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LIRIS Calibrations

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

BIASes and DARKs

First, move all blanks into the light path. Then call one of the last two commands to get N BIASes or DARKs:

SYS> lblanks

SYS> multbias liris <N> "Bias"

SYS> multdark liris <N> <int time> "Dark"

Note that in principle you do not need to take BIASes since each image has its own BIAS (the pre-read) already subtracted. The same holds for DARKs in the sense that when you calculate a sky background model, this model already contains the DARK component which is then automatically subtracted. Yet of course feel free to experiment with it. However, bear in mind that the detector might be in a slightly different equilibrium state during a later exposure series (with the telescope moving in between), than at the beginning of the night. A master DARK or BIAS therefore does (likely) not reflect the later state of the detector.

Imaging flat fields

Domeflats: ask the observing support assistant (OSA) to point the telescope to a suitable position. Use the top ring lamps. Ask the OSA for assistance if you need help. 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. Skyflats should always be your first option.

Skyflats: ask the telescope operator to point the telescope to a suitable sky position. You can start taking flatfields in narrow band filters at around sunset, in broad band filters a few minutes later. Do not forget to offset the telescope between exposures, or ask the OSA to simply 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.
Do not forget to take the flat fields in the obseving mode you use later at night (imaging, coronography, polarimetry).

A number of N flat fields can be obtained with the command

SYS> multflat liris <N> <int time> "Flatfield"

Spectroscopic flat fields

For spectroscopic flats in the zJ grism you can use the Wolfram lamp that is integrated in the A&G box. Spectroscopic flats in the HK grism are better done as dome flats due to an otherwise present broad absorption feature. The following is for the zJ grism.
In order to bring the light from the lamp to the detector, we need to move a mirror in the light path, include a neutral density filter, and switch the Wolfram lamp on. This is done in this order by the three commands

SYS> agcomp
SYS> compfilta i
(with i=4 or 6 for ND=0.8 or ND=3.0)
SYS> complamps w

In the schematic display of the A&G box the changes you have done are drawn. In this representation the light from the sky enters from above, and LIRIS sits below the lower exit (not shown).

Now bring LIRIS into spectroscopic mode and do not forget to insert the slit mask you want as well (these slit masks and grisms are currently available). Take N spectroscopic flats.

SYS> lspec <grism name> <slit mask name>
SYS> multflat liris <N> <int time> "Specflat"

Once you are done, switch off the lamp, remove the neutral filter and move the mirror out of the beam so that LIRIS can see the sky again:

SYS> complamps off
SYS> compfilta clear
SYS> agmirror out

Hint: The A&G Box can also be configured by means of the Instrument Control Box, instead of from the command line. To this end, in the A&G menu, select the Main ND Filter you desire. In the A&G Observer window, (un-)select the "W" for the Wolfram lamp, then click the gears symbol next to it to switch the lamp on (off). In the Mirror Control section, select Acq/Comp to move the mirror in, or Out to remove it again. Do not forget to remove the ND filter once you are done with the flats.

The following is for the HK and K grisms. Telescope pointing to zenith. Open the mirror petals and take the pick-off mirror out of the way

SYS> petals open
SYS> agmirror out

To switch on the dome flat lamps

SYS> fflamp all on


SYS> fflamp <N> on

where N is the dome flat lamp number: 1 = 9W, 2 = 25W, 3 = 150W, 4 = 500W and 5 = 500W.

To switch off the dome flat lamps one can use

SYS> fflamp all off


SYS> fflamp <N> off

To close the mirror petals

SYS> petals close

Guidelines for specflat exposure times:

Grism Slit Light source Neutral density filter Exposure time
lr_zj l0p65 W ND=0.9 (compfiltb 4) 0.9 s
lr_hk l0p65 Dome flat lamps 1-5 - 1.5 s
mr_k l0p65 Dome flat lamps 1-5 - 10 s
hr_j l0p65 W ND=0.6 (compfilta 3) 1 s
hr_h l0p65 W ND=0.9 (compfilta 3) 1 s
hr_k l0p65 Dome flat lamps 1-5 - 7 s
lr_zj l0p75 W ND=1.2 (compfiltb 5) 1 s
lr_hk l0p75 Dome flat lamps 1-5 - 1.3 s
mr_k l0p75 Dome flat lamps 1-5 - 7.5 s
hr_j l0p75 W ND=0.7 (compnd 0.7) 1 s
hr_h l0p75 W ND=0.6 (compfilta 3) 1 s
hr_k l0p75 Dome flat lamps 1-5 - 5.5 s
lr_zj l1 W ND=1.8 (compfilta 5) 1.5 s
lr_hk l1 Dome flat lamps 1-5 - 1 s
mr_k l1 Dome flat lamps 1-5 - 5 s
hr_j l1 W ND=0.8 (compfilta 4) 1 s
hr_h l1 W ND=0.9 (compfiltb 4) 1.5 s
hr_k l1 Dome flat lamps 1-5 - 4 s
lr_zj l2p5 W ND=2.3 (compnd 2.3) 1 s
lr_hk l2p5 Dome flat lamps 1-3 - 1.5 s
mr_k l2p5 Dome flat lamps 1-5 - 1.5 s
hr_j l2p5 W ND=1.8 (compfilta 5) 0.9 s
hr_h l2p5 W ND=2.0 (compfiltb 6) 1 s
hr_k l2p5 Dome flat lamps 1-5 - 1 s
lr_zj l5 W ND=2.6 (compnd 2.6) 0.9 s
lr_hk l5 Dome flat lamps 3 - 1 s
mr_k l5 Dome flat lamps 4-5 - 1 s
hr_j l5 W ND=2.2 (compnd 2.2) 0.9 s
hr_h l5 W ND=2.3 (compnd 2.3) 1.3 s
hr_k l5 Dome flat lamps 1-4 - 1 s
lr_zj l10 W ND=3.0 (compfilta 6) 1 s
lr_hk l10 Dome flat lamps 1-2 - 2.2 s
mr_k l10 Dome flat lamps 4 - 1.1 s
hr_j l10 W ND=2.6 (compnd 2.6) 1 s
hr_h l10 W ND=3.0 (compfilta 6) 1.5 s
hr_k l10 Dome flat lamps 1-3 - 1.5 s

This gives you a level of about 20 000 counts.

Spectroscopic arcs

For wavelength calibration LIRIS offers you an Argon lamp (lamp 1) and a Xenon lamp (lamp 2). First, bring LIRIS into spectroscopic mode and do not forget to insert your desired slit mask (these slit masks and grisms are currently available):

SYS> lspec <grism name> <slit mask name>

Then switch on one of the calibration lamps. If you take arcs in the HK bands, switch both lamps on since the number of lines is low. It takes a short while until the lamps are warmed up and you get the prompt back. Move the mirror into the light path:

SYS> llampon 1
SYS> llampon 2
SYS> agcomp

Take N arc spectra, switch the lamps off and move the mirror out again:

SYS> multarc liris <N> <int time> "Arcs"
SYS> llampoff ALL
SYS> agmirror out

Guidelines for arc exposure times:
grism slit exposure time
Argon lamp
exposure time
Xenon lamp
lr_zj l1 2.5 s (low)
10 s (high)
4 s (low)
40 s (high)
lr_hk l1 60 s 60 s
mr_k l1 600 s 600 s

For the lr_zj grism we give two exposure times since there is a small number of bright lines that saturate quickly.

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Contact:  (LIRIS Instrument Specialist)
Last modified: 18 November 2013