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Leica Zoom 25x-50x ASPH (17.8-8.9mm)


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Summary

The new Leica ASPH Zoom was designed for their spotting scopes (82mm and 65mm) and has been adapted for astronomical use by means of a 2“ adapter. Its focal range of 17.8-8.9mm makes it a versatile choice for the amateur astronomer. Besides the general advantage of zoom eyepieces to enable fine-tuning of the magnification according to the needs this zoom for the first time combines no-compromise quality with a generous apparent field of view (AFOV) of 60° to 80°. Other features are comfortable eye relief of 18mm with a twist-up eyecup. In addition it is waterproof up to 5m/17ft through its nitrogen filling. The designation "ASPH" refers to the aspheric lens(es?) employed. "25x-50x" refers to the available magnifications together with the spotting scopes.

This review describes how well it works with an astronomical telescope. All statements made apply directly to using it with my TEC140 (f/7) refractor, but most of them can of course be applied to any telescope. Particular attention is directed to the zoom's use together with a barlow lens which extends the focal range and thus it can substitute almost a whole eyepiece collection.

I've no affiliation with Leica and have purchased this eyepiece through regular channels.

Introduction

My acquaintance with zoom eyepieces dates back in 2001 with the purchase of my Leica 22-7.3mm Zoom for my Leica Apo-Televid 77. In 2003 I got my first astronomical telescope, a 115/805mm TMB/APM apochromat. The Leica Zoom was adapted with a 2" adapter and it was used from that time on as my main eyepiece. During the years I very often did comparisons against other high quality fixed focal length eyepieces (Pentax XW, Nagler T6, Nagler 6-3mm Zoom, Takahashi Orthos, and Takahashi LE). I always found the Leica Zoom to be on par and due to the viewing comfort of being able to dial the appropriate magnification without sacrificing contrast, definition, and lack of straylight, left only one thing that I desired: more AFOV at the longer focal length end. Over the years I did learn to handle this handicap and to concentrate on the positive sides of this zoom.

The area where zoom eyepieces shine are high magnifications where they enable the user to choose just the right magnification to cope with the target and the seeing conditions. This I'd like to elaborate a bit more. High magnifications are those where the resolving power of the telescope is utilized. This means exit pupils from 1.2mm down to about 0.6mm (21x/inch of aperture to 42x/inch of aperture). Therefore, with the typical focal ranges of 21-7mm or 24-8mm a zoom eyepiece can only be used to its best if the telescope's focal ratio is around f/12 or higher. For the large number of telescopes with f/8 to f/4 a zoom will not cover the sweet spots. Now barlow lenses come into the game and enable almost any telescope to use a zoom at the high magnification focal range where the fine-tuning ability is of utmost importance. Thus for most telescopes the use of a zoom eyepiece is really successful only if combined with a barlow lens.

At the low magnification end the zooming mechanism can be handy but it is not as important as at the high magnification end. This area can often be better covered by fixed focal length widefield eyepieces where magnification steps of 2x are sufficient. If of course the zoom eyepiece enables 2x or 1,5x changes at the low power end as well it can also replace fixed eyepieces. But even a zoom fan like me has for very low powers a widefield fixed focal length eyepiece.

All zoom eyepieces have a smaller AFOV at the long focal length end. This is due to the fact that a zoom consists of a variable focal length eyepiece and a variable barlow lens acting together. The diameter of the internal field stop of the variable focal length eyepiece is fixed and does not change when the focal length varies. Therefore, the AFOV as seen by the observer will increase towards shorter focal lengths since the field stop diameter will stay constant.

The new Leica ASPH Zoom combines for the first time - to my knowledge - wide to ultrawide apparent fields of view with an imaging quality which favorably competes with even the very best fixed focal length eyepieces.

Specification

- Focal range: 17.8-8.9mm

There has been some confusion about this zoom's focal range. At first it was announced as "18-9mm" eyepiece for the new Leica spotting scopes (440mm focal length). But the likewise stated magnification range from 25x to 50x would have resulted in focal lengths of 17.6mm to 8.8mm. Later Leica has published the data as "17.9-9mm" (17.9mm +/- 0.15mm - 9mm +/- 0.1mm). I've chosen to call it 17.8-8.9mm as a compromise. Anyway, it's not important since these differences are only marginal. The magnification figures 30x and 40x correspond with focal lengths of 14.8mm and 11.1mm.

The white markings seen on the eyepiece body are little pieces cut from a nylon cable binder and glued onto small strips of transparent selfadhesive tape. They enable "feeling" the chosen magnification and are placed such that they give with my TEC140 magnifications of 60x, 80x, and 100x unbarlowed, and the corresponding magnifications multiplied with the barlow factors. Thus click-stops aren't missed.

- number of elements: 8 lenses/5 groups

- focal plane: 4.5mm within the body of the eyepiece

- virtually flat field (within my measurement uncertainty of 0.05mm)

- no astigmatism up to the field stop

- twist-up eyecup (3 positions, 9mm difference)

- eye relief: 18mm

- diameter of the lenses: eye lens 26mm, field lens 18mm

- waterproof up to 5m/17ft, nitrogen filled

- smooth zoom-ring movement

- field stop measurements and AFOV calculations: given are the measured field stop diameters and the calculated AFOVs at the different zoom settings.

25x/18.2mm/58.6°; 30x/15.9mm/61.6°; 40x/13.75mm/71.0°; 50x/12.6mm/81.1°

- parfocality: this zoom has been found to be parfocal through its focal range. However, judging parfocality the following thoughts have to be taken into consideration. If a zoom eyepiece is designed to be parfocal through its focal range it can only behave parfocally if the observer is neither nearsighted nor farsighted. If he is one or the other and does not wear glasses, his eye will need diverging or converging light beams from out of the eyepiece. This will be accomplished by moving the eyepiece out of its nominal position. And this move will depend upon the focal length of the eyepiece. The longer the focal length the larger amount of adjustment will be neccessary. This consideration is of course valid also for fixed focal length eyepieces.

Now an example for minus 1 diopter (slight nearsightedness/myopia): 17.8mm focal length equals 56.2 diopters. Adding 1 diopter (positive diopters would have to be subtracted) give 57.2 diopters which in turn equal 17.5mm focal length. In order to focus the eyepiece there will be 17.8mm-17.5mm=0.3mm intrafocal movement required as compared to its nominal position. At 8.9mm focal length the equivalent diopters are 112.4 and again plus 1 diopter now result in 113.4 diopters to be reconverted into 8.8mm resulting focal length (eye/eyepiece-combination). The neccessary focal shift now is only 0.1mm. The difference between the two focal shifts is 0.2mm which will be misinterpreted as "not being parfocal". For minus 2 diopters this difference would be 0.4mm.

If on the other hand the observer is farsighted (positive diopters) he can fortunately use his eye's accomodation power to compensate more or less the neccessity of refocusing resulting in the impression of parfocality of the zoom even if the particular zoom is not parfocal.

- Body dimensions: 59mm/2.3" diameter, 87mm/3.4" body length (without bayonet barrel)

- weight: approx. 435g/15oz (plus 50g/1.8oz for the 2“ adapter)

Performance

- General: In October 2008 I got a complete set of ZAOIIs (Zeiss Abbe Ortho II) plus the 2x Zeiss Abbe Barlow. Having ordered my Leica ASPH Zoom much earlier I intended to use the ZAOIIs as an ultimate quality reference against my future ASPH Zoom and of course also as the eyepieces of choice for those rare occasions where the sky would really cooperate.

At that time I still had my former Leica Zoom 22-7.3mm which I had compared over the years against other high quality eyepieces. For the first time I now had my own reference set and started my comparisons with the old Leica Zoom. All comparisons I've done yielded a tie. This of course doesn't neccessarily mean there were no differences but may be I just couldn't detect them or they were too subtle to be noticed. After taking delivery of the Leica ASPH Zoom in February 2009 I of course immediatly started comparisons against my ZAOIIs.

Therefore, this review about the Leica ASPH Zoom often mentions the ZAOIIs as a reference when reporting about my findings.

- Contrast and definition: First light.

Fortunately the skies were cooperative at the very first evening showing a 12 day's moon with good seeing conditions. I did compare the new Leica ASPH Zoom against the ZAOIIs and the old Leica Zoom at different focal lengths of 16mm, 10mm, 8mm, 6mm, and 5mm. The main target for the higher magnifications was crater Gassendi. As experienced before the old zoom and the ZAOIIs again were so close that it wasn't possible to see differences. The new ASPH Zoom instantly could be seen as being on par with both. And after the comparisons had been performed for more than one hour it even seemed to excede both contenders for coming slightly better into focus, not by a large margin but repeatedly so. The ASPH Zoom seemed to look a tad "sharper" which I did interpret as being may be even more contrasty. At any rate it could easily compete for contrast, definition, and absence of stray light (see below).

Since February I've done many comparison observations under the skies and also tests using my artificial test targets (money bills, the "Variable Contrast USAF Target" (Edmund Optics #V53-714), and "IEEE Resolution Target").

I've also used my method to simulate lunar structures which is important for me since the moon is by far my most favorite target. The narrow (about 8mm) grey gaps (face joints) between the wall tiles in our kitchen are kind of rough consisting of a variety of very fine grainy material and cement structures. At the chosen distance of about 10m there are lots of details above and below the angular resolution of my TEC140. Using an oblique illumination with about 45° inclination these gaps through the scope look very much like the crumbled impact structures on the lunar surface being exposed to sun light or being in the shadow. I conducted this comparison from 16mm (2.3mm exit pupil) in all available focal lengths down to 5mm (0.7mm exit pupil) and had a hard time to detect differences. As far as contrast and definition are concerned both eyepieces are that much equal that my conclusion is an almost perfect tie. No matter whether the ZAOIIs were barlowed versus the unbarlowed Leica or the other way round or both equally barlowed or not they appeared to be alike. There were no structures however difficult which could not be seen in both equally easy or equally difficult. Also the focussing was not different.

All tests performed so far have not shown differences in favour of the ZAOIIs. As mentioned before this isn't sound proof for differences not to exist. But if there are any they will be that subtle that only very experienced observers under the most favorable seeing conditions could see may be a hint of differences.

As I've learned from other discussions here on CN my 140mm aperture may be too small to do the ZAOIIs ultimate justice for deep sky viewing. This of course could apply as well to the Leica ASPH Zoom. But with lunar observations my scope should be able to yield different results – if there are differences. And as a final thought: may be my northern lattitude of 52.3° will make the neccessary seeing conditions rather unlikely.

- Stray light: At first I've used the sun as a very tough test target for stray light. The setup was my Baader 2" Herschel wedge with ND3 neutral filter and 2" Solar Continuum filter. With a resulting attenuation of 100 000 times (density 5) from combining the Herschel wedge (ND1.4) and the ND3 filter plus about another 0.6 density (weighted from the bandpass characteristics with only a narrow peak at 540nm) from the Solar Continuum filter the apparent brightness of the solar disk is still 1.6 magnitudes brighter than the full moon. Putting the sun just outside the field stop there could not be seen even the faintest glow. This is equal to the ZAOIIs where the approaching solar disk can be seen already through the triangular cuttings without generating stray light within the field of view.

Repeating this test with a bright moon just outside the field of view showed the same results. This was also the case with Jupiter just outside the field of view. Jupiter has also been used to detect ghosting but there was none. Notwithstanding the multiple lenses (8 lenses + 2 barlow lenses) as compared to the ZAOIIs (4 lenses + 2 barlow lenses) this zoom is as free of stray light and ghosting as the ZAOIIs.

There has been quite some discussion about multiple lens eyepieces to be inferior to 4 lens eyepieces. My testing does not confirm this. The amount of stray light is much more depending upon the execution of the eyepieces - design, polishing, coating, and baffling - and not so upon the number of lenses.

- Color rendition: very neutral color rendition. To evaluate this a bit better I've made a little test. Holding a piece of pure white paper onto a window pane lit from the back by the bright shining 11 a.m. sun I positioned the Leica ASPH Zoom and the ZAOII 16mm side to side on the paper. I did compare the "color" of the brightly illuminated light output of both eyepieces.

At first both seemed to be purely white. But after some time of looking back and forth the light from out of the Leica seemed to have an extremely subtle yellow hue as compared to the Zeiss. I doubt anybody would see it with real targets, but this "laboratory" test revealed an ever so slightly difference.

- Barlowing: This is a most important topic for me. Being mainly interested in lunar, solar and planetary observations, in open and globular clusters, and of course in some highlights like M42, I'm using the zoom very often barlowed (mostly 2x). From all the discussions here on the forums it can be taken that a high quality barlow does not degrade the image. Even the unquestionably highest quality eyepieces like the ZAOIIs rely heavily on the use of their 2x barlow. The 4 physical focal lengths of 16mm, 10mm, 6mm, and 4mm would not give a practical set without the 2x barlow for the other focal lengths of 8mm, 5mm, 3mm, and 2mm.

My barlow of choice is the Baader/Zeiss VIP Barlow (comprising the lens element #4D, the 1.25" nosepiece #14, two 15mm extension rings #25A, the M48/T2 adapter #29, and the 2"/1.25" reducer #15). With only the lens element (inserted into the nosepiece #14) screwed directly into the T2 thread of the 2" nosepiece of the zoom a barlow factor of 1.5x will result. Introducing 32mm T2 extension (the two #25A 15mm rings with 2mm distance rings inbetween) yield a barlow factor of 2x. Finally I can add the 2"/1.25" reducer #15 as a T2 extension of 32mm length to achieve a 2.5x barlow factor.



As can be seen from the picture the 1.5x and the 2.0x configurations fit within the 2" eyepiece holder of my Baader 2" Maxbright diagonal. For the (seldom used) 2.5x configuration the #15 (2"/1.25" reducer) working as 32mm T2 extension is placed outside the Maxbright 2" eyepiece holder.

The resulting focal lengths are given together with the resulting magnifications, exit pupils, and true fields of view.

Without barlow: 17.8-8.9mm, 55x-110x, 2.54mm-1.27mm, 64'-45'

Barlow factor 1.5x: 11.9-5.9mm, 83x-165x, 1.70mm-0.85mm, 43'-30'

Barlow factor 2.0x: 8.9-4.5mm, 110x-220x, 1.27mm-0.64mm, 32'-22'

Barlow factor 2.5x: 7.1-3.6mm, 138x-275x, 1.02mm-0.51mm, 26'-18'

The Baader VIP barlow lens has a focal length of approximately -64mm. Using it in the 1.5x configuration places it quite near to the field lens of the zoom which introduces slight astigmatism near the edge of the field. If this barlow factor is very important a longer focal length barlow lens like the Antares 1.6x or the AP Barcon should be considered to reduce the astigmatism.

On the other hand, for me the 1.5x with its up to 43' true field of view (framing nicely the double cluster at 83x) is too useful to reject it for some edge astigmatism. And zooming in up to 165x and still maintaining a true field of view of 30' (framing one or the other of both clusters) with even less astigmatism is very attractive. At 2.0x and 2.5x barlow factors no astigmatism has been detected.

The total focus range of 17.8-3.6mm enables the Leica ASPH Zoom together with the quality modular barlow lens to act as an almost complete eyepiece collection. For an extra wide field and correspondingly low magnification only one widefield should be added. I've chosen this to be the WO UWAN 28mm giving 35x magnifcation with an exit pupil of 4.0mm and a true field of view of 2.27° (136').

Conclusion

There have been high quality zooms before used by amateur astronomers. But never did they have also widefield characteristics. For the first time now there is a zoom available which leaves nothing to desire when used as the main - even the only - eyepiece.

Is it the "ultimate" eyepiece? For me: YES. Together with a premium barlow lens (Baader VIP Modular) and a two-stage scheme of adding extension rings I have available 3 barlow factors of 1.5x, 2x, and 2.5x. Thus a focal range of 17.8mm down to 3.6mm gives me everything I would need. For widest true fields of view I'm using my WO UWAN 28mm which gives incredible views with my TEC140.



I'm not of the opinion that my observations, my testing, and my conclusions so far are thorough or even exhaustive. But they show that a general purpose zoom eyepiece if designed and manufactured with utmost care and without monetary constraints can compete with even the most demanding planetary eyepieces of today.

As a final note: the Leica ASPH Zoom 17.8-8.9mm isn't cheap. I've paid 800 Euros (about 1040 USD) and the Baader VIP Barlow was another 185 Euros (about 240 USD). But for less than 1000 Euros (1300 USD) it is a complete eyepiece system of the highest possible quality together with an ease of use and viewing comfort that vastly surpasses a set of fixed focal length eyepieces. And a complete set of fixed focal length eyepieces of comparable quality will be even more expensive.





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