Borg Fluorite APO 90 FL F3.9 PLUS - Flatfield Refractor with Objective from Canon Optron

Borg 90 mm f/3.9 FL Flatfield APO Refractor

This wonderful apochromatic refractor with fluorite optics offers an incredible f3.9 focal ratio and it has a very large 55 mm flat field to allow the use of cameras also equipped with very large sensors, even larger than full frame. The carbon tube makes the Borg fluorite 90FL f3.9 with ESATTO 3" one of the most compact and lightweight 90 mm apochromatic refractors on the market, very easy to transport!

Borg fluorite apochromatic refractor 90FL f3.9 comes with ESATTO focuser that offers high load capacity, great focusing precision without backlash! The focuser has a rear M57 thread to connect cameras by adding the proper photographic adapter. You can directly attach and also rotate the camera via the M57 connector and also rotate it, for the right image section.

Attaching accessories to the telescope:

Forget about the uncomfortable and weak support rings that are often found in cheap telescopes! Borg fluorite apochromatic refractor 90FL f3.9 with ESATTO 3" comes with the PLUS (Prima Luce Universal System) rings, aluminum, made with high precision CNC machines. This gives you maximum freedom in connecting other mechanical accessories (dovetail plates, guide rings, dovetail clamps) always with simple M6 screws and with 2 screws for every element (to avoid flexures). This way you can also connect the EAGLE directory on the telescope obtaining a fantastic system for astrophotography that does not require external computers to allow you to perform great quality astrophotography!

Attaching a camera to the apochromat:

The necessary backfocus for best sharpness in the field is 60 mm from the M57 thread. If you remove the 360° rotation with the M57 thread, then an M68.8 thread with 80 mm backfocus is at your disposal.

For a DSLR camera, you need the # 7000 photo adapter and one of the M49.8 photo adapters with the matching camera mount. With these adapters, the camera sensor of your SLR is automatically in the correct distance.

If you use an astro camera, then you need the adapter # M57-T2 7522 which changes the M57 thead into a universal T2 thread. From this T2-thread you have exactly 55 mm working distance (backfocus) to the optimal camera position.

The special optics of the Borg 90FL 3.9:

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 also in the blue little from it.

Artificial fluorspar, also known as fluorite, is considered to be even better. This is not glass, but a crystal of CaF₂, that is 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 Apo 90FL F/3.9 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 a very fast, extremely sharp optics with high transparency - a perfect telescope for deep-sky photography, but also a wonderful tool 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

Technical Information

Teleskop-Service has collected a lot of helpful information about refractors and provided it in English as a PDF file: refractors.pdf

Astrophotographic results obtained with this telescope on Astrobin

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