Sharpstar HNT, SCA and Z4 astrographs: f/2.8 Newtonians, Cassegrains and image circles

Current and discontinued

Sharpstar’s discontinued list now holds most of the refractors the brand was known for: the 61EDPH III and II, 76EDPH, 94EDPH, 50EDPH and AL-140PH, along with the carbon-tube 13028HNT and 15028HNT. What remains under the Sharpstar name is the aluminium-tube HNT-AL pair, the SCA260 v2 and SCA310, and the Z4. The company’s other brand, Askar, carries the SQA, FRA and PHQ refractors, covered on the SQA, FRA and PHQ page.

For anyone buying an EDPH second-hand, the reducer pages are still online and publish their results directly: 270 mm at f/4.4 for the 61EDPH III, 342 mm at f/4.5 for the 76EDPH, 414 mm at f/4.4 with a 50 mm circle for the 94EDPH, and 230 mm at f/4.6 for the 50EDPH. The 61EDPH III reducer’s own page calls it f/4.5 in one heading and f/4.4 in its table and text.

HNT: f/2.8 with the corrector built in

The 13028HNT-AL (130 mm, 364 mm) and 15028HNT-AL (150 mm, 420 mm) are hyperbolic Newtonians whose correctors sit in the focuser: two elements on the 130, three on the 150, one ED element in each. Both cover a 44 mm circle at 55 mm back focus, enough for a full-frame sensor. With an ASI6200MC Pro the 130 frames 5.66° × 3.78° at 2.13″ per pixel.

Sharpstar’s “field of view” figures, 6.9° for the 130 and 6° for the 150, are the angular size of that 44 mm circle: 2·atan(22 ÷ 364) = 6.92° and 2·atan(22 ÷ 420) = 6.00°. The rectangular sensor inside it always sees less.

The speed comes with a large secondary. Sharpstar gives secondary minor axes of 65 mm and 70 mm and says each is sized to illuminate a full frame. That is half the aperture by diameter and 25% of the area on the 130, 47% and 22% on the 150. Compared with the f/5.5 Z4, f/2.8 raises the per-pixel signal rate on extended objects by (5.5 ÷ 2.8)² = 3.86 at the same pixel size. Filters behave differently in a beam this fast; see the narrowband and fast optics guide.

SCA260 and SCA310: two different Cassegrains

The SCA260 v2 (260 mm, 1300 mm, f/5) pairs an aspherical PZ33 primary with a spherical secondary and a three-element corrector, and Sharpstar rates it for an 80 mm circle with the M48 face 70 mm from the focal plane. That circle spans 3.52° on the sky, far more than a full-frame sensor uses: 1.59° × 1.06° at 0.60″ per pixel.

The SCA310 (310 mm, 1178 mm, f/3.8) is faster on paper and covers 55 mm, which Sharpstar describes as optimized for medium format with full coverage of full frame. Its 184 mm secondary is the catch. Computed from the published diameters:

Telescope	Nominal	Blocked by area	Same clear area as	Focal length ÷ that aperture
SCA260 v2	f/5	21%	231 mm	5.64
SCA310	f/3.8	35%	250 mm	4.72

By nominal focal ratio the SCA310 looks (5 ÷ 3.8)² = 1.73 times faster than the SCA260; by clear area it is (5.64 ÷ 4.72)² = 1.43 times. It still collects more light per pixel, but the gap is smaller than the f-numbers suggest.

Z4

The Z4 is a 100 mm, 550 mm f/5.5 refractor with six air-spaced elements, three at the front and three at the rear, two of them ED. Sharpstar calls it self-flattened, with no back-focus calculation needed, and rates it for a 36 mm circle, labelled “APS frame”. An APS-C sensor (28.3 mm diagonal) fits with margin: 2.45° × 1.64° on an ASI2600MC Pro. A 43.3 mm full-frame sensor does not.