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Loral 1200 x 800 CCD for Spectroscopy at 1.5-m (1Sep95) (from CTIO, NOAO Newsletter No. 43, September 1995) Initial tests of the Loral 1200 x 800 CCD (hereafter Loral 1K CCD) plus 1.5-m spectrograph combination were made during an engineering night in May. The science grade chip had only been installed a few days before this engineering run, so characterization of its properties was far from complete. However, the CCD was judged to be working well enough to allow initial evaluation of its performance at the telescope. The engineering run was highly successful and, as reported below, the results are encouraging. Unfortunately, following the engineering run, the CCD had to be returned to the US to have a broken bond wire repaired. This operation has now been performed successfully and work on the laboratory characterization of the detector will soon be under way again. Provided there are no unforeseen problems, we expect that the Loral 1K CCD will be made available for visitor use following a second engineering run in November. Thereafter it will be the only CCD offered with the 1.5-m spectrograph. The Loral 1K CCD is a thinned front side illuminated CCD with a single layer AR coating. The table below compares the QE of this CCD to that of the current GEC CCD. QE(%) Wavelength(A) GEC Loral 1K 3000 20 25 3500 19 48 4000 17 65 5000 22 83 6000 35 93 7000 45 91 8000 30 83 9000 14 59 10000 3 10 Unfortunately, on the engineering night the skies where heavily overcast so that it was not possible to obtain measurements of the absolute sensitivity. However, observations of standard stars confirmed that the sensitivity peaks at approximately 6000AŹ, where the QE curve peaks, and that there is significant sensitivity down to the atmospheric cutoff at 3000AŹ. Scaling previous measurements of the system efficiency with the GEC CCD by the above QE numbers, suggests that the peak throughput of the telescope plus spectrograph plus detector combination should approach 18% at 6000AŹ. Because of the broken bond wire only one of the two on chip amplifiers was operational at the time of the engineering run. Operating this amplifier at a gain of 2e-/ADU the readout noise was ~7.2 e- , similar to that obtained with CTIO's Loral 3K. The readout time was 35 seconds. An important goal of the engineering run was to measure image quality. The Loral CCD is 1.4 times longer than the GEC device currently in use and has smaller pixels (15 um compared to 22 um). It was thus anticipated that the camera optics might limit the resolution somewhat, especially at the extreme edges of the field. In addition diffusion of photoelectrons within the CCD may blur the images further, a phenomenon seen in other Loral devices. This latter effect is greatest at blue wavelengths since higher energy photons are absorbed closer to the surface of the CCD. While it was found that the camera and/or detector do degrade the images somewhat, the resolution obtained with the Loral, nonetheless, exceeds or is comparable to that obtained with the GEC CCD using a given grating. With a two arcsecond slit (110 um, which projects to 1.5 pixels on the detector) comparison lines near the center of the CCD have ~2.5 pixel FWHM for a wavelength of 4000AŹ and about 2.0 pixels FWHM for a wavelength of 9000A . The images are somewhat poorer in the corners of the detector, but, with this slit width, are everywhere narrower than 3.0 pixels FWHM. Grating l/mm blaze^1 Dispersion Coverage (A) (A) (A) 13 150 5000 5.73 6820 11^2 158 8000 5.45 530 09 300 4000 2.87 3410 32 300 6750 2.87 3410 22^2 300 10000 2.87 3410 58 400 8000 2.10 2560 16 527 5500 1.61 1920 26 600 4000 1.44 1705 35 600 6750 1.44 1705 56 600 11000 1.44 1705 47 831 8000 1.02 1220 36^3 1200 7500 0.72 850 (1) Littrow Value: for the actual 1.5-m spectrograph configuration the effective blaze wavelength is 0.89 of the Littrow value. (2) This grating is silver coated and does not reflect light below ~4000AŹ. (3) This grating cannot be tilted far enough to be used in second order. The table above gives the wavelength coverage and dispersion (AŹ/pix) obtained with the various gratings. One drawback of the Loral CCDs is that they suffer from substantial fringing at red wavelengths. For this device the peak-to-peak fringe amplitude is ~2.5% at 7500AŹ, rising to 10% at 8400AŹ and reaching a peak of 20.6% at 9300AŹ. It subsequently falls, reaching 10% again at 10,200AŹ. We do not yet know how well this fringing is corrected by flat fielding techniques. However, it seems likely that at wavelengths shortward of about 8000AŹ, where the amplitude is below 5%, fringing will be unimportant or easily correctable. Redward of this it will likely be necessary to obtain quartz flats for each object and take great care in flatfielding the data. Even then, observations requiring high S/N at wavelengths near 9300AŹ may not be possible with this CCD. Updated information on this system will be posted on the CTIO mosaic page, http://ctio.noao.edu, as it becomes available. Steve Heathcote, Mark Phillips
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