Ken Hough's Website

Field Of View

The field of view of a telescope (or lens) depends on the focal length of the telescope, AND on the focal length of the eyepiece being used, or dimensions of the sensor of the camera.

Field of view is often expressed terms of degrees of angle. This allows users to relate the field of view of a particular telescope plus eyepiece or camera to the angular size of the area of sky to be viewed

Ideally, the field of view of a telescope system should be slightly larger than the area to be viewed. No single optical setup can match all possible objects wrt field of view. Compromises must be made.

Some typical scenarios are described below along with suggestions for suitable optical equipment.

Requirements for mounting/supporting telescopes are also discussed.

and also for the requirements of mounts/supports. Unless exposure periods are to be relatively short, good quality motor driven equatorial mounts are essential. Refer to the section about Equatorial Mounts.

Wide Sky:
Wide sky photography is taken to mean the recording of relatively large angular views of the sky such as constellations and even the Milky Way. For this kind of subject, relatively short focal length photographic lenses might be to more appropriate than telescopes. Top quality lenses are needed for this job. Astrophotography will show up limitations of mediochre/poor lenses. Old style zoom lenses that use a push/pull zoom adjustment are to be avoided. If possible use fixed focus (ie non-zoom) lenses which are of APO (Apochromatic) design.

Lenses of focal lengths of 300mm down to 16mm or even less would be used for wide sky work.

Medium Angle:
Many gaseous nebulae (including the stunning Orion Nebula) and the Moon fall into this category. These items appear to be relatively small so longer focus telescopes are needed. Telescopes of around 700mm focal length are often used successfully for direct viewing of some nebula and of the moon, but typically images will occupy only a small part of the total field of view. Better to use a telescope having a focal length of 1000mm to 1500mm. My 250mm diameter/1200mm focal length Newtonian telescope is excellent for this kind of work. Being a "fast" telescope (aperture number is F4.8), exposure periods can be kept fairly short.

Some astrophotographers prefer to use high quality refractor telescopes for this kind of work and indeed, results can be very good. But the aperture numbers of long focus refractors is relatively low (typically F8 or greater), so exposure periods can be quite long.

Small Angle/High Magnification:
Even the bigger planets (eg Jupiter and Saturn) appear to be quite small. To photograph these items, it is necessary to employ very high magnification and this means using telescopes with focal lengths of 2000mm or more. Without doubt, the most popular kind of telescope for this duty is the Schmidt Cassigrain having a diameter of 200mm or more. It is also practical to use a large fast Newtonian telescope with a x2 Barlow lens which effectively doubles the effective focal length from say 1200mm to 2400mm. By doubling the focal length, of course the aperture number is increased by two stops from F4.8 to F9.6. But, this compares well with Schmidt Cassigrain telescopes which usually have aperture numbers of F10.

Mounting Telescopes For Astrophotography:
The apparent positions of celestial objects appear to revolve around the Earth at 15 degrees per hour (ie 360 degrees in 24 hours). Unless using very short exposures, as might be the case for Lunar photography, it is necessary for the telescope/camera to follow the movement of the celestial objects. To do this an EQUATORIAL MOUNT is essencial (refer to section Equatorial Mounts).

For wide sky photography, exposure periods of up to 5 minutes can be managed by mounting a camera and lens on a manually driven BARN DOOR MOUNT (refer section Astro Projects). At higher magnifications, a motor driven equatorial mounts is necessary.

Return To Astrophotography Index Page