A lens corrector near focus lets a Cassegrain work at f/3.8
Charles Harmer and Charles Wynne published the design in 1976, for wide-field survey work. Moving most of the correction into a group of lenses close to focus lets the mirrors stay simple, a spherical secondary in particular, while the field stays flat and sharp to its edge at a focal ratio fast enough for short exposures of faint nebulae.
The price of the speed is a very large secondary mirror. On Officina Stellare's RiFAst 600 it blocks 55% of the aperture's diameter, which costs some light and contrast but leaves the wide field intact.
Officina Stellare's RiFAst line is the Harmer-Wynne astrograph on the market
| Model | Maker | Aperture | Focal length | Price |
|---|---|---|---|---|
| RiFAst 300 | Officina Stellare (Italy) | 300 mm | 1,140 mm (f/3.8) | special order |
| RiFAst 400 to 800 | Officina Stellare (Italy) | 400 mm to 800 mm | f/3.8 | special order |
| RiFAst 600 | Officina Stellare (Italy) | 600 mm | 2,280 mm (f/3.8) | $95,095 |
Prices are US list prices checked in November 2026. Special-order instruments are quoted by the dealer.
Officina Stellare describes the RiFAst as a modified Harmer-Wynne with a 220 mm to 240 mm back focus, room for a filter wheel and a large-format camera. It is built for permanent observatories, not portable setups.
A RiFAst 300 frames galaxies and nebulae at f/3.8
On an APS-C camera with the Sony IMX571 sensor (23.5 x 15.7 mm, 3.76-micron pixels), a RiFAst 300 at 1,140 mm gives a field of 1.18 x 0.79 degrees at 0.68 arcseconds per pixel. 100 of the 110 Messier objects fit in the frame, and 42 of them fill at least a quarter of its short side, among them M22 (Sagittarius Cluster), M67 (King Cobra Cluster), M101 (Pinwheel Galaxy) and M20 (Trifid Nebula).
Most backyard skies blur stars to 2 to 3 arcseconds. Image scales below about 0.7 arcseconds per pixel record no extra detail, so cameras on long focal lengths are usually binned 2x2.
A Harmer-Wynne astrograph cannot be used with an eyepiece
Its corrector is designed for a camera at a fixed distance, and there is no visual back. For amateur wide-field imaging at a fraction of the cost, look at the RASA or a fast apochromatic refractor.