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 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.

Setup and Procedure

The default instrument setup for taking polarimeter focus data is spectroscopic mode on both red and blue arms with either the 1.0 or 1.5 arcsec wide polarimeter long slits in the beam:

Configure the instrument for focusing.

trapdoor	closed
slitmaks	long_0.7, long_1.0, long_1.5
dichroic	as desired
grism	as desired
grating	as desired
wavelen	as desired
red filter	clear
blue filter	clear
blue focus start	-2800
red focus start	2300
blue/red exp. time	1 sec

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.
Start the focus loop for spectroscopy
From 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.

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.
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 either the "last 7 red" or "last 7 blue"
- use your cursor to mark the regions encompasing the spectrum. 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 Best fit.
- Enter the best focus value into the XLRIS focus setting.
In a punaluu window type:

focus_table | lpr -Plw4

The focus table will be printed on the laser printer located in the same room, close to the entrance.
Just typing focus_table in the punaluu xterm will print the table on screen. See below for example output from focus_table

Setup and Procedure

focus_table Example output