Borg Fluorite APO 90 FL F5.6 - Objective from Canon Optron - Kasai R&P Focuser

Borg 90 mm f/5.6 FL APO refractor

This apochromatic refractor with fluorite optics is one of the most compact and lightweight 90 mm apochromats on the market. The focal length of 500 mm makes the 90FL a universal telescope that can be used for a wide range of aastronomical observation.

The focuser:

The heavy-duty Kasai 2" Crayford focuser is equipped with large roller bearings allow to handle the heavy equipment without flexure. And large knobs make focusing quite smooth. It can be loaded with up to 8 kg of accessories. With the stable 2" connection with brass ring, even heavy accessories can be reliably clamped.

The special optics of the Borg 90FL 5.6:

Most high-quality telescope lenses today contain fluoride-containing glasses, such as the well-known FPL53 from Ohara, Japan. These have a small dispersion and deviate little even in the blue region of the spectrum.

Artificial fluorspar, also known as fluorite, is considered to be even better. This is not glass, but a crystal of CaF₂, i.e. calcium fluoride.

This material has a very low dispersion over virtually the entire visual spectrum, hardly any deviations in the blue and a high transparency. With a suitable counterglass, an apochromatic correction process is already obtained with two-element lenses, and a Petzval system such as that of the Borg Fluorite Apo 90FL f/5.6 mainly consists of two-element lenses.

Then why is fluorite so rarely used? Quite simple: it is expensive and not so easy to process. Only a few companies can handle this material and exploit the possibilities it offers. Borg is one of them!

The result is an extremely sharp optic with high transparency - a perfect telescope for visual observations.

Astrophotographic results obtained with this telescope on Astrobin

Here you can find some astrophotographs made with this telescope model: Link to Astrobin

Astrophotographic results obtained with this telescope on Astrobin

Here you can find some astrophotographs made with this telescope model: Link to Astrobin