Refactor Telescope: William Optics FLT120

Name: William Optics Fluorostar 120 (FLT 120)

Price: $3298

Specs: Focal Length 780 mm, aperture 120 mm, f 6.5, weight 7.2 kg, length 680 mm with dew shield fully retracted, 850 mm with dew shield fully extended, image circle 43 mm (with separate field flattener).

Fluorostar 120 Features

The Fluorostar series is one of several apochromatic quality refractor telescopes by William Optics. The company is well-known in the astrophotography community for quality equipment at a reasonable price. The FLT series comes in 4 different aperture sizes: 91mm, 120mm, 132mm and 156 mm. Refractor lenses are curved by nature, bringing different wavelengths of light to focus at different points. Although visual star images might be described as “pretty to look at”, the colorful rings around the stars are not visually accurate. Refractors with three or more lenses correct for this by combining those multiple lenses in a configuration that brings light of differing wavelengths to focus at nearly the same point. These are referred to as apochromatic telescopes. The Fluorostar 120 uses three lenses to achieve this effect.

The telescope is attractive and is obviously high quality. It comes in three different anodized colors: red, gray and gold. The retractable dew shield which makes it resistant to the typical dew formation that occurs as the night cools and can be pulled back on nights that are not prone to dew. One of its unique features is a built in Bahtinov mask in the lens cap. Bahtinov masks are highly useful in achieving the most precise focus. The Bahtinov mask was developed by a Russian amateur astronomer. It consists of 3 grids at an angle from one another. The mask is placed on the end of the telescope, and the scope is pointed at a bright star. What is seen is a three line crisscrossing pattern. The focus is then adjusted until all three lines intersect at the same point. When this happens, the focus is as good as possible.

Typical Bahtinov mask pattern
Image using a Bahtinov mask. The center image is perfectly focused. The images on each side are slightly out of focu

It comes with a handle bar for easy carrying. The handle bar also functions to accept accessories with Vixen bases, like a view finder or separate guide telescope. It is mounted on its own Losmandy style base for easy attachment to almost any mount. Losmandy and Vixen bases are two common standard types of bases for telescope and other accessories.

By itself, it will produce a flat photographic field for cameras with crop sensor, APS-C or smaller. For cameras with full frame sensors, a separate field flattener will be required. Without the field flattener the stars at the corners will look out of focus and can be described as little comets. This phenomenon is known as coma.

It also has a built-in rotator that doesn’t change focus when you rotate the camera. This is very useful to achieve the perfect framing of an object without changing the focus.

Pros

  • Very high quality and not outrageously expensive
  • Built-in dew shield
  • Built-in Bahtinov mask
  • Three lens apochromatic refractor
  • Very large 3 inch rack-and-pinion focuser results in large diameter light path with stable focus
  • Built-in camera rotator with degree markings

Cons

  • Price may be outside of many budgets
  • Separate field flattener required for full-frame cameras to achieve flat field across the full 43mm diameter
  • Precise back-focus requirement for the separate field flattener is difficult to achieve precisely
  • Even with a nearly perfect flat field (once the flattener is in the precise position), images may still suffer from vignetting (light intensity fall-off at the edges of the image). This can easily be corrected with proper image calibration. A similar competitive scope is the ZWO FF130, which is a little more expensive, but does not need a separate field flattener and has an aperture of 130mm as opposed to 120mm.

Conclusion

This is a very high quality refractor suitable for both visual and photographic uses. It is my go-to scope for larger celestial objects. It is a little pricey, but not nearly as expensive as some other very high quality refractors and the quality is just as good. William Optics is known for producing high quality astronomy equipment that won’t break the bank. The Fluorostar 120 is no exception.

Verdict: 4.5/5 stars. Subtraction for vignetting at edges

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