Refractor optical tube Skywatcher BK 80/400 OTA — 80 mm aperture, 400 mm focal length

The Skywatcher BK 80/400 OTA is a refractor optical tube with an 80 mm aperture and 400 mm focal length, supplied as an optical tube assembly (OTA) for use on compatible mounts. Its compact, lightweight construction and short focal ratio make it suitable for wide-field observation of star fields, nebulae and large deep-sky objects, as well as for lunar and planetary viewing at low-to-moderate magnifications. As an OTA, it is intended for users who want a portable, entry-level instrument to pair with a variety of mounts and basic imaging setups.

Advantages

This 80 mm refractor provides a well-corrected view across a wide field due to its optical design and moderate aperture, delivering sharp stars and good contrast for many targets. The 400 mm focal length and resulting fast focal ratio allow shorter exposure times for basic astrophotography and ease of framing large objects. The OTA format keeps the unit versatile: it can be combined with different mounts, finders and accessories, and it is convenient to transport and set up for visual observing or quick imaging sessions.

Suitable for

• Wide-field observation of star clusters, large nebulae and Milky Way regions
• Moon and planetary viewing at low to medium magnifications
• Entry-level astrophotography with short-exposure imaging and DSLR or mirrorless cameras
• Portable observing sessions and travel setups when paired with a suitable mount

Key specifications

• Optical type: Achromatic refractor optical tube assembly
• Clear aperture: 80 mm
• Focal length: 400 mm
• Focal ratio: f/5
• Suitable for visual use and short-exposure imaging
• Compact and lightweight OTA format for mounting on compatible tripods and mounts

How to use

Attach the Skywatcher BK 80/400 OTA to a compatible equatorial or alt-azimuth mount using the dovetail or mounting bracket provided by the mount manufacturer. Install an appropriate diagonal and eyepiece for visual observation, or a camera adapter and imaging camera for basic astrophotography. Balance the mount after fitting the OTA, perform polar alignment if using an equatorial mount, and use low-power eyepieces to locate targets before switching to higher magnification if needed.

Usage recommendations

Use this refractor OTA for wide-field targets and short-exposure imaging; pair it with a stable mount, a small guide scope if longer exposures are required, and a field flattener or reducer designed for refractors to improve edge sharpness when imaging with larger sensors. Regularly check collimation and clean only with proper optical tools to maintain image quality.