Sky Instruments Antares Elite 105 f/15 Long Focal Length Refractor Review: Back to Basics

PREMISE

With the arrival of Chinese-made low-dispersion doublets, the glorious long-focal-length achromatic refractors, which for years accompanied dozens of enthusiasts in lunar and planetary observations in Italy, have almost completely disappeared. These instruments also allowed many advanced amateurs to study double stars through those wonderful accessories, now unknown to young amateur astronomers, called filar micrometers.

 
Amateur astronomers and the legendary achromatic refractors

Amateur astronomers in their forties will certainly remember with a certain nostalgia the legendary Zeiss Telementors, the Polarex Unitrons and other purely Italian solutions, often designed with extreme meticulousness and precision by Italian optical manufacturers, to enable some amateur astronomy research.

The Italian company SkyPoint decided to import a series of instruments from the Canadian company Sky Instruments, consisting of an achromatic doublet made in Japan and offering excellent performance. We decided to test the most powerful version, the 105 mm, with a focal length of 1575 mm and a focal ratio of f/15.

 

FOCAL RATIO

An achromatic doublet is formed by two lenses equipped with glasses of different composition, with dissimilar dispersion: the converging lens of type crown, which makes up the “classic” lens of the Antares Elite, should be composed of calcium oxide, potassium oxide and silica, while the rear lens, known by amateurs as “flint”, in addition to silica, should contain both lead oxide and calcium oxide.

Unfortunately, it is currently almost impossible to obtain from manufacturers the exact "recipe" with which they created their instrument; however, since it is an air-spaced Fraunhofer doublet with four different radii of curvature, we believe that the composition may be the one commonly used by manufacturers over the years, although the manufacturing process and anti-reflection treatment are constantly evolving.

The use of two lenses involves the presence of chromatic aberration, which should not be judged as a mere construction defect, but as a real peculiarity deriving from the disproportion between the dispersion at the various wavelengths of the two lenses.

To contain this problem, there is a golden rule that all designers and DIYers keep in mind when designing their own instrument, and this optical commandment is formulated as follows: F=1.12XD^2

 

Put in practical terms, this means that to reduce chromatic residue using an achromatic doublet, the focal length must be at least equal to the diameter squared.

For this reason, a 10cm refractor should, at the very least, have a focal ratio of f/11.2, a value well exceeded by the instrument under test, being an f/15.

We must remember that, understandably, in addition to this rule, countless other factors will come into play, such as the quality of the optics, their workmanship, their composition, the presence of a treatment capable of eliminating or reducing the deviation of out-of-phase rays, the use of excellent eyepieces and so on.

Explained this way, it might be that the amateur astronomer, after reading these simple considerations, will be convinced to design a 20-cm achromatic refractor with a focal ratio of at least f/22.4, but this would cause serious inconvenience due to the bulk it would take up. Personally, we have been fortunate to observe with some of these 13- and 15-cm refractors designed by reputable Italian manufacturers, featuring long focal ratios; however, the weight, bulk, and large investment in a specialized mount made the incredible images they provided ultimately a disadvantage.

Even though this 10 cm is not one of the most compact, we believe that this diameter is still usable by the average amateur astronomer, who will be able to obtain performance similar to that of a renowned apochromat at a lower cost, provided that he follows very specific rules.

You'll need a sturdy mount; we used a Synta EQ6. The instrument must be perfectly balanced on both axes, and for some types of observation, you'll need a high-quality mirror diagonal to avoid having to lie down when aiming the telescope near the zenith.

There are also other little tricks that, at a modest cost, can significantly reduce the vibrations produced by a tube of this length. Personally, we use small sandbags placed along the tube to absorb the vibrations. Weighted armbands and anklets, which also have a Velcro strap for fastening, are perfect. They can be found for a few euros in major Italian supermarkets.

These, ultimately, are the defects and precautions to remember if you want to enter the world of long focal length achromatic refractors.

The Antares Elite 105mm is also available in two other versions: open at f/10 and open at f/13. The model we tested is quite long… which becomes so with the focuser fully extended. This accessory required precise calibration to allow us to use it at zenith with heavy accessories; we also tried attaching an “old and homemade” filter slide for studying the lunar albedo.

All in all, even considering the purchase price, we found this focusing system, when properly adjusted using the numerous screws, to work very well during visual observations. The focusing reduction system is always a valuable aid, as it counteracts the increased vibrations caused by such a long tube when adjusting at high magnifications.

The tube, while bulky, is certainly lightweight, allowing it to be easily assembled by a single person. The standard finder scope was very bright and easy to collimate, thanks in part to the oversized screws that allow for quick collimation with the main optical tube. We would have preferred a removable lens hood that would reduce the length when not in use, but the manufacturer opted for a fixed system.

Overall, this telescope has a spartan and essential design; let's be clear: it has everything, but without the aesthetic frills that often only increase the purchase price, while maintaining the optical performance unchanged. It seems like a telescope for those who value substance over form. Such a sophisticated instrument could only generate excellent star testing. During various observing sessions, a couple of amateur astronomers who participated in the test noted that, although the diffraction image was perfect, almost textbook, a slight bluish halo appeared when simply observing the lunar limb. This presence, however, was attributable to the average quality of the eyepieces they were using, dissimilar to the excellent ones we usually use for star analysis.

Another factor to consider, often overlooked by novice amateur astronomers, is seeing: turbulence can further shift the aforementioned out-of-phase frequencies. This condition could create an increase in chromatic aberration during the star test, generating a nonexistent spherical aberration. We have noted that even in a telescope with such a high focal ratio, in the presence of strong turbulence, a faint secondary spectrum, invisible under normal conditions, becomes apparent both during the star test and during purely visual observations.

Under perfect atmospheric conditions, we noticed no traces of coma or astigmatism, even when bringing the diffraction image to the outer edge of the circle of minimum aberration.

This leads to the usual conclusion: with high-performance telescopes, it's essential to use excellent accessories to maximize the optical performance of the entire system, as well as perfect atmospheric conditions. Just as it's pointless to buy a diagonal mirrored to 1/12 lambda for use on a cheap refractor, it's unthinkable to be satisfied with high-resolution images using a good instrument but poor eyepieces. Indeed, it's important to remember that these accessories, in particular, suffer from lateral chromatic aberration.

We observed how the Antares Elite refractor appreciated the use of high-quality orthoscopic eyepieces, providing truly sharp and contrasting lunar and planetary images. Thanks to their optical design, which allows for excellent spherical and chromatic correction, we were able to obtain a perfectly flat field over 45°, a factor impossible to achieve with a similar lens with half the focal length.

The use of a mirror diagonal was necessary for observations close to the zenith, since, using a fixed column to support the EQ6, it was not possible to benefit from a greater height.

In practical observation, we really have no objections: theory is no slouch in this case. With these focal ratios, lunar and planetary images are truly excellent, similar to those provided by a much more expensive apochromatic telescope. Furthermore, it may seem strange, but a simple achromatic doublet is less affected by atmospheric turbulence than a sophisticated multi-lens system.

During our lunar observation, on a velvety evening, we exceeded 300x magnification; the light loss was evident, but we fondly recall the morphology of Rupes Recta, which can only be observed at high magnifications. A visual detail that has often driven us crazy over the years is the Vallis Alpes rim, which we have rarely admired with a 10-cm refractor. Indeed, in addition to good resolving power, proper illumination and excellent eyepieces capable of ensuring extreme sharpness are required. Well, perhaps thanks to the combination of all these factors, it was possible to admire, between slight micro-twitches, at about 350x, the sinuosity of the rim, which took on the shape of a dark gray snake that seemed to be hiding in the lunar surface. This was an immense satisfaction for someone who, having been observing for years, has almost always achieved these performances with instruments at least 20 cm in diameter.

The medium-magnification images of the lunar surface are extremely "dry," similar to those obtained with a graphics program and an unsharp mask. This quality reminded us very much of the view offered by an old 90mm f/13 Kenko, which we were fortunate enough to test years ago, although the brightness was significantly lower then.

Even observations in early December, even though viewing conditions were not yet optimal, provided beautiful views, with countless details visible from 200X to 350X magnification, such as Miliacus Laucs, Hellas, Mare Cimmerium, and Sirenum. Furthermore, using various filters, from W80 to W25, it was possible to sometimes exclude atmospheric details and sometimes highlight the details of the desert regions more.

However, the undisputed realm of this refractor seems to be the observation of double stars: it is incredible how easily it reveals very unbalanced pairs even at low magnifications, such as the very famous Rigel, which has a companion at 9" that, however, turns out to be about 400 times fainter. Observed in a reflecting telescope, the image is often blurry, and the companion is hidden in the brightness of the background sky and the blue supergiant. With the Antares 105 mm, however, we observed it even in poor seeing conditions, something impossible in a 20 cm diameter catadioptric telescope. Moreover, it is known that a reflecting system generates twice the error on the wavefront, precisely because of the double direction-reflection transition, compared to the mere refraction obtainable with a telescope similar to the one being tested.

Castor, in the constellation Gemini, is truly stunning at about 350x magnification; the two components are perfectly separated and the point-like quality is still remarkable. With excellent seeing, it was possible to perfectly admire, at a full 400x magnification, the double star Zeta Orionis, which currently dominates the winter sky along with the constellation Orion.

The ability to provide nearly perfect stellar images at the focal plane and its resolving power, just over 1.2", could allow enthusiasts of this type of observation dozens and dozens of pleasant nights, without the fear of necessarily having to search for dark skies, as happens with enthusiasts of deep-sky objects. These are ideal observations even from the center of the city. We personally believe that a "purist" of visual observations should prefer a neutral system, free of mirror diagonals and anything else that could cause degradation of the wavefront. For this reason, it would be preferable to invest in a column support or an oversized tripod. These assertions, well known even to lunar and planetary observers of years ago, are now completely forgotten by the younger generation, who seek greater comfort, the aesthetics of the instrument, and perhaps the use of a good webcam, rather than pushing the extreme limits afforded by personal and visual use of their instrument.

We also see this refractor as a valid instrument for solar observations with the highest-performance filters currently available on the market. In short, the Antares 105 from Sky Instruments certainly can't be called a compact telescope, but its optical performance and price might convince the purely visual amateur astronomer to prefer it to the more expensive apochromatic refractors or the less performing semi-apochromatic doublets made in China.