The Celestron C90 Maksutov Cassegrain Telescope
Mar 02 2015 05:00 PM by skyaddict
William Optics Zenithstar 71ED and Twilight I M...
Feb 28 2015 10:53 AM by Ahab
If you want to Master your craft, read Lessons...
Jan 07 2015 11:29 AM by AstroDad
Celestron Ultima 80
Dec 16 2014 05:54 PM by Gianluca67
Categories See All →
- CN Reports
- User Reviews
- How to . . .
- Observing Skills
- Astronomical History
- Optical Theory
- Vision and Related Experiments
- How to Gain the Support of your Family for your Astronomical Pursuits
- Evaluation Tips
- Special Events
- The Elements
- New Articles in [!monthname!]
- Telescope Articles
- Submit a Review / Article
- Monthly Guides
- Behind the Scenes
- About Us
- Copyright ©
- Terms & Conditions
- Tiny Eyes on the Skies
- From the Editor's Desk
- What's Up . . .
- The Light Cup Journals
- Who is this Super Light Cup?
- Cloudy Nights T-Shirts
- Imaging Contest
- Small Wonders
- Previous Imaging Contest Winners
- This Month's Skies
- Mike's Corner
- The Cloudy Nights Friends and Family Discount
- Uncle Rod's Astro Blog
- Fishing for Photons
- Binocular Universe
- Article Submissions
TeleVue 76 Doublet Apochromat Refractor
Voice your opinion about this subject in our forums
Tele Vue has enjoyed great success throughout the United States as producers of high quality optical products for the discriminating visual observer and more recently for the advanced astro-imager. Even after 30 years, Tele Vue eyepieces are made to a standard by which all other designs are compared and are deservedly the acknowledged world leaders in this arena of visual observing. But, if we’re honest, their telescopes haven’t enjoyed the same universal acclaim that say Astro Physics or Takahashi has enjoyed. Yes, they are very popular in the US, but here in the UK they enjoy a love-hate relationship with amateur astronomers. Some folk adore them. Other’s think they are too expensive and over-rated.
As a case in point, my path crossed with Anthony McEwan, a discriminating visual observer who once owned a lovely Tele Vue 85 and used it on most clear nights near his home on the Murray Firth in North East Scotland. But after experiencing the views through an economy-priced Sky Watcher ED 80, he told me he was unable to justify keeping the considerably more expensive Tele Vue, as the ED 80 was serving up very comparable images. McEwan’s story is not unique. My own experiences with a Tele Vue 102 were that, while it is mechanically in a different league to cheaper ED doublets, there was little difference between the views they delivered. No if or buts about it.
That said, owning a Tele Vue refractor is special. For me, they strike a near perfect balance between form and function, exuding quality from the tip of their threaded lens cells to the exquisite engineering of their rack and pinion focusers. And if looked after, they can last several lifetimes. So even though the Tele Vue 102 didn’t tick all the boxes for me, I still hankered after a high quality, uncompromising refractor for use by day and night; an instrument that I could take anywhere at a moment’s notice and which could deliver the readies when called for. The 102 is simply too big to be considered a grab and go scope, so when a used Tele Vue 76 came up for sale at a reasonable price I took a chance and purchased it. I now know I made the right decision, as it really has exceeded all my expectations.
The TV 76 goes anywhere at a moment’s notice
Introduced in 2001, the Tele Vue (TV) 76 was the replacement for their older ED scopes – the Ranger and Pronto – both of which were splendid 70mm F/7 doublets with good but not Apo quality colour correction. The TV 76 (F/6.3) has a slightly larger aperture but the same focal length as the older scopes. Like the Tele Vue Pronto, it has a beautiful rack and pinion focuser in a 2-inch format. Bought new, the package includes a custom soft case, a screw-on lens cover and sliding dew shield, a 20mm Tele Vue Plossl eyepiece, and 2-inch Everbrite diagonal, 1 ¼" adapter (all with clamp ring fittings) and manual signed by Uncle Al himself. When outfitted with an eyepiece and diagonal, it tips the scale at just over six pounds. That’s significantly heavier than some top-of the range spotting scopes but not enough in my opinion to present problems in the field. Any loss of portability though is made up for by the TV 76’s amazing versatility. A three inch aperture is just about large enough to make high resolution visual observing worthwhile, and its short focal length (480mm) coupled to a big, wide angled eyepiece means that you get majestic, five-degree views of the night sky.
I received a forest green, pre-2005 model, as it has the chromed focus barrel and single focus lock screw located just above the focus wheels. Personally, I’ve always preferred the look of the older focuser to the newer version. It also came with a balancing adaptor that enables you to adjust the pivoting point of the scope quickly and effectively when using eyepieces and other accessories that vary in weight.
It’s easy to test good optics and you don’t need an optical test bench to do it. The optics on this scope must really be experienced to be believed. Having owned and looked through several lower-cost ED doublets of similar specification, I can say, hand on heart, that the TV 76 bested them all. The difference was more dramatic in my opinion, than those I had experienced in comparing the 4-inch TV 102 with the Orion/Sky Watcher 100ED. Star testing this scope at 120x shows how superbly crafted the optics are. Vega displays a bone white Airy disk surrounded by a single diffraction ring. No colour error was noted, safe for the occasional sparkle from atmospheric turbulence. The diffraction patterns both inside and outside focus were the nearest I’ve seen to perfection in any telescope. They’re cleaner and easier to see compared to the slightly fuzzier patterns I’d usually observe with cheaper ED doublets. Like other two-element Apos, it does display a small amount of colour on the rim of the diffraction pattern both inside (magenta) and outside (green) focus but that’s normal behaviour for an instrument with an ED doublet objective.
The diminutive TV 76 atop a sturdy Gibraltar Alt-azimuth mount
The very well figured doublet objective of the TV 76
If you look carefully at the objective at various angles, you’ll notice that not all the air to glass surfaces have the same anti-reflection coatings applied. The coatings seem to be individually tuned to maximise light transmission. Although I think knife edge baffles do a slightly better job, stray light is effectively dealt with using the company’s matt black flocking material. The TV 76 comes with a dedicated clamshell and mounting bracket that enables you to attach the instrument to a Gibraltar or Telepod mount using two wing knobs. You can also attach it to a heavy-duty photographic tripod for more portable terrestrial projects.
Using a Vernier callipers I measured the colour error (see my previous Cloudy Nights article, “A little piece of Russian magic” for details) of this scope to be about 0.018% which I believe to be very good considering it has such a short focal ratio (F/6.3). A well figured lens ought to able to take very high magnifications before noticeable image breakdown occurs. My daylight and night time tests with high quality eyepieces and image amplifiers show that the TV 76 can take amazingly high powers and I can confidently attest that this little scope can hold 100x per inch of aperture. It has very low spherical aberration (at least 1/8th wave measured) and is devoid of astigmatism and coma. I consider that extraordinary too. And it’s no accident either. It’s down to both the excellent figure of the lens and the employment of a large air gap between the objective elements. Nagler is clearly a fan of the late American astronomer James Gilbert Baker (1914-2005), who introduced large air spaces between the objective elements in order to reduce spherochromatism (the change of spherical correction with wavelength) in the design of his refractors. And Uncle Al’s been doing it for quite some time now. My old 1991 Genesis F/5 had an air gap measured in inches between the front doublet elements!
The superlative rack and pinion focuser on the TV 76
One for the Birds
I must admit, though I’m no birder, I look through scopes as frequently by day as by night. The great advantage of using a ‘cross-over scope’ like the TV 76 compared with a conventional spotting scope, is that it can be purchased as an optical tube assemby and you can carefully choose a diagonal and eyepiece combination tailored to your needs. This, in my opinion, makes them far more versatile than dedicated spotting scopes. One can choose either a 1.25 inch diagonal or a 2-inch diagonal depending on the eyepiece you want to use. Most birders make do with spotting scopes that use relatively lightweight 1.25-inch eyepieces. 2-inch eyepieces deliver greater fields of view, which is great for astronomy but normally overkill if you’re trying to concentrate on the variegated feathers of a nesting kestrel. Besides, 2-inch eyepieces add a lot of weight to the scope too.
By purchasing an optical tube assembly, you get to choose the kind of viewing you want to experience. Having observed through traditional spotting scopes for many years, with their dedicated, non-interchangeable zoom eyepieces, I find the freedom the TV 76 gives me to be a great liberation. If for example, you wish to experience the prismatic world with its correctly orientated view, a number of companies including William Optics produce both 1.25 and 2-inch prismatic diagonals angled for 45 degree viewing. They were designed to give very good images over typical daylight magnifications for their small ED scopes. Alternatively, you can purchase the 60-degree Everbrite diagonal for 1.25 inch eyepieces which delivers the finest views this instrument can offer -although you’ll still have to get used to left-right reversed images!
The Tele Vue 60 degree Everbrite Diagonal
Night time observing shows up many of the strengths and a few of the weaknesses of this scope. With a 31mm Nagler, the little refractor serves up five degree views more comparable to giant binoculars than to an astronomical telescope. Many economical eyepieces work well with its modest focal ratio, especially orthoscopics. I noted some field curvature when used with low power eyepieces of various genres (including plossls, Pentax XWs and Naglers). Stars sharply focused at the centre have to be slightly re-focused at the edge of the field. Little or no field curvature was detected using my high power eyepieces.
The Moon and bright planets are gorgeous through this instrument. Creamy Jupiter showed me four or five bands on good nights with a palid Great Red Spot and hints of low contrast swirls and ovals in the giant planet’s atmosphere. The four ‘Medicean stars’ resolve to tiny globes at 196x. Banding on the Saturnian globe was also evident at powers at 120x and above but the rings were too ‘edge-on’ for me to test this scope on them. The full Moon was sharp and colour free with the prominent rays craters clearly in evidence. The background sky showed up the pinpoint light of faint stars near the limb – all good signs that stray light was being kept at bay. First quarter luna is a sight for sore eyes at powers up to and in excess of 200x with no colour fringing evident near crater rims. I did note that, while observing our satellite at high powers, the lunar regolith seemed to be imparted with a very pale ‘coffee’ coloured hue compared to the view served up by a Newtonian reflector set up next to it.
The TV 76 really rocks when it comes to resolving double stars. The instrument’s excellent colour correction makes seeking out variegated doubles a joyous adventure. Albireo, 61 Cygni and Almach unveil their austere beauty at moderate and high powers. Forget Polaris and Rigel: these high-contrast companions usually cited as tests for a 3-inch refractor are too easy for this refractor. More challenging (and more fun) are the lovely triple system of Iota Cassiopeiae, and close binaries such as delta Cygni and epsilon Aurigae, all of which the TV 76 managed to resolve. And though it’s not the hardest binary system to discern with a good 3-inch refractor, Epsilon Bootes (Izar) is arguably one of the most compelling sights to see in a small telescope in all the heavens. Steady skies and high magnifications are required to elucidate its lovely secret; a magnitude +4.6 blue green companion separated from its primary by just 2.9 arc seconds of sky. Now my little telescope can resolve pairs as close as 1.5 arc seconds provided they are of fairly equal brightness. But the near sevenfold difference in brilliance between Izar and its main sequence companion renders the secondary hard to see, overwhelmed as it is by the light of its primary. Optics plays a role with this system too – I've looked through 4-inch instruments that consistently struggled with this system but a high quality 60mm refractor should just do the job under good conditions. And though I've looked at Izar with all sorts of telescopes, from small portable 'scopes to humongous Dobs measuring fully two feet across, I must say that the fondest view of Izar I've ever enjoyed was with this 3-inch refractor. During a recent vacation to a tiny coastal resort on the north west coast of Scotland, I chanced upon some fair weather. Tucked away in a shallow inlet, the early evening winds subsided gradually to a dead calm after midnight, allowing me to take advantage of exceptional observing conditions with dark magnitude +6.5 skies. On two successive nights, I was able to rack up the power on my 'scope to 276x to get a razor sharp separation of the system. Like a budding yeast cell seen under a microscope, the pale blue ball of the secondary sat on an otherwise perfect first diffraction ring of a golden orange primary. Small wonder the famous double star observer Otto Struve was impelled to call it Pulcherimma!
The Tele Vue 76 waits patiently for darkness at a remote Scottish resort.
The TV 76 is arguably among the most perfectly made, ultra-compact, and highly versatile refracting telescopes ever designed. And even though many other instruments of similar specification to the TV 76 are now available at much lower cost than what this instrument retails for ($1725), I believe an experienced observer will notice the difference even during casual viewing, either by day or by night. While being an excellent astronomical instrument (with limited aperture of course), it also provides a real alternative to birders and other nature enthusists who require a super-quality, durable, no-nonsense instrument that will deliver bright, high contrast images and with better colour correction than any traditional spotting scope can currently deliver. Uncle Al has really done his homework with this instrument and, not having looked through a Tele Vue 85, I consider it to be the real jewel in Tele Vue’s telescopic crown.
The author has no affiliation, commercial or otherwise with Tele Vue Optics.
You can hear more about my telescopic musings in his forthcoming book, Choosing and Using a Refracting Telescope, which will be published by Springer in 2010.