SharpStar 15028HNT-AL 150mm f/2.8 Hyperbolic Newtonian Astrograph

150mm f/2.8 Hyperbolic Optics: A 44mm Flat-Field System

The optical core of the 15028HNT-AL is its 150mm hyperbolic primary mirror, a more advanced design than the parabolic mirrors found in standard Newtonians. This specific figure is engineered to work as a complete system with the built-in corrector, enabling superior off-axis correction across a large 44mm image circle. The mirror is made from PZ33, a low-expansion Pyrex-type glass that ensures thermal stability as ambient temperatures change during an imaging session.

To deliver its expansive 6-degree field of view, a three-element, air-spaced corrector is integrated directly into the focuser assembly. This design, which includes one ED (Extra-low Dispersion) glass element, is optimized for the hyperbolic primary, eliminating coma and ensuring pinpoint stars to the edge of a full-frame sensor. By integrating the corrector, the system removes the complexity of matching a third-party coma corrector and guarantees the specified 55mm back-focus distance.

Mechanical Design: A 4.8kg OTA with Rigid Secondary Support

The 15028HNT-AL is built from all-metal components, with an aluminum main tube 443mm long and an OTA weight of 4.8kg. The secondary mirror bracket is CNC-machined in one integrated piece with no misalignment, a rigid design that ensures clean diffraction spikes. The primary mirror cell features three sets of adjustment screws, each with a locking screw, for precise and stable collimation.

The astrograph uses a 70mm secondary mirror, which SharpStar sizes to give sufficient illumination for full-frame imaging. This results in a central obstruction of 46.7%, a necessary trade-off for achieving this level of speed and field coverage. This mainly lowers contrast for visual observation; for imaging, flat frames take care of any uneven illumination across the field.

3kg Capacity Focuser: 20mm of Travel and 360° Rotation

An astrograph's focuser must be rigid enough to carry a modern imaging train without flexure. The 15028HNT-AL is equipped with a dual-speed rack-and-pinion focuser capable of supporting a 3kg payload. The focuser provides 20mm of travel for achieving sharp focus and is compatible with most electronic auto-focusers.

A built-in 360° camera rotator is positioned at the front of the focuser, allowing for effortless framing of your target without disturbing focus or alignment. The camera connects via a standard M48x0.75 thread, and the optical design provides a standard 55mm of back-focus from this mounting surface, simplifying camera and accessory spacing. SharpStar ships the focuser and rotator locked, so loosen the locking screws before first use to avoid damaging them.

Mounting & Accessories: 200mm Losmandy Plate and Handle Bar

Designed for portability and rapid deployment, the SharpStar 15028HNT-AL has a total weight of just 6kg, including its sturdy tube ring (with enlarged hand screws that open and close without tools) and a 200mm Losmandy-style dovetail plate. The compact 443mm tube length makes it easy to balance on a wide range of equatorial mounts. The package also includes a 200mm handle bar, and a finder base on the side of the tube provides an additional mounting point for a finder scope or other astronomical accessories.

Collimation Tools: Precision Alignment for a 0.77" Resolution System

To maintain its theoretical 0.77 arc second resolution, precise collimation is essential. SharpStar collimates the telescope at the factory, but transport knocks or cleaning can shift the optical axis. The box contains a collimating adapter, a collimating eyepiece, a 1.25" adapter and a T8 Torx wrench, allowing you to accurately align the optical axis; the mirror adjustment and locking screws take 2.5mm, 3mm and 4mm Allen wrenches, which are not in the box list. The manual provides a detailed, step-by-step visual guide for adjusting both the secondary and primary mirrors, ensuring you can achieve peak performance from the hyperbolic optical system.

15028HNT-AL vs. 6" f/4 Newtonians: The f/2.8 Advantage

While a standard 6" f/4 Newtonian is a capable imaging instrument, the SharpStar 15028HNT-AL is purpose-built for a higher tier of astrophotography. The primary differences lie in optical speed and field correction.

• Exposure Time: At f/2.8, the 15028HNT-AL gathers light approximately twice as fast as an f/4 system. This means a 5-minute exposure on the 15028HNT-AL collects the same signal as a 10-minute exposure on an f/4 scope, effectively doubling your imaging efficiency.
• Integrated Correction: The 15028HNT-AL's hyperbolic mirror and built-in corrector are designed as a single optical system to produce a flat 44mm field. A standard f/4 Newtonian requires a separate coma corrector that may not fully correct the field or illuminate a full-frame sensor without vignetting.
• Optical Design: The hyperbolic design allows for sharper stars at the edge of the 6-degree field compared to what a standard parabolic mirror and separate corrector can typically achieve, especially on large sensors.