Photography through the telescope
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 acting 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 the camera lens does not need to be removable. The method is suitable for lightweight compact digital cameras. Brackets are available for mounting like this.
In all cases the telescope needs to be on a motorised mount for photographing stars and nebulae. The considerations on the previous page about the need for multiple exposures still apply, only more so: alignment and drive errors are magnified and the amount of light collected is greater.
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.
The Meade telescope shown below has excellent optics for lunar and planetary photography but, in common with all catadioptric telescopes, its photographic aperture ratio is poor: f/15. So it is excellent for photographing bright objects such as the moon and planets but it is not so good for stars and nebulae.
Here is a Canon SLR mounted at the prime focus of a Meade ETX-125 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.
Newtonian reflectors have larger photographic apertures and so are more suitable for photographing nebulae. I use a 254mm (10 inch) aperture SkyWatcher Newtonian:
Its focal length is 1200mm, so the aperture is f/4.8. That is probably as large a focal ratio as possible because it does cause some distortion at the edge of the field of view, of a kind called coma. It is possible to use a coma corrector and I have evaluated one (as described here).
Details of how I attach the camera at prime focus are given here.
As a rough indication of cost for this set-up: the telescope plus equatorial mount together cost only half as much as the Canon camera body (all purchased retail, brand new).