Wright Camera
Encyclopedia
In astrophotography
, the Wright Camera design, presented by Franklin Wright (Berkley, California) in 1935, just a few years after the introduction of the Schmidt camera
, was his "short" alternative to the original arrangement.
at the focal plane, and had the mirror aspherised to cancel coma
resulting from the altered aperture
stop position (the aperture stop effectively coinciding with the corrector).
remains present in the Wright camera, it combines with the mirror's Petzval field curvature
to result in a flat best image surface. In effect, the only monochromatic fourth order (wavefront
) 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
diameter and F the focal ratio 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, 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.
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
, the Wright Camera design, presented by Franklin Wright (Berkley, California) in 1935, just a few years after the introduction of the Schmidt camera
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. Other similar designs are the Wright Camera and Lurie-Houghton telescope....
, was his "short" alternative to the original arrangement.
Design
Wright placed the Schmidt correctorSchmidt corrector plate
A Schmidt corrector plate is an aspheric lens which is designed to correct the spherical aberration in the spherical primary mirror it is combined with. It was invented by Bernhard Schmidt in 1931, although it may have been independently invented by Finnish astronomer Yrjö Väisälä in 1924...
at the focal plane, and had the mirror aspherised to cancel coma
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...
resulting from the altered aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
stop position (the aperture stop effectively coinciding with the corrector).
Image quality
While astigmatismAstigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
remains present in the Wright camera, it combines with the mirror's Petzval field curvature
Petzval field curvature
Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane....
to result in a flat best image surface. In effect, the only monochromatic fourth order (wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
) 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
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
diameter and F the focal ratio number).
However, while the off-axis error
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...
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, 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
Aspheric lens
An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....
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.