Photography through the telescope
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There are 3 distinct ways of attaching a camera to look through an astronomical telescope:
- At the prime focus, without any eyepiece or camera lens. In this case the main mirror or objective lens of the telescope is used as the lens of an SLR camera.
- On the eyepiece holder, with a telescope eyepiece but without a camera lens (so we mean SLR again). The telescope's final image is projected directly onto the film or detector chip of the camera. This is not using the telescope quite as designed because its output rays are not parallel, as they would be for observation by human eye. The advantage over the next method is that there are fewer glass surfaces to lose light by absorbtion and reflection. Camera lenses typically contain many such surfaces.
- On the eyepiece holder, with both telescope eyepiece and camera lens. Focussing is arranged such that between the eyepiece output and the camera lens the light rays are parallel. The telescope is in its optimal setting and the camera lens is focussed on infinity. This is known as the parfocal method. An attraction of this method is that it can be used with any type of camera because its lens does not need to be removable. It is suitable for lightweight compact digital cameras. Brackets are available for mounting like this.
A misconception about astronomical telescopes is that their main purpose is to provide a high degree of magnification, as a super-telephoto lens in photographic terms. While that is useful for studying the moon and planets and perhaps the sun, for other subjects it is more important to collect as much light as possible. So aperture is significant. My Meade telescope, shown above, has excellent optics for lunar and planetary photography but, in common with all catadioptric telescopes, its photographic aperture ratio is poor: f/15. So I can photograph the moon and planets through it but it is not so good for stars and nebulas. Many years ago I had a Newtonian telescope. It had an aperture diameter of 150mm, similar to the Meade, but it was f/8, which is typical for the simpler Newtonian design. So these days I do star photography in the piggy-back mode described previously and I mainly photograph through the telescope for the moon, planets and sun (warning: the sun requires a special filter, to be described later).
Prime focus photography
Here is my camera mounted at the prime focus of my Meade telescope through a port on the back of the scope:
That uses a T-mount adaptor, having a Canon fitting at one end to fix to the camera body and a T-mount screw thread for attaching to the telescope. Telescopes and microscopes commonly use the T-mount standard for fitting accessories. The little white spot at the back of the telescope is a lever which flips a 45-degree mirror out of the way to give a clear path to the camera instead of reflecting up to the eyepiece on the top of the instrument.
Photography through an eyepiece
