Tele Vue Zoom 8-24 mm
Do you like flying near the Moon surface or diving into the Milky Way ? Yes, this is quasi the impression I have felt using the Tele Vue zoom 8-24 mm !
For years zoom have gotten a bad reputation due to their poor optical quality. They yield very dim images and they were tall and non-ergonomic. Hopefully with years things have changed a lot. The best proof is that brand as Tele Vue, Meade or Vixen, not the worst on the place, on the contrary, decided to sell a 8-24 mm version which gradually forced observers to change the perception they had of this forgotten accessory.
Brands and models
Three models of zoom eyepieces share similar performances. The Vixen, sold by Telescopes in the U.S.A. or Interoptic in Belgium, is the cheapest model ($170) and uses famous Lanthanum lens-elements. It is very similar to the Tele Vue or Meade model ($210 or 229 €). For one time Meade relabeled a Swarovski zoom but dropped the unit. Their current version is a Vixen although the Swarovski was a superior unit. Tele Vue released in 2002 a new version, a 8-24 mm click-stop zoom; it is similar to the previous model excepted that it includes stops at the 4 focals marks, a feature suited for binocular viewers.
The three models have a "JAPAN" engraved label. They are in fact designed by the same Taiwanese manufacturer, rebranded and customized on request and thus presenting minor cosmetic differences as an apparent field of view (AFOV) from 40° at 24 mm to 55 or 60° at 8 mm (Vixen), more or less focal marks or a better rubber eyecup (Tele Vue).
Another model is sold by Orion Telescope, the Ultrazoom. This is a 7-21 mm zoom which yields an AFOV from 35 to 65°. Scopetronix also sells a cheap 7.4-22 mm zoom probably of lower optical quality. At last, the Meade Deluxe zoom is the only one to provide a 6.5-18 mm zoom but it performs not as good as the other models.
Right out of the box, like a precious gem, the Tele Vue zoom is quite heavy and tall in hand. It looks like a professional accessory, a nice black and silver piece of craftsmanship presenting a large black rip ring which motion is very smooth. At cold its resistance is more noticeable but it works still fine at temperature as low as -15°C.
Without entering the God's secrets, the layout of such zoom is well preserved. Such zoom use 7 lens-elements in four groups. At least 6 surfaces are in contact with the air but all are fully multicoated, the Tele Vue reflecting a green and purple light. None of the zoom's I inspected presented the least smudge or default.
When moving the mechanism in the light, we can see an internal element moving. In fact all zoom use a moving Barlow at the tail end of the optics to insure the magnification. This is when the Barlow in nearest the frontal lens that the focal is the shortest (8 mm).
The inside of the barrel is flat and painted black to reduce internal reflections. A corrugated internal barrel as one can often see it in discrete eyepieces should have been more useful, but practically impossible to use due to the moving elements.
As many good accessories, this design works pretty darn well, the coating avoiding ghosting in usual conditions while aberrations are almost totally corrected.
As all the new generation of eyepieces, these zoom's are also threaded to accept standard 1.25" filters.
All zoom owners use the zoom with small scopes from 60 to 125 mm of aperture at focal ratios ranging from f/8 to f/12, some with a 2x Barlow, successfully. Even Tele Vue suggests to use its zoom with its Barlow's. But we have to remember that the eyepiece or zoom performance can change dramatically at shorter focal ratio.
The disadvantage of this kind of accessory are numerous but I can personally live with them as advantages balanced rather well these constraints.
A zoom will never reaches the performance of a discrete (fixed) eyepiece, even if their design deeply progressed with years.
A zoom will always be limited in field size and overall performances due to its relative complexity. The field of view for example is quite narrow at low power : at 24 mm the AFOV is only 40°. This yields a true field of view near 1° in a 125 mm SCT, but that already drops to 28.8' in a 200 mm f/10 and only 19.2' in a 300 mm f/10 scope. Hopefully the AFOV gets wider as you zoom in.
For comparison, at only 12 mm - but at nearly twice that price too - the Tele Vue Nagler IV yields the same true field as the zoom at 24 mm, but with twice that magnification ! Consequently the discrete eyepiece gains numerous faint stars for an identical field of view. In a similar way, the 22 mm Nagler IV yields a true field twice wider than that of the zoom at that focal !
The zoom is not parfocal with itself ! That means that as you zoom from one end to another end, you will need to refocus by a few tens. This is more apparent at short focals but there are some "bands" which are nearly parfocals.
The image quality, without speaking of price and design but only brightness, sharpness and contrast in the field, is a subjective matter we can however circled. Whatever your scope f/ratio, for a same aperture the brightness is a constant for a same magnification. But we have to remember that a dark background might yield the feeling of better contrast, and less intense in another circumstance in which a bright background seems reduce the overall contrast giving the feeling the picture is less sharp.
Nowadays these 8-24 mm zoom are on par with most Plössl's which yield an AFOV of 50°. But the comparison stops here. In nearly all circumstances, due to their simpler design, even Orthoscopic's eyepieces (AFOV of around 40°) give better contrast, mainly at high power on planets. The zoom has also some pincushion (when panning the sky the star field seems magnified), more pronounced to the edge, giving the feeling that the frame is bent.
Although the focal change is linear, problems occurs at high magnification. Below 10 mm of focal, the image is sensibly dim and not very pleasant even on brigth subjects as the Sun. I personally tried many times to use the zoom at its maximum to study sunspots and planets but without much success. May be it was a bad day for turbulence. Other amateurs using a 90 mm SCT with the Vixen zoom at 8 mm and a Celestron 2x Ultima Barlow were however able to discern detail on Mars during favorable oppositions. But despite of good seeings, for the image quality Ortho's win; you will always see more clearly small details through an Ortho than using a zoom. Therefore, the practical range of focals is between 24 and 10 mm, the turbulence and an usually medium seeing prevailing to push the zoom to the maximum.
At last, the Orion Ultrazoom is noticeably sharper than all other models but lost some light in the numerous lenses it uses. It has also less eye relief. So it has its own compromises.
Even taking account of the caveats, the 8-24 mm zoom is an extremely versatile eyepiece, as one might expect; it gathers in one piece all the ones you need, from the wide field to the shortest at your fingertips. This zoom performs thus with success on a variety of targets. This is a true "all-in-one" accessory suited for all purposes, asking for a minimalist setup and storage place. When you have to move to a remote observation site, one accessory hiding several discrete eyepieces is very useful. More, being sometimes your unique eyepiece you don't easily lost it in the dark !
Optically speaking, if we dare to compare this zoom to discrete eyepieces, whatever the zoom model at low power (20-24 mm) the image is brighter and crisper to the edge than in a Meade Series 4000 Plössl's for example which only use 4 lens-elements and also sharper to the edge than a Celestron 25 mm SMA (modified Kellner). The contrast on a bright subject as the Moon or its resolution on Jupiter cloudy bands is also better than using a Celestron Plössl, but not as clear as in their Ultima Plössl's.
With a 68° AFOV, the Tele Vue 19 mm Panoptic gives however a flatter field, with a bit of pincushion, but the image is cleaner in the zoom, specially in the Vixen one. According to Darren Henning from Canada who uses a Celestron G3 (90 mm), at 8 mm the Vixen zoom presents also a flatter and brighter field than the Tele Vue zoom.
What is also enjoyable and pleasant to observe is the darkening of the sky background while your zoom in and the feeling of flying over your subject. At low power on a 60-125 mm scope, your target is only a small spot hanging in a bright and clear sky everywhere present. But gradually, going to higher magnifications, you have the feeling to fly towards your subject, the contrast improves and features appears. A pure relaxing and exciting moment of observation.
In that regard, among the numerous advantages of a zoom over a discrete eyepiece, you have understood that there is one major benefit to use it : the fact you can quickly change your power. With its continuous range of magnifications, you can't never miss your subject and it can't never fall through the focal marks !
I first used the TV 8-24 mm zoom on a 60 mm refractor - the zoom being more expensive than the scope and at the limit of the weight supported by the drawtube (216 g) - then on larger scopes. Each time the feeling was very funny and gave me a surprising large range of magnifications. Indeed, its main advantage is to offer you the possibility to select your magnification according to the subject you are looking at, and to optimize your subject aspect.
A too low magnification on the Moon for example, reduces drastically the resolution and the contrast of details, so zooming in, you can select on fly the appropriate field of view to correctly circle the area you want to observe. Idem on nebulae. Placing your zoom at the lowest focal, you are sure to never loose your subject. Then you can gradually zooming in until you reach an optimum field of view, which will gives you the best contrast against the background in order to improve details on its low brightness surface. Better, using scope over 200 mm of aperture, this zoom offers you another challenge : if the DSO is plainly visible, it allows you to zoom in in order to bring out finest details. A discrete eyepiece could never offer you such versatility.
Double stars hunting is the favorite task for a zoom. Once in target you only need to magnify a bit more your image to split the system in its components, et voilà ! The zoom performs also well on bright globular clusters, wherein, by changing your zoom setting you can gradually resolve more faint stars. The contrast is best at low power but features are enhanced at shorter focals (around 10 mm). An irresistible and funny play.
About the design itself, excepted the subjective feeling for such or such brand, what is really welcome on such a large and heavy zoom is the groove or "shoulder" to let the screw hold the zoom to the focuser and prevent it to fall down. Even if the screw loosens a bit, it makes very difficult for the zoom to fall out. The Vixen and Tele Vue have this feature.
Another interesting and well-thinked detail is the large rubber eyecup that stops parasitic light and cold wind near your eye. You can also wear glasses while observing what is not possible with the Orion Ultrazoom.
At last, at short focal, the zoom uses even fewer lens-elements than similar discrete eyepieces (7 or more). In fact, for each subject there is a small range of focal that emphases details. Therefore, in nearly all circumstances, using a zoom you maximize your chances of detecting faintest objects and see details, excepted if we compare such a zoom with ultra wide eyepieces which are designed for that specific purpose (less lenses and better sharpness on-axis).
Comparison with an ideal competitor
Now, we can compare the Tele Vue zoom with a discrete eyepiece of same apparent field of view, may be specifications, and similar price. There are some. The Tele Vue 14 mm Radian is the ideal competitor. The Pentax 14 mm SMC XL is another one but it presents a simpler design (4 lens-elements, AFOV of 65°).
Some purists will argue "it is a bit silly to compare the zoom to a Radian or a Pentax as these last are in a whole different ballpark as far as quality is concerned. There would be something serious amiss if such high-end eyepieces didn't give a better image than the zoom !" But the comparison is useful to appreciate how much the zoom design improved with years.
The famous Radian yields an AFOV of 60° wide, a 20 mm eye relief, cost $228 (24% more expensive than the Vixen) and presents a similar weight (225 g) but it is one third shorter (7.5 cm). The Radian has also one less lens-element than the zoom. But the number of glasses and interfaces is not the only criterion that defines the quality of an optic.
As expected, in a f/10 scope the Radian wins on each run : at the same magnification, the discrete eyepiece using a different design looses less light in the glass and coating. In urban location or under exceptional seeing conditions the quality of views it provides is always better than that of the zoom : it yields brighter images and a sharper view without pincushion. Comparing to Pentax XL's the advantage goes always to the discrete eyepiece but not in a huge margin. That means that the zoom 8-24 mm evolved much with times and tends to be a good challenger to discrete eyepieces.
Now, we have to insist on the fact that there are not many short focal discrete eyepieces presenting a tremendous eye relief of 15-20 mm. Tele Vue Radian's, Pentax SMC XL and Vixen/Celestron LVW are the only one I know. But even including other brands, not all keep an AFOV of around 50° in medium focals, excepted high-ends and expensive models. So you would have to buy numerous Tele Vue (Plössl) to get the same overall performance as the Tele Vue zoom, including the pleasant eye relief and AFOV at all focals.
I personally uses the Tele Vue model. Tele Vue first designed this zoom to provide an ideal 20x-60x spotting scope power range with the Tele Vue Pronto and Ranger 70 mm aperture refractors.
The TV zoom has a harder and thicker rubber eyecup than the other models which does not fall down easily as the Meade, it has less focal marks (Meade marks each millimeter of focal while Tele Vue only marks at 8, 12, 16 and 24 mm) but the first has a much better support from Tele Vue which is in my humble opinion the most important thing after the product quality. These details worth well the few dollars more of the Tele Vue zoom.
Even if I use discrete eyepieces too, the zoom is the one I use the most because its power fills in the gap between low and high power discrete eyepieces, the intermediate powers where all scopes are most the times used under usual seeing conditions.
Knowing that the zoom costs about the same as two or more decent Plössl's, if we add its versatility, the calculation is quickly done and is in favor of the zoom. Of course purists will not have the same opinion, and I accept their argument as well.
Whatever your selection, all these arguments should convince you, and mainly the casual observer, that this zoom is not at all a bad compromise. For what it is worth this is a must that I warmly recommend you.
Users' comments extracted from SAA :
"SAA" states for sci.astro.amateur, the main English-speaking newsgroup dedicated to amateur astronomy.
Joe Hartley : "There are two differences that I've noted which lead me to recommend the Tele Vue over the Meade version. First is the rubber eyecup [...] then I regularly see the price of the Tele Vue unit $20-$30 cheaper than the Meade unit [...] The Tele Vue zoom has remained my most-used eyepiece".
Brian Tung : "I have a Vixen 8-24 mm zoom, and while I wouldn't say it is my most frequently used eyepiece, it is up there. I tend to use it mostly when I'm hunting down difficult targets and I suspect there might be a small range of magnifications where the object will be best seen".
Chris Marriott : "I've got the TV 8-24 mm zoom [...] I'd recommend it thoroughly - the variable magnification it gives makes it a very versatile eyepiece".
Jerry Bame : "I'm extremely happy with the Vixen zoom. The convenience of not having to switch eyepieces and hunt for your target make observing a lot more enjoyable".
Jim Idone : "I own the TV zoom, have owned TV Plössl's, Celestron Ultima and some of the Meade 4000 series and this zoom compares very close to all with better FOV".
Jay Reynolds Freeman : "I have a Vixen lanthanum 8-24 mm zoom eyepiece [...] It works well, and is invaluable for chasing down low surface-brightness deep-sky objects: some of these are best seen over only a small range of exit pupil size - I can change magnification in tiny increments with the zoom eyepiece and see what works best, and often as little as a 10 percents change will make a considerable difference".
Andre "PhotoKing" : "When I compare the Vixen zoom, no doubt the Pentax XL's win every time. But not by a huge margin. It is by a noticeable margin. The zoom delivers perfectly enjoyable, sharp, detailed contrasty views [...] I like to use the zoom when I know I'm going to quickly be looking at several different types of objects, and don't want to keep switching eyepieces [...] The $160 zoom is a good way to immediately have a broad spectrum of eyepieces sizes that bring immediate pleasure to using the telescope".
For more information
Sky & Telescope test report, Alan Dyer, Sep 2002.