Borg 90FL F4 Astrograph, Complete Set with Helical Focuser

Borg 90FL f/4 Astrograph as Complete Set with Helical Focuser

This wonderful apochromatic refractor with fluorite optics and separate focal reducer offers a fast focal ratio of f/4.0 and has a very large flat field to allow the use of cameras equipped with very large sensors - up to full frame. At the same time, the instrument is suitable for travel, because at 2.5 kg and 390 mm in length, it comes along light and compact.

The Borg 90FL f4 apochromatic fluorite refractor comes with a helical focuser that provides high focusing accuracy. The focuser has a rear M57 thread to connect cameras by adding the correct photo adapter. You can directly attach the camera via the M57 connection and also rotate it for the right image composition.

Attaching a camera to the apo:

The required backfocus for best sharpness in the field is 60 mm from the M57 thread.

For a DSLR SLR camera, you need the supplied # Borg7000 photo adapter and one of the M49.8 photo adapters with the matching camera mount. With these adapters, the camera sensor of your SLR camera is automatically at the correct distance.

If you use an astro camera, then you need the adapter # Borg7522, which changes the M57 thread 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 f4:

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 F4 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