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INT Wide Field CCD Mosaic Camera
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The INT Prime Focus Camera (INT PFC) is designed to provide a large field survey and supernovae search capability for the prime focus of the 2.5m Isaac Newton Telescope on La Palma. It is a joint collaboration between the Royal Greenwich Observatory (United Kingdom), Kapteyn Sterrenwacht Werkgroep (Holland) and the Lawerence Berkeley Laboratories (California). The camera was upgraded in April 1998 using large format EEV CCDs. Some test images from the commissioning run are shown below.

Messier 13 Messier 81 Messier 8 Messier 101

The 4-chip mosaic The INT PFC consists of a 4 Chip Mosaic assembly. utilising thinned and anti-reflection coated CCDs. These are EEV devices each measuring 2K x 4K pixels. They are operated cryogenically in a purpose-built Liquid nitrogen cooled cryostat assembly. A fifth CCD, a Lesser thinned Loral device, is co-mounted with the science array in the cryostat to provide autoguiding functions.The image on the left shows both the original and upgraded detector mosaic. This cryostat then mounts to the main camera assembly at the prime focus. camera assembly This assembly will include standard filters mounted in a filter wheel and a novel shutter wheel which has been specifically designed to give a very flat uniform exposure time profile across the detectors - compare this with a standard IRIS type shutter which exposes for longer near its centre. The camera will have an unvignetted field of 40 arcminutes and a focal ratio of f/3.29. This results in a very tight mechanical specification for coplanarity and flatness of the array of CCDs and also quite stringent flexure tolerance of the camera assembly.

The coplanarity of the mosaic was verified at the RGO using purpose built optical scanner. This consisted of an optical head mounted on an XY translation stage. This head contained an LED illuminated pinhole and a 6 element lens system that imaged the pinhole onto the mosaic from one of two angles. Translating the head whilst integrating an image onto the mosaic produced line images whose subsequent analysis yielded a vertical profile with a resolution of a few microns. Initial coplanarity errors between the autoguider and science CCDs were corrected by the insertion of foil shims beneath the autoguider package. The final coplanarity error was +/-15microns. A cardboard model of the mosaic with an exaggerated vertical scale was produced from the scanner data to assist in final focussing of the camera at the telescope. The scanner was also used to determine the gaps between the mosaic elements. The design and operation of the coplanarity scanner is described elsewhere or can be downloaded from here.

The camera reads out its 32M pixels in 2.5 minutes with a read noise of less than 10e RMS. The full well of the detectors is 200,000e. The sky coverage is 34' x 34' with a plate scale of 0.333"/pixel. The camera is therefore capable of sampling 80% of the unvignetted field of the Isaac Newton Telescope.

A particular point concerning the camera is the fact that the default state of our shutter is always open, even when idling, this is to allow guiding between exposures. The CCDs are therefore prone to remenance type effects caused by bright stars. The net effect of this is seeing ghost images of these bright stars in follow on images. A new remenance removal routine has been developed which removes these effects. This is transparent to the user. A new anti blooming routine has also been developed which removes/reduces the effects of saturated stars blooming up and down a column. A more detailed description of these modes of operation are given elsewhere.

The camera was initially commissioned on the telescope in May 1997 and upgraded in April 1998. A picture of the camera can be seen here as mounted on the telescope. camera on telescope The camera is able to rotate to the four cardinal points to search for guide stars, although with such a large guide CCD, this is not usually necessary. We have constructed a cable twister arrangement to fix the cables and twist them when the rotator moves. A mosaic!! of pictures which were taken during commissioning are shown here.

The camera is controlled from a GUI interface which runs on the SUN 20 observing system. The observer need only need click on the GUI buttons buttons to set exposures going and change filters etc.