Imagers are astronomical instruments that have the capability of forming images of sources. The simplest form of imager is when a multi-pixel detector, such as a CCD, is placed at the focal plane of a telescope, in front of which is usually placed a filter in a filter wheel, as shown in figure 69.

figure 69:

A photograph of the simple imager used by Sheffield astronomy undergraduates at the Cassegrain focus of the 0.5 m Mons telescope on Tenerife. From right-to-left are shown: the CCD detector, the filter wheel, the "flipper" (a diagonal mirror housed in the aluminium box which can be moved into the beam to direct the light upwards to the eyepiece) and the bottom of the primary mirror cell.

Since the filter in a simple imager is in the converging beam from the telescope, it causes the focal plane to move away from the primary, as shown in figure 70. To compensate for this when using a small telescope, the detector is usually moved outwards. On large telescopes, however, which generally have multiple instruments that require focusing, the position of the detector and filter is usually fixed and the focal plane is moved back towards the primary by moving the secondary mirror away from the primary. Inserting filters of different thicknesses and/or refractive indices causes the focal plane to move by different amounts. This can be compensated for by adjusting the telescope focus each time, or eliminated entirely by ensuring that the filters all have the same optical thickness, i.e. the product of the thickness and refractive index of each filter in the wheel is the same.

figure 70:

Left-top: A simple imager - parallel beams of light from an on- and off-axis star are brought to focus by a telescope onto an array detector placed at the focal plane. Left-bottom: the insertion of a filter forces the telescope focal plane outwards. Right: the QSI 532 CCD camera with a 5-position filter wheel, as used on Sheffield's 10" robotic telescope, ROSA.