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Introduction.

At present we use the EEV-05-30 or the Tek-1024 CCD for most applications; these sensors offer a size of or respectively. Nearly all of our focal stations can use detectors of size in excess of 25mm, at least in one dimension. Table 1 shows a summary of the requirements for most instruments.

It is also evident from this table that the spectrographs require a rectangular sensor with dimensions of . In contrast, the direct imaging focii require detectors exceeding 60mm in size. There are a few locations where embedded CCDs are used (listed later in table 2). The INT Prime Focus (PF) is soon to be equipped with a mosaic array of Ford/Loral CCDs. This work is being undertaken in collaboration with the LBL/Berkeley group, and so provision of it's sensors is not discussed here.

We propose that all new detectors for the ING need to be of the thinned, back-side illuminated type because of the clearly superior quantum efficiency at all wavelengths. Figure 1 illustrates the spectral response achieved with different types of CCD.

To achieve the highest efficiency in different wavelength regions, for example, the ISIS red & blue arms, one could consider using CCDs optimised for each of these wavelength regions. For a thinned device the AR coating can be selected to optimise the red or blue spectral response. Furthermore, since thinned CCDs can exhibit interference fringes at , a thick, front-side illuminated device might be preferable for observations at longer wavelengths. In practice a wide range of detector options is not realistic and on balance we would aim to provide a single detector which exhibits the best overall response.

We have adopted a seeing figure of , since this is the best achieved to date on ING telescopes; this leads to an ideal pixel size corresponding to for imaging instruments (see table 2). The implementation of an adaptive optics system, planned for the WHT, will require pixels of perhaps half this size; devices providing pixels of this size will need to be considered at a later date.

Hence to provide good image sampling, pixels sizes in the range will suffice in most cases. If programmes require larger pixels, then on-chip binning can be used; this permits a single (small-pixel) chip design to be used for a wide variety of applications.

On ING telescopes, CCD heads are frequently moved from one focus to another, and it is expected that this degree of sharing of camera heads will continue in the future. If the camera heads are equipped with CCDs from the same source, then the provision of spares is also more practicable. This means that we need to choose sensors to serve a range of requirements where possible. An overview of the requirements for all telescopes will now be presented.