Borg Fluorite APO 107 FL F3.9 PLUS - Flatfield Refractor with Objective from Canon Optron

Borg 107 mm f/3.9 FL Flatfield APO Refractor

This wonderful apochromatic refractor with fluorite optics offers an incredible f/3.9 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 107FL f3.9 with FTF-3215B-A one of the most compact and lightweight 107 mm apochromatic refractors on the market, very easy to transport!

The Borg fluorite apochromatic refractor 107FL f3.9 comes with a Feather Touch FTF-3215B-A TRUE 3" focuser that offers high load capacity, great focusing precision without backlash: this is the perfect focuser for astrophotography! The focuser has a rear manual rotator with a M57 thread to connect cameras by adding the proper photographic adapter. Back focus from the rear M57 thread is 60 mm and you can use the rotator to easily rotate the camera in order to find the best object framing.

Attaching a camera to the apochromat:

In order to do astrophotography, you can use both DSLR cameras and CCD/CMOS cooled cameras with the Borg fluorite apochromatic refractor 107FL f3.9 with FTF-3215B-A TRUE 3". The requested backfocus (the working distance from the sensor in order to have the best performance) is 60 mm from the M57 thread on the rear of the rotator (if you want, you can even remove the rotator and have a M68.8 thread with a back focus of 80 mm).

If you want to use a DSLR camera, you need to add the Borg7000 photo adapter and one of the M49.8 photo adapters with the bayonet of the different DSLR cameras manufacturers. By connecting the DSLR camera with these adapters, the camera sensor will automatically be at the perfect backfocus.

If you want to use a cooled CCD or CMOS camera, you need to add the M57-T2 photographic adapter Borg7522 that converts the M57 thread in the rotator to an universal T2 male thread. From this T2 thread the backfocus will be 55 mm and, since most of the cooled cameras have a lower backfocus (unless you use also filter wheels or off-axis guiders), you need to add a T2 extension between Borg7522 and the camera.

The special optics of the Borg 107FL 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, 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 107FL 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 optic 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

- Borg 107FL f/3.9 PLUS OTA
- Feather Touch Focuser FTF-3215B-A
- M57 Rotator

Astrophotographic results obtained with this telescope on Astrobin

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

Safety informations: PDF Download