{{Short description, Astrophotography camera design
In
astrophotography
Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
, the Wright camera design, presented by Franklin Wright (
Berkeley, California
Berkeley ( ) is a city on the eastern shore of San Francisco Bay in northern Alameda County, California, United States. It is named after the 18th-century Irish bishop and philosopher George Berkeley. It borders the cities of Oakland and E ...
) in 1935, just a few years after the introduction of the
Schmidt camera
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. The design was invented by Bernhard Schmidt in 1930.
Some notable e ...
, was his "short" alternative to the original arrangement.
Wright placed the Schmidt corrector at the focal plane, and had the mirror aspherised to cancel coma resulting from the altered
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An ...
stop position (the aperture stop effectively coinciding with the corrector).
Image quality
While
astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at n ...
remains present in the Wright camera, it combines with the mirror's
Petzval field curvature
Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image pl ...
to result in a flat best image surface. In effect, the only monochromatic fourth order (
wavefront
In physics, the wavefront of a time-varying '' wave field'' is the set ( locus) of all points having the same '' phase''. The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal fr ...
) aberration of the Wright camera is relatively strong astigmatism.
In terms of the wavefront error, the flat-field P–V errors are identical in both, Schmidt and Wright camera, given by W=h2/16DF3 (h being the height in the image plane, D the
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An ...
diameter and F the
focal ratio
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
number).
However, while the off-axis error in the flat-field Schmidt results from defocus, in the Wright camera it is caused by astigmatism. Since the RMS/P–V error ratio is smaller by a factor of 0.51/2 for astigmatism, the actual quality flat-field radius in the latter is larger by a factor of 1.4.
On the other hand, the best (curved) image surface of the Schmidt is practically free from off-axis aberrations, hence clearly superior in quality to a best (flat) field in the Wright camera. Also, since the Wright camera requires twice as strong corrector to cancel spherical aberration of the mirror, its chromatic error is double that in a comparable Schmidt camera.
Both, more strongly aspherized corrector and, especially, strongly aspherized fast mirror (into a rather unpopular type of aspheric shape) of the Wright are a fabrication disadvantage. On the plus side, the Wright camera is only about half as long as an equivalent diameter Schmidt. Also, since the corrector in Wright's arrangement nearly coincides with the image plane, it can support the film/detector assembly, clearing the optical path from supporting vanes.