LRIS: Instrument Focus | |||
IntroductionThe 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 ProcedureFollow these steps to obtain a sequence of focus images on the red and blue sides using the dome flatfield lamps:
Data Analysis Procedure
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 TableThe 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:
Sample Focus TableThis 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. | |||
Last modified: 01/03/2020 22:24 Send questions or comments to:LRIS Support |