150 F/5 Refractor with Binocular Viewer: Is It Really an Affordable Alternative to Giant Binoculars?

PREMISE

In this article, I share some thoughts on an instrument I use often and which can be an excellent, affordable alternative to giant binoculars. The focus is on a 150mm short-focal-length (F/5) refractor from Celestron, which replaces the classic 100mm angled binoculars. This setup can satisfy both astronomy enthusiasts and landscape enthusiasts, offering an immersive experience thanks to the use of a binocular viewer.

 

CELESTRON OMNI XLT 150 f/5 REFRACTOR

This achromatic refractor has a focal length of 750 mm (F/5) and is a Celestron model (OMNI XLT 150), easily found on the used market at affordable prices (between 300 and 350 euros). With a lower cost than 100 mm angled binoculars, this configuration meets the needs of those looking for high quality without excessive investments. It is equipped with a two-inch focuser and a support for the finderscope. The previous owner had disassembled it, sawing the optical tube to allow focusing with binocular viewers. Since there was some dirt left on the back of the doublet, I decided to disassemble the cell and during cleaning I noticed that on the edges of the lenses there were references for the correct positioning of the two lenses.

BAADER MAXBRIGHT BINOCULAR VIEWER

A peculiarity of my setup is the cut tube to allow the use of the binocular viewer without the classic optical correctors (Glass Path). As a "combat" setup I often use an old Baader MaxBright binocular viewer, a model that is also popular used and available starting at around 120 euros.

I often add a Baader prism to my setup, which I find ideal for safety and optical quality, although there are less expensive diagonal prisms available. With a total budget of around €550, you can build an excellent system for astronomical observation at medium-low magnifications, compared to €1,400–€1,600 for a pair of 100 mm angled binoculars.

 

CHOICE OF EYEPIECES

Personally, I use Vixen LVW or the newer 14mm SSW eyepieces, which have an apparent field of view of 83°. However, you can use existing eyepieces or opt for cheaper models.

The telescope is mounted on the AOK SWISS altazimuth mount with an Oberwerk maple tripod, which I often use for giant binoculars, but it is also compatible with other mounts.

PERFORMANCES AND BENEFITS

One of the main advantages of this setup is the brightness, comparable to 100mm binoculars, if not even superior. Stars appear pinpoint across the entire field of view, thanks in part to the eyepieces you choose. This setup allows for easy orientation in the sky, similar to astronomical binoculars, and offers image quality similar to larger, more expensive instruments, without compromising brightness or definition.

Additionally, you can remove the binocular viewer and use the refractor with a mirror diagonal and a 2-inch eyepiece for brighter observing. Compared to a Celestron C8, this setup is significantly brighter for observing galaxies and star clusters, comparable in fine detail perception to a C9.25, thanks in part to the renowned Celestron XLT anti-reflection coating.

PLANETARY AND TERRESTRIAL OBSERVATION WITH AN F/5 ACHROMATIC?

The OMNI XTL 150 F/5 refractor isn't the ideal choice for high-resolution planetary observations, and I say this as a fan of F/15s. However, when equipped with a binocular viewer, it often shows more detail than I can see with the Celestron C8 thanks to its lower sensitivity to turbulence. Jupiter is obviously surrounded by a blue halo and the planet is yellow-green, but the details are there, sharp and visible as a 6-inch "lens" can do.

For landscapes, this setup can work, although the terrestrial image will be tilted left and right without an erecting prism, and above 30-40x, you'll definitely start to notice residual coloration. Discerning terrestrial observers will undoubtedly prefer apochromatic binoculars, which are sharper and more accurate.

DISADVANTAGES AND CONSIDERATIONS

This setup has some disadvantages: it's bulky compared to 100mm binoculars, requires a stable stand, and requires a lot of care when moving it quickly, for example, around the garden. Furthermore, the image is flipped left and right without an erecting prism, which is a factor to consider for terrestrial use. The weight and size require care when moving it, making the setup more challenging than with binoculars.

CONCLUSIONS

A 150 mm short focal length refractor, sought after on the used market, such as the Celestron OMNI XLT 150 F/5 when equipped with a binocular viewer, can be an excellent alternative to the classic 100 mm angled binoculars. If you already have a good set of accessories (eyepieces, binocular viewer, and prism), such a refractor, in good condition, could be an economical choice for observing galaxies and star clusters. While not perfect for every use, this setup offers interesting results at a competitive price.