120x to 140x, which translates to a little better than 30 power per inch of aperture, but my eyes match textbook performance (see references), which suggest that the average eye will see the most amount of detail available with about this ratio (magnification at 1.3 times diameter of objective in millimeters). My experience supports that. I can often easily push good telescopes up to powers higher than this ratio but I can’t say that I actually ever see more detail past this magnification ratio. It was the same with my 4” APO, where about 140x presented the most detail available to my eyes. Magnifications any higher than this presented a more pleasing image scale, but I don’t think I could really detect more detail in either the Achromat or the APO when going much beyond these powers. My 140mm refractor does best at about 160x to 180x, so I find the 1.3 power per mm of aperture rule to be a reasonable predictor of optimum magnification available with an instrument. Because of my local seeing conditions, it is extremely rare that I can support powers higher than about 180x for lunar and planetary observing anyway, and often, 160x to 180x is the best I can really do, so the small scope isn’t really at that great a disadvantage.


On the gas giants, the Vixen does quite beautifully. I got this scope too late to catch Saturn at opposition, but as of May 2004, when this is being written, I can detect no secondary color on Saturn when viewing with the Vixen. Saturn presents a light golden hue on the planet’s disk, with a smoky graduation starting about half way from the rings to the pole. My NexStar 11 actually shows something like a detectable stepping or a couple of dark bands in this graduation, and I think I can detect some hint of this in the 4”, but it could just be because I know that that they are there. Detecting Cassini’s division is trivial. The Vixen easily shows it crisply. The rings themselves present beautiful shading differences, and the shadow of the planet on the rings is distinct. I owned the 4” APO before acquiring this scope, and Saturn was much closer then, so I am reluctant to say that the APO gave a better performance because I didn’t compare them directly. Saturn looks terrific in the Vixen.

On Jupiter, the Vixen easily shows the NSGRS (Not-So-Great Red Spot…it just seems to be a misnomer to call it the Great Red Spot anymore, because it more closely resembles a pale pink blur). Easily visible are the equatorial belts, with a hint of detail along the edges of the bands and a hint of festoons (though in recent years, Jupiter seems to be disappointing here as well). I sometimes see small ovals south of the GRS with my NexStar 11, but to date these have been undetectable in the Vixen 102, though I cannot say that I ever really saw them with the 4” APO either. Still, on this one target, I would say that I think that the APO was perhaps a tiny bit sharper. With the Vixen, secondary color is present as a barely distinguishable fringe of Jupiter though ONLY just so, and I mean ONLY. The APO showed none though, and I believe that I could see just the tiniest bit more detail at the edges of the equatorial belts with the APO. I could also raise the magnification maybe 10X to 20X before hitting the cliff, where no more magnification will produce additional detail (to MY eyes). The differences, in my opinion, would be difficult to detect for all but the most patient and experienced observers, and only under the most extremely good seeing conditions. Jovian moon transit shadows are sharp and clean, and the Jovian moons themselves appear as perfect, tiny disks. I would not hesitate to say that you would struggle to achieve equal planetary performance with less than a 6” reflector or better performance at any 4” aperture, unless you are willing to spend a very great deal more money for an APO.

Now while this was intended as a review of the Orion/Vixen 102, I did include several comparisons to a 4” APO. I only did this provide a perspective as to what you could expect from an excellent 4” Achromat. It is my belief that you would have to spend between 3 and 5 times as much money on any other 4” refractor to get a even a barely detectable improvement in the view. What you can expect from the Orion/Vixen 102 is a GREAT little refractor. If you are in the market for an excellent quality 4” scope, and work really hard for your money and need or desire exceptional value, I would say that this OTA would be one of the very best 4” telescope that a really hard earned dollar could buy.
My regards,
Ed

For further reading to support my comments regarding optimum ratio of magnification per inch of aperture, please consult the following references:

H. S. Coleman. “The Influence of magnification on Resolving Power of Telescopic systems for Foucault Test Objects of Different Inherent Contrast”

Rutten & van Venrooij “Telescope Optics, A Comprehensive Manual for Amateur Astronomers”


Click here for more about the current Vixen line of telescopes. -Ed.