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Sky-Watcher Esprit 120 0.77x Reducer/Flattener
The Sky-Watcher Esprit 120 0.77x Reducer/Flattener is a dedicated 3-element corrector that transforms the Esprit 120 from its native 840 mm f/7 optics into a faster 647 mm f/5.4 astrograph. This accessory provides a 34 mm flattened and illuminated field while offering two distinct back focus distances of 95 mm and 55 mm for versatile camera integration. Weighing just 910 g, it threads directly onto the Esprit 120's focuser drawtube.
From f/7 to f/5.4 with 0.77x Reduction
By applying a 0.77x reduction factor, this accessory shortens the Esprit 120's focal length to 647 mm and increases its photographic speed to f/5.4. The shorter focal length gives a wider field of view.
A 34mm Flattened Field for the Esprit 120
Engineered as both a reducer and a flattener, this 3-element optical accessory is designed specifically for the Esprit 120. It corrects for field curvature to ensure sharp, point-like stars across a 34 mm image circle. It is a full-frame reducer/flattener with full illumination and minimized color fringing across the 34 mm imaging plane.
Dual 95mm/55mm Back Focus and Filter Integration
The reducer provides two standard back focus distances to accommodate different imaging setups. The native 95 mm back focus is measured from the M78 x 1 male thread; the supplied rings give 95 mm with any DSLR, and astronomy cameras need a custom ring. The back focus spacer and M48 ring also give 55 mm. For filtering, a 48 mm (2") filter can be mounted inside the DSLR extender tube's closing ring, which has a 48 x 0.75 thread.
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Can I use the Esprit 120 0.77x Reducer on a telescope other than the Sky-Watcher Esprit 120?
No. This is a dedicated optical accessory designed exclusively for the Sky-Watcher Esprit 120 (840 mm f/7). It is not compatible with other telescopes, even refractors with similar apertures or focal lengths, as its corrective properties are matched precisely to the Esprit 120's optical design.
What sensor sizes will the Sky-Watcher Esprit 120 Reducer's 34mm image circle cover?
It gives full illumination and field flatness across a 34 mm imaging plane. Compare 34 mm with your sensor's diagonal before buying.
How do I install 2" filters with the Esprit 120 0.77x Reducer?
A 48 mm (2") filter can be mounted inside the DSLR extender tube: unscrew the closing ring at the rear, insert the filter without overtightening and refit the ring, which has a 48 x 0.75 thread inside.
What are the 95mm and 55mm back focus options for on the Esprit 120 Reducer?
95 mm is the distance from the start of the M78 x 1 male thread to the sensor. The supplied rings give 95 mm with any DSLR; astronomy cameras need a custom ring with a female M78 x 1 thread. The 55 mm figure applies with the back focus spacer and M48 ring.
How do I attach a camera with M48 threads to the Esprit 120 0.77x Reducer?
The reducer includes an M48 adapter ring. This adapter threads onto the reducer's primary M78 x 1 exit thread, converting it to an M48 x 0.75 thread. This allows you to directly connect camera-specific M48 T-rings or other M48-threaded accessories.
Does the Sky-Watcher 0.77x Reducer change the Esprit 120's focal length?
Yes. The 0.77x reduction factor changes the telescope's optical characteristics for imaging. It reduces the native 840 mm focal length to 647 mm and increases the focal ratio from f/7 to a faster f/5.4. This gives a wider field of view.
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| 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 |
| Illuminated & Flattened Field |
34 mm |
| 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 |
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0.77x reducer/flattener
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- Sky-Watcher 2-Year Warranty
