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
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:
- 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°.
- Launch the acquisition script from the desktop menu
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
rfocNNNN.fits where NNNN is the current
frame number for blue and red.
- Reconfigure LRIS for imaging in the (blue) G and (red)
R-band filters with the focus_holes slitmask.
- Turn on the spectral flatfield lamp.
- Open the LRIS trapdoor to view the dome.
- Window both detectors and set exposure times
- Acquire a sequence of 7 images with varying focus.
- 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.
- 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
Repeat for other setups.
- 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.
- 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.
- 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:
- Note the best fit value, which is the optimal focus for the
chosen instrument setup.
- 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.
- 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!
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:
- From the desktop menu, select
LRIS Utilities > Print Focus Table
The focus table will be printed on the printer in Remote Ops
- 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
- The saved focus table can be seen by selecting
LRIS Utilities > View Focus Table
- You can also view the focus table by typing the following
command in any lrisserver xterm window:
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 Imaging Longslit
+ multislit spectroscopy
clear 2287 2246
B 2182 2141
V 2387 2346
R 2227 2186
Rs 2252 2211
I 2233 2192
GG495 2281 2240
OG570 2175 2134
RG850 2279 2238
Dichroic add offset:
B L U E S I D E F O C U S
Filter Imaging Longslit
+ multislit spectroscopy
clear -2408 -2373
U -2383 -2348
B -2428 -2393
G -2408 -2373
V -2407 -2372
NB4170 -2433 -2398
There is no dichroic offset for the blue side