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.

To facilitate rapid and efficient measurements of instrument focus, Keck provides software to configure the instrument, to gather focus images and to analyze them.

Setup and Procedure

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:

1. Configure the instrument for focusing.

   trapdoor	closed
   slitmaks	long_0.7, long_1.0, long_1.5 or multi slit mask
   dichroic	as desired
   grism	as desired
   grating	as desired
   wavelen	as desired
   red filter	clear
   blue filter	clear
   blue focus start	-2450 (or current default)
   red focus start	-0.61 (or current default)
   blue/red exp. time	1 sec

2. Turn on arc lamps:
   • Red side: Switch on the Ne, Ar lamps.
   • Blue side: Switch on the Hg, Cd and Zn lamps. Wait ~3 min for lamps to warm up.
   • If using both cameras, turn on all arc lamps.
3. Start the focus loop for spectroscopy

   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).

Examples of focus data for the red and blue sides are shown below.

Blue side image of long slit used to focus the instrument.

Red side image of long slit used to focus the instrument.

4. The script will take the requested number of images and save them in the default data directory as bfocXXXX.fits and rfocXXXX.fits, where XXXX is the current frame number for blue and red.
5. Analyze the focus images
   • Launch IRAF: Right-click on the background pane and select from the desktop menu:

     IRAF and IDL -> IRAF -> cl + ds9

   • Load keck.lris package
   • cdata to change to the data directory
   • run lspecfocus. A prompt will request which images to analyze. You can indicate the images using the syntax last 7 red or last 7 blue, for example.
   • use your cursor to mark up to 2 regions to analyze:
     • Place your cursor at the left edge of the first region
     • Press the spacebar to mark the starting position
     • Move cursor to the right
     • Press spacebar again to mark the ending position.
     • If you only want to analyze that region, press the Q key to quit; otherwise, mark a second region using the cursor and pressing the spacebar as described above.
     • After the second region is marked, the routine will move to the analysis phase.
     Mark the region
     
   • The routine will next plot the FWHM of the lines and the best fits at each of the focus positions and fit a quadratic to the data Press Q key to proceed to the next region. Best fit.
     
   • When finished, lspecfocus averages the derived focus values and prints the average best focus.
   • Enter the best focus value into the XLRIS focus setting.
6. From the pulldown menu, select

   LRIS Utilities --> Print Focus Table

   The 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
   Do not change the instrument configuration before printing the focus table. The focus_table reads the current setup before calculating the focus table. If the instrument configuration is changed after focusing, the focus_table script will report incorrect values.

focus_table Example output

  >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