Refractor/Reflector telescope Skywatcher BKMAK 90 SP OTA — 90 mm aperture, 1250 mm focal length

Skywatcher BKMAK 90 SP OTA is a compact Maksutov–Cassegrain optical tube assembly with a 90 mm aperture and 1250 mm focal length. The design combines a long focal length in a short tube, delivering high-contrast, sharp images of the Moon, planets and brighter deep-sky objects. As an OTA, it contains the optical elements and focuser but requires a compatible mount and eyepieces to form a complete observing setup. Its portable size and rugged construction make it suitable for both backyard observing and field use.

Advantages

The Maksutov–Cassegrain configuration provides good correction of aberrations and a long effective focal length in a compact tube, producing high-contrast images with a relatively narrow field of view ideal for lunar and planetary detail. The 90 mm aperture balances portability and light-gathering capability, while the 1250 mm focal length yields a focal ratio favorable for high-magnification visual observing and basic planetary imaging. The OTA format allows users to pair the optical tube with a variety of mounts and accessories to suit different observing needs.

Suitable for

• Visual observation of the Moon and planets
• Observation of bright deep-sky objects (nebulae, globular clusters) under moderate skies
• Portable field astronomy and backyard sessions
• Basic astrophotography of planets and the Moon when mounted on a stable tracking mount

Key specifications

• Optical design: Maksutov–Cassegrain (catadioptric)
• Aperture: 90 mm
• Focal length: 1250 mm
• Focal ratio: f/13.9 (approx.)
• Optical tube assembly (OTA) format — requires separate mount and eyepieces
• Compact, portable tube suitable for transport

How to use

Mount the BKMAK 90 SP OTA on a stable equatorial or alt-azimuth mount rated for the OTA mass and intended accessories. Install a compatible diagonal and eyepiece into the focuser for visual use, or fit an appropriate T‑ring/adapter and camera for basic planetary imaging. Collimation is generally stable for Maksutov designs, but inspect and adjust according to the mount and accessories used. Balance the assembly on the mount, perform polar alignment or level alignment as required by the mount type, and use low- to medium-power eyepieces for locating targets before switching to higher magnifications for detail work.

Usage recommendations

For best visual results, observe from a stable mount on nights with steady seeing; use low-humidity locations to improve contrast. When attempting basic astrophotography, use a motorized tracking mount and short exposure techniques for planetary imaging. Protect the optics with a dust cap during storage and allow the OTA to reach ambient temperature before high-magnification observing to reduce thermal currents.