102 mm Refractor versus 140 mm Maksutov; Vixen ED102 vs OMC140
by John Baars
102mm refractor versus 140mm Maksutov
Vixen ED102S vs. OMC140
Some years ago I bought an older Japanese Vixen ED 102 f / 9 refractor from the Nineties. Reason: I wanted to get rid of the long cooling time of my OMC Maksutov, but at least retain the image quality of the Maksutov. Preferably with the same weight. The telescope comparison below has as its main objective: visual perception. Issues that I value less like opportunities for webcamming, a dazzling finish or focusing are not taken into consideration. The mechanical finish of both instruments is at level 2000, whatever that may be, at least good enough for the purpose: visual observation.
The Vixen is originally from Japan, has a 102mm ED doublet lens and is surprisingly well corrected for chromatic aberration due to its relatively long focal length of 920mm, actually amazingly well. Fields of view of more than 2.5 degrees are possible. More modern and shorter focal length versions are produced in the Synta factories in China. Star tests have shown an accuracy of at least 0,96 Strehl or better. The scope has five light baffles and is very black inside. Furthermore, it possesses a relatively simple focuser.
The OMC (Orion Maksutov Cassegrain D140mm, F2000mm, from the UK) was purchased new in 2001 from Astrotechniek Holland to replace an older 102mm f / 9 achromat. At the time, the achromat was excessively impacted by the wind. I was looking for a shorter telescope, so I chose a Maksutov. Skywatcher was not yet in the picture with high quality equipment. Orion Optics UK was one of the few outlets advertising at time with a guaranteed ¼ Lambda. At that time there were few brands who dared to make such a claim and were afraid of not being able to meet expectations. Of course nowadays this is different.
Upon arrival, the mirror appeared to meet the specifications, however, it suffered from some surface roughness. So I sent it back, they returned it with a new mirror. Actually the new primary mirror turned out to be better. Strehl 0,97. The OMC was designed by H. Rutten as an aspherical Gregory- Maksutov and is internally equipped with a sophisticated baffle system. The instrument is originally not made to handle 2 "eyepieces, but an adapter makes it possible. The focusser is a standard SCT- one with some play. However, it does not suffer from the much feared mirror shift. Also, a micrometer focuser is available on the later types. It has a very long focal length so the field of view is restricted to 1.25 degrees.
The comparison was made during three clear nights at the end of September 2011. A direct side-by-side comparison was not possible, because I only had one mount, a GPDX on a column. The previous mount I had used was broken. So the telescopes had to be swapped out. This was quite easy to do because of the Vixen dovetail. Usually I adjust it ½ to 2 degrees. A Televue 1.25" zenith mirror was mounted in the Vixen. The OMC included a Williams Optics 2 ”zenith mirror. Both were of unquestioned quality and performance. If a barlow lens was required I used the same Vixen DX from 1991, which is at least of the same quality as a Televue I that I used in other comparisons. The eyepieces used were a Panoptic 24 mm, a Nagler 16 and 7 mm, a Brandon 12 mm, a Pentax XF 8.5 mm, a Pentax XW 5 mm, a Pentax XO5 and in one case even the Pentax XO 2.5 mm.
I started out each evening of testing by allowing the telescopes to cool off. The big advantage of the refractor, with its faster cooling down time, was clear. Within 15 minutes it provided good views. The images on double stars were acceptable almost immediately. The Maksutov, with its thick corrector and primary mirror in the back, took more than 1 ¾ hours to cool down. Acceptable planetary images were obtained immediately, however they improved greatly after 2.5 hours. Optimal images of the planet Jupiter were visible after 3 hours. A trick to speed up the cooling process was to open the holes for the collimating bolts at the back as well as straightening up on a cool surface without the zenith mirror, the hole pointing upwards. And of course, putting the scope outside before dinner time. Also, acceptable images can be attained within half an hour by isolating the tube, which decreases the amount of turbulence inside.
Airy and Seeing Sensitivity
Both telescopes produced textbook Airy discs as a star image within a thin diffraction ring and some much fainter and thinner rings. The Vixen gives a "clean split" on double stars based on the fact that it is less sensitive to seeing conditions than the OMC. A neat disc with an almost still mini diffraction ring was visible. The incoming light therefore goes down the tube in convergence, away from the tube-wall. So it travels down the scope only once.
Images were clearly visible in the OMC, but the first and slightly thicker diffraction ring kept being degraded as a result of the seeing conditions. Even with a cooled telescope, the OMC was able to hold the entire diffraction ring with difficulty, unlike the refractor. Light goes up and down the tube three times and the first time alongside the tube wall itself. This made for a slightly sloppy sight, but par for the course for these optical systems. In part because of this it seemed that the OMC was worse at separating double stars. However the truth, based on the opening, the OMC should do even better. In practice, they turned out to be well matched, with a visible advantage for the Mak. Actual seeing conditions means that greater resolving power of the OMC is often not achieved.
Tests were made on bright double stars; they were the first objects visible and if possible I repeated the observations at the end of the session. On closely paired double stars such as Delta Cygni, in the Vixen 368X (3.7 X D, objective diameter in mm) splitting the stars was still possible, but then the faint companion became less visible. The OMC was able to sustain this game a little longer, but got stuck around 470X (3.4XD). I tried even more in both, but this did not improve the quality of the image. Stars became fainter (more stretched out) and more vague in aspect. I have not determined the exact resolution of both telescopes, but in both instruments you can separate Epsilon Lyrae 1 or 2 with the greatest ease. Or you can separate SAO 84572 in Hercules:
Star SAO84572 Mags. 7.0 and 7.9 Separation 1,2“ Image in three scopes. From left to right: 102mm Vixen, 120mm Evostar and 140mm OMC140
Airy disk with first diffraction ring. This is the double star Delta Cygni. The faint double is in the first diffraction ring. Sketch by author through a 70mm refractor.
But there is more. An obstructed telescope such as the OMC produces less starlight in the airy disk and more in the first diffraction ring than an instrument without obstruction. (65% light in the disk and 22% in the first ring) An unobstructed refractor such as the Vixen produces more light in the star point (84%) and less in the first ring. (7%) This is a given fact in high-end optics. The poorer the optical quality, the more light is visible in the rings.
Diameter of the Airy disc in mm: N / 735. In the OMC, therefore, 0.02 mm. And 0.012 mm for the Vixen. It is not difficult to see that the OMC handles its collected light a bit more "sloppily". Just use the calculator and it turns out that the Vixen definitely puts more light in the Airy disk than the OMC resulting in smaller and brighter pinpoint stars! This was actually seen on Delta Cygni's small companion: the OMC unmistakably spreads it out more.
My apologies for the previous intermediate step, but this brings me to attempting to achieve the limiting magnitude of the scopes. The OMC met the requirements by staying at magnitude 13 on the small star next to M57. With an observed magnitude of 13 on the same star, the refractor penetrates more than 0.8 magnitude above the 12.2 as described in the brochures. Rather, the difference lies in the number of times a star is seen at that limiting magnitude with averted vision. For example 10X per 60 (time) seconds in the OMC versus 3X per minute (time) in the Vixen. Those subtleties disappear when a much larger aperture is used.
Limiting magnitude stars were seen in the OMC at 333X with the barlowed Brandon and in the Vixen at 216X with the barlowed Pentax XF. Increasing magnification even higher did not actually result in any further visible improvements, only larger and dimmer star discs. If I generalize this, it means 2.4D in mm for a Mak and 2.1D in mm for a refractor.
Colors at high magnification are visible with great difficulty in the Vixen. A deep dark purple glow is perceptible around Vega; when the sky is transparent, barely distinguishable from the sky background. You really have to sit down for that. No color error worth mentioning appeared on Jupiter. But there is a tiny, tiny edge. It is questionable if this is due to atmospheric dispersion. Like all ED refractors of this class slightly colored rims around a defocussed star are visible. In focus none. Natural behaviour for an apo-like doublet. The OMC naturally has no discernible color error. Atmospheric dispersion is seen in the OMC too of course.
We switch to Deep Sky; my expectations were higher there. The higher absolute light gain of the OMC did its job here. But, also, the Vixen turned out to perform better than I expected. As expected, there was higher contrast in a telescope without obstruction compared with an obstructed system.
Globular clusters were slightly brighter in the OMC but definitely not so dramatic for a gain of 2X such as a beginner might expect. The differences are far more subtle. Although a bit dimmer, in the Vixen they appear with a tad more contrast. The very faint NGC7008 was just as evident in the 10 cm refractor as in the 14 cm Maksutov. Globulars in particular scored almost the same. The smaller star dots in the refractor may be attributed to that fact. I saw the same behavior in a direct comparison between a C9.25 and a 130mm APO.
Smaller DSO’s tolerate a higher magnification in both telescopes than larger ones. As is a well-known fact; small planetaries can take much higher magnifications than large galaxies. Aperture rules in the end, for slightly more subtle differences and details can be noticed with the OMC in nebulae like M42 for instance. In contrary to common belief, they don’t become any brighter.
It is obvious that the F900mm of this refractor is capable of showing much larger widefield views than the F2000 mm of this reflector. Enthusiasts will appreciate this very much and in many cases this can be a tipping point in the decision-making process about whether or not to purchase. And let's be honest: a wide rich field view with multiple deep sky objects in it is simply breathtaking.
It was striking to me that at equal magnifications in the refractor the open clusters and to a certain extent also the globular clusters had just a little more "taste". The contrast with the sky background was higher and although the image was slightly fainter, the greater degree of contrast and "finer" star pinpoints in the Vixen gave a more pleasant, but also more brilliant appearance. In short: in the Mak the sight was “softer”.
NGC 6888 Crescent nebula through OMC140 on a dark location. Sketch by author.
Planets, in this case Jupiter…
Jupiter sketch through 120mm refractor. Watch the subtleties in the yellow Equatorial Zone (2019) which were seen in the 140mm Mak too, during superior seeing. The equatorial zone turned yellow in 2019. The equatorial belts became narrower. Here we see the transition of Ganymede and Io's shadow. Io is still outside the Jupiter-disc. Sketch by author.
Planets are another story. The capabilities of a refractor in relation to a larger mirrored telescope are fairly well known. With the same average seeing, the smaller Vixen refractor scores better than the OMC telescope with its larger mirrored objective. For example: 95% of the time you will see more details in a refractor. During the other, and best 5% of your observation time, the larger mirror telescope scores better. The accuracy of your optics also plays a very decisive role in this. Basically you sometimes have to wait a long time for these moments of superior seeing, occasionally over a period of several evenings. Certainly this is enough time to engender all sorts of myths around the performance of refractors versus reflecting telescopes ... I have no illusions about that.
During the test evenings I experienced one superior moment with the OMC and Jupiter. The image appeared exactly as predicted: a super flawless, very detailed one, so finely drawn and laced with garlands and hues that no photo or drawing can match it. It only took a few seconds ..... I have not experienced such a moment with the refractor yet. Needless to say, the image in the refractor is on average sufficiently better than in the OMC, and provides a fountain of details for the connoisseur, even a luxury. And that is how the refractor gets its name as a planet killer… But when the moment of superior seeing arrives, the larger aperture outperforms it.
Jupiter sketch through 102 mm Vixen by author. (2017) Here the GRS, a turbulent area near it and two festoons are clear. The start of a third festoon is also visible.
Which magnifications for Jupiter?
More important for forum members is the question of which magnifications may be used for attaining these beautiful images. In the Maksutov, 167X (Brandon) was usually the most beautiful and brightest. (1.2 x D) If the seeïng was very good I could go up to 235X (1.7 X D with Pentax XF), but those were exceptions. During moments of superior seeing I could go until …… I don't know…..When it happened I had 167X in it. I suspect around 250-280X, so about 2 X D. As I have said, atmospheric conditions are our limiting factor. In all my observing years I have only experienced these kind of moments a few times per year and in those cases always the lower magnification is in the scope ……
In the Vixen refractor, 153X provided an excellent magnification (1.5 XD with the barlowed Brandon). With the good seeïng of Sunday morning 02-10-2011, 184X was very nice (1.8 XD) with the Pentax XW5, Pentax XO5 and the refractor gave images like we all want from refractors: clear, sharp and brilliant. 216X was tried but was less brilliant.
At even higher magnifications, the exit pupil becomes so small that you see the protein strings floating in your eye fluid across the Jupiter bulb (mouches volantes). Vivid, but not conducive to distinguishing low-contrast details on the slightly faint and fading Jupiter disc.
140 mm OMC Maksutov or 102 mm Vixen ED refractor? Neither of these instruments are inferior to each other. The OMC is a nice all-around scope for DSO observation and excels especially at small planetary nebulae and during the rare moments of excellent seeing with the planets. There is no chromatic aberration. There is a visible, but not a dramatic gain in brightness, but this does not necessarily translate into observing fainter stars. Such gains must be attained with averted vision. On nebulae there is an increase in visible details.
The Vixen is more than a good all-around scope on the planets and excels in its cool down period, seeing, star images and for the viewing of larger open clusters. The high contrast transmission and sharper stars largely compensate for the lower light gain. Also, it achieves a higher limit magnitude than you might initially expect.
Did we not already know this? Somehow, I just wanted to see this confirmed.
These telescopes are evenly matched. Because of my personal preference for rapid cooling, tighter star images, and the beautiful planetary views and large well-contrasted star fields, I give my vote to the Vixen. However in the final assessment this is entirely my perspective, perhaps someone else would weigh things differently and come out with another choice.
Thanks for reading,