With the advent of larger-format array detectors at the AAO (e.g., the MIT/LL 4K x 2K CCDs), the process of arc line identification and spectrograph setup has in some ways become more complicated than before, owing to the bewildering number of arc lines and spectral orders captured in one image. Although a variety of arc line atlases are available for the ThAr lamp (e.g., D'Odorico et al. 1987, Bessell & Pettini 1991, Meurer et al. 1991, and Lipman et al. 1993), most of them are available in hardcopy only, every arc lamp produces slightly different line ratios, and even the line ratios from any given lamp will change with time. This on-line atlas is not intended to replace these useful references, but rather to serve as a complement, particularly when using UCLES with the MITLL CCDs.
The data was obtained on 1999 October 22, using the MITLL2 CCD (binned x2 in the spatial direction) and 2 settings of the 31.6 grooves/mm grating: one centred on 3946.0 Å covering 3522-4653 Å (orders 160-123), and the other on 6182.6 Å covering 4824-9386 Å (orders 61-117). Note that the detector was not centred on the peak of any one order, but since the extent of each order recorded in one MITLL CCD image extends way beyond the free spectral range, there is still almost complete wavelength coverage throughout. The exposure time in the blue was 10 seconds, and 2 seconds in the red (which unavoidably results in some extremely saturated lines beyond 7000 Å). The line identification and fitting was carried out using ecidentify within IRAF, relying mainly on the line identifications in Lipman et al. (1993) and the IRAF line list in linelists$thar.dat. Generally a 4th order fit in x should be sufficient to remove any trend in the residuals with pixel number, and a 5th order fit in y should remove any trend with order number.
The arc atlas is presented as a series of Encapsulated Postscript images. To view them, just click on the order number (the wavelength ranges shown are those covered by the detector, which is generally much greater than the FSR listed in Appendix A.2 of the UCLES manual). If this does not launch a Postscript viewer such as ghostview, then check that the application is set properly (for Netscape, go to Edit -> Preferences -> Navigator -> Applications and set "Postscript Document" to "ghostview %s"). The plots are small, but you can change the "Magstep" setting, or use the Zoom (middle mouse) button to enlarge them. Alternatively, a complete hardcopy of the atlas is available in gzipped Postscript format.
The image on the left shows the raw XMEM display for the MITLL2 CCD when the grating is set to 6182.6 Å (the MITLL3 display will be similar, except that it will appear flipped left to right). In this case, wavelength decreases (order number increases) going from left to right (or from right to left with the MITLL3 CCD), and redder wavelengths are at the bottom of each order. Note that this is the reverse of the Tek CCD orientation, and when the files are saved in FITS format, they will be then reversed about the x-axis. Thus, it is not necessary to flip MITLL2/MITLL3 CCD images about the x-axis prior to reduction (as is usually done with Tek CCD images) if you wish to have wavelength increasing with pixel number along each order. The leftmost order which does not run off the edge of this image is order 61; the rightmost complete order is 117. The most heavily saturated line (which has another saturated line just above it, and three more in a right-angle triangle just below and to the right) is at 8115 Å in order 70.
The image on the left shows the raw XMEM display for the MITLL2 CCD, when the grating is set to 3946.0 Å. Once again, wavelength decreases (order number increases) going from left to right (or right to left for the MITLL3 CCD), and redder wavelengths are at the bottom of each order. The leftmost order which does not run off the edge of this image is order 123; the rightmost complete order is 160.
Last Updated: 19 December 1999 by Stuart Ryder