The new model series "Onyx"



The "Onyx" model series was named after a precious gemstone whose black color is as mystical as the dark night sky with its thousands of stars.

In the new series, many elements of the two previous series "Infinity NL" and "Horizon" have been woven together to combine their respective advantages.

The very short focal length of f/3.3 allows the 16-inch model (ON400) to look through the telescope very comfortably from a sitting position, while the 20-inch version (ON500) requires neither a step nor a ladder, even for objects high up in the sky. Further improvements are a lower weight, a smaller pack size and a simplified set-up of the telescopes at the observation site.

The main mirrors, which are only 32 mm thick, are manufactured in-house and not bought from anywhere else.



1) The Telescope


1.1) Secondary Cage

The octagonal secondary cage consists of a composite of lightweight aluminum profiles and is designed as a mono ring in the 16 inch version. The 20 inch model uses two of these rings, which are connected to each other by robust multiplex plates. The spider is constructed eccentrically, which makes it particularly torsion-resistant and insensitive to rotational movements.

As a focuser, a 2" Baader Diamond Steeltrack is used, which, thanks to its solid and mechanically flawless construction, also handles heavy eyepieces very well and, thanks to the 1:11 reduction, allows very accurate focusing.

Suitable holes for a standard finder shoe and a Telrad finder are provided.


1.2) Truss Tube

The truss tube consists of four pairs of black powder-coated tubes. Thanks to a matching screw, the upper tube mounts allow the secondary cage to be positioned easily and accurately even in the dark.

In the lower part, the tubes are screwed to both the mirror box and the side bearings, which makes the construction very torsion-resistant and ensures that vibrations are reduced.


1.3) Mirror box und mirror cell

The mirror box houses the primary mirror. It consists of a square base frame made of lightweight, black powder-coated aluminum profiles. It can be easily placed on the floor because of its four stands with attached protective caps.

The front panel made of double-painted multiplex gives the mirror box even more rigidity and covers the mirror when not in use.

The mirror cell is firmly attached to the base frame and can be adjusted from above using easy-to-grip star knob screws. Two strong springs ensure a high pre-tension. The main mirror itself rests on 9 freely movable bearing points with the 16 inch model and on 18 points with the 20 inch model. The mirror is also supported in its line of gravity by two ball bearings which reduce astigmatism to a minimum.


1.4) The rockerbox

Like the mirror box, the rocker box consists of a combination of lightweight aluminum profiles that are screwed to a solid base plate made of impact-resistant multiplex. The Teflon-Ebony pairing, which has been used thousands of times, allows the telescope to move smoothly and easily.


1.5) Transportation modes

Both models can be assembled very compactly for transportation and therefore fit easily into almost any car trunk. Nevertheless, it only takes around 5-10 minutes to assemble the telescope without tools; a result of the well designed construction of the “Onyx” model series.

The dimensions can be found in the following figures. In the first picture, the smaller number refers to the mirror box with the mounted side bearings, whereas the second number also includes the rocker box into the calculation.



If you want to assemble the telescope even more compactly, remove the side bearings and the front struts. This allows the mirror box to be lowered into the rocker box together with the attached secondary cage. This transport mode significantly reduces the height even further.



2) Primary and secondary mirror

2.1) The primary mirror

Each primary mirror of the "Onyx" model series is unique - this means that the built-in 400mm and 500mm parabolic mirrors are ground, polished, parabolized and finally measured interferometrically at "Spacewalk Telescopes". This guarantees a consistently high optical quality of the parabolic mirrors - resulting in high-contrast, sharp images and wonderful views of the moon, planets and deep-sky objects.



The mirrors are made from a special glass that is purchased directly from the company "Schott" and is known under the brand name "Supremax". This substrate is not only characterized by a minimal edge thickness of just 32 mm, but also by a very low expansion coefficient of only 3.2x10e-6/K. A telescope mirror made of "Supremax" deforms only half as much as the very widely used mirrors made of BK7 when adjusting to the ambient temperature.

The optical quality is ensured throughout the entire production process using technical aids. At the end, the finished mirror is measured several times in different positions using a laser interferometer. Errors such as astigmatism, steep zones or a central peak can be detected and corrected just as reliably as a wrong overall correction. The results are included in a detailed test report, which not only contains elementary data such as the mirror diameter and the focal length, but also the measured Strehl ratio, a 3D representation of the deviations from the ideal shape of a parabola and the corresponding MTF curve.

Depending on the budget, the parabolic mirrors are available in two quality levels (Standard + Premium), whereby the Strehl ratio measured here on site is always decisive for the assessment:

1) Standard: Strehl Ratio S>0,80
2) Premium: Strehl Ratio S>0,90 (surcharge)

Some selected measurements of "premium mirrors" are shown below:



All mirrors are supplied with an aluminum coating, which is made more durable with a special, water-repellent protective layer. The reflectance of the coating is 92% in the UV (380nm), 93% at 550nm and 90% in the infrared (850nm). This means that the mirrors achieve a very high reflectivity in the entire spectral range and therefore provide bright and high-contrast images.


2.2) The secondary mirror

The secondary mirrors are obtained from two sources - one from Taiwan and one from "Antares Optics" in the USA. The latter have a very high surface accuracy of better lambda/10 and are delivered with a Zygo protocol.

With the smaller "ON400" model, you can choose between a 100mm secondary mirror from Taiwan and (for an extra charge) a secondary mirror from "Antares Optics", while the "ON500" model is fitted with a 120mm secondary mirror from Antares from the start.




3) Telescope Accessories

3.1) Secondary Heater

On a very humid night, it can happen from time to time that the secondary mirror becomes covered with dew and the observation night comes to an abrupt end. To prevent this from happening and to ensure that the night can be used to its full extent, a suitable secondary mirror heater is available.

It consists of three high-power resistors that are glued to the rear of the secondary mirror. The power supply cables run down along the spider and the truss tubes, where they are connected to a battery box when in use. It uses 8 AA (Mignon) batteries. As a rule, one battery charge is sufficient for a complete night.


3.2) Filter Slide

When observing gas nebulae as well as the planets, it can be an advantage to use different nebula or color filters in order to see as many details of the object as possible. As these filters are screwed into the eyepiece, changing the filter is tedious and time-consuming.

However, changing filters is much quicker with a filter slide, with which the filters can be pushed directly in front of the eyepiece in a matter of seconds.


3.3) Bag

To prevent the truss tubes and the front struts from lying around loose during transport, they can be stored in a custom-made bag made of high-strength nylon fabric, which can be closed with a drawstring.


3.4) Transportation Box for Primary

If the mirror is transported or not used for a long time, it can be stored in a specially designed box made of multiplex. The mirror rests on large felt pads that prevent direct contact between the mirror and the wood and allow it to be easily taken out of the transport box.


3.5) Counter Weight

If very heavy eyepieces are used, a 1 kg counterweight can be integrated into the mirror cell.



4) Technical Data




The most important optical and mechanical properties of the "Onyx" model series are listed in the table below:


Model Onyx 400 Onyx 500
 
Primary Mirror 400/1300mm 500/1600mm
f/3.3 f/3.2
Thickness 31mm 31mm
Weight Primary 7.8 kg 11.6 kg
 
Strehl Ratio (Standard) S > 0,80 S > 0,80
Strehl Ratio (Premium) S > 0,90 S > 0,90
interferom. protocoll yes yes
 
Limit at V=200x 15.9mag 16.3mag
Limit at V=350x 16.3mag 16.7mag
Dawes Limit 0.29" 0.23"
 
Secondary Mirror 102mm 120mm
 
Obstruction (linear) 25.5% 26.0%
Obstruction (area) 06.5% 06.8%


Model Onyx 400 Onyx 500
 
Total weight of telescope ~ 22 kg ~ 33 kg
Mirror box + Mirror ~ 12 kg ~ 19 kg
 
Thickness of truss tubes 18 mm 20 mm
Max. length of truss tubes 105 cm 110 cm
 
Viewing Height Zenith 115cm 145cm


5) The Universe in a 20 inch dobsonian telescope


Drawings best reflect the visual impression in the eyepiece. Experience the universe as you see it in a 20" telescope of the "Onyx" model series under a dark sky and discover a multitude of star clusters, gas nebulae and galaxies in drawn form!