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by WilliamPaolini

Fig 1: The CZAS Binoviewer with included accessories: Baader 1.25" ClickLock eyepiece holders, Zeiss Bayonet, Baader T-2 Quick Changer, and Baader 1.25" nosepiece. Image Credit: Author.

1. Overview

[NOTE - This product evaluation was conducted in 2021] This astronomical binoviewer is offered by Denis Levatić of Croatia (denis0007dl@gmail.com) as an after-market modification of new Zeiss binoviewer bodies. Mr. Levatić's value-added modifications to these Zeiss binoviewer bodies are then offered under the moniker of "The Carl Zeiss Apochromatic & Sharpest Binoviewer". To the main body is added a Zeiss Bayonet mount for telescope side accessories, and on eyepiece-side he adds specially designed T-2 accessory connectors and Baader T-2 eyepiece holders (either their ClickLock or Helical Microfocuser model). Mr. Levatić also precision collimates each binoviewer at 360x magnification prior to shipping to customers and will soon offer a custom fitting hard shell ABS case with each binoviewer (Fig 3).

Some of the distinctive characteristics of this astronomical binoviewer are:

  • T-2 eyepiece connectors to accept T-2 compatible eyepiece holders
  • Preconfigured with either the Baader ClickLock 1.25" T-2 eyepiece holder (#T2-08/ALT208) or the Baader Helical Microfocuser 1.25" T-2 eyepiece holder (#T2-8A/EYEHOLD-2/2458125)
  • Zeiss Bayonet mount (for quick connect-disconnect from diagonals and telescope focusers) and a Baader T-2 Quick Changer and 1.25" T-2 nosepiece
  • Easy adaptability with the wide variety of aftermarket T-2 optical accessories
  • Inter-Pupil Distance (IPD) adjustment over a large 46mm - 77.5mm range which is also bi-directional (i.e., the binoviewer can be used in right-side-up or up-side-down orientations)
  • No need for special optical correctors to correct for spherical aberration or chromatic aberration due to long optical paths through prisms -- instead uses high precision first surface aluminized mirrors and a small prism for beam splitting
  • Collimation can be performed by the user so no need to send back to the factory
  • Bright View; Low Scatter; High Contrast; No Polarization; Light Weight; Metal Housing

NOTE - Throughout this review you will notice that some images show the CZAS Binoviewer with a raised Blue & White Zeiss logo (e.g., upper images in Fig 3) whereas others show a CZAS Binoviewer with a flat plate Black & White Zeiss logo (e.g., lower images in Fig 3). The unit used for testing and reported here was the unit pictured with the Raised Blue & White Zeiss logo. During the course of this review the vendor made exterior updates to the CZAS Binoviewer and some stock pictures from the vendor of this updated version of the binoviewer are also included in this review.

A summary of the CZAS Binoviewer's specifications as supplied by the vendor (in black text; blue text are measures by the Author):



Internal Prism & Mirrors

Zeiss Fine Polish Scratch Resistant Coated Mirrors;
Zeiss Fully Multi-Coated Beam Splitter Prism

Clear Aperture

21mm (Telescope-Side); 25.5mm (Eyepiece-Side)

Eyepiece Holders

Self-Centering ClickLock Holders with Diopter Adjustments

-or- Helical Microfocuser Compression Ring Holders

Telescope Connectors

Dovetail Ring (Zeiss Micro Bayonet) with 1.25" T-2 Nosepiece

Light Path

138mm (bottom of Zeiss Bayonet Plate to top of ClickLock holder)
(measured - 139mm to top of ClickLock holder)

135mm (bottom of Zeiss Bayonet Plate to top of Helical holder)

Interpupillary Distance

46mm - 77.5mm
(measured - 49mm-77mm in right-side-up orientation w/Click Lock)
(measured* - 46.5mm-77mm in right-side-up orientation w/Helical)

(measured - 54mm-77mm in up-side-down orientation w/either holder)


User Accessible for Collimation


Diecast Metal Housing and T-2 Connectors


20.9oz/592g (ClickLock); 19.7oz/558g (Helical)

Included with Package

CZAS Binoviewer

1.25" Eyepiece Holders (Baader ClickLock or Helical)

Zeiss Bayonet Connector

Baader T-2 Quick Change Connector

Baader 1.25" Nosepiece

End Caps for Eyepiece Holders and Nosepiece

Custom Hard-Shelled ABS Geoptik Case (new feature)

Paperwork: 1) Collimation Instructions, 2) Specifications Certificate,
3) Use/Cleaning/Storage Instructions, 4) Certificate of Sale


$1,399 USD (with ClickLock eyepiece holders)

$1,339 USD (with Helical Focuser eyepiece holders)

Fig 2: Vendor supplied specifications (Author measurements shown in blue text).
* - See Fig 13 and discussion under Fig 13 for further information.

A complete list of features and purchasing information is available from the vendor, Denis Levatić of Croatia (denis0007dl@gmail.com) .

2. Product Inspection and Impressions

Fig 3: Eyepiece-side (top-left) and telescope-side (top-right) views of the CZAS Binoviewer.
(lower-left) the new branding graphics and (lower-right) case that will shortly be available on these binoviewers. Top Images Credit: Author; Bottom Images Credit: Denis Levatić.

Handling the CZAS Binoviewer conveys a solid sense of quality. The fit and finish of the unit appears precise and well made. Operation of all components were smooth and similarly precise. Adjustment of the folding mechanism to adjust IPD distance was smooth, not being too tight or too loose. The various T-2 components on the eyepiece side and telescope side of the unit also conveyed high quality. All outwardly visible components of the binoviewer are metal with high-quality build and finish.

Fig 4: CZAS Binoviewer showing the 1.25" nosepiece with T-2 Quick Changer and one 1.25" T-2 ClickLock eyepiece holder removed. The author's Helical T-2 Microfocuser eyepiece holder is also pictured; the binoviewer can be ordered with a pair of these instead of the ClickLock holders if requested. Image Credit: Author.

A very appreciated aspect of this binoviewer were the T-2 connectors that are used on the eyepiece-side and telescope-side of the unit. T-2 is a physical inter-connection approach that was originally created by the optical manufacturer Tamron of Japan. It was later adopted as an industry standard for cameras and various other optical equipment. The ability to utilize universal T-2 connections allows the use of multiple T-2 capable eyepiece holders and T-2 capable diagonal/focuser connections for the CZAS Binoviewer, maximizing its flexibility and customization capability for the user. The use of the Zeiss Bayonet mounting on the telescope-side of the binoviewer along with the included Baader T-2 QuickChanger was also very much welcomed as using these connectors allowed me to change from binoviewer to conventional monoviewing in a matter of seconds. Removal and changing of these T-2 components was quck and easy. The binoviewer as purchased comes configured with the Baader T-2 eyepiece holders and the Baader 1.25" T-2 nosepiece.

Fig 5: Close-up of the Baader 1.25" ClickLock holders on the CZAS Binoviewer.
Image Credit: Author.

The Baader 1.25" ClickLock eyepiece holders that came configured with the binoviewer I tested are self-centering and with a short turn of the larger rotating collar on the holder unlock or lock the eyepiece in the holder. The smaller rotating collar at the top of the holder is used for diopter adjustment of the eyepieces. This adjustment is used to equalize the focus point of each eyepiece so they can be separately set to come to precise focus with each eye of the observer. The Diopter Collar has a range of adjustment of 0-6mm, more than sufficient to equalize focus due to any difference in vision between the observer's individual eyes or variation in the placement of the field stops in the two eyepieces being used.

The customer has the option to request the unit be configured with either the Baader 1.25" ClickLock eyepiece holders (pictured in Fig 4 & 5), or the Baader 1.25" Helical Microfocuser eyepiece holders (pictured in Fig 4). These eyepiece holders attach directly to a custom adapter plate on the eyepiece-side of the binoviewer which has universal T-2 male threads. The ClickLock holder has no protrusions and uses two rotating collars for locking and diopter adjustment. Diopter adjustment of the eyepiece only functions when the eyepiece is unlocked in the ClickLock mechanism. The Helical Microfocuser holder has three set screws engaging a compression ring to lock the eyepiece, and a fourth set screw that locks the diopter adjusting helical focusing housing.

Fig 6: Diagram of the mirrors and prism layout in the CZAS Binoviewer.
Illustration Credit: Author.

The CZAS Binoviewer is a little unconventional compared to other astronomical binoviewers in that it uses both prisms and mirrors to guide the light path. Typical astronomical binoviewers use only prisms to split and direct the light path from the telescope's main objective to the eyepieces. As a consequence of this design utilizing mirrors, the resulting reduction in light path though prism glass greatly reduces the Chromatic Aberration (CA) and Spherical Aberration (SA) caused by extensive light paths through prisms. This CA and SA that can occur through long prism paths is why a Barlow or optical corrector is typically bundled with prism binoviewers as this optical element is needed to eliminate the CA and SA that the extensive prism path can produce.

The amount of CA and SA induced by prism-only binoviewers without the use of Barlows or special optical correctors is often greater in telescopes with faster focal ratios. In my observing experience using my apochromatic telescopes, which are all near f/8 focal ratio, I find that any CA or SA induced is minimal and of no consequence unless I am at very high magnifications approaching 50x/inch of aperture or more. In my testing the lack of induced CA was easily noticed when I used the CZAS Binoviewer with 4mm eyepieces without Barlows or optical correctors in the path compared to other all-prism binoviewers I own. This CA was especially noticeable on star points if they were not precisely in focus. As a consequence of this, over the years I have developed the habit of always using a Barlow or other optical corrector when performing high magnification planetary observation with my binoviewers on my apochromatic telescopes, as I can see the impact to planetary detail without the correctors.

The late optical designer Thomas Back once pointed out this CA and SA as a result of prism path in forum postings. He stated that there is no general rule for how much CA or SA will be induced with large prism paths in the optical train with a refractor, unless one models it with the exact optical prescription of the main objective.

"Indeed. a prism will add its own aberration (overcorrected spherical and color), but until you raytrace a system with a prism (I have – with ZEMAX), your just guessing at the aberrational residuals. (Ref: groups.google on 12/1996, Thomas Back, Subj: Prism Diagonals Pros & Cons)"

His point has been born out in the postings of other amateur astronomers I have come across as some can visually see the CA or SA induced when using a prism-only binoviewer with their refractors without Barlow of optical correctors, while others report they can see no induced CA or SA even at high magnifications with their refractors. With the CZAS Binoviewer using its mirror design however, this appears to be a non-issue given that my testing for this specific issue showed no induced CA from the CZAS Binoviewer at high magnifications with no optical correctors, but it did show induced CA with my other all-prism binoviewers.

Fig 7: The view inside the left side of the CZAS Binoviewer with eyepiece holder removed showing one of the internal mirrors. Image Credit: Author.

When the T-2 eyepiece holders are unscrewed from the body of the binoviewer, looking down into each eyepiece holder opening you can better see the precision diagonal mirror that directs the light rays from the beam splitter to the eyepiece. The left mirror is inset near the bottom of the binoviewer while the right mirror is near the top of the binoviewer. The mirrors are at 45 degree angles directing the light through the binoviewer. The light path is well baffled with flat black microbaffles (not pictured). The image in Fig 7 is taken from an angle to best highlight the internal mirror and not from the angle that the light path would actually take when the binoviewer is attached to the telescope. The view of the mirror in Fig 7 therefore reflects other internal portions of the binoviewer outside the baffled light path which is why the microbaffles are not visible. For cleaning, any dust that may accumulate on the mirrors can be easily removed with an optical brush or with forced air from a squeezable air bulb or a filtered airbrush compressor. Note - when cleaning never use canned compressed air as it contains chemical propellants in addition to the air that leave deposits, and never blow with your breath as this deposits microscopic droplets of saliva onto the components.

Fig 8: Views of the Zeiss Bayonet mount on the telescope-side of the CZAS Binoviewer. Note that the lower image shows the beamsplitter prism in the binoviewer. This prism then directs the light toward multiple precision diagonal mirrors that then send the light rays to each eyepiece. Image Credit: Author.

The Zeiss Bayonet accessory on the CZAS Binoviewer makes using this device in the field extremely convenient, facilitating the enjoyment of my observing situations by letting the equipment get out of the way. Prior to using this connection method I would either monoview exclusively or binoview exclusively during an observing session. By adding the Zeiss Bayonet connector to my binoviewer, diagonal, and eyepiece holder, moving from binoviewing to monoviewing now takes me just seconds with no screwing and unscrewing of components needed. While binoviewers can connect to the telescope in multiple ways, I would encourage those contemplating binoviewers to consider whether the binoviewer they are considering has this capability, as it has been for me a godsend for my observing habit. In addition to allowing easy switching between monoviewing and binoviewing with the telescope, when the diagonal also uses a direct T-2 connector to attach its eyepiece holder, it then allows the binoviewer to be directly connected to the diagonal without using the eyepiece holder on the diagonal or the 1.25" nosepiece on the binoviewer. The result is that it frees up some of the light path used by the binoviewer and thereby saves precious backfocus of the telescope (see discussion that follows Fig 13 for more on this topic). In many instances this saving of backfocus can mean the difference between needing a magnifying Barlow or optical corrector to reach focus, or not needing these magnifying components thereby allowing wider TFOV views possible with the observer's long focal length eyepieces.

Fig 9: The CZAS Binoviewer can fold in both directions to adjust IPD distance. It can achieve an IPD as close as 46.5mm (or closer*) when folded downward (left) and as close as 54mm when folded upward (right). Image Credit: Author.

A somewhat unique feature of the CZSA Binoviewer is that it is "ambidextrous" in that it can be folded either upward or downward to adjust the IPD distance for the observer's eyes. I did not find this of any particular advantage in the field during tests but I can imagine that it might make some situations easier, especially at darkest sites, as there would be little need to determine in the dark if I had the binoviewer in the correct orientation since it can work either way.

* - Note that the vendor states that the minimum IPD distance is 46mm whereas my measure was 46.5mm when the Baader Helical Focuser eyepiece holders are used on the binoviewer. This discrepancy is a result of the positioning of the T-2 thread adapters that are on the eyepiece-side of the CZAS Binoviewer. The vendor explained that these adapters can be loosened and repositioned inwards slightly should an observer need the absolute minimum IPD of 46mm. However, when doing this, the user will need to re-collimate the internal mirrors. The vendor can walk through the process for any customer that wishes to do this, or they can request that the vendor to do this prior to shipping the binoviewer they purchase.

Fig 10: When requested, the product creator will sign and date the CZAS Binoviewer for the customer. Image Credit: Denis Levatić.

The vendor, Mr. Denis Levatić , indicated that he often does special customizations to the binoviewer or how it interconnects to the telescope when requested by customers to meet any unusual needs they may have. This is rare if not impossible for more mainstream mass produced astronomical products, but often feasible when dealing with components made or customized by hand from the more 'boutique" vendors. As is popular in the astronomical community, customers sometimes ask that the makers of a product sign the equipment they purchase from them. Fig 10 shows one such instance for a binoviewer build done by Mr. Levatić in 2020.

3. Field Test

Fig 11: Takahashi TSA-102 Super Apochromat with Takahashi 18mm LE eyepieces in a CZAS Binoviewer using the included Zeiss Bayonet to directly connect to a Baader T-2 Quick Changer on a Baader Zeiss 1.25" T-2 Prism Diagonal. Image Credit: Author.

Location --

Field testing was conducted for approximately 4 weeks during December 2020 through January 2021 in forested rural Virginia (Yellow Zone) located approximately 50 miles southwest of Washington, D.C. Sky Quality Meter readings at this location range between 20.5 to 21.3 mag/arcsec2 on moonless nights. Outdoor temperatures during field testing were typically around 40° F with humidity averaging 50-60%. Elevation of the observing site is approximately 300 feet above sea level.

Process --

All outcomes are generally recorded at the time of occurrence at the telescope using a voice recorder. Each performance test is replicated multiple times to ensure they are consistent and accurate. When results are compiled, if there are any discrepancies or conflicting test results, then those tests are redone until the root cause of the initial discrepancy is identified and eliminated. Any test related to assessment of perceived contrast, brightness, background field of view uniformity are only conducted under darkest conditions on moonless nights.

Equipment --

Fig 12: Some of the eyepieces used in pairs for celestial object observations with the CZAS Binoviewer. Form left-to-right: Explore Scientific 24mm 68° Series, Takahashi 18mm LEs, Baader 17.5mm & 9mm Morpheus, vintage Celestron volcano top 4mm Abbe Ortho.
Image Credit: Author.

Testing was accomplished primarily in my Takahashi TSA-102 f/8 Triplet Super Apochromatic refractor. For testing of vignetting with Maximum True Field of View (MaxTFOV) eyepieces the following telescopes were used: f/8 TSA-102, f/7.7 Vixen 81S Apochromat, and f/6.25 Celestron 80mm Onyx Apochromat. The diagonal used for all testing was the Baader Zeiss 1.25” T-2 Prism Diagonal. Eyepieces used for celestial object observation shown in Fig 12. Eyepieces used for MaxTFOV testing are shown in Fig 17.

Every telescope is designed with a given amount of what is called backfocus. This is the amount of distance from the edge of the visual back on the focuser to where the main objective comes to focus (which is at the Field Stop of the eyepiece inserted into the focuser). The operating manual or the marketing material for your telescope may tell you the amount of backfocus your telescopes is designed with.

Every component you add to the visual back of your telescope therefore uses some of the available backfocus of the telescope. So when you add a diagonal between the visual back of the focuser and the eyepiece, then that diagonal uses some of the available backfocus. When you add a binoviewer on top of the diagonal then it uses even more of the available backfocus between the visual back of the focuser and the eyepiece. Depending on the amount of backfocus your telescope is designed with, you can quickly need more backfocus than is available from your telescope and then be unable to reach focus. The good news is that if this happens it can be remedied for most telescopes my adding a Barlow, Glass Path Corrector (GPC), or Optical Corrector Assembly (OCA) under the binoviewer because these devices alter and extend the available backfocus of the telescope. The downside of this is that these devices generally also add a magnification factor to the eyepiece, so the magnification is more from the eyepiece you are using and therefore the True Field of View (TFOV) will be less with that particular eyepiece than if no Barlow, GPC, or OCA was used. Many observers therefore often strive to find ways to reduce the backfocus used by components in an effort to get their telescope to work with a binoviewer without the need for Barlows, GPCs, or OCAs so they can get the widest celestial views possible.

Fig 13 illustrates how one can use components in a way that reduces the amount of backfocus they use from their refractive telescope. The most obvious way is to not use a diagonal. This however is literally a pain-in-the-neck to observe this way with a refractor. So a diagonal is always a must. To reduce the light path used from the diagonal, choosing a 1.25" diagonal will do the trick, and often a prism diagonal will use less light path than a mirror or dielectric diagonal. As example, a Takahashi 1.25" prism diagonal uses about 64mm of light path whereas a Tele Vue 1.25" Everbright Dielectric uses 78mm of light path. And 2" diagonals use much more; as example the Astro-Physics 2" MaxBright Dielectric uses about 103mm of light path. So choosing the diagonal wisely will save you substantial backfocus light path that the binoviewer may need.

For those observers who plan to binoview with their telescope, it is a good idea to first determine how much backfocus their telescope is designed with so they can reach focus with their binoviewer without needing a Barlow, GPC, or OCA. In this way they can enjoy the widest views possible with their longer focal length eyepieces. As example, when binoviewing with my TSA-102 using 24mm Explore Scientific 68° eyepieces, if I can binoview without a Barlow, GPC, or OCA then my TFOV will be approximately 1.9°, plenty to frame most of the largest celestial objects. However, if I was not able to reach focus with my setup and had to use a 1.5x GPC as example, then my TFOV would be only 1.3°, losing more that the width of the Full Moon from my TFOV!

Celestial Objects --

A range of different celestial object types were observed to assess the general performance of CZAS Binoviewer. Objects observed included:

  • The Moon
  • Planets: Mars, Jupiter, Saturn
  • Multiple Star: Iota Ori
  • Multiple Star: Lambda Ori (Meissa)
  • Multiple Star: Delta Ori (Mintaka)
  • Multiple Star: Beta Ori (Rigel)
  • Multiple Star: Sigma Ori/Struve 761
  • Multiple Star: Theta Ori (Trapezium)
  • Nebula: M42 (Orion)
  • Nebula: M43
  • Nebula: NGC 1973/1975/1977 (Running Man)
  • Open Cluster: M37
  • Open Cluster: M38/NGC1907
  • Open Cluster: Melotte 25 (Hyades)
  • Open Cluster: M45 (Pleaides)
  • Open Cluster: NGC1980
  • Open Cluster: NGC1981
  • Supernova Remnant: M1 (Crab)
  • Star: Alpha Ori (Betelgeuse)
  • Star: Alpha Aur (Capella)

Operational Assessments --

After approximately 4 weeks of field testing, the following are my impressions of the CZAS Binoviewer for each of the listed performance categories:

* Sharpness / Contrast / Transmission *

To assess how crisply the views were rendered with the CZAS Binoviewer I observed the Moon, Mars, Jupiter, and Saturn at magnifications up to 200x, which was 50x/inch aperture (0.5mm Exit Pupil) in my TSA-102. Views of the Moon were literally jaw dropping. The Moon is actually my favorite target for binoviewing as the observation is very natural and relaxed when using two eyes. When I binoview the Moon I also often go well beyond a magnification of 50x/inch aperture. Since the Moon is so very bright and high contrast, it can easily take the more extreme magnification. I can then enjoy what often feels as if I am actually walking along the surface of the Moon and I can also linger for a long time observing as there is no eye fatigue when observing with two eyes like there often is when just monoviewing.

Fig 14: Schröter's Valley on the Moon. The "Cobra head" volcanic vent pictured fed a river of lava that flowed down the Aristarchus plateau before spilling out onto the lava plains of Oceanus Procellarum. To the east (left) of the Cobra head is Aristarchus crater and to the west (right) is Herodotus crater; Marius crater (40 kilometer diameter) is in the far distance very near the limb. Apollo 15 Metric Photograph AS15-M-2610. Image Credit: NASA/JSC/ASU.

The most memorable lunar views for me during testing were those of Schröter's Valley (Fig 14). During these observations the valley was right at the Lunar Terminator so the lighting was stark and dramatic. Indeed, the entire region looked magically three dimensional while binoviewing with the crater walls and other higher relief features casting dramatically stark shadows. The various dark shadings contrasted with the brilliant ejecta streaks from crater Aristarchus were similarly quite mesmerizing while the valley was so close to the Lunar Terminator. Fine details were also abounding -- I especially liked the small craters of Aristarchus Z (5 mile diameter), Aristarchus B (4m mile diameter), Vaisala (5 mile diameter), and Herodotus H (4 mile diameter). Aristarchus was ablaze with bright ejecta while Herodotus contrasted it with its darker crater floor with only a small section having lighter ejecta crossing it.

After observing numerous features on the Moon I turned the binoviewer to the Lunar Limb in an effort to see if any Chromatic Aberration (CA) could be induced by its limited prism path. I used the Celestron 4mm Abbe Orthos without any Barlow or Glass Path Corrector (GPC), but no CA was to be seen even at these high magnifications. I also moved the Moon throughout the field of view using longer focal length eyepieces, including outside of the field of view in all directions in an effort to detect any stray light artifacts from the very bright Moon, but none were seen.

Fig 15: Sketch by author of Mars during the 2020 Opposition observed with the APM/Lunt 152-ED Apochromat with Baader MaxBright-II Binoviewer at 200x magnification. Note - this observation was prior to receipt or testing of the CZAS Binoviewer. Illustration Credit: Author.

To assess contrast I chose what I consider the most challenging objects in this regard, the planets. While I did observe Jupiter and Saturn during testing, their positioning lower on the horizon meant that I could not use them for any credible assessments. Mars however, was nicely placed higher above the horizon and afforded many excellent views during testing. Overall the views were what I consider to be outstanding with many details being revealed, as well as the fainter contrast edge details of the Martian maria. During many observations a most prominent feature was the darker basaltic volcanic rock of Syrtis Major Planitia, followed by a very small but still quite bright South Polar Cap. The dark rim around the South Polar Cap, the receding Lowell Band, was also nicely evident.

During evenings of very best seeing for Mars I also compared the performance of the CZAS Binoviewers with my own Baader MaxBright-II Binoviewers. The views between the two were exceedingly close with the prominence of the details on the Red Planet being just ever so slightly more distinct through the CZAS Binoviewers. The difference was very subtle, too subtle to really quantify, but still slightly noticeable with very careful observation. I replicated this very subtle difference in performance through multiple observations on Mars.

Fig 16: Spitzer Space Telescope infrared image of the Crab Nebula supernova remnant located in the constellation Taurus (Messier 1/NGC 1952). The blue-white shows the energetic electrons trapped within the central neutron star's magnetic field, emitting "synchrotron" radiation. The red shows the well-known filamentary structures that permeate the nebula. The nebula is ~ 6,500 light-years distant and 5 light-years across.
Image Credit: NASA/JPL-Caltech/R. Gehrz (University of Minnesota)

I observed multiple Deep Sky Objects (DSO), including Nebula, Supernova Remnants, and Open Clusters to assess apparent brightness/throughput of the CZSA Binoviewer. These celestial objects were used to assess perceived transmission of the binoviewer. For each of these target observations I also compared the view of the CZAS Binoviewer to that of my Baader MaxBright-II Binoviewer to assess any visual differences between the two.

Using the Great Orion Nebula (Messier 42), I examined the subtleties of its mottled structure as well as the length or extent of the Wings. For The Crab Nebula (Messier 1), De Mairan's Nebula (Messier 43), and the Running Man Nebula (NGC 1973/1975/1977), all of which are fairly faint at my location through my 102mm Apochromat, I simply looked for the apparent prominence and extent of their visible nebulosity. Finally, for Open Clusters Messier 36, Messier 37, and Messier 38, I examined faintest stars, especially those that were visible only with adverted vision. In all cases and in all tests I could not discern any performance difference relative to perceived brightness or visibility between the CZAS and MaxBright-II Binoviewers. The views through either were bright, well contrasted, and very pleasing.

* Scatter / Stray Light Suppression *

During my observation of brighter stars, planets, and the Moon I positioned these bright objects at various locations throughout the field of view, and also placed these objects both just outside the field of view and more distantly outside the field of view to see if I could generate any stray light ghosts, reflections, flare, or other stray light artifacts. In all tests but one I could not generate any stray light artifacts. In one test using Mars however, I was able to generate a very small semicircle bloom of reflected light when Mars just exited the field of view at the very bottom of the view. I was surprised to see this given that it did not occur on any other test target. I then kept all components on the telescope the same and substituted the MaxBright-II binoviewer. To my surprise, since I did not record any stray light artifacts in my testing of that binoviewer, I saw exactly the same artifact. Given that these two binoviewers are very different in design, one being all-prism and the other a hybrid of prism and mirrors, this did not make sense to me so I examined the rest of the optical chain very carefully. I was eventually able to trace the stray light artifact as coming from not the binoviewers but from a small Barlow cell I was using on the 1.25" nosepiece of the binoviewers for some of my higher magnification planetary observing. I removed that component and repeated the test on both binoviewers and the stray light artifact vanished. I also employed several other Barlows and none of those showed the light artifact either when used with the binoviewers.

With the elimination of the offending Barlow cell in the optical chain that was causing stray light artifacts, the CZAS Binoviewer passed all stray light tests not showing any stray light artifacts in any of the tests, with or without other Barlows, and regardless of eyepieces used. Similarly, scatter around planets and bright stars was judged as low, again being on-par with the performance of my Baader MaxBright-II Binoviewer.

* Field of View *

Fig 17: Eyepieces with Field Stops producing widest True Field of View (TFOV) used singly in the CZAS Binoviewer for vignette tests. From left-to-right: Orion 35mm Ultrascopic, Celestron 30mm Pre-Ultima Plossl, Explore Scientific 24mm 68° Series. Image Credit: Author.

To assess whether the 21mm telescope-side clear aperture or the 25.5mm eyepiece-side clear aperture of the CZAS Binoviewer would show any readily discernable dimming around the periphery of the FOV when using eyepieces with field stops larger than 25.5mm, I selected an array of single eyepieces to test in one of the eyepiece ports of the CZAS Binoviewer. These eyepieces were the: Orion 35mm Ultrascopic, Celestron 30mm Pre-Ultima Plossl, and the Explore Scientific 24mm 68° Series. All these eyepieces have Field Stops at or near 27mm, except for the Orion 35mm Ultrascopic which has a slightly larger Field Stop of approximately 29mm.

With the larger Field Stop of the Orion 35mm Ultrascopic, or using the more standard MaxTFOV eyepieces with 27mm field stops (Explore Scientific 24mm 68° Series and 30mm Celestron Pre-Ultima Plossl), they all showed nicely illuminated views from center to edge during nighttime observing through the CZAS Binoviewer. Switching to daytime observing, a very ethereal darkening no more than 2-3 degrees from the field stop was detected. This very slight darkening was only perceivable during daylight viewing. During nighttime astronomical observing, no visual dimming was visible near the field stop and stars appeared as brightly at the field stop as they did at the center of the field of view. Even when faint adverted vision stars in Open Clusters were moved from center to adjacent to the field stops of these eyepieces, those stars would remain visible with adverted-vision. For the many observers who use eyepieces like the 24mm Tele Vue Panoptic or the 24mm Explore Scientific 68° or a general purpose 30-32mm Plossl to obtain the largest TFOV with a 1.25" eyepiece, the CZAS Binoviewer will provide a uniformly illuminated visual view edge to edge for celestial observation.

With the CZAS Binoviewer directly connected to my 1.25" Zeiss Prism Diagonal to reduce backfocus needs, the binoviewer was able to reach focus without the use of Barlows, GPCs, or OCAs in my TSA-102. In this configuration I could then take advantage of the CZAS Binoviewer's ability to show bright edge-to-edge views even with MaxTFOV 1.25" eyepieces. I always find it pleasurably engaging when a binoviewer allows the observer to enjoy the full TFOV available from eyepieces like the 24mm Explore Scientific 68° or Tele Vue 24mm Panoptic, which yield approximately 1.9° TFOV in my TSA-102. This large vista of sky permitted me spectacular binoviews of large celestial showcase objects like the entire Sword of Orion having the grandeur of the Great Orion Nebula (M42) at its center, and the gloriously bright young blue stars of the Pleiades Cluster (M45). In my opinion a critical function of a binoviewer is to be able to show a bright edge-to-edge view using 1.25" eyepieces with largest field stops like the 24mm Explore Scientific 68° or Tele Vue 24mm Panoptic. With other binoviewers I have owned that did not allow this, the experience was always disappointing as I felt a critical capability was lacking in addition to not being able to use some of my most enjoyable 1.25" eyepieces.

* Mechanical Operation / Ergonomics *

Throughout testing all mechanical operations of the CZAS Binoviewer were smooth and precise. At no time did any feature bind, get stuck, or not operate as it was intended (note - for the majority of testings the temperature was approximately 40° F / 4.4° C). At no time did any eyepiece not seat properly in the eyepiece holder so there was never any issue with attaining a merged image when viewing with both eyes. Instead of using the 1.25" nosepiece, the quick release aspect of the Zeiss Bayonet connector to attach the binoviewer to T-2 diagonals configured with a bayonet receiver made switching between binoviewing and monoviewing seamless and effortless.

Operation of the eyepiece locking collar on the eyepiece holder for the Baader ClickLock eyepiece holder was fast and positive and there was never an issue with correct centering of the eyepiece so the views were easily merged. When turning the upper collar of the holder clockwise to lower the diopter height of the eyepiece, rotating this collar clockwise would also turn the ClickLock collar clockwise, locking the eyepiece and not allowing it to move down with the lowering of the diopter collar. As a result I would either have to unlock the ClickLock so the eyepiece would seat and see the results of my adjustment, or I would have to hold the ClickLock collar in the open position with one hand while lowering the eyepiece with the diopter collar using my other hand so I could visually watch the diopter focus change through the eyepiece as I made the adjustment. This two handed operation for diopter adjustment was not necessary if I needed to raise the diopter collar with a counter-clockwise turn as this automatically unlocked the ClickLock allowing the eyepiece to freely rise with the diopter collar.

When using the Baader Helical Focuser eyepiece holder, three set screws around the housing move a compression ring that then locks the eyepiece. If I chose to only tighten one set screw then the eyepiece would not center properly and merging the view was not possible. However, when I tightened the three set screws just approximately equally the eyepiece correctly centered and image merging was never an issue. Alternatively I could also leave the eyepieces unlocked and they would stay properly centered for image merging. The helical focusing mechanism for diopter adjustment of either eyepiece could also be rotated freely whether the eyepiece was locked or unlocked in the holder (unlike teh ClickLock holder). There is no special collar to focus the Helical Microfocuser holder as the entire holder rotates in its base for diopter adjustment. The helical focusing is firm, not loose, and the threading is very fine so the adjustment is very precise. The eyepiece will stay at its set height regardless of the weight of any eyepiece I used. I have used these particular Baader eyepiece holders on diagonals for many years and their function has not changed over time and they remain very precise.

4. Summary Impressions

Fig 18: CZAS Binoviewer with Takahashi 18mm LE eyepieces on the Takahashi TSA-102.
Image Credit: Author.

The CZAS Binoviewer was a joy to use in the field, providing high contrast views with excellent image brightness. All eyepieces had no issue when being inserted or removed from the eyepiece holders regardless of their undercut style using the Baader ClickLock eyepiece holders that came configured on my test unit. Mechanical operation of all features was smooth and precise in their function. Image brightness and lack of scatter was excellent and on-par with my Baader MaxBight-II Binoviewers, showing even dimmer Messier Objects well like the Crab Nebula (Messier 1). Unlike a number of 1.25" binoviewers, the use of maximum true field of view 1.25" eyepieces like the 24mm Explore Scientific 68° Series (similar to a Tele Vue 24 Panoptic) or a standard 30-35mm Plossl showed no visual vignetting for astronomical observing, so I was able to enjoy low magnification wide vista views of 1.9° True Field of View with my 24mm Explore Scientific 68° Series eyepieces in my TSA-102 Apochromat. While the glass path of this binoviewer is not the shortest, it was still short enough for me to observe without the use of Barlows, Glass Path Correctors, or Optical Corrector Assembly devices which would have lessened the true field capability of my bino-friendly TSA-102 Apochromat. The CZAS's use of internal mirrors, while unconventional compared to most respected binoviewers, performed excellently as one would expect from a Zeiss branded product, showing none of the Chromatic Aberration or Spherical Aberration often shown by full prism diagonals when used at high magnification without Barlows, Glass Path Correctors, or Optical Corrector Assemblies. The CZAS Binoviewer also provided a very subtly better defined planetary image when viewing Mars so shows itself quite adept in that role. Finally, in addition to feeling solid and of high quality, these binoviewers were also nicely light not being weighed down by large internal glass prisms. Overall I found their performance very satisfying in the field. Highly recommended!

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About the Author

William "Bill" Paolini has been actively involved in optics and amateur astronomy for more than 50 years, is author of the desk reference on astronomical eyepieces: Choosing and Using Astronomical Eyepieces which is part of the Patrick Moore Practical Astronomy Series published by Springer of New York, has published numerous product reviews on major online amateur astronomy boards, and has worked as a tour guide volunteer for the United States Naval Observatory's historic 1873 26" and 12" Alvan Graham Clark refractors.

Bill has been observing as an amateur astronomer since the mid-1960's, grinding mirrors for homemade Newtonian telescopes during the 1970's and eventually owning, using, and testing over 400 eyepieces in a wide variety of telescopes including Achromatic Refractive, Apochromatic Refractive, Newtonian, Maksutov-Cassegrain, and Schmidt-Cassegrain optical designs. Today he enjoys observing from his rural home an hour's drive southwest of Washington, D.C., where his primary amateur astronomy pursuits are lunar, planetary, bright nebula, open cluster, globular cluster, and double star observing.


This article is placed into the Public Domain by William Paolini 2021; Author images © William Paolini 2021. For permission to use author images separate from the full article, submit your request to wapaolini@hotmail.com.

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