------------------------------------------------------------------------------- Air Schmidt Plus Loral 3K on the 4-m Echelle (1Sep95) (from CTIO, NOAO Newsletter No. 43, September 1995) The Blue Air Schmidt plus Loral 3K CCD was tested, in combination with the 4-m Echelle spectrograph, during an engineering run in May. From experience with the Reticon CCD, we anticipated that variations in image quality would limit the usable field to only a small portion of the detector. We were, therefore, delighted to find that acceptable images were obtained over most of the CCD. Indeed the image quality was very similar to that obtained when the same camera detector combination is used on the R-C spectrograph. With a very narrow slit, the measured FWHM of comparison lines was ~2.6 pixels at best focus for a wavelength of 3700A, while for a slit width of 1.3" (200 um) it was 3.0 pixels. This corresponds to R~17,400 when using either the 31.6 l/mm or 79 l/mm echelle gratings. With a 1.3" slit, the resolution was 3.5 pixels FWHM or better corresponding to R > 15,000 over two-thirds of the area of the CCD, and was better than 4.0 pixels even in the corners of the detector. Charge smearing in the Loral 3K CCD causes blurring of the images that is more severe at shorter wavelengths (see NOAO Newsletter No. 42 p.29). Thus slightly higher resolution will be achieved when working in the red, and slightly poorer resolution at extreme UV wavelengths. As used on the Echelle, the long axis of the Loral CCD runs along the echelle dispersion direction. The entire free spectral range of the 79 l/mm echelle fits on the CCD out to beyond 1 mm (the FSR fills 80% of the CCD's size at this wavelength). The FSR of the 31.6 l/mm echelle occupies less than one third of the detector at 1 mm. The table below shows the approximate coverage obtained in the cross dispersion direction for some commonly used cross-dispersion gratings. Grating l/mm Order Blaze (A) Coverage(A) G250 158 1 4000 4210 G400 158 1 8000 4210 G226-2 226 1 8000 2940 G226-3 226 1 6300 2940 G510 300 1 10000 2217 KPGL2 316 1 4400 2105 G181 316 1 7500 2105 KPGL3 527 1 5500 1260 G510 300 2 5000 1108 G181 316 2 3750 1052 KPGL1 632 1 4200 1052 Note that the need to select appropriate order sorting filters may limit the usable cross dispersion coverage in some cases. In particular, note that when using a cross disperser in second order, it may be necessary to guard against contamination from both first and third orders. For further information users are referred to the CTIO World Wide Web page (http://ctio.noao.edu). This features an interactive tool to help determine the layout of the echelle spectrum on the CCD. Figure 1 shows sample output from this program for two cases: (1) a blue setup using cross disperser KPGL2; and (2) a red setup using cross disperser G400. The World Wide Web manual also contains a copy of the Observer Support Questionnaire for the echelle, and (hopefully, by the time this Newsletter comes out) an exposure time calculator to help you plan your observations. Figure 2 shows the overall system efficiency (the fraction of photons striking the primary mirror, that are detected by the CCD) derived from measurements of standard stars. The plotted quantity is the OSE at the peak of the echelle blaze in each order. Note that the measurements were made with a wide (10") spectrograph slit. Curves are shown for two cross dispersers, G400 and KPGL2, used together with the 79 l/mm echelle and various order sorting filters. Two points of reference may be of use to those preparing TAC proposals. Firstly, these numbers for the throughput of the Echelle plus Air Schmidt plus Loral 3K CCD are roughly 3-3.5 times lower than the throughput of the 4-m R-C spectrograph used with the same detector, camera and grating. Resolving powers up to R~6000 can be obtained with the R-C spectrograph. Secondly, these numbers are 1.5-2.0 times higher than equivalent numbers for the Echelle when used with the Long Camera and Tek2048 CCD. This latter setup delivers R ~30,000. [Figures not included] Figure 1. Output from the echelle simulator tool "ECHMAP": (top) blue setup using cross disperser KPGL2 (wavelength range 3150-5175A); and (bottom) red setup using cross disperser G400 (wavelength range 5700-9800A). The heavy rectangle marks the outline of the CCD, while the roughly-vertical converging lines mark the edges of the free spectral range. The roughly-horizontal slanting lines are individual orders, and "Min. Order Separation" is the separation in arcsec of the most closely spaced orders (i.e. the usable slit length). Figure 2. Overall system efficiency for the 4-m Echelle spectrograph with the Blue Air Schmidt and Loral 3K CCD. Curves are shown for two cross dispersers, G400 158 l/mm blazed at 8000A and KPGL2 316 l/mm blazed at 4400A, used together with the 79 l/mm echelle. Steve Heathcote, Jack Baldwin