Do you like flying near the Moon surface or diving into the Milky Way ? Yes,
this is quasi the impression I feel using the Tele-Vue 8-24 mm zoom.

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 contrary, decided to sale
a 8-24 mm version which gradually forced observers to change the perception
they had of this forgotten accessory.

The brands and models

At least three models gives me the same feeling and share similar
performances. The Vixen, sold by Interoptic in Belgium is the cheapest model
($160) and uses famous Lanthanum lenses elements. It is very similar to the
Tele-Vue or Meade  model ($210). 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 just released a new version, a
8-24mm click-stop zoom but similar to the previous model excepting stops at
the 4 focals marks to be 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 sales also 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.

Zoom Pictures : Scopetronix,  Vixen,  Tele-Vue,  Tele-Vue (click-stop)
 

Right out of the box, like a precious gem, the Tele-Vue zoom for example 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 lenses-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 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.

Disadvantages

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 already drops to 28.8' in a 200mm f/10 and only
19.2' in a 300mm 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 ! By consequence 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 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 Plossl's which yield an
AFOV of 50°. Due to their simpler design, in nearly all circumstances
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 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. I
personally tried many times to use the zoom at its maximum on the Sun 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
the Ortho wins. 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.

Advantages

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, whatever the zoom model, at low power (20-24 mm) the
image is brighter and crisper to the edge than a Meade Series 4000 Plossl's
for example which only use 4 lenses-elements and also much better 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 Plossl, but not as clear as in their Ultima Plossl'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 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 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 focals 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 the subject you are looking 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 of these zoom, excepting 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 one.

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 lenses-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, excepting 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 characteristics 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 lenses-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 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, excepting
high-ends and expensive models. So you would have to buy numerous Tele-Vue
(Plossl) 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 also, 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 Plossl's,
if we add its versatility, the calculation is quickly done and is in favor
of the zoom.

Whatever your selection, all these arguments should convince you and mainly
the casual observer this zoom is not at all a bad compromise. For what it
worth this is a must I warmly recommend you.

Users's comments extracted from SAA :

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 Plossl'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".

Thierry LOMBRY
http://www.astrosurf.com/lombry/
email : lombry@pt.lu
Luxorion