A Sextuplet Refractor: The Z4 from Sharpstar
This flat-field sextuplet from Sharpstar is a high performance refracting astrograph. Featuring a 100 mm aperture and a 550 mm focal length, the Z4 has a focal ratio of f/5.5.
The optical design of this astrograph is a sextuplet in a 3+3 configuration. One lens in the front, and one lens in the rear are made from extra-low dispersion (ED) glass to minimize chromatic aberration. The optical design creates a flat field, eliminating the need for a separate field flattener accessory or having to calculate backfocus. The Z4 has a 36 mm, APS frame image circle. It is not recommended to use full frame cameras with the Sharpstar Z4 astrograph.
A Powerful and Attractive Refracting Astrograph
Unlike other Sharpstar designs, the Z4 comes in an attractive, two-tone matte gray. The tube and retractible dew shield is a light gray and the lens cover, handle, tube ring, and dovetail provide a nice darker gray accent.
The Z4 includes the optical tube, tube ring, carry handle, lens cap, a dovetail, a four-piece photographic adapter, manual, and carrying case.
Sextuplet Optical Design
The sextuplet design of this refractor is composed of two lens groupings in a 3+3 configuration. One lens from the front, and one from the rear are made of extra low-dispersion (ED) glass to minimize chromatic aberration. This design also provides a flat field, with no additional field flattener required. Also, there is no need to calculate backfocus.
Focuser and Rotator
The Z4 features a large 3.4″ rack-and-pinion focuser with a 1:10 fine adjustment knob. Attached to the focuser is a convenient 360° rotator, which allows an attached camera or accessory to be rotated to the desired orientation.




















JAMES C. Verified Buyer –
The overall quality is amazing! Much better than I was expecting. When you consider the specs and price point, it was a definite green light to purchase! No more back focus issues. And the Askar M54 OAG and Filter drawer I added added were of equal quality and connected perfectly! Rock Solid.