One Mirror Reflector
Introduction
The Newtonian is the simplest and probably the most popular type of telescope for amateur astronomers. It is, without a doubt, the most commonly home-built amateur instrument. As such, the Newtonian is the standard against which the performance of other types of telescope is measured.
The Newtonian optical system consists of a paraboloidal primary mirror and a flat secondary mirror which directs the converging cone of light out of the tube for visual or photographic applications. The ray tracing and nomenclature are shown bellow:
The focal ratios of the Newtonian generally lie between f/4 and f/12. F/4 is a practical lower limit because of the rapid increase of coma away from the optical axis; f/12 is the upper limit because the length of the telescope becomes excessive. Those with slow primaries (between f/7 and f/12) are generally considered best for lunar and planetary observing, while those with fast primary (f/6 to f/4) are better suited for observing deep-sky objects and for prime-focus astrophotography or imaging (CCD).
For relatively slow and small Newtonians, the primary mirror need not be paraboloidal. The most Newtonians today do have a spherical mirror, because the sphere is quite easy to make (for example the 4.5“ Meade). Slow Newtonians need not be parabolized if the spherical aberration blur is smaller than Airy disk.
For the 150 mm Newtonian, a spherical mirror is acceptable for visual use in f/12 and slower systems; parabolizing is not needed for photographic instruments f/8 and slower.
The dominant image aberration of the Newtonian is coma. Astigmatism plays a role at relatively large image angles, and curvature of field is present. When the entrance pupil is at the mirror, which is usually the case, the best focal surface lies midway between the tangential and sagittal focal surfaces, and has aradius of curvature equal to the focal length.
Strong coma and astigmatism show at large off-axis distances, especially for fast primaries. Coma is the primary aberration near the axis; farther away astigmatism is visible as a second tail in the comatic blur figure.
When the spherical aberration is not equal to zero and coma is present, coma can be eliminated by changing the location of the entrance pupil. For Newtonian with spherical primary mirror the coma can be full eliminated if entrance pupil is located at the primary mirror radius of curvature.
The Newtonian‘s field is limited by coma. Coma occurs in both spherical and paraboloidal Newtonians because an oblique beam of parallel rays has no axis of symmetry with respect to the mirror. The image of this beam is not formed on the axis of the beam – but is instead formed to one side. The image, therefore, will be not be symetrical.
The underlyng cause of this asymmetry is that the entrance pupil, which forms the limiting stop for all entering bundles of light, coincides with the mirror itself. When the stop is moved from the mirror to its center of curvature, symmetry is obteined. It is clear that every beam of light passing through the stop, whether it is parallel to the optical axis or not, has its own axis of symmetry. This is not true when the stop lies at the mirror. The symmetry principle holds, of course, only when the mirror is spherical. But the image formed by the spherical mirror still suffers from spherical aberration.
Design and Optimization
Bellow is displayed the Newtonian design and analysis applet and here you can input/change the general data, like clear diameter, focal length, and the back borking bistance (BWD – this is the distance from secondary mirror center to the focal plane).
After all general data is imputted the design is followed by Solve button pressing. Newtonian data and Seidel aberration coefficients are calculated. Finaly detailed spot diagram is ploted.
The Newtonian design can be customized and optimized by using the scrolls from the optimization part. Five parameters can be changed (mirror diameter Diam, focal length F1, eccentricity E1, back focal length BFL and back working distance BWD), until acceptable image quallity is reached. The change of BWD takes effect in secondary mirror size only (i.e. central obstruction).
The Newtonian is the simplest and probably the most popular type of telescope for amateur astronomers. It is, without a doubt, the most commonly home-built amateur instrument. As such, the Newtonian is the standard against which the performance of other types of telescope is measured.
The Newtonian optical system consists of a paraboloidal primary mirror and a flat secondary mirror which directs the converging cone of light out of the tube for visual or photographic applications. The ray tracing and nomenclature are shown bellow:
The focal ratios of the Newtonian generally lie between f/4 and f/12. F/4 is a practical lower limit because of the rapid increase of coma away from the optical axis; f/12 is the upper limit because the length of the telescope becomes excessive. Those with slow primaries (between f/7 and f/12) are generally considered best for lunar and planetary observing, while those with fast primary (f/6 to f/4) are better suited for observing deep-sky objects and for prime-focus astrophotography or imaging (CCD).
For relatively slow and small Newtonians, the primary mirror need not be paraboloidal. The most Newtonians today do have a spherical mirror, because the sphere is quite easy to make (for example the 4.5“ Meade). Slow Newtonians need not be parabolized if the spherical aberration blur is smaller than Airy disk.
For the 150 mm Newtonian, a spherical mirror is acceptable for visual use in f/12 and slower systems; parabolizing is not needed for photographic instruments f/8 and slower.
The dominant image aberration of the Newtonian is coma. Astigmatism plays a role at relatively large image angles, and curvature of field is present. When the entrance pupil is at the mirror, which is usually the case, the best focal surface lies midway between the tangential and sagittal focal surfaces, and has aradius of curvature equal to the focal length.
Strong coma and astigmatism show at large off-axis distances, especially for fast primaries. Coma is the primary aberration near the axis; farther away astigmatism is visible as a second tail in the comatic blur figure.
When the spherical aberration is not equal to zero and coma is present, coma can be eliminated by changing the location of the entrance pupil. For Newtonian with spherical primary mirror the coma can be full eliminated if entrance pupil is located at the primary mirror radius of curvature.
The Newtonian‘s field is limited by coma. Coma occurs in both spherical and paraboloidal Newtonians because an oblique beam of parallel rays has no axis of symmetry with respect to the mirror. The image of this beam is not formed on the axis of the beam – but is instead formed to one side. The image, therefore, will be not be symetrical.
The underlyng cause of this asymmetry is that the entrance pupil, which forms the limiting stop for all entering bundles of light, coincides with the mirror itself. When the stop is moved from the mirror to its center of curvature, symmetry is obteined. It is clear that every beam of light passing through the stop, whether it is parallel to the optical axis or not, has its own axis of symmetry. This is not true when the stop lies at the mirror. The symmetry principle holds, of course, only when the mirror is spherical. But the image formed by the spherical mirror still suffers from spherical aberration.
Design and Optimization
Bellow is displayed the Newtonian design and analysis applet and here you can input/change the general data, like clear diameter, focal length, and the back borking bistance (BWD – this is the distance from secondary mirror center to the focal plane).
After all general data is imputted the design is followed by Solve button pressing. Newtonian data and Seidel aberration coefficients are calculated. Finaly detailed spot diagram is ploted.
The Newtonian design can be customized and optimized by using the scrolls from the optimization part. Five parameters can be changed (mirror diameter Diam, focal length F1, eccentricity E1, back focal length BFL and back working distance BWD), until acceptable image quallity is reached. The change of BWD takes effect in secondary mirror size only (i.e. central obstruction).
Applet Tag:
- param name = "Title" value = "One Mirror: Newtonian Telescope"
- param name = "Mirror Diameter" value = "150.0"
- param name = "Mirror Focal Length" value = "1250.0"
- param name = "Mirror Eccentricity" value = "1.000"
- param name = "Back Working Distance" value = "200.00"
- param name = "Half Field Angle" value = "0.45"
- param name = "Central Obstruction" value = "0.00"
- param name = "Plot Scale" value = "0.10"
- param name = "TF Start Angle" value = "0.00"
- param name = "TF End Angle" value = "0.45"
- param name = "TF Defocus" value = "0.05"
- param name = "Number of Arms" value = "36"
- param name = "Number of Rings" value = "10"
- param name = "Ray Density" value = "10"
- param name = "Scale Type Index" value = "3"
- param name = "Ray Pattern Index" value = "0"
- param name = "Merrit Function Index" value = "2"
- param name = "Glass Catalog Index" value = "2"
- param name = "Correction Index" value = "0"
- param name = "Monochr. Color Index" value = "10"
- param name = "WL1 Color Index" value = "1"
- param name = "WL2 Color Index" value = "4"
- param name = "WL3 Color Index" value = "9"
- param name = "Text Color Index" value = "10"