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.

- order 61 (9241-9386 Å)
- order 62 (9092-9234 Å)
- order 63 (8948-9088 Å)
- order 64 (8809-8945 Å)
- order 65 (8673-8808 Å)
- order 66 (8542-8674 Å)
- order 67 (8415-8545 Å)
- order 68 (8291-8421 Å)
- order 69 (8171-8298 Å)
- order 70 (8055-8180 Å)
- order 71 (7942-8067 Å)
- order 72 (7832-7954 Å)
- order 73 (7725-7845 Å)
- order 74 (7621-7741 Å)
- order 75 (7519-7636 Å)
- order 76 (7421-7538 Å)
- order 77 (7324-7439 Å)
- order 78 (7231-7343 Å)
- order 79 (7139-7251 Å)
- order 80 (7050-7160 Å)
- order 81 (6963-7073 Å)
- order 82 (6879-6986 Å)
- order 83 (6796-6903 Å)
- order 84 (6715-6820 Å)
- order 85 (6636-6741 Å)
- order 86 (6559-6661 Å)
- order 87 (6484-6586 Å)
- order 88 (6411-6511 Å)
- order 89 (6339-6439 Å)
- order 90 (6268-6365 Å)
- order 91 (6200-6297 Å)
- order 92 (6132-6229 Å)
- order 93 (6067-6162 Å)
- order 94 (6002-6097 Å)
- order 95 (5939-6031 Å)
- order 96 (5877-5969 Å)
- order 97 (5817-5909 Å)
- order 98 (5758-5848 Å)
- order 99 (5700-5790 Å)
- order 100 (5643-5730 Å)
- order 101 (5587-5674 Å)
- order 102 (5532-5619 Å)
- order 103 (5479-5564 Å)
- order 104 (5426-5511 Å)
- order 105 (5375-5460 Å)
- order 106 (5324-5406 Å)
- order 107 (5274-5356 Å)
- order 108 (5226-5308 Å)
- order 109 (5178-5260 Å)
- order 110 (5131-5211 Å)
- order 111 (5084-5164 Å)
- order 112 (5039-5119 Å)
- order 113 (4995-5072 Å)
- order 114 (4951-5028 Å)
- order 115 (4908-4985 Å)
- order 116 (4866-4943 Å)
- order 117 (4824-4899 Å)

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.

- order 123 (4581-4653 Å)
- order 124 (4544-4616 Å)
- order 125 (4508-4578 Å)
- order 126 (4472-4542 Å)
- order 127 (4437-4507 Å)
- order 128 (4402-4472 Å)
- order 129 (4368-4438 Å)
- order 130 (4335-4402 Å)
- order 131 (4302-4369 Å)
- order 132 (4269-4336 Å)
- order 133 (4237-4304 Å)
- order 134 (4206-4273 Å)
- order 135 (4175-4240 Å)
- order 136 (4144-4209 Å)
- order 137 (4114-4179 Å)
- order 138 (4084-4149 Å)
- order 139 (4055-4120 Å)
- order 140 (4026-4088 Å)
- order 141 (3997-4059 Å)
- order 142 (3969-4031 Å)
- order 143 (3941-4003 Å)
- order 144 (3914-3976 Å)
- order 145 (3887-3949 Å)
- order 146 (3861-3921 Å)
- order 147 (3834-3894 Å)
- order 148 (3808-3868 Å)
- order 149 (3783-3843 Å)
- order 150 (3758-3818 Å)
- order 151 (3733-3793 Å)
- order 152 (3708-3768 Å)
- order 153 (3684-3741 Å)
- order 154 (3660-3717 Å)
- order 155 (3636-3693 Å)
- order 156 (3613-3670 Å)
- order 157 (3590-3647 Å)
- order 158 (3567-3624 Å)
- order 159 (3544-3599 Å)
- order 160 (3522-3577 Å)

- Bessell, M. S., & Pettini, M. 1991,
*UCLES Spectrum of the Thorium-Argon Hollow-Cathode Lamp. I. 79 grooves/mm echelle grating and IPCS detector*, AAO User Manual 28.1 (Sydney: AAO). - D'Odorico, S., Ghigo, M., & Ponz, D. 1987,
*An Atlas of the Thorium-Argon Spectrum for the ESO Echelle Spectrograph in the 3400-9000 Å region*ESO Scientific Report No. 6 (Garching: ESO). - Lipman, K., Pettini, M., Wall, M., & Walton, N. 1993,
*UES Spectrum of Thorium-Argon Hollow-Cathode Lamp*ING La Palma Technical Note No. 91 (Cambridge: RGO). - Meurer, G. R., Pettini, M., & Stathakis, R. A. 1991,
*UCLES Spectrum of the Thorium-Argon Hollow-Cathode Lamp. II. 31.6 grooves/mm echelle grating and Thomson CCD*, AAO User Manual 34.1 (Sydney: AAO).

Last Updated: 19 December 1999 by Stuart Ryder