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Sky-Watcher Esprit 120 0.77x Reducer/Flattener

by Sky-Watcher
SKU SW-S20209
$525.00 - $525.00
$525.00
$525.00 - $525.00
$525.00
Price Match Policy!
  • Converts the Esprit 120 to a 647mm f/5.4 astrograph
  • Provides a 0.77x focal reduction factor
  • Cuts required exposure times by 68% compared to the native f/7
  • Delivers a 34mm fully illuminated and flattened image circle
  • Offers dual back focus options of 95mm and 55mm
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Check which filter and adapter sizes clear the sensor with the Sky-Watcher Esprit 120 0.77x Reducer/Flattener, and how much light the corners lose.

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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Downloads
  • Warranty

  • Sky-Watcher Esprit 120 0.77x Reducer/Flattener

    This dedicated 3-element Sky-Watcher Esprit 120 0.77x Reducer/Flattener converts the native 840mm f/7 optical system into a 647mm f/5.4 configuration, cutting exposure times by about 68%. It is a full-frame reducer with a fully illuminated 34mm imaging plane, giving a 3.18 x 2.12 degree field of view on a full-frame sensor. Back focus is 95mm from the start of the M78 x 1 male thread, or 55mm with the back focus spacer and M48 ring installed.

    From f/7 to f/5.4: Cut Exposure Times by 68%

    By increasing the optical speed to f/5.4, this reducer dramatically shortens the sub-exposure times needed to capture faint detail. This 68% reduction in acquisition time means you gather significantly more signal in a single imaging session, improving your signal-to-noise ratio and revealing fainter structures in nebulae and galaxies.

    The 0.77x reduction also widens your field of view: a full-frame sensor captures a 3.18 x 2.12 degree patch of sky, while an APS-C sensor covers 2.08 x 1.39 degrees.

    34mm Corrected Field: Full-Frame Illumination for the Esprit 120

    Designed specifically for the Esprit 120, this 3-element accessory is both a reducer and a flattener. Sky-Watcher states it delivers excellent field flatness and full illumination with minimized color fringing across the entire 34mm imaging plane.

    According to Sky-Watcher, no universal reducer flattener will deliver the results of this dedicated reducer on the Esprit 120. It threads directly onto the focuser drawtube once the 2" ring adaptor has been unscrewed and removed.

    Back Focus: 95mm Native or 55mm with Spacer and M48 Ring

    For best results, the distance between the reducer's last lens and the sensor should be exactly 95mm, measured from the edge marking the start of the M78 x 1 male thread. With a DSLR, Sky-Watcher's method is to screw the extender tube onto the reducer, then the enlarged 48 mm T-ring and the camera. For astronomical cameras, a coupling ring with a female M78 x 1 thread places the sensor at the correct 95mm distance.

    Retailers also give a 55mm back focus with the back focus spacer and M48 ring installed. A built-in 2" filter cell provides a location for light pollution or narrowband filters.

  • What is the main benefit of the Esprit 120 0.77x Reducer?

    The primary benefit is speed. By converting the telescope from f/7 to f/5.4, it reduces the required exposure time for any given target by 68%. It also flattens the field for sharp stars across a full-frame sensor and provides a wider field of view for imaging large celestial objects.

    Can I use this 0.77x reducer on a telescope other than the Sky-Watcher Esprit 120?

    No. This reducer/flattener is designed specifically for the Sky-Watcher Esprit 120's 840mm f/7 system. Sky-Watcher states it is not compatible with other telescopes, even refractors with the same aperture and focal length.

    How do I achieve the correct 55mm back focus with the Esprit 120 reducer?

    Retailers state the 55mm back focus applies with the back focus spacer and M48 adapter ring installed. Without them, the reducer's back focus is 95mm, measured from the start of the M78 x 1 male thread.

    Can I use 2" filters with the Esprit 120 0.77x Reducer?

    Yes, there are two options for using 2" (48mm) filters. The reducer features a built-in 2" filter cell for direct installation. Additionally, if you are using a DSLR extender tube, a 48mm filter can be mounted inside it via its 48 x 0.75mm threads.

    What is the 95mm back focus option for on the Esprit 120 Reducer?

    The native 95mm back focus is measured from the start of the reducer's M78 x 1 male thread to the sensor. Sky-Watcher's DSLR setup reaches it with the extender tube and enlarged 48 mm T-ring; for astronomical cameras, a coupling ring with a female M78 x 1 thread is used to place the sensor at 95mm.

  • Manufacturer SKU S20209
    Compatible Telescope Sky-Watcher Esprit 120 (840 mm f/7)
    Optical Design 3-element Reducer/Flattener
    Reduction Factor 0.77x
    Reduced Focal Length 647 mm
    Reduced Focal Ratio f/5.4
    Exposure Time Reduction 68% (vs. native f/7)
    Illuminated & Flattened Field 34 mm
    Field of View (Full Frame) 3.18 x 2.12 degrees
    Field of View (APS-C) 2.08 x 1.39 degrees
    Native Back Focus 95 mm from M78 x 1 male thread
    Standard Back Focus 55 mm with back focus spacer and M48 ring
    Telescope Connection Threads directly onto Esprit 120 focuser drawtube
    Exit Thread M78 x 1 male
    Camera Adapter Thread M48 x 0.75 (with supplied adapter)
    Filter Support Built-in 2" filter cell; 48 mm (2") filter can also mount inside the DSLR extender tube (48 x 0.75 thread)
    Weight 910 g (2.0 lb) including end caps

    • 0.77x reducer/flattener

      × 1

    • End caps

      × 1 set

    • M48 adapter ring

      × 1

  • Sky-Watcher 2-Year Warranty