Introduction

The LRIS red and blue cameras' optimal focus can change significantly from day to day, presumably due to thermal variations in the cameras. Hence, the careful observer will be sure to focus the telescope every afternoon to ensure that good data are obtained at night.

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.

Note: if you are NOT going to do any imaging, you can focus the blue side only, and forget about the red side. This saves quite a lot of time.

Data Acquisition Procedure

Follow these steps to obtain a sequence of focus images on the red and blue sides using the dome flatfield lamps:
  1. Wait for the telescope to be released. At this point, verify on FACSUM that the following are true:
  2. Turn on the Flat Field lamps. From the background menu, K1 Telescope Status Menu, Dome Lamp control GUI. Slider the power control to 0, then click Set Power and then On. Remember to turn the flat lamps off at the end of the procedure.
  3. Launch the acquisition script from the desktop menu by selecting
      LRIS Utilities > Run Focusloops > DOME lamps, blue & red (DEFAULT)
    Change this to blue only if you are focusing the blue camera only.
    This script will: 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.
  4. When the first images are displayed, 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.
  5. 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.
  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 arcsec in the red and 0.21-0.24 arcsec 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!
  • Repeat for other setups.

    Special note regarding V filter on red side: The V filter on the red side has been observed to show relatively large focus variations from night to night and the focus value derived from the focus table is probably not correct. If you want to use this filter, please contact your Support Astronomer and be prepared to focus this filter independently. The average focus values to use as a starting point is -0.8.

    Generate Focus Table

    The printed focus table lists the focus values for each red and blue setup. Refer to these values every time you change instrument modes or filters. To generate this table:
    1. From the desktop menu, select
      LRIS Utilities > Print Focus Table
      The focus table will be printed on the printer in Remote Ops I.
    2. We recommend that you also save your focus table (especially if you are running LRIS remotely). From the desktop menu, select
      LRIS Utilities > Save Focus Table
    3. The saved focus table can be seen by selecting
      LRIS Utilities > View Focus Table
    4. You can also view the focus table by typing the following command in any lrisserver xterm window:
      focus_table
      or print it by executing either
      focus_table | lpr -Plw4
      focus_table | lpr -Plw6

    Sample Focus Table

    This table is only valid if you are starting in a configuration known
    to be in good focus.  Typicaly you would run xfocus in some
    configuration, set to the recommended focus and then run "focus_table".
    
    The current setup is read, tabulated offsets calculated from the
    red and blue filters, red and blue focus values, the dichroic, and
    whether there is a long slit in or not.  Then new values are calculated
    for all other combinations.
    
            R E D   S I D E   F O C U S
    
    Filter/Grism    Imaging/multislit     Longslit
    =============================================================
    mirror                     -0.660       -0.620
    150/7500                   -0.644       -0.604
    300/5000                   -0.623       -0.583
    400/8500                   -0.662       -0.622
    600/5000                   99.340       99.380
    600/7500                   -0.690       -0.650
    600/10000                  -0.649       -0.609
    831/8200                   -1.020       -0.980
    900/5500                   -0.690       -0.650
    1200/7500                  -0.663       -0.623
    clear                      -0.700
    B                          99.340
    V                          -0.825
    R                          -0.660
    Rs                         -0.659
    I                          -0.700
    GG495                      -0.667
    OG570                      -0.589
    RG850                      -0.700
    
           B L U E   S I D E   F O C U S
    
    Filter/Grism    Imaging/multislit     Longslit
    =============================================================
    clear                       -3443        -3523
    300/5000                    -3425        -3505
    600/4000                    -3415        -3495
    400/3400                    -3403        -3483
    1200/3400                   -3357        -3437
    U                           -3423
    B                           -3463
    G                           -3443
    V                           -3408
    NB4170                      -3588
    NB4040                      -3370
    NB4430                      -3409
    NB3875                      -3501
    NB3985                      -3417
    NB3640                      -3433
    
    NOTE: There are no dichroic offsets for the red or blue side.
    Focus values of order 100 indicate no offset has been measured for that optic.