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LIRIS Calibrations

In the following we summarise how to obtain imaging polarimetry calibrations for LIRIS.

Imaging polarimetry flat fields

Domeflats: can be done with telescope pointing to the zenigh. Bear in mind that domeflats can have unwanted spectral characteristics due to the lamp and/or the reflected light from the painting in the dome.

SYS> petals open

There are five topring lamps 1) 9w, 2) 25w, 3) 150w 4-5) 500w. To switch them all on

SYS> fflamp all on

to switch them all off

SYS> fflamp all off

or if one needs less light, there is a possibility to switch on only the desired lamps

SYS> fflamp 1 on

the same way one can switch off the topring lamps. The number of lamp can be 1 to 5 (see the power of the lamps above).

SYS> fflamp 1 off

To close the mirror petals

SYS> petals close

Orientative exposure time values. Always test before running the hwflats script.
  Ks fflamp 1,2 on, exposure time 1.5 s
  H   fflamp 1,2 on, exposure time 0.9 s
  J   fflamp 1,2,3 on, exposure time 0.9 s
  Y   fflamp 1,2,3 on, exposure time 0.8 s
  z   fflamp 1,2,3,4 on, exposure time 0.8 s

  Hc   fflamp 4 on, exposure time 1 s
  Jc   fflamp 4,5 on, exposure time 0.8 s
  Kc   fflamp all on, exposure time 1 s

Skyflats: Before sunset ask the OSA to point the telescope away from the Sun and at elevation of 20 degrees. Ask the OSA to switch off tracking. Since the read-out time of LIRIS is short you can easily afford to take a sufficiently large number of flats, such as 20 per filter.

For all kinds of flat fields, choose the exposure time such that you reach about 20000 counts in the image (post-read minus pre-read).

It is important that you take flat fields since the effective quantum efficiency of the LIRIS detector shows variations of up to 20% across the field. The flat field itself, however, is very stable with time.

Put the desired polarimetric setup

SYS> lipol <filter>

Flat fields can be obtained with the command

SYS> hwflats.bsh <int time> title <nmult>

nmult is the number of flats per one setup i.e. 1) with orthogonal half-wave plate and 2) with parallel half-wave plate in use. Thus using 5 for nmult will give 10 flats.

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Contact:  (LIRIS Instrument Specialist)
Last modified: 22 October 2018