Sky-Watcher Explorer 250PDS

Designed for prime-focus astrophotography while retaining excellent visual performance, the Sky-Watcher Explorer 250PDS uses a 254 mm parabolic Newtonian optical system with a 1200 mm focal length and f/4.7 focal ratio.

Its low-expansion borosilicate mirrors, dual-speed 2-inch Crayford focuser and rigid internally darkened tube are among the main features of this imaging-oriented PDS design.

254 mm Parabolic Primary Mirror

The primary mirror has a diameter of 254 mm and a parabolic surface.

This geometry brings incoming light rays to the same focal point and eliminates the spherical aberration associated with spherical mirrors used at fast focal ratios.

Sky-Watcher specifies diffraction-limited optics designed to produce sharp, high-contrast and detailed images.

Low-Expansion Borosilicate Mirrors

Both primary and secondary mirrors are manufactured from low-expansion borosilicate glass.

This material promotes faster thermal equilibrium and helps the mirrors retain their correct shape during the cooling process.

1200 mm Focal Length and f/4.7 Focal Ratio

With a 1200 mm focal length and 254 mm aperture, the Explorer 250PDS operates at f/4.7.

This configuration is particularly suitable for astronomical imaging while retaining excellent visual observing capabilities.

2-Inch Dual-Speed Crayford Focuser

Focusing is provided by a 2-inch dual-speed Crayford focuser.

The 10:1 reduction system allows rapid coarse adjustment followed by much more precise fine focusing, particularly useful for astrophotography.

The focuser also incorporates an M54 internal thread.

1.25-Inch and 2-Inch Compatibility

Adapters for both 1.25-inch and 2-inch eyepieces are supplied.

This provides compatibility with a wide range of astronomical eyepieces and accessories.

Radiant Aluminum Quartz RAQ Coating

The primary and secondary mirrors feature Sky-Watcher Radiant Aluminum Quartz RAQ coatings.

The coating system combines reflective aluminium, a dielectric tantalum pentoxide layer designed to improve reflectivity and durability, and a protective silicon dioxide layer.

Sky-Watcher specifies 94% reflectivity.

63 mm Secondary Mirror

The secondary mirror has a 63 mm minor axis.

It is supported by four 0.5 mm thin spider vanes designed to minimise obstruction of the optical path and reduce diffraction spikes.

Fully Collimatable Optics

Both primary and secondary mirrors are fully collimatable.

The primary mirror cell incorporates large knurled adjustment knobs for convenient collimation.

Rigid Internally Darkened Tube

The optical tube is manufactured from rolled steel and features an internally darkened finish.

This construction helps reduce unwanted internal reflections while maintaining good mechanical rigidity.

Optional Coma Corrector

Sky-Watcher offers an optically matched M48 coma corrector for this range to improve star correction towards the edge of the field during imaging.

The coma corrector is available separately.

28 mm LET Eyepiece

A 28 mm 2-inch LET eyepiece is supplied with the optical tube.

It provides approximately 43× magnification with the Explorer 250PDS's 1200 mm focal length.

9 × 50 Finderscope

A fully multi-coated 9 × 50 optical finderscope is supplied.

It incorporates a crosshair reticle and bracket for alignment with the main optical tube.

Camera Adapter

A piggyback DSLR camera mounting adapter is supplied.

It complements the imaging capabilities of the PDS optical tube.

Mounting

The optical tube is supplied with tube rings and a 45 mm dovetail bar.

With a stated weight of 14.4 kg, Sky-Watcher recommends suitably rated mounts including the EQ6-R Pro, AZ-EQ6 Pro, CQ350i Pro, EQ8-R Pro and Wave-150i.

Technical Specifications

• Manufacturer : Sky-Watcher
• Model : Explorer 250PDS
• Product code : 10221
• Type : Newtonian telescope optimised for imaging
• Primary mirror diameter : 254 mm
• Primary mirror : Parabolic
• Focal length : 1200 mm
• Focal ratio : f/4.7
• Mirror material : Low-expansion borosilicate glass
• Secondary mirror diameter : 63 mm
• Mirror coating : Radiant Aluminum Quartz RAQ
• Stated reflectivity : 94%
• Secondary mirror supports : Four 0.5 mm vanes
• Focuser : Crayford
• Focuser size : 2 inch
• Reduction ratio : 10:1
• Focuser internal thread : M54
• Eyepiece compatibility : 1.25 inch and 2 inch
• Supplied eyepiece : 28 mm 2-inch LET
• Magnification with supplied eyepiece : 43×
• Highest practical power stated : 500×
• Finder : 9 × 50 with crosshair reticle
• Collimation : Adjustable primary and secondary mirrors
• Optical tube : Internally darkened rolled steel
• Dovetail : 45 mm
• Camera adapter : Included
• Optical tube weight : 14.4 kg
• Stated light gathering compared with 200 mm : +56%

Package Contents

• Sky-Watcher Explorer 250PDS optical tube
• Tube rings
• 45 mm dovetail bar
• 28 mm 2-inch LET eyepiece
• 9 × 50 finderscope with crosshair reticle
• 1.25-inch eyepiece adapter
• 2-inch eyepiece adapter
• Camera adapter