Disadvantages of The Newtonian Design
- Newtonians, like other reflecting telescope designs using parabolic mirrors, suffer from coma, an off-axis aberration which causes imagery to flare inward and towards the optical axis (stars towards edge of the field of view take on a "comet-like" shape). This flare is zero on-axis, and is linear with increasing field angle and inversely proportional to the square of the mirror focal ratio (the mirror focal length divided by the mirror diameter). The formula for third order tangential coma is 3θ / 16F², where θ is the angle off axis to the image in radians and F is the focal ratio. Newtonians with a focal ratio of f/6 or lower (f/5 for example) are considered to have increasingly serious coma for visual or photographic use. Low focal ratio primary mirrors can be combined with lenses that correct for coma to increase image sharpness over the field.
- Newtonians have a central obstruction due to the secondary mirror in the light path. This obstruction and also the diffraction spikes caused by the support structure (called the spider) of the secondary mirror reduce contrast. Visually, these effects can be reduced by using a two or three-legged curved spider. This reduces the diffraction sidelobe intensities by a factor of about four and helps to improve image contrast, with the potential penalty that circular spiders are more prone to wind-induced vibration.
- For portable Newtonians collimation can be a problem. The primary and secondary can get out of alignment from the shocks associated with transportation and handling. This means the telescope may need to be re-aligned (collimated) every time it is set up. Other designs such as refractors and catadioptrics (specifically Maksutov cassegrains) have fixed collimation.
- The focal plane is at an asymmetrical point and at the top of the optical tube assembly. For visual observing, most notably on equatorial telescope mounts, tube orientation can put the eyepiece in a very poor viewing position, and larger telescopes require ladders or support structures to access it. Some designs provide mechanisms for rotating the eyepiece mount or the entire tube assembly to a better position. For research telescopes, counterbalancing very heavy instruments mounted at this focus has to be taken into consideration.
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