LIRIS is focused by means of the WHT itself. Once a focus has been found at the beginning of the night, it is unlike that it has to be readjusted throughout the rest of the night. If, however, the seeing is excellent and if you have tight constraints concerning the PSF, then you might want to refocus the telescope.
The best telescope focus value for observing with LIRIS is usually within 98.10 +/- 0.05mm. Check the focus in steps of 0.05. Only if the seeing is really excellent (say 0.5" or better), a smaller step size makes sense. To focus the telescope, take images of 5 seconds for the different focus values and compare the image seeing you get.
There is also the following automated procedure available. However, most observers determine the focus manually since it is faster.
- Select a star of magnitude ~10-11 and tell the observing support assistant (OSA) to slew the telescope to it.
- Select a filter. Since no focus offset has been observed between different broad band filters, use either filter H or J to avoid high background levels in the images:
TO> limage h
- Take a sequence of images for different values of the telescope focus using the command focusrun followed by:
- the number of images (i.e. number of telescope focus movements),
- the exposure time,
- the telescope focus value for the first image in the sequence (in mm) and
- the focus increment between each image (in mm)
TO> coave liris 2
This sets to 2 the number of exposures averaged for each image of the sequence (optional).
TO> focusrun liris 9 8 97.9 0.05
In this case, nine files (e.g. r559681.fit, ..., r559689.fit) would be saved to disk, corresponding each to a telescope focus value (starting at 97.9mm with steps of 0.05mm) and obtained after coaveraging two 8 exposures.
Expose at least 8-10s in order to average out seeings effects. If the seeing is not very good, you can use a larger focus step of 0.1mm instead.
- Find the best telescope focus value: on whtdrpc1, in /home/whtguest/liris/iraf/ start an IRAF session and within the INGRID quick look reduction package (ingrid_ql), type:
in> istarfocus r559681 nim=9
and follow the instructions (i.e. mark with m one or more stars in the image, type q and wait until an xgterm window appears showing the FWHM and ellipticity plotted versus the telescope focus value (in meters, m). Finally, type q again to find the best value). NOTE: The best telescope focus value is given also in m.
- IMPORTANT: Set the telescope focus value to the best value determined above (in mm), e.g.
TO> focus 98.10
since the telescope focus is left to the last value of the sequence after the focus procedure.
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