Performance graph of Tele Vue optics (zoom 18-24mm and TV eyepieces)
Abstract
Experienced users state zoom cannot reach the performances of fixed focals eyepieces. Although they actually use better designs than in the past and better multicoated, the result in far from perfect. To its discharge, the price of course which is very attractive for what I would qualify of "Pseudo Plossl" or better, "Super Ortho" optic.
Diagram :
Apparent True field
field
0 10' 20' 30' 40' 50' 1¡ 1¡10
| | | | | | | |
|
| 8mm 12mm 18mm 24mm
|
|
| 4.8 7 9 12 (13) 16 17 (20) 22
82¡| ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡
80¡| Nagler's
|
|
|
|
|
70¡| 15 19 22 27 35
68¡| ¡ ¡ ¡ ¡ ¡
| Panoptic's
|
|
| 8mm LV
60¡| 34 5 6 + 8 10.5 12 14
57,3¡| ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡
| Radian's
| +8mm TV
|
| 8 11 15 20 25 32 55
50¡| ¡ ¡ ¡ ¡ ¡ ¡ ->¡
| Plossl's
|
43¡| 40
| ¡
|
40¡ | + 24mm TV/LV
| Ortho's
|
|______|______|______|______|______|______|______|
0 10' 20' 30' 40' 50' 1¡ 1¡10
True field
(c)T.LOMBRY,07/1999
Interpretation :
These trends line are objectives results and has to be consider as such.
They only display the ratio between the improvement in design and the resulting increase of true field of Tele Vue series of high-end eyepieces.
They ignore the subjectivity, the emotion a user feels once he looks at wide planetaries fields (stars, moon or planets), the brightness (gain or lost), aberrations that could subsist in such optics.
Also important, the contrast and brightness feeling; whatever the aperture ratio f/ for a same aperture, the brightness is a constant for a same magnification. But a dark background might yield the feeling of better contrast and less intense in another circonstance in which a bright background seems reduce the overall contrast and give the feeling the picture is less sharp.
So, all these parameters are not taken in account.
The goal of this chart is to show the quality of several zoom versus fixed focal eyepieces offering a similar field of view or focal lenght.
Material :
Tele Vue zoom 8 - 24 mm (55-40¡ from 8 to 24mm, eye relief from 15-19mm)
Vixen LV serie zoom 8 - 24 mm (60-40¡ from 8 to 24mm, eye relief from 15-20mm)
Meade serie 4000 zoom 8 - 24mm (similar to Tele Vue)
Appreciation
First, to qualify a zoom "as good as" a serie of fixed focals eyepieces, its slope must trend toward the up and right of the diagram (wider field and better quality). For comparison needs, nowadays the Nagler's serie of eyepieces (over 82¡ wide) are considered as one of the best optics on the market.
With a true field of 13.7', at its shortest focal Vixen LV Zoom reaches near the Radian's true field (15'), while the Tele Vue model is "only better" than Plossl's (11.7') due to its 55¡ apparent field at that focal.
Although the focal change is linear problems occur some stops lower.
At 16mm of focal for Vixen LV zoom and 12mm for Tele Vue the two models cross the Plossl's level of quality. At that focal they yield the same results as a 12mm Plossl eyepiece, but the Tele Vue already presents a true field a bit shorter (18') than the Plossl (20') and the Vixen LV zoom (21'). Then both quickly degrade down to reach the Orthoscopic performances at 24mm.
Their most significant degradation is the size of the true field which reaches just the one of the 26mm Orthoscopic at their lower focal of 24mm (30.2').
On the other side of the scale, at only 12mm (but at nearly twice that price too) the new Nagler type 4 yield the same true field of 30.9' as the zoom at 24mm !, but with twice that magnification (158x vs 79x). By consequence it gains numerous faint stars for an identical field of view ! In a similar way, the 22mm Nagler 4 yield a true field of 56.2', twice wider that the zoom at that focal !
At last, in the range 8 - 12 or 16mm the Tele Vue and Meade zoom's seem also less sharp than a Radian eyepiece, yield probably no more aberrations under "normal" circonstances but both models are dimmer than Plossl's at all focals due to the number of elements.
So their best usage stays between focals 8-16mm but for longer focals they are both equivalents to high corrected Orthoscopic's but no more.
Regarding their price.
In a range 160 - 259$, depending the dealers discount, this useful optic is really cheap for results finally quite correct.
It's only a pity their optical quality tends globally down rather than toward the Radian or Panoptic zone.
I'm sure with some imagination they can become war machines, ready to fight on par and surely for some run with so called high-end fixed focal optics.
I wonder why Nagler and his team haven't scheduled yet a more performed design.
Part of the answer could well be found in the volume and weight of such optic...
Fixed eyepieces in the same range of focals are for example (not exhaustive list) :
8mm and 14mm Radian, 19mm Panoptic and 22mm Nagler4. All are parfocal except the Nagler. This set cost at least 1095$ versus "only" 160$ (259$ list) for the zoom.
The calculation of my budget is quickly done.
If I buy such a zoom, I'm sure to get the same quality as a "super" Ortho, but in sparing the 4/5 of the full set price I can reinvest in two or more fixed focal eyepieces to complete both end of magnifications (7mm Nagler and 35mm Panoptic for example) and there'll be still money !
T.LOMBRY, Luxembourg, 30 July 1999.