New Loral 3K CCD Available for Spectroscopy at the 4-m (1Jun95)

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New Loral 3K CCD Available for Spectroscopy at the 4-m (1Jun95) (from CTIO, NOAO Newsletter No. 42, June 1995) We have obtained a second Loral 3K X 1K CCD for use in the Blue Air Schmidt camera. This replaces the first of these devices, which was found to be defective when operated for the first time at CTIO (see NOAO Newsletter No. 40 p.25). This new CCD was installed in the camera and successfully tested with the 4-m R-C Spectrograph during an engineering run in February, and has since been used by several visitors. The Blue Air Schmidt plus Loral 3K will be the system of choice for most programs with the R-C spectrograph. The only exceptions will be work in the near IR where fringing of the Loral CCD may make the Folded Schmidt plus Tek 1K combination the better choice. The Blue Air Schmidt + Loral 3K has also been used successfully with the Argus Fiber fed spectrograph and will be the only camera/detector combination supported on that instrument (the Folded Schmidt cannot be used with Argus). We have yet to try the Blue Air Schmidt + Loral 3K on the echelle spectrograph (engineering time is scheduled in early May). We anticipate that useable image quality will only be maintained over the central one third of the CCD in this case. However, in the blue/UV where entire orders of the echelle can be fitted on this section of the CCD, the very high QE of the Loral CCD may still make this an attractive choice for some programs. Further information will be posted on the CTIO Mosaic page shortly after the engineering run. The Loral 3K X 1K CCD is a thinned front side illuminated device with a two layer AR coating designed to maximize its sensitivity over a wide wavelength range, while reducing fringing in the red. The table below gives the QE as measured in the laboratory at KPNO. Also included are values for the overall system efficiency (OSE: the fraction of photons striking the primary mirror that are detected by the CCD) obtained when using the Blue Air Schmidt + Loral 3K with the 4-m R-C spectrograph and grating KPGL1 (632 l/mm 4200A blaze) derived from measurements of standard stars. Both measures show that the sensitivity is up to a factor 1.8 greater than that obtained with the Reticon CCD. Because of the wide wavelength coverage and high QE of the Loral CCD, it is important to pay attention to the selection of order sorting filters when planning observations. For instance, when taking intermediate dispersion spectra using a grating in second order, it is important to guard against possible contamination from both first and third orders. Wavelength () QE (%) OSE (%) 3200 78.9 2.4 3500 76.7 10.6 4000 73.0 14.1 5000 86.6 18.6 6000 93.0 14.3 7000 93.9 8000 73.9 9000 41.8 Like NOAO's other Loral CCDs, this particular device has, by modern standards, rather high read out noise (~7.3e- RMS) but quite low dark current (~0.5e-/pixel/hour). Unfortunately, it has only one working amplifier so that, although it is being operated with an Arcon, the read out time is a glacially slow 136s. Work at KPNO has uncovered a problem with the Loral CCDs that limits the best resolution that can be obtained, which has been attributed to diffusion of photoelectrons within the CCD. The effect is largest at UV/blue wavelengths, since such photons are absorbed close to the surface of the CCD and thus have more time to diffuse before being trapped in the wells of the CCD. When a very narrow slit is used on the R-C spectrograph, the measured FWHM of comparison lines is ~2.3 pixels at best focus for a wavelength of 4000A. For a slit width of 1" (150 um, which should project to 2 pixels on the detector), the best images have ~2.6 pixel FWHM, while for a 1.5" slit (225 um projecting to 3 pixels) line widths grow to ~3.0 pixels FWHM. There is slight curvature of the focal plane that results in some variation of the FWHM with position on the CCD. With a 1" slit the images are 3.3 pix FWHM or better over most of the chip (growing to ~4 pix in the extreme corners), while with a 1.5" slit the images are 4.5 pixels or better over most of the chip (~5 pixels worst case). Even the worst-case images are symmetrical and do not show the very broad asymmetric wings seen in out-of-focus images obtained with the Reticon CCD and the same camera, or the Tek 1K CCD on the Folded Schmidt. In general, the images obtained with the Loral are much more uniform than with these other CCD's. With Argus, where the fibers project to 3 pixels on the detector, comparison lines have widths of 3 pixels FWHM at best focus. The table below gives the wavelength coverage and dispersion (A/pixel) obtained with the various gratings. This information is valid for both the R-C spectrograph and for Argus, since the parameters of the two spectrographs have deliberately been made the same. For reference, values are also given for the Folded Schmidt plus Tek 1K CCD; note that this camera/CCD combination is not available with Argus. B ASch + Loral 3K FSch + Tek 1K Grating l/mm Blaze^1(A) Coverage(A) Dispn.(A/pix) Coverage(A) Dispn.(A/pix) 250 158 4000 11431 3.75 6096 6.00 400^2 158 8000 11431 3.75 6096 6.00 510^2,3 300 10000 5999 2.01 3199 3.22 181 316 7500 5708 1.91 3044 3.05 kpgl2 316 4400 5708 1.91 3044 3.05 kpgl3 527 5500 3417 1.16 1822 1.85 420^3 600 8000 2981 1.02 1589 1.64 kpgl^1 632 4200 2872 0.95 1531 1.53 kpglf 632 8200 2872 0.95 1531 1.53 450 632 11000 2872 0.95 1531 1.53 kpgld 790 8500 2290 0.75 1221 1.20 kpglg 860 11000 2101 0.68 1120 1.09 380^3 1200 8000 1563 0.48 833 0.76 ^1 Littow value: for the actual R-C spectrograph configuration the effective blaze wavelength is 0.92 of the Littrow value. ^2 This grating is silver coated and so does not reflect light below ~4000A. ^3 This grating is not very efficient when used in second order. Despite the two-layer AR coating, the Loral 3K CCD does fringe at wavelengths longward of about 7500A. We are still in the process of evaluating the behavior of the CCD in this respect, so the following should be considered preliminary: People planning to use the Loral 3K at extreme red wavelengths should contact us (or see the CTIO Mosaic page) for updated information on its fringing properties. Because of the fast beam of the camera, the fringe amplitude depends not only on the wavelength of the light, but also on how far off-axis that wavelength is imaged. For instance in a spectrum covering 7050-9900A the largest fringe amplitude was 2% peak-to-peak, which occurred at 8500A close to the center of the detector; the fringe amplitude at 9000A was below 1%. Conversely, when the grating tilt was changed so that 9000A was centered, the fringe amplitude rose to 12% peak-to-peak at 900A0 and was greater than 5% at 8500A. Onthe R-C spectrograph, the Folded Schmidt plus Tek 1K may be a better choice when working exclusively at such red wavelengths. Please note that with the successful commissioning of the Loral 3K CCD, the Reticon CCD, previously installed in the Blue Air Schmidt, has been withdrawn from service. Steve Heathcote (sheathcote@noao.edu), Mark Phillips (mphillips@noao.edu), Bob Schommer (rschommer@noao.edu), Ricardo Schmidt (rschmidt@noao.edu), Alistair Walker (awalker@noao.edu)

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