Introduction

The proper focus of each camera varies with filter and observing mode (e.g., direct imaging and slitmask spectroscopy vs. longslit spectroscopy) and, for the red side, with the dichroic in use. Even though the zero point for the focus changes daily, the relative offsets between various focus modes on LRIS are believed to be stable. Hence, if the observer focuses the instrument in the nominal imaging mode then the focus for all other instrument modes can be derived from that measurement.

This document describes the officially-endorsed procedure for daily focusing of LRIS. The procedure involves using a pinhole mask in the LRIS focal plane to obtain a series of images with varying focus values, then analyzing these images to derive the optimal focus for imaging mode. Once the instrument is focused, the observer generates a table showing the corresponding focus values in other modes.

Data Acquisition Procedure

1. Wait for the telescope to be released. At this point, verify on FACSUM that the following are true:
   • Telescope elevation is 45°.
   • Dome azimuth and telescope azimuth differ by 90°.
2. Launch the acquisition script from the desktop menu by selecting:
3. Set up the blue and red side for your imaging science setup.
4. Turn on the spectroscopic dome lamps using the utility under LRIS Utilities -> Dome Lamps.
5. Run the "Focus Loop" script available from the CMD ... on the Xpose GUI. The script will save these images in your data directory as bfocNNNN.fits and rfocNNNN.fits where NNNN is the current frame number for blue and red.
6. When the first images are displayed, use the Stats Box mode of FIGDISP to check that the signal level within the pinholes is at least 1000 counts above the background level. If the illumination is inadequate, please contact your support astronomer immediately for troubleshooting assistance.
7. The data acquisition scripts for the red and blue sides run in parallel in separate xterm windows. When these complete, the xterm windows will disappear and a master xterm window will display the message

     Press <Enter> to exit...

   Press the Enter key to terminate this window.

Data Analysis Procedure

1. Determine the best focus using the IDL Xfocus tool. Note that there are two different versions, one for the red side and one for the blue.
   • From the central monitor select from the desktop menu:

     LRIS Utilities > Xfocus > Xfocus red

   • From the right-hand monitor select from the desktop menu:

     LRIS Utilities > Xfocus > Xfocus blue

2. When the widget appears, use the File > Open function to get a file chooser window. The window will display a list of all FITS images in your current data directory. Select the last set of images beginning with the prefix rfoc (for red focus data) or bfoc (for blue data). Click on the first image and drag the mouse downward to highlight multiple images. With your seven images selected, click the OK button to proceed.
3. The program will locate the pinholes and measure the median pinhole width on each image. The results are fit with an hyperbola and plotted.
   Click on the images below which show what the plots should look like:
4. Note the best fit value, which is the optimal focus for the chosen instrument setup.
5. Note: check that the calculated FWHM for the minimum of the plot is 0.31-0.35 μm in the red and 0.21-0.24 in the blue. When using internal lamps to focus, expect the FWHM to be slightly larger. The above values reference data taken with the dome. If the FWHM for data taken using the dome lie outside the range, something may have gone wrong in either data taking or analysis and you should contact your support astronomer for assistance.
6. Use XLRIS to set the red and blue focus to these best focus values. Important: Do not change any other instrument settings until the focus table has been printed!