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- My experience using two 80-millimeter long-focus refractors
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My experience using two 80-millimeter long-focus refractors
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My experience using two 80-millimeter long-focus refractors
By Armando Caussade, November 7, 2019.
Copyright © 2019 Armando Caussade, CC–BY–NC–ND–4.0. Some rights reserved.
After almost four years as a member of Cloudy Nights, it is my pleasure to share my first ever telescope review. The equipment to be assessed comprises the following:
(1) The 80-millimeter f/11 Orion achromatic refractor (henceforth "Orion refractor"), which I purchased new from Orion Telescopes and Binoculars in May 2004; then known as item #7380 and sold as an optical tube assembly, this was discontinued around 2010, but similar models are currently offered by various retailers under diverse brandings. As part of this purchase I also acquired a pair of mounting rings, and in May 2005 would go on to upgrade the factory-installed focuser (more on this later).
(2) The 80-millimeter f/11 TMB / Burgess "Planet Hunter" achromatic refractor (henceforth "TMB refractor"), which I purchased in the used market in February 2010. This telescope was conceived by the celebrated optical designer Thomas M. Back (1957–2007) and later placed in the market by Burgess Optical in 2006; with a limited production run of only 50 units, it completely sold out by 2007. The TMB refractor was then offered as an optical tube assembly with mounting rings and a red-dot finder.
In contrast to most reviews which are written soon after the purchase of new equipment, my analysis of this pair of astronomical telescopes will convey a broader perspective that only time can provide.
The goal of this review is two-fold: (1) To share with the Cloudy Nights community my impressions of the aforementioned instrumentation in both the mechanical and optical domains; on this last count I will provide results obtained by means of a star test; and (2) to provide a first-hand account of astronomical observations that will give a clear idea of the optical capabilities of these two refractors. I intended this assessment to be as comprehensive as possible, but have to acknowledge one specific omission: a side-by-side comparison between the telescopes, for the reason that I can only mount one each, at a time.
For the record, I should state that my ultimate purpose with this review is only to provide a personal opinion. I have never had a business interest with any optical manufacturer or retailer, and the items discussed here were obtained through regular commercial channels and via the used telescope market.
1. My background in amateur astronomy.
I will start by summarizing my qualifications in amateur astronomy. After 36 continuous years in the field I have owned ten telescopes of various designs, and have used well over a hundred different units belonging to friends and acquaintances, again of different designs and varying optical quality.
Starting in 1983, Patrick Moore's books were highly influential in teaching me the basics of visual astronomy, a topic I have written about twice in Cloudy Nights forum postings; at the time I used 8×40 binoculars together with a 50-millimeter refractor. John Dobson was also an influence via his "sidewalk telescopes", a cost-effective design that I enthusiastically adopted in 1993 by way of Coulter Optical and which introduced me to serious deep-sky observing. From 2003 onwards, medium-sized catadioptrics would allow me to delve into lunar and planetary observing, an optimal kind of work for my light-polluted backyard (class 8 in the Bortle Dark-Sky Scale) located in suburban San Juan, Puerto Rico.
At present I use the two 80-millimeter refractors that are the subject of this review, plus a 150-millimeter Maksutov-Cassegrain that provides a stronger light grasp, but I also enjoy minimalist astronomy using only the naked eye. Learning the 84 constellations that are visible from my latitude of 18 degrees north got me into a habit of regular observation in both shallow-sky and deep-sky modes, with the latter being more challenging because of ruinous levels of light pollution. Extensive travel from the Arctic to Antarctica allowed me to see and compare the changing geometry of the celestial sphere from different latitudes, a powerful experience that has vastly enriched my comprehension of visual astronomy.
The Puerto Rico Astronomy Society (PRAS), the oldest and largest amateur organization in the island, has been a major factor throughout my life. Helping establish PRAS back in 1985, then chairing the group for six years, and now serving as president emeritus, are all highlights of my astronomy career. My years as chairman were highly productive, and with ample support from members I began directing large projects of public outreach that still thrive, with yearly live audiences surpassing the 5,000 mark.
Professionally a BS helped me to establish myself as a computer systems administrator, and a GCSc allowed me to launch a second career as an instructor of observational astronomy at local colleges with over 100 yearly enrollments. In 2015 I traveled with PolarTREC to the Amundsen–Scott South Pole Station where I successfully conducted ten days of work at the IceCube Neutrino Observatory. Also worthy of mention is my recent work as the PRAS affiliate representative before NASA Puerto Rico Space Grant Consortium, one of the 52 university-led consortia now operating throughout the nation.
Recent events such as the Puerto Rican debt default of 2015, Hurricanes Irma and Maria in 2017, plus the Puerto Rican political crisis of 2019, have successively conspired to bring increasing hardship to every resident in the island; but my commitment to astronomy has never wavered, nor has that of my colleagues. Together with PRAS I remain active in public outreach, lecturing at both academic and community venues and sharing the views from my telescopes with thousands every year.
Five sky viewing events marked my life as an amateur astronomer and will stay forever etched in my memory: (1) Totality during the Caribbean solar eclipse of 1998, watched for three and a half minutes under a perfectly clear sky from Curaçao; (2 and 3) the Venus transits of 2004 and 2012, both seen telescopically, together with PRAS colleagues under cloudless morning and afternoon skies in Puerto Rico, where I reside; (4) fifteen seconds of annularity during the hybrid eclipse of 2005, watched through high clouds from Panama; and (5) the midnight sun, enjoyed for several days in 2015 from both the Antarctic coast and the Antarctic interior, the latter under the bluest and most pristine sky ever.
And in speaking about the Sun, never attempt to look at the Sun without proper eye protection! I practice safe solar viewing using only approved equipment and filters, and always take the utmost care and precautions when observing with public audiences to avoid even the slightest chance of eye injury.
2. Motivation behind an 80-millimeter refractor; first impressions.
In the months following the perihelic opposition of Mars in August 2003 I began to mull over the possibility of obtaining a small refractor. Carrying out a 200-millimeter Schmidt-Cassegrain every few nights and setting up this telescope—as I diligently did for two months right around opposition—was starting to feel like a burden, even when such outings only meant a walk to my own backyard. After all, the hobby did not have to be that involved, and I went on to recall memories from my early years in amateur astronomy when all I used was grab-and-go optical equipment, together with a simple star atlas. Neither the 330-millimeter Dobsonian that was my main fixture from the 1990's, was an option in this regard; only rarely would I bother about setting-up this mammoth or even moving it around.
I was desiring three specific features for my new telescope: (1) portability and ease of setup, (2) minimal cooldown time, and (3) a wider absolute field of view. Feature #1 was my top concern, as explained before, but #2 and #3 also mattered. As good and enjoyable as my larger telescopes were, I was becoming increasingly frustrated with the inherent thermal issues of the reflector, and with the restrictive fields provided by the catadioptric. Having owned a small refractor in the past was influential in my choice, and I set my sights on the market for an affordable 75 to 100-millimeter long-tube achromat.
The author standing next to the 80-millimeter f/11 Orion achromatic refractor.
Venus transit of June 5, 2012, 17:09 UTC−04 — Toa Baja, Puerto Rico.
Credit: © 2012 Armando Caussade. All rights reserved.
One sunny afternoon on May 2004, a large and narrow box showed up on my doorstep. I was now the owner of an Orion 80-millimeter ƒ/11 refractor, and like every telescope that arrived to me in the past, the optical tube appeared larger in real life than expected. Three hours later the evening twilight came to an end, leaving me under a cloudless and steady sky on what normally would be the start of the rainy season. Jupiter was optimally located in Leo, high in the south, while a crescent Venus and a distant Mars lingered low in the west. Saturn, in Gemini, was better placed higher in the west, and a full Moon would soon start its slow ascent in the east. I was thrilled, as this particular night marked my return to the world of refractors, after my initial experience 20 years before with the tiny 50-millimeter achromat.
First light was Jupiter, and "Wow!" I interjected. The view at 90× using a 10-millimeter Plössl eyepiece was very satisfying, with visible features along the equatorial belts of the planet. Saturn, with its rings then tilted at near maximum, showed atmospheric and ring detail. The color rendition given by this telescope was generally good, displaying only a hint of the violet fringing which refractors suffer from due to inherent chromatic aberration; in this regard, my new long-tube achromat clearly gave a cleaner view than the short-tube achromats that I had been given the opportunity of looking through in the past.
"First-class optics on this one," I told myself, while still wondering that such performance came for the unbelievable price of US$120, plus $20 for mounting rings; but this accounts for the optical tube alone, as I had planned to reuse an old, lightweight EQ-2 equatorial mount with the new refractor. While I was seeking superior optics, I cannot deny that the low price point was what really catched my attention.
As the nights went on, the expanding shadows on the Moon from the receding terminator made lunar observation possible, with distinctly sharp views; in addition, the waning Moon which rose later every night brought darker skies that allowed improved deep-sky observation. The 1.8-degree field displayed by a 32-millimeter Plössl eyepiece not only gave panoramic views of sprawling star fields that are unobtainable with larger telescopes, but also fulfilled my long-standing desire of watching close lunar and planetary pairings, which I have since enjoyed immensely. A neutral density solar filter, also from Orion, came in soon after; this allowed me to directly see sunspots for the first time in years, as I had never bothered to purchase the complicated and costly filters that my heavier instruments would require.
The ease of operation of the Orion refractor made it a joy to use, and its arrival marked a crucial change in my approach to amateur astronomy, as I could now enjoy quick and frequent access to the night sky.
The 80-millimeter f/11 TMB / Burgess "Planet Hunter" achromatic refractor.
Mounting rings were
replaced, and the red-fot finder has been removed.
Credit: © 2019 Armando Caussade. All rights reserved.
The growth in the refractor market during those years directed my attention to new offerings, and in 2006 I became aware of plans by Thomas M. Back (TMB) and Burgess Optical to produce and sell a premium 80-millimeter ƒ/11 "enhanced achromat" using proprietary glass—with better optical properties than the crown and flint glasses used in conventional achromats—to improve correction of chromatic aberration and yield near-total suppression of false color. This new "Planet Hunter" telescope seemed like if it were custom made for me, and I resolved to get hold of a unit at the earliest; but life sometimes gets in the way of one's desires, and starting a new job as a computer systems manager at a large clinic, as I did in 2007, meant that free time to pursue the hobby was minimal. Later that same year, I became disheartened to learn that the entire production run for the refractor had already sold out.
For years I kept a watchful eye on the used telescope market. One morning in February 2010 an unexpected telephone call came into my office. A PRAS colleague, José E. Torres, who is also a member of Cloudy Nights, had just spotted an advertisement for a used TMB optical tube which was being offered for US$425. "Sorry to bother you," my friend said, "but I know you're looking for one of these; they sell fast, so don't waste time; I suggest that you allow me to answer the ad, right now, on your behalf." An offer like that could not be refused, and I agreed. He knew how important this telescope was for me not only because of our friendship, but also because he owned one of these himself.
By March 2010 the TMB refractor—the now legendary Planet Hunter telescope, considered by many to be among the finest achromats ever made—had arrived in like-new condition, and the package included mounting rings, dovetail plate, a red-dot finder and a soft padded case. Back in those days I was so busy setting up an electronic health record system for the clinic that José offered to go all the way with the purchase, something for which I am forever grateful; he arranged everything with the seller and even volunteered to pay in advance from his own money, for which I later reimbursed him in full.
3. Mechanics, upgrades and accessories.
While the optics on the TMB truly live up to its reputation, it was the mechanics of the instrument what left me quite impressed. The Planet Hunter exemplifies what a real telescope should look and feel like: well-constructed with an all-metal body, where every component is held firmly in place without even the slightest play. The optical tube feels substantial but not heavy, and no plastic parts are used anywhere, excluding the white endcap that seals the focuser. The tube is elegantly painted in a white coat that seems resistant to chips and cracks, with the interior painted in flat black; the focuser and the lens cell are made out of black anodized aluminum, with only the focus knobs and drawtube left as bare metal.
While I cannot say that construction quality on the Orion refractor is up to the level of its counterpart, in reality it does not lag far behind and stands on its own. This telescope also features an all-metal body, but suffers from a couple of loose ends here and there such as the aluminum dewshield whose internal diameter happens to be a bit larger than needed. The weakest part on the Orion refractor is undoubtedly the cheap plastic cell that holds the objective lens, which I was shocked to discover when I first examined the optical tube upon arrival; but to be fair I cannot ask more for the money, and in practice the cell has remained intact after heavy use day and night, and frequent travel, for over a decade now.
By the way, neither telescope allows the lens cell to be adjusted for collimation. Yet another example of a design flaw which in real life becomes largely inconsequential, as long-focus refractors rarely need collimation, and after years of use I have yet to see any misalignment from the objectives. I did find a minor deviation of the focuser on the Orion refractor, which I fixed by inserting a tiny metal shim.
On both the Orion and TMB refractors, the dewshield that keeps nighttime dew away from the objective falls short of the recommended length of three times the aperture, which equals 240 millimeters; my measurements gave 80 millimeters and 140 millimeters, respectively. Fortunately for me humidity is not a problem, and in practice the dewshields seem to perform as I rarely see any fogging of the objective.
The total length for the optical tube—measured from the edge of the dewshield, with the focuser fully retracted—amounts to 895 millimeters for the Orion refractor, and 910 millimeters for the TMB refractor. I found this size to be convenient for a refracting telescope because of two reasons: (1) long enough to allow the mounting rings and dovetail to ride across the full stretch of the optical tube, in order to precisely balance the instrument; and (2) short enough to allow for a comfortable position of the eyepiece at all heights above the horizon, and particularly when looking high up near the zenith. Years of use with a variety of popular mounts (EQ-2, EQ-3, EQ-5 aka Super Polaris, SkyView Altazimuth, etc.) tell me that this tube length is optimal for essentially any type of mount in the amateur market.
The contemporary market is flooded with inexpensive refracting telescopes that have had their effective aperture diminished by 10% or even 20% due to constricting lens cells or aggressive baffling designs, but I can gladly report that this is not the case with either the Orion or TMB refractors. The optical tube is hefty and visibly oversized on both units, with an external diameter of 91 millimeters which allows for unobstructed cell designs and optimal baffling. In fact, the presence of two accurate ring baffles on the Orion refractor, to control unwanted internal reflections, was a surprise given its low price-point. The TMB refractor also features two ring baffles, in addition to microbaffling on the inside of the drawtube.
Maximum focuser travel on the TMB and Orion refractors, compared side-by-side.
The Orion refractor
is featured here with its upgraded 2-inch focuser.
Credit: © 2019 Armando Caussade. All rights reserved.
The one remaining gripe I had with the Orion refractor was the original factory-installed focuser, a long-travel, sturdy rack-and-pinion of the 1.25-inch variety. Mechanically it was pleasing, but the feeling came that I was cutting myself short with the field of view that this focuser gave, versus the larger view on a 2-inch unit; and let us recall that part of my motivation behind an 80-millimeter telescope was a wider absolute field of view for low-power scanning of the night sky. Not being able to use the bigger 2-inch eyepieces became increasingly frustrating as time went on, until one year later when I ordered an aftermarket 2-inch focuser that I saw advertised on ScopeStuff. I had compelling reasons to believe that this upgrade was being sourced from the same Chinese manufacturer where the telescope itself came from (i.e., Suzhou Synta), and upon arrival the fit to the Orion optical tube was as perfect as could be.
One feature that I appreciated with the new 2-inch focuser is its long drawtube that extends upwards to 150 millimeters, exactly like the old 1.25-inch unit did. Long-focus refractors benefit from lengthy drawtubes, and this is where the TMB refractor fails to meet expectations. With only 80 millimeters of travel, it is undeniable that the pre-installed Crayford focuser was designed with shorter refractors in mind. Mechanically excellent, it will achieve proper focus whenever a diagonal viewer is present, although some eyepieces will eat up almost the full stretch of inner focus. Further testing revealed that certain eyepiece combinations are impossible without a diagonal—or a tube extension—and although I rarely do astrophotography, the light path could be insufficient for prime focus or negative projection.
A diagonal viewer is a mandatory component for any refracting telescope, and for use with the Orion refractor I purchased an Orion-branded 2-inch dielectric mirror diagonal that has never disappointed. For the TMB refractor I later acquired the Baader clicklock 2-inch dielectric mirror diagonal, a product which in addition to superb optics also provides the largest unvignetted aperture in the 2-inch market. An 8×40 finderscope—borrowed from my 150-millimeter catadioptric—rides atop the Orion refractor, and while not a fanatic of the red-dot finder that came with the TMB refractor, I occasionally use it.
Speaking of eyepieces, until 2006 I used mainly the symmetrical Plössl type. Nowadays I use primarily the Orion Ultrascopics, a discontinued set of Japanese-made oculars belonging to the so-called pseudo Masuyama design that was sold under a variety of brands. While their apparent field of view of 50 degrees may not be the widest ever, what little field there is, appears flawless. Together with the Orion refractor the Ultrascopics yield an outstanding level of performance in regard to field flatness, distorsion and many etceteras. Off-axis astigmatism, the prevailing aberration of astronomical eyepieces, is barely noticeable with these, and the crispness of stars throughout the telescopic field is unbelievable with the Ultrascopics, perhaps the finest that I have ever seen out of any combination of eyepiece and objective.
4. Performance on Solar System objects.
Living in the tropics—with the ecliptic high up in the sky—allows me to watch the planets near the zenith, where atmospheric turbulence is minimal; additionally, being surrounded by the ocean brings consistent steadiness to the air. I benefit greatly from these two conditions, and the observations to be described all took place near the windward coast of the island, and at altitudes over 45 degrees.
Mars, a vivid orange globe through my refractors, shows prominent dark areas like Syrtis Major and Mare Erythraeum, in addition to whitish polar caps and occasional clouds along the limb of the disc. The high surface brightness of the planet allows effective use of extreme magnification up to three times the aperture; for example, 200× and even 250× still provide a respectable level of brightness and color saturation. It has been said that spotting meaningful surface detail on Mars will require an angular diameter of 14 seconds of arc, at least, and from my experience I can tell this is true for 80-millimeter-class telescopes. I also find that color filters work better with this planet than any other, and a Wratten #15 yellow filter fitted to an 80-millimeter instrument will enhance contrast in the Martian surface.
Jupiter, the king of planets, has invariably been my favorite target for planetary observations, due to two main reasons: (1) Its large apparent diameter against the sky, which on average surpasses that of all other planets; and (2) the wealth of atmospheric detail that can potentially be displayed by any telescope.
My refractors show the following on Jupiter: (1) A large, cream-colored disc displaying a decent level of brightness and color saturation upwards to 100×, and a more subdued although still sharp image at higher powers. (2) Multiple tan-colored atmospheric belts, with meaningful structure discerned on the two equatorial belts starting at 75×; bluish festoons are frequently noticed flowing from the North Equatorial Belt towards the Jovian equator. (3) The Great Red Spot, marginally visible at 45× but easily observed as a reddish oval at 100×, revealing an intense tint has lately become obvious, even with small apertures; but sadly, I have never seen any of the white ovals nor the salmon-colored oval BA, perhaps because I have not been attentive enough. (4) The four Galilean moons, with the impression of a disc on both Ganymede and Callisto when working at high magnification, but I am unsure on this last point.
Does the king of planets fare equally with the Orion refractor and the TMB refractor? Visibility of low-contrast planetary detail may be used as a benchmark of optical quality, and Jupiter can be particularly suitable in this regard due to its subtle atmospheric features. My assessment is that both telescopes will accurately portray the Jovian atmosphere upwards to a magnification of 130×, beyond which the Orion refractor stalls, while the TMB refractor will keep delivering fine detail until 180×, at the least. But Jupiter represents a stringent test for any optical system. And moreover, the reader should recall that I have never conducted a side-by-side comparison, meaning that my impressions are from memory alone.
Shadow transits by the Galilean moons of Jupiter are plainly visible with small telescopes. I greatly enjoy these transits and have witnessed many such events of Io, the innermost moon, whose shadow against the Jovian atmosphere is detectable at only 45× and becomes an obvious sight at 100×. Europa, being farther away from Jupiter, casts a smaller shadow that requires at least 80×, with 150× yielding a clearer view. Ganymede comes third, but its larger size allows detection of the shadow using only 50×. But I have not yet been lucky enough to watch an event involving Callisto, the fourth of the Galilean moons, whose transits are less frequent as it misses Jupiter because of its slight orbital inclination.
Saturn always affords an amazing sight through any telescope, irrespective of aperture. My refractors show the following: (1) A yellow-tinted disc hinting at orange, which marks a contrast with the ice-white of the ring when seen at moderate powers around 80×, but the yellowish disc is less brightly lit than Jupiter when compared using similar magnifications. (2) A dark atmospheric belt near the equator, obvious at 100×. (3) A well-defined ring system, with the Cassini Division that separates rings A and B discernible at only 60× across the entire length of the system on years near the Saturnian solstice. (4) The planet's shadow over the ring is evident, as is also the dark C ring against the globe of the planet. (5) The large moon Titan, an easy target even under poor seeing conditions, as well as the four Cassinian moons, i.e., Tethys, Dione, Rhea and Iapetus, with the latter being the most challenging to find.
Uranus, a neglected planet by amateur astronomers, is one that I enjoy observing each year around its opposition. Both refractors show a small bluish disk that is evident at 50× and obvious at 100×. No details are revealed, but its cooler tint makes for an interesting contrast with the warmer-colored planets.
The Moon is also an engaging telescopic target, but it is the flatness of the lunar images rendered by the long focal ratio on my refractors what makes them so enjoyable to use. Three observations from 2005, with the Orion refractor, were particularly gratifying: (1) The four craterlets inside the large crater Plato, spotted using 100× on separate occasions around the full moon, when the illumination angle is optimal. (2) the lunar dome Kies Pi. (3) Four out of six lunar domes near the crater Hortensius, seen exactly three days after a first quarter moon. These old volcanic cones were all detectable using only 100×, but better seen at 130× and 165×. The Orion refractor holds up well at high magnification with the Moon, delivering crisp, pleasing views to about 165×, but the TMB will keep delivering upwards to 250×.
Let us not forget the Sun, with me being one among many who enjoy following not only intrinsic solar activity but also extrinsic events like an eclipse or a transit. Smaller telescopes are well suited for this kind of work and, in good measure, my purchase of the Orion refractor in May 2004 was prompted by the impending transit of Venus about to take place in June 2004. To this day, I continue to use the Orion refractor in combination with a neutral density glass filter, as I did then, but find that the sharpness of the image leaves much to be desired, with the filter and not the telescope clearly being the weak link in the optical chain. There are better solutions in the market, like a Herschel wedge—now item #1 on my shopping list—which provides improved levels of resolution for solar observation in white light.
5. Performance on deep-sky objects.
By definition, my geographic location should be ideal for deep-sky astronomy. The entire Milky Way is well presented in its progression throughout the year, down to its southernmost reaches in the constellation Crux, and the Galactic Center in Sagittarius is prominently displayed each summer at some 45 degrees above the horizon. But in reality, the entire island of Puerto Rico is rated at greater or equal to class 4 in the Bortle Dark-Sky Scale, meaning that dark rural skies no longer exist. Undeterred by this, I have still managed to record hundreds of non-stellar bodies including the complete Messier Catalog, and the observations quoted here were done mostly under class 8 skies. Occasional ferry trips to offshore Puerto Rican islands Vieques and Culebra allow improved views under darker, class 3 skies.
Messier 42, the Orion Nebula, is my favorite ever deep-sky object. Both telescopes show a huge, bird-shaped, green patch of nebulosity spanning one half of a degree, in turn bounded north and south by rich clusters of stars that appear sharp to the very field stop; precisely, it is with targets like these where the flatness of a telescopic field becomes noticeable. The fifth and sixth stars of the Trapezium star cluster—the star nursery at the heart of the nebula—are not too difficult with either the Orion or TMB refractor, and I can see both stars consistently at 100× on nights with good atmospheric steadiness.
Messier 45, the Pleiades star cluster, comes next in my list. Spanning over a full degree of sky, this is a difficult object to fit in a single field while, at the same time, maintaining crisp stars to the edge. But the Orion and TMB refractors easily brave this challenge, framing the entire cluster at a power of 16×, with ample space to spare and pinpoint stars everywhere across the telescopic field of view. I cannot deny that I have spent hours, on countless autumn and winter nights, admiring this exact view of the Pleiades.
Small refractors excel with low-power, panoramic views of the night sky, where deep-sky objects can be discerned within the broader context of their surrounding stellar fields. From my experience, an 80-millimeter telescope fitted with 2-inch eyepieces is unsurpassed for wide-field scanning of the night sky.
Open clusters are the best deep-sky targets for small telescopes, not only because they resolve easily into individual stars, but also because they hold up well against light pollution. NGC 869 and NGC 884 (the Double Cluster, in Perseus) are both beautifully framed by the 1.1-degree field provided at 45× by a 20-millimeter Ultrascopic eyepiece, with more than 100 colorful stars strewn over half the telescopic field of view. This impressive sight of the Double Cluster, obtained at home under class 8 skies, makes me revisit this object more than any other in its category. From this same location I have recorded many other groups, including NGC 4755 (The Jewel Box, another colorful cluster in Crux), NGC 6231 (in Scorpius), Messier 41 (in Canis Major), plus the splendid pair of Messier 46 and 47 (both in Puppis).
Planetary nebula Messier 57 (Ring Nebula, in Lyra) looks very star-like at lower powers, with a minimal disc starting to emerge around 22× and a distinct ring seen at 45×. Messier 27 (Dumbell Nebula, in Vulpecula) is a large, greenish cloud immersed within a plentiful stellar area, and wide-field views at 45× are amazing even under class 8 skies. Three other planetaries, namely NGC 2392 (Eskimo Nebula, in Gemini), NGC 3242 (Ghost of Jupiter, in Hydra) and NGC 7662 (Blue Snowball, in Andromeda) are seen as tiny globes at 100×, each glowing with a striking blue-green tint that is impossible to miss. But as explained before, the charm of these views lies not as much in the sparse detail revealed by the nebulae per se, but more in the overall appearance of these little orbs in the midst of dense star fields.
Globular clusters can be a challenge in 80-millimeter telescopes, but are still worthwhile to watch. As the renowned visual astronomer Stephen James O'Meara aptly conveys in his book The Messier Objects, dozens of clusters in this category are all within reach of small refractors. A few large globulars will start showing granularity through the Orion refractor with perhaps a dozen stars noted, even under urban class 8 skies. Messier 22 (in Sagittarius) can be resolved using only 28×, becoming progressively easier at 45× and 60×, with optimal detail appearing at powers around 100×. Messier 4 (in Scorpius) is resolved at 100× and will clearly display the famous bar that runs along its core. In contrast, the great globular NGC 5139 (Omega Centauri) will only hint at granularity in spite of its overall brightness.
Galaxies are not only challenging, but they can be reduced to invisibility by even minor levels of light pollution. Yet, I recall one spectacular view of Messier 31, the Andromeda Galaxy, exceeding the 1.8-degree field provided at 28× by a 32-millimeter Plössl eyepiece, and even hinting at structure through the Orion refractor under class 3 skies, during a December 2004 outing. Two satellite galaxies were seen in the same field, namely Messier 32 and Messier 110, and the nearby Messier 33, the Triangulum Galaxy (a spiral system in the constellation Triangulum) easily covered over half the field of view, as seen that same night. Other recurrent targets of mine with the refractors are Messier 81 and 82 (both in Ursa Major), as well as the Leo Triplet consisting of Messier 65 and 66, together with NGC 3628.
While binary stars are not exactly my specialty, I sporadically like to watch them, and the observations summarized here all took place between 2017 and 2019. Beta Cygni (Albireo) is my all-time favorite, with its orange and blue components providing a colorful vista at 45×. Beta Orionis (Rigel) and Alpha Geminorum (Castor) are easy to resolve at 60×, while Epsilon Lyrae (the so-called "double-double") yields at 75×. Increased magnification helps a lot on tighter pairs, and at 130× Gamma Virginis (Porrima) and Epsilon Boötis (Izar) are both separated with clear black space, while at 150× Epsilon Canis Majoris (Adhara) is cleanly split in spite of its huge 6-magnitude difference. The notoriously difficult pair of Eta Orionis is just resolved at 150× using the TMB refractor, with the stellar diffraction discs touching one another, but this was a quick impression obtained at barely 24 degrees of elevation.
As explained before, my suburban neighborhood is affected by unrestrained light pollution from greater San Juan, a one-million-people metropolitan area. My backyard sky (rated at Bortle class 8) allows me to easily see stars of magnitude 10.5 through the TMB refractor, with magnitude 11.5 being attained via direct vision, under moderate to high magnification, during nights of optimal transparency and seeing. These results suggest that 12th magnitude stars should be observable from darker locations, without an unreasonable effort, and I know of credible reports on the Cloudy Nights forums of stars well into the 13th magnitude being detected by observers with 80-millimeter-class telescopes, under pristine skies.
6. Impressions of optical quality following a star test.
With all the signs that both units were optically excellent, I still desired to perform actual tests that would provide quantifiable results. I have read and studied the standard work on the subject, Harold R. Suiter's Star Testing Astronomical Telescopes, but anyway felt the need to seek out expert opinion.
To that end I requested the assistance of a PRAS colleague and friend, Guido Santacana, who also happens to be a longtime member of Cloudy Nights and a refractor enthusiast himself. What made him the right person for the job is his expertise in conducting and correctly interpreting a star test. We met at his home in a nearby suburb at seven in the evening, one moonless, cloudless and dark night in January 2018, right in the aftermath of hurricane Maria and in the midst of a months-long, island-wide blackout. If I say that this particular evening was perfect, I am not exaggerating: the air was both transparent and steady, two desirable atmospheric conditions that seldom come together during the same night.
My main optical concern was the possibility of residual spherical aberration—and more specifically, undercorrection—which I consider the prevailing optical error in mass-produced telescopes, and the cause of fuzzy and indistinct images even after rounds of rigorous collimation. As I shared this belief with my friend, we both came into complete agreement. Considering that an impeccable optical figure is unachievable, a certain amount of spherical aberration should be expected out of any fabrication process; the key issue for any manufacturer would be minimizing this flaw to a level below the threshold of detectability. Would this be the case with the two refracting telescopes that I was wanting to assess?
I was also interested in evaluating field curvature and chromatic aberration, both of which are inherent defects of refracting telescopes that become less noticeable at longer focal ratios. Personal preferences influence the selection of an astronomical telescope, and I have to disclose a bias against curved fields in refractors and comatic fields in reflectors, which explains my tendency towards higher ƒ-numbers.
Guido had suggested that I bring only the optical tubes, kindly offering to provide his own EQ-3 mount which provides adequate support for telescopes of this size. He then proceeded to test each unit separately, starting with the Orion refractor which he aimed at the first-magnitude star Capella. We used a 4-millimeter Abbe orthoscopic eyepiece that yielded 228×, together with a medium amount of defocus for the star test, as recommended by H. R. Suiter in his book. And in keeping with the rigors of such an experiment we also gave each telescope 15 minutes to acclimate to ambient temperature, although in theory small achromats barely need any cooldown time, and in practice the modest temperature differentials of the tropics make thermal acclimation superfluous for casual astronomical observing.
Separately with each optical tube, Guido spent a good 30 minutes at the eyepiece. Our expectations ran high and the results did not disappoint: we found precise collimation on both objectives, with no pinched optics and no indication of intrinsic astigmatism, zonal errors or turned-down edges, which led Guido to offer an initial, tentative verdict: "I see absolutely no reason to envy any other telescope," he said.
The results for chromatic aberration were consistent with the expectations: we indeed noticed a fair amount of residual color in the Orion refractor, but not more than what theory would predict for an achromatic objective of this size and focal ratio. The TMB refractor, on the other hand, did display a trace amount of color which was judged to be inconsequential for regular astronomical observation. Optical theory also predicts a minimal amount of field curvature, an aberration that can be evaluated without recourse to the star test and which after some scrutiny was never noticed or even implied.
Finally, I got the official verdict on spherical aberration: The Orion refractor was found to suffer from a slight case of undercorrection, "but not much, and indeed you own a good-quality achromat," Guido said. On the other hand, the TMB refractor came out of the star test as a winner, bearing an essentially perfect achromatic doublet; spherical aberration was barely detectable, with a trace of undercorrection being suspected but unconfirmed. "This is one of the best corrected objectives I have ever seen," Guido explained, "and it truly is an optical gem." While expected, those words brought relief. "Can you quantify?" I asked, and he offered some interesting numbers: the Orion refractor can be rated to about one-sixth wave, peak-to-valley (PV), while the TMB refractor rates at one-eighth wave PV, at the least.
Nonetheless, Guido stressed that his values for spherical correction are only approximate, and he further cautions that the star test method, while reliable, is a modest technique which bears its own constraints.
To make a long story short, what was intended as a one-hour visit evolved into a four-hour observing session. At eleven in the night I said thanks, shook my friend's hand and walked out to my car. I raised my eyes to the sky one last time, realizing that not a single cloud had rolled-by so far in the evening, something unusual for the Caribbean where clouds appear out of nowhere and when least expected.
7. Additional considerations on refractive optics.
I am sometimes asked—in particular by owners of expensive equipment—why use a long-focus refractor, when shorter telescopes can now be made with better optical correction thanks to newer types of glass? This argument is mistaken for a number of reasons, but primarily because it overlooks the deleterious effect of field curvature, an inherent aberration of refractive telescopes that is agnostic of glass type and worsens linearly at shorter focal ratios. I suspect that quite a few refractor owners, falling prey to this misconception, ended up shelling out large amounts of cash with the hope that some exotic, rare-earth glass would provide them a nice flat field with point-like stars out to the eyepiece edge.
My answer: Long-tube refractors possess many advantages for visual use, asides from the mere control of chromatic aberration. A partial list follows: (1) A linear reduction of field curvature, as explained before; (2) greatly reduced defocus aberration, via an exponential increase in depth of field that makes easier to achieve a precise focus; (3) minimized effect from misalignments in the optical train, and ease of collimation should the need arise; and (4) improved performance with eyepiece designs that remain popular in amateur astronomy, such as Abbe orthoscopics, Plössls, Königs and Erfles. Statements #2, #3 and #4, plus an exponential decrease of coma, also apply to most long-focus reflective systems.
Someone will insist that expensive glass implies superior optics that go beyond the suppression of color. In part, yes, but no amount of optical figuring or polishing can remove intrinsic aberrations like field curvature in refractors, or coma in reflectors. What needs to be understood is that off-axis aberrations act irrespectively of glass type and quality control, but can be lessened at nominal cost by switching to longer focal designs, i.e., higher ƒ-numbers. Granted, field flatteners are commercially available to achieve a true flat field in short refractors, but with the disadvantages of increased cost and greater difficulty of collimation; the same goes for the Petzval four-element objective used in astrophotography.
I have brought up the issue of curved fields because for some reason it does not appear to be a regular topic in discussions concerning refractors, which nowadays focus mainly on chromatic aberration.
But field curvature requires panoramic fields of view to become evident, and this brings me to a related issue: Are wide fields of view possible at all, using telescopes this long? Higher focal ratios are known to be challenged in this regard, and an ƒ/11 system is generally considered to belong in this category. At 900 millimeters of focal length using a 2-inch focuser, theory predicts that a true visual field of slightly over 3 degrees should be delivered. But is this the actual case with the Orion and TMB refractors?
The TeleVue 55-millimeter Plössl—an amazing 2-inch eyepiece, which I obtained as a gift from a good friend at PRAS—coupled with the Baader 2-inch diagonal viewer, make perhaps the ideal pairing for an experiment like this; the lens with its 46-millimeter field stop, plus the diagonal with its 48-millimeter unvignetted aperture, offer the largest possible fields in the contemporary 2-inch market. After repeated use with both refractors it became evident that the Plössl yields a true field of exactly 3 degrees, which I believe is more than adequate for casual telescopic observation. This ocular will display the entire Belt of Orion in one single view, and will also frame the Pleiades star cluster with room to spare. Large clusters like Messier 7 (in Scorpius) and Messier 44 (Praesepe, in Cancer) are optimally presented.
The Orion 32-millimeter Optiluxe—another fine 2-inch eyepiece, a Japanese import that was sadly discontinued back in 2007—is also a personal favorite, achieving a true field of slightly over 2 degrees. The Optiluxe performs well with both refractors, and in spite of its inherent off-axis astigmatism the tightness of stars across the entire area remains impressive, explaining why it is my most used 2-inch ocular. Regions like Orion's Sword, the Virgo Cluster of Galaxies, and the dense stellar patches that populate the Galactic plane, offer memorable views with this combination of eyepiece and objective.
Together with the refractors, these two eyepieces give low-power views at 16× and 28×, respectively, in turn yielding telescopic exit pupils of 4.9 and 2.8 millimeters. This implies that no light is being wasted.
To recap, long-focus refractors are not items of nostalgia, but a sensible application of the laws of optics to reduce intrinsic errors; off-axis aberrations, in particular, can be lessened by increasing the focal ratio of an optical system. For purely visual use, the main added benefit of a longer refractor is a flat field of view, with the potential of tack-sharp stars across the entire area when using well-corrected eyepieces.
8. Final thoughts and a summary.
The Orion and TMB refracting telescopes are both a joy to use, and will undoubtedly stay with me for life. I sometimes get offers to sell but have politely declined; as the reader will suspect, the thought of a sale has never entered my mind. The TMB refractor, in particular, is the most satisfying instrument I have ever owned, both mechanically and optically. After nine years its performance under the night sky still amazes me, and even a quick peek through this telescope feels like peering out from a spaceship.
Interestingly, my most used telescope is nowadays the Orion refractor. With an optical tube weighting less than 2 kilograms, this instrument is unparalleled in convenience and portability. Particularly in the Caribbean, where unpredictable rain showers give only seconds of notice to seek shelter, portability is a valuable asset. This makes the Orion refractor my preferred telescope for public outreach, both during night and day, as I am frequently requested to do solar observing events. On the other hand, the TMB refractor remains my backyard instrument of choice for quieter nights of lunar and planetary observing.
As explained, an amateur astronomer must seek telescopes that are specialized for his interests, and my extolling of the virtues of small refractors should not be interpreted as dismissive of other sizes and optical configurations. I also own a fine 150-millimeter ƒ/12 Maksutov-Cassegrain which, despite a few mechanical quirks, has never failed to deliver fantastic views; naturally, this larger instrument will provide better resolution and richer planetary detail than any 80-millimeter refractor. I also enjoy Newtonian reflectors and would eventually like to acquire a premium 150-millimeter unit in the ƒ/6 to ƒ/8 range. Occasionally I am granted access to larger telescopes from PRAS friends and colleagues, but my current limitations of space and budget preclude me from indulging in tempting purchases like those.
My impressions after using both the Orion refractor and the TMB refractor, for 15 years and 9 years respectively, are summarized below.
Summary of advantages:
• Outstanding optical quality, with minimal to zero residual spherical aberration.
• Focal ratio of ƒ/11 allows near-perfect correction of both field curvature and chromatic aberration.
• Improved glass type on the TMB refractor provides additional reduction of chromatic aberration.
• Generous field of view allowing upwards to 3 degrees, in spite of long focal ratio.
• Solid construction and adequate baffling, particularly with the TMB refractor.
• Moderate length of tube (900 mm) provides excellent ergonomics.
• Lightweight (1.8 kg and 3.2 kg); easy to transport and mount.
• Focuser on the Orion refractor features adequate travel length (150 mm).
• Elegant, classic appearance that makes a statement.
• Affordable price for both models.
Summary of shortcomings:
• Aperture of only 80 mm imposes an onerous limit on light grasp and optical resolution.
• Fixed, non-collimatable lens cell on both units; unappealing plastic cell on the Orion refractor.
• Dewshield too small on both units; passable on TMB refractor, unacceptable in the Orion refractor.
• Focuser on the TMB refractor lacks sufficient travel length (only 80 mm, need about 150 mm).
• Factory-installed focuser on the Orion refractor is 1.25-inch, but can be upgraded to 2-inch.
As noted before, both the Orion and TMB refractors are now discontinued, but likely to be found in the used market. Over the years and due to its reputation and its low production, the TMB refractor has evolved into a collector's item, but units still turn up on classified advertisements once in a while. Other brands are also in the market, like Vixen with its 80-millimeter ƒ/11 A80Mf refractor. I have never seen one of these in person, but the reviews are generally positive; however, there is a consensus that older units are preferable to more recent ones, as the former A80M featured superior Japanese optics.
I submit that a telescope similar to the ones that I have reviewed here will offer the aspiring amateur astronomer a solid introduction to the night sky, at a very affordable price. People frequently approach me to ask what to buy as a first-time telescope; it is the top question that I receive and a prominent topic in my public talks. But my advice is straightforward: get either a 75 to 100-millimeter long-tube refractor, or a 150 to 200-millimeter Dobson-type reflector. An 80-millimeter refractor is a good choice for a beginner, and it is my belief that the preceding review offers solid arguments to support this advice.
This concludes my review, but a thread will be started on the forums to allow further discussion of this topic. If you enjoyed my story, you are invited to chime in. I would like to express my appreciation to José E. Torres and Guido Santacana, fellow members of Cloudy Nights who took the time to read the final draft and to convey their thoughts. And if you made it this far, thanks for reading! ■
The author's English-language book containing the memories of his work with the IceCube South Pole Neutrino Observatory is available for free download from the PolarTREC website, at the following address: http://www.polartrec.com/resources/article/a-puerto-rican-in-the-south-pole
The author's debut commercial title, "Astronomía descriptiva", is a 2019 Spanish-language astronomy textbook which is suitable for both introductory college courses and as a general reference for amateur astronomers. For details visit the author's website at: http://armandocaussade.org/
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