It is important to determine the correct instrument focus before performing the telescope focus procedures (a.k.a. MIRA) to avoid incorrect results. The instrument focus is determined by measuring the FWHMs of comparison arc lines along the spectrum.Long slit and multi-slit focus data are obtained with arc lamp exposures on both ordinary and extraordinary beams.
To facilitate rapid and efficient measurements of instrument focus, Keck provides software to configure the instrument, to gather focus images and to analyze them.See below for setup and procedures.
The default instrument setup for taking focus data is spectroscopic mode on both red and blue arms with either the 1.0 or 1.5 arcsec wide long slits in the beam:
|slitmaks||long_0.7, long_1.0, long_1.5 or multi slit mask|
|blue focus start||-2450 (or current default)|
|red focus start||-0.61 (or current default)|
|blue/red exp. time||1 sec|
From the XPOSE GUIs the CMD... menu select Focus loop Spectroscopy
This will bring up a gui with default parameters. Click the GO button to initiate the loop.
When the first images are displayed check that the signal level of the "slit" is well above the background (at least 4000 counts above).
Blue side image of long slit used to focus the instrument.
Red side image of long slit used to focus the instrument.
IRAF and IDL -> IRAF -> cl + ds9
LRIS Utilities --> Print Focus TableThe focus table will be printed on the Remote Ops laser printer. Note: type focus_table in the lrisserver xterm will print the table on screen. See below for example output from focus_table
>focus_table R E D S I D E F O C U S Filter Imaging Longslit + multislit spectroscopy + superlong ============================================== clear 2342 2301 B 2237 2196 V 2442 2401 R 2282 2241 Rs 2307 2266 I 2288 2247 GG495 2336 2295 OG570 2230 2189 RG850 2334 2293 Dichroic add offset: mwmwmwmwmwmwmwmwmwmwmw clear 0 mirror 0 460 -7 500 11 560 16 680 -9 B L U E S I D E F O C U S Filter Imaging Longslit + multislit spectroscopy + superlong ============================================== clear -2834 -2799 U -2809 -2774 B -2854 -2819 G -2834 -2799 V -2833 -2798 NB4170 -2859 -2824 There is no dichroic offset for the blue side