Tele Vue TV-76 Review

Pre-2005 TV-76 on Tele Vue Panoramic mount.

When Tele Vue replaced the Pronto with the TV-76, nothing changed; from the outside at least. In terms of the lens, though, the TV-76 is a proper apochromat whilst the Pronto was really an achromat, despite using a cheap ED glass for marketing purposes. Does this justify the price hike that went with the upgrade? Yes, it does, because apochromatic lenses are much more expensive to make than achromats, both in terms of raw materials and labour. So exactly what is an apochromat anyway?

APOs, Apples and Blueberries

As usual, if you know this stuff, skip to the review …

You will find many precise definitions of apochromat out there, usually couched in terms of crossings and airy disk sizes and wavelengths. Unfortunately there seem to be as many precise definitions as there are definers.

The reason for the ambiguity is that there is a continuum in refractor telescopes from fast achromats like the short tubes 80s, through slower achromats, then ED glass doublets, fluorite doublets, all the way to ED glass triplet “super APOs”, which are effectively as free from false colour as a reflector. So here’s my (deliberately vague) definition of apochromatic:

 “A level of false colour too low to be noticeable in normal use.”

I’ll tell you up front that the Tele Vue TV-76, a 76mm (!), F6.3 ED glass doublet, meets this criterion for visual use at least; the Pronto certainly didn’t.

Quantifying chromatic aberration is difficult on the night sky. My preferred way is to look at tree branches silhouetted against a bright daytime sky using about 100x magnification. This way, any false colour evident to the eye will show up (for aberrations deep into the violet or beyond the visual range you would need to take CCD images).

Under this (admittedly harsh) test, a Pronto shows strong violet and green fringing in or out of focus; a TV-76 shows little false colour in focus, but a noticeable apple or blueberry haze either side. By comparison, a Takahashi FS-78, a longer focal length fluorite doublet, shows just the slightest hint of colour out of focus only. Trying the same test on the finest triplet super APO (e.g. a Takahashi TOA 130 – the most APO of them all) reveals no visible false colour at all, in or out of focus.

Would these differences make much difference is use? Well, for casual viewing, no. On the other hand if you want to take images (either at night or during the day) they might.

There is a further complexity. With a fast doublet like the TV-76, designers have to choose which wavelengths (i.e. colours) to correct – like life, you can’t have it all. So fast doublets are often poorly corrected at longer (red) wavelengths. You can see this effect in these two graphs of Strehl (an indicator of overall aberration level – 1.0 is perfect) against wavelength for two (actually Takahashi) APOs – one an F6 doublet, the other an F8 doublet:

 

Strehl at different wavelengths for an F6 doublet

Strehl at different wavelengths for an F8 doublet

These graphs (copied from a report by W. Rohr) are specific to the lens designs to some extent, but you get the idea. Certainly the TV-76 is an APO, but its performance will approximately equate to the first graph. What difference does this make in use? Wait til we get to Mars …

Design and Build

Lens

I have said that the TV-76 is an ED glass doublet and done the APO thing to death, so I won’t dwell further on that aspect of the ‘scope. Suffice to say that in the TV-76 the 76mm F6.3 (480mm F.L.) lens is permanently collimated in a basic, narrow cell which keeps helps keep the tube slim. The cell is held on by three dome-head fasteners and factory collimation is achieved by nudging the cell into perfect alignment. It’s a simple system that’s used on all TVs, but it works.

The lens coatings, as you would expect, are top quality.

Tube

The other way in which Tele Vue go about making their scopes compact (and they are, the TV-76 is much smaller than the FS-78, or just about any other 3” APO – 14.5” long and a bare 3” diameter) is to use a flocking material in the tube to combat stray light, rather than the traditional knife-edge baffles which need a fatter tube. As far as I can see, the material Tele Vue use is essentially matte black painted coarse sand paper, but again … it works!

The TV-76 is certainly compact, but it isn’t really lightweight for its size. Though the basic OTA weighs about 2.7 Kg, once you add the tube rings it comes close to the weight of some much larger scopes (the FS-78 is much bigger but similar in weight). The reason is that the TV-76 is extremely solidly and ruggedly built. When you pick it up it feels, as a friend vividly expressed it, “like a solid chunk”.

TV-76 alongside Tak’ FS-78: both are 3” APOs, the TV-76 is slightly heavier!

The general build quality and style of the TV-76 is the same as other Tele Vue refractors of the last 25 years or so. The tube is powder coated in an attractive (and durable) knobbly finish and the focuser and sliding dew-shield are black anodised (later versions are satin black). The lens cap is another TeleVue standard: heavy, solid metal and screw-on; it looks great but can be fiddly (and screechy) to fit at the end of an observing session.

I reckon the TV-76 is one of the most attractive ‘scopes out there and certainly the heft, palpable build quality and simplicity give it an “heirloom” feel. It may not have the very finest lens, the very best Crayford focuser, or any gold leaf or brightly coloured anodising, but overall quality is as good as it gets and highly functional.

The OTA is 14.5”, the case carry-on at 21”.

The TV-76 comes with a carry-on soft case with room for accessories. If you opt for the package you get a clamshell, a 2” dielectric diagonal (of the highest quality) and a 20mm Plossl as well.

Focuser

The focuser on my TV-76 has been out of production for many years. It is the original-style Tele Vue focuser with a chrome draw-tube and glossy anodising. It comes with the classic and much copied Tele Vue ‘Mag Wheels’. A basic rack and pinion unit it might be, but this old-style TV focuser is incredibly smooth and free from play or image shift. It also looks good.

The modern units are more solidly constructed to take heavier cameras and have dual tensioners which create less image shift on lock-up. My experience is that they have a different feel that is not as fluid, but perhaps more solid and less likely to rack out under load. The most recent versions come with a dual-speed Feathertouch pinion unit built in. I should also point out that a conversion to a 2” Feathertouch, that replaces the main focuser body with an adapter, is available and looks attractive (though you would lose the serial number plate).

Tele Vue make an after-market dual speed focuser pinion called the ‘FocusMate’. It works well, but I don’t recommend it: I know of people (no, not me) who have damaged draw-tubes, cross-threaded screws and lost vital shims (creating image shift) trying to fit it.

Mounting

Mounting is via a clamshell which has three ¼-20 threaded holes. The centre one is for a photo tripod (the TV-76 is small enough), whilst the others are for Tele Vue’s own mounts. If you want to use the TV-76 on a Vixen or Synta mount then Tele Vue make a dovetail which bolts straight on the clamshell and which has the clever feature of replicating the Tele Vue pattern threads on its base so you can easily move from one type of mount to another.

The clamshell is super-slim and intended to stay-on. You can adjust tension via a winged nut, but it doesn’t hinge and you need an Allen key to remove it.

Mounting ring base with ¼-20 threads.

For these tests I mounted the TV-76 on Tele Vue’s Panoramic and TelePod mounts and a Vixen GP.

In Use - Daytime

The TV-76 works superbly as a daytime spotting or birding scope, delivering beautifully crisp, bright, wide views with no significant in-focus false colour. It’s quite compact enough too, though sufficiently heavy that you’d need a good solid tripod and head with a large (preferably fixed) plate (most quick-release plates will be too small and not stable enough).

TV-76 with dew-shield extended.

Since Better View Desired said nice things about the TV-85 many years ago, TVs have developed quite a following amongst birders and with reason – it’s hard (if not impossible) to get a scope containing roof or porro prisms to perform to this level and the ability to deliver critical detail for astronomy makes them ideal for spotting that last nuance in plumage. Of course, the TV-76 isn’t waterproof and a standard diagonal gives a mirror-reversed image.

In Use – The Night Sky

Cool Down

The TV-76 cools very quickly and benignly so you can use it straight from a warm house up to moderate powers. This surprised a friend when we did a test against his Questar (which takes much longer being a Maksutov). The small size of the TV-76 helps to keep tube currents down and it takes noticeably less time to cool than the larger FS-78. Even the best triplet would take much longer. Quick cool-down makes the TV-76  particularly good for grab-n-go.

Star Test

My TV-76 has a decent star test with a bit of under-correction. It tends to confirm the test reports I have seen that suggest Tele Vue achieve a good solid ‘better than diffraction limited’ quality with their lenses – maybe 1/5th wave PV. All the Tele Vue’s I have had have achieved this kind of quality. Takahashis are, I think, usually a bit better – often up to 1/8th wave PV. Can you notice the difference? Generally I think not, but for the most critical high-power planetary use – possibly.

The Moon

The TV-76 does deliver excellent views of the Moon. The good correction for false colour means you get true a lunar monochrome - greys and whites and buffs. At similar powers an achromat like the Pronto spoils with a false-colour purple wash. A Nagler 5mm gives just under 100x and makes an ideal Lunar eyepiece for the TV-76 - the whole Moon fits pleasingly into the field whilst revealing a lot of detail. Plug in a 2x or 2.5x Barlow (or even better, Tele Vue Powermate) and your DSLR will give great photos of the Moon as well, at fast exposures so that tracking isn’t required.

When I tested the TV-76 against a Sky-Watcher Equinox 80, they were generally very similar, but I found that the Equinox produced a fringe of diffuse light around the bright lunar limb at high power that the TV-76 avoids. This (subtle) effect was repeated by the larger Equinox I tested – due, I suspect, to a finer level of polish or perhaps better glass in the Tele Vue.

Planets

I have had very fine views of Jupiter with the TV-76, especially when mounted and tracking on a good equatorial mount. I’ll say it again – a stable mount really matters for seeing planetary detail. In good seeing a 3.5mm Nagler at 137x yields much Jovian pleasure: multiple belts, dark festoons and storms, the Great Red Spot (now faded to a pale buff) and shadow transits are all within the TV-76’s grasp.

Saturn looks similarly fine on nights of good seeing, crisp and aberration free at high power: a perfect tiny ringed planet floating in a black void with subtle shading on the planet and ring shadow in stark relief. A fine 3” APO is the smallest size that will reliably show you the Cassini Division in my experience.

Why then have I never been able to get very satisfactory views of Mars with the TV-76 (and believe me I’ve tried when for years it was my main grab-n-go)? The answer goes back to those graphs at the beginning. Fast ED doublets like the TV-76 tend not to be so well corrected at the red-end of the visual spectrum. Consequently, Mars always looks a bit bloated, with a bit of colour fringing and I can never quite seem to get perfect high-power focus. Compared with the Takahashi FS-78 (which is a great small scope for Mars) this is particularly noticeable.

If you love Mars, then an F6 doublet – TV-76 included - may not be your ideal APO; go for an F8 fluorite doublet or a premium triplet instead.

Deep Sky

The TV-76 works particularly well for low power sweeping and open clusters, with a stunningly wide field (up to 5.5 degrees with a 2” eyepiece like a Nagler 31mm or Pentax 40mm), perfect pin-point stars and no false colour. Unlike some fast APOs, field curvature and coma are very well controlled in the TV-76 and never intrusive. Tele Vue’s push-pull Panoramic mount is great for this kind of viewing.

A 3” scope may have a modest aperture, but it’s already enough to be enjoyable on deep sky (in a way that a 60mm is not) and the TV-76 allows good medium-power views of planetary nebulae, but with a wide enough low-power field for really extended targets, such as the Andromeda Galaxy and the North American nebula.

If you want to image with the TV-76 (and its short focal length and lack of false colour are ideal), Tele Vue make a dedicated field flattener, the TRF-2008, which converts it to F5.

Summary

The used value of TV-76s has slumped here recently and I’m pretty sure I know why. People view ED doublets as being “old hat and inferior” (I’m quoting from a recent conversation) to the “newer” triplets and no better than cheap Chinese mass-market doublets. The same effect killed off the FS Takahashis a few years ago. It’s a shame really, because there are some false assumptions in there.

Triplets are not necessarily “better” than doublets at this size and F ratio (and certainly not at slower F ratios), unless you are doing very critical astrophotography with an expensive CCD, or critical planetary viewing. Even then, only the best triplets will be an improvement, cheap ones won’t. What’s more, doublets let a bit more light through, cool more quickly and are lighter and less front-heavy.

If you buy a cheap Chinese small APO you simply don’t get the guarantee of high optical quality you get with Tele Vue and for me this negates the cost benefit somewhat. The lenses in some Chinese APOs are good to excellent, others mediocre and even poor; they still vary a lot. Even if you buy a good one, chances are it has a lower level of hand finishing; it definitely won’t have been individually checked like a Tele Vue.

That said, Tele Vues are expensive, especially over here. Whilst I certainly don’t think Tele Vue should convert their range to triplets, I do think they will need to move towards the kind of absolutely premium optical quality of Tak’, TEC and AP if they want to keep charging high prices for their scopes.

To sum up, consider what else you might buy if you need a highly portable small do-everything visual telescope in the 80mm class:

1)      A super-shiny Chinese APO? No, because you’ll risk poor optical quality and they tend to be bigger and heavier anyhow.

2)      A triplet super APO? No, because it will be heavy and slow to cool.

3)      A longer focal length doublet (FS78, FL80, ED80 etc)? No, because they are much larger and less portable.

4)      A Questar? Perhaps, but it’s slow to cool and the long focal length isn’t much good for open clusters, star fields etc.

5)      A Sky 90? Possibly, but they are expensive and the objective design is very sensitive to collimation– hardly ideal in a travel scope.

 

In fact the main competitors for the TV-76 remain the TV-85 and TV-60!

 

The TV-76 is highly recommended as a grab-no-go or travel scope, but not at exorbitant UK new prices – buy used or on your Stateside holiday!