dimensional photon counter

A two -dimensional photon counterStephen A. ShectmanMount Wilson and Las Campanas Observatories, Carnegie Institution of Washington,813 Santa Barbara Street, Pasadena, California 91101 -1292AbstractThe 2D- Frutti instrument is a spectroscopic detector used at the cassegrain spectrographof the Palomar 5 -meter telescope. A Fairchild 380 x 488 element CCD array is used to detectphoton event scintillations in a high -gain image intensifier. The diodes are inspectedserially at a 15 MHz rate. As little as 1/8 of the CCD vertical format can be inspected ina minimum frame time of 1 msec. The event centers are located to 0.25 diode horizontallyand 0.5 diode vertically. Up to 600 FWHM resolution elements are produced in the horizontaldirection. Coincidence corrections are no more than a few percent for count rates up to 105/sec when the array is uniformly illuminated.IntroductionThe 2D- Frutti detector extends the capabilities of the one dimensional Reticon spectro-meters) to the fully two -dimensional regime. The Reticon array is replaced by a Fairchild380 x 488 element CCD, which is used to detect photon event scintillations in a high -gainimage intensifier. Event centering is carried out in both the horizontal and verticaldirections. The two -dimensional format permits long -slit spectra to be taken, both for theefficient study of extended objects and for the more accurate determination of sky back-ground when measuring point sources. The 2D- Frutti is similar to the instrument describedby Stapinski, Podgers, and Ellis2.Intensifier chainThe two -dimensional photon counter uses the same spectrograph and image intensifier con-figuration as the Reticon detector. The double spectrograph for the Palomar 5 -m telescopecassegrain focus is described by Oke and Gunn3. A three -stage EMI magnetically- focussedimage intensifier is placed at the focal plane of the folded Schmidt camera of the bluespectrograph. The 50 -mm diameter EMI tube can cover the entire 40 -mm format of the camera,and provides excellent resolution and blue response. The EMI intensifier is housed in awater -cooled solenoid which provides the magnetic field for 3 -loop focus at a power dissipa-tion of less than 300 watts. The combination of camera and intensifier provides roughly1000 FWHM resolution elements across the spectrograph focal plane.Four additional stages of Varo electrostatic intensifier are placed at the focus of atransfer lens operating at a magnification of 0.75. The spacing of the transfer lens, thelow- carbon steel flux return surrounding the solenoid, and a magnetic shield around the Varotubes serve to completely isolate the electrostatic intensifiers from the magnetic intensi-fier focus field.A transfer lens with 0.5 magnification couples the final phosphor to the Fairchild 380 x488 element CCD. Because its readout noise is lower, the CCD is about 10 times more sensi-tive to photon events than the Reticon. The improved sensitivity of the CCD compensates forthe lower efficiency of the transfer lens, compared to the fiber -optic coupling of theReticon. The horizontal dimension of the CCD is 11.2 mm, which corresponds to 22.4 mm atthe last stage of the intensifier, or 30 mm projected back to the spectrograph focal plane.ElectronicsThe Fairchild CCD is constructed with an interlaced pattern of odd and even rows, whichin the normal format would make event centering in the vertical direction difficult. How-ever, both halves of the interlace can be combined on the chip to yield a fully serialformat of 380 x 244 elements. This is just as well since it is difficult to read out thelarge number of diodes in a time short enough to give reasonable coincidence rates.