Sky-Watcher HAC 125 f/2 Honders Advanced Catadioptric Mini Astrograph
The Sky-Watcher HAC 125 is a groundbreaking compact astrograph that delivers an unprecedented combination of optical performance and portability. With a 125 mm aperture and an ultra-fast f/2 focal ratio, it ranks among the fastest production astrographs available anywhere and opens up new possibilities for mobile astrophotography - whether from a balcony, a garden or a dark-sky site far from city lights.
Honders Advanced Catadioptric Optical Design
At the heart of the telescope is the innovative Honders Advanced Catadioptric (HAC) optical system, which combines refractive and reflective surfaces into a single, highly efficient optical unit. The core is an aspherical Mangin-type primary mirror with a high-reflectivity RAQ (Radiant Aluminum Quartz) coating achieving 94% reflectivity, paired with a corrector carrying multi-layer HTMC anti-reflection coatings. This combination effectively eliminates chromatic aberration, coma and astigmatism, delivering pinpoint stars and high contrast across the entire 16 mm image circle - with no need for an external field corrector. Optical quality is diffraction-limited (1/4 wave).
f/2 Focal Ratio - The Key Advantage for Astrophotography
An f/2 focal ratio means the HAC 125 collects light four to five times faster than typical compact astrographs at f/4.9, and approximately ten times faster than telescopes at f/6.3. The practical impact is significant: a 60-second exposure with the HAC 125 captures the same amount of light as a 10-minute exposure through an f/6.3 telescope. This translates directly into a higher signal-to-noise ratio, brighter images of nebulae and galaxies, and considerably shorter total imaging sessions. The short 250 mm focal length also eliminates the need for autoguiding in most situations, substantially simplifying the whole setup.
Field of View and Suitable Objects
The 16 mm image circle projects to a 3.6° field of view on the sky, making the HAC 125 an outstanding instrument for large extended objects: the full disc of the Andromeda Galaxy (M31), large emission nebulae such as the Orion Nebula or the Rosette Nebula, sweeping Milky Way star fields, and wide-field mosaic panels. For best sensor coverage, cameras with an APS-C or smaller chip are recommended - sensors such as the IMX533 or IMX585 are particularly well matched to this focal length.
Focuser and Camera Compatibility
Focusing is achieved via a helical focuser built into the corrector assembly, with the camera sitting statically at the front of the tube. This arrangement keeps cable routing simple and eliminates any camera rotation when repositioning the telescope. The instrument accepts cameras with a body diameter up to 60 mm without cooling shroud, connecting via a standard 1.25" barrel or T2 thread (M42x0.75). The primary mirror cell is collimatable, allowing optical axis alignment to be verified and corrected in the field if necessary.
Portability and Mounting
The compact tube, approximately 40 cm in length and weighing 3.7 kg, is ideally suited to lightweight portable equatorial mounts. The included clamshell rings with a Vixen/CG5-compatible dovetail bar allow the HAC 125 to be mounted on most standard saddles without additional adapters. Two accessory mounting points accommodate a finder scope or guide camera for a complete imaging rig. A removable dew shield is included, which also acts as a lens shade and can be detached for direct camera connection.
Technical Specifications
Optical design: Honders Advanced Catadioptric (HAC)
Aperture: 125 mm
Focal length: 250 mm
Focal ratio: f/2
Central obstruction: 53 mm (diameter) / 42.5% by area
Image circle: 16 mm (field of view 3.6°)
Primary mirror coating: RAQ (Radiant Aluminum Quartz), reflectivity 94%
Corrector coating: HTMC multi-layer anti-reflection
Optical quality: diffraction-limited (1/4 wave)
Focuser: helical, single-speed
Camera connection: body diameter up to 60 mm, 1.25" barrel or T2 thread (M42x0.75)
Dovetail bar: Vixen/CG5 compatible
Weight: approx. 3.7 kg
In the box: optical tube, clamshell rings with dovetail bar, dew shield, 1.25" adapter and M42x0.75 adapter
