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LIRIS: Long-slit Intermediate Resolution Infrared Spectrograph.

LIRIS is an Instituto de Astrofisica de Canarias (IAC) project consisting of a near-infrared (0.9-2.4 microns) intermediate resolution spectrograph (R=1000-3000) for both long-slit and multi-object spectroscopy, in addition to imaging, coronography and polarimetry modes.

The optical system is based on a classical collimator/camera design. Grisms are used as the dispersion elements. The plate scale (0.25 arsec/pixels) matches the median ORM seeing (0.5 arcsec in the K band). The detector is a Hawaii 1024x1024 HgCdTe array operating at 65 K. LIRIS has no acquistion TV: instead use continuous readout mode of the detector to acquire the objects. This is done by the observer.

Startup & Checks

Vacuum Pump: Before moving the Telescope for the first time, make sure the LIRIS pump is switched off (since the telescope MUST be at zenith while the pump is running). The telescope power should be "locked off" by padlock while the LIRIS pump is running. If it needs to be switched off, the switch for the pump is clearly identified inside the "Enclosure B" cabinet.

Cryostat filling: LIRIS needs filling just once a day (except possibly after it has just been cooled, or in case of problems). You no longer need to fill in the sheet in the LIRIS logbook, since DEWARS records temperature and pressure continuously. However DO check the vacuum level is low enough (10E-3 or less) BEFORE filling. Some alarms may go off while filling with LN2, in the first few days after mounting the instrument at CASS, because the closed cycle cooler has been off for a while. Provided the vacuum is not larger than 10-3, then this is OK.

There are two pipes for LN2 coming out of LIRIS: the left one is IN and the right one is OUT. The LN2 pipe is attached with an O-ring and a clamp to the IN pipe. Wait until you see liquid LN2 spilling out of the OUT pipe (it is NOT full if it is spilling out of only the IN pipe at the join) and then turn off the LN2 at the dewar.

  • Zeroset AZ, ALT and ROT
  • USER> FOCUS 98.05-98.10 (typical, as determined in 2011)

AG MIRROR: for observing with LIRIS, the agmirror should be OUT. This is normally controlled by the observer. The mirror position can be seen on the A&G tab of the mimic. If it is not already out, do

SYS> agmirror out

NEW! Make sure that the TV scale is set to DIRECT (not SLIT as you would use for ISIS observing):

SYS> tvscale direct

Instrument temperature control:

Check that LIRIS temperature control is active at the start of each night. It can be checked in DEWARS and also be seen in a small window called "LIRIS_THERMAL_STATUS" and you should see "Control Loop" ON. If it is OFF, enable it using the command

SYS> ltempsetp

At the END OF THE NIGHT, switch it off with:

SYS> lsleep

This ensures that temperature is maintained steady at a slightly higher value (72K) than normal.

First Night

Rotator centre:

  • SYS> startudasrtd (this works like continuous readout from an acquisition TV). If this becomes slow or crashes, just restart it.
  • SYS> multglance liris 100 0.5 (takes a series of 100 0.5-second exposures).
  • USER> gocat star (11mag star from the system catalogue)
  • USER> rot sky 0 (or whatever is convenient)
  • Note the star position (x,y)
  • USER> rot sky 180
  • Measure the star position again (x,y)
  • Calculate the midpoint, which is the rotator centre. Make a circle on the RTD at this position and do the calibrate on this point. See the logbook for the most recent value found.


  • USER> ROT MOUNT xxx (Calibrate is done with the rotator tracking turned off)
  • USER>cal faint
  • Use handset in X_Y and centre star on rotator centre
  • When centred hit handset again to go to next star
  • Repeat for 7 stars
  • When done answer "Y" to the question Were all Stars Centred Correctly?
  • Answer "Y" to use this new calibrate result

Telescope focus:

The telescope focus is measured by the observer. At the start of each night (maybe unless the seeing is realy bad) they measure the best telescope focus taking exposures at different focus positions of a standard star. They usually use focusrun to do this.
There is a script under development that estimates the best focus obtained from the mentioned focusrun. It is possible to use it from the data directory in iraf (e.g. /obsdata/whta/20170207) as follows:
ecl> !python /home/whtobs/focus/ r{first_image} -nima {number_of_images}
The optional parameter -nima gives you the chance to use only some of the focusrun images in case you want to abort it before it's completed or if you know the first image was not properly taken (i.e. you choose the first image to analyse and how many consecutive images you want to analyse)

Handset alignment (XY and APOFF):
       +------+          +--------------+
| | |detector |
| ^ | | |
| | | | |
| | | +----> X |
+------+------+------+ | | |
| | | | | | |
| <-- | | | --> | | v |
| | v | | | Y |
| | | | | |
+------+------+------+ +--------------+

Sky directions (PA=0 and PA=90):

+--------------+    +--------------+
|detector | |detector |
|PA=0 | |PA=90 |
| | | |
| N | | E |
| ^ | | ^ |
| | | | | |
| | | | | |
| E <---+ | | +---> N |
| | | |
+--------------+ +--------------+

AUTOGUIDER: Use the standard autoguider for CASS. See the WHT logbook for the most recent value of AUTOFOCUS (it could be around 2000-2500). Start with 'obssys', 'startobssys' and 'startag AG6'. No window is required here. GSS2 is used for finding guidestars, with instrument set to LIRIS. If an aperture to offset is used to place the object in a particular position in the field, enter X and Y into GSS2. Handset movements move the guide star in the usual direction.

Choose a 12-14 mag star from the output list, and send the guideprobe to the required position using the usual:

SYS> prag r theta

For notes on the size of the guide window, see the sections on imaging and spectroscopy.

Autoguider Filter

If guiding is needed (it often isn't for LIRIS imaging), try to use the i autoguider filter:

SYS> autofilt ag-i

This minimises the difference in position between the guide star and target changing with airmass due to differential refraction. With imaging, which usually involves very frequent dithering and jittering and short exposures, it doesnt matter too much as the fact that the target will move between these short exposures isn't really important. It's much more important for spectroscopy, more important the longer the exposure and it's very important for MOS especially. With the i filter, stars tend to look a lot fainter so it may not be possible to find a bright enough star at the required sky PA. In this case, consider rotating the sky PA by 180 degrees, if the observer doesn't mind (obviously this can't be done for MOS), to see if there is a brighter guide star there instead.


Sky Flats

Method 1: do at zenith with tracking off.

Method 2: see printed sheet under keyboards and choose a blank field with RA near the ST:
  • USER> gocat blankx (go to blank sky field)
  • USER> rot vfloat (i.e. any sky PA is OK)
Offsets are needed between each exposure and can be done in 2 ways:

a) using the OFFSET command (this can be done by the observer, or TO if the observer doesn't want to do it!). Offsets are absolute, not from the previous one, e.g.:
  • USER> offset arc 10 10
  • USER> offset arc -10 -10
  • USER> offset 0 10 ..... etc, etc
b) use the mdither script, which will automatically move the telescope between exposures. This is the most efficient way of doing it. The observer runs a script such as:
  • mdither 9 5 (takes 5-second exposures at 9 different dither positions)

Imaging Fields
  • USER> gocat target
  • USER> rot sky 0 (usually stays at 0 always, but observer may request something different)
For LIRIS imaging, probably no guiding is needed as exposure times are usually well under a minute. If guiding is required, almost certainly a dither pattern will be used for IR imaging (unlike optical imaging when it is rarer). Ask the observer what size dither pattern will be used. The default is 12 arcsec, measured from the start position in different directions, so in this case the size of the guide star box needs to be about 300 so that the guide star remains in the box in all the dither positions. The A/G exposure time should not be made longer than about 5 seconds when dithering, otherwise the Autoguider response will be slow and could affect telescope position.


Usually a 12 arcsec movement along the slit is used for LIRIS long-slit spectroscopy, with spectra taken at alternating A and B positions. This is to help with sky subtraction. For this size offset a 300 pixel guide star box is needed. During observing, usually the "ag_spec_nod" script is used for long-slit spectroscopy, which stops the guiding at the end of an exposure, offsets the t/s along the slit and resumes guiding before beginning the next exposure.
  • USER> ENTER APERTURE 0 5 35 (Apr 2016). It is advisable to enter an aperture to point closer to the slit
  • USER> gocat target
  • USER> rot sky parallactic (usually)
  • Find a guide star in the usual way
Centre the guide star roughly in the autoguider field, so that when the t/s is moved to put the target in the slit, the guide star will still be visible no matter which direction the t/s moves in.

The observer will take acquistion images, to see where the target is with respect to an overlay of the slit. Then they will calculate the offset required to put the object in the slit, using the lobject_inslit script. From here there are 2 ways to proceed:

1) If the movement is then done using the lobject_inslit script, you will see a TWEAK command appear in the TCS after movements using the script, and the telescope and hence the guide star will move, probably by quite a long way - maybe even so far that the guide star will be lost. If necessary, recentre the guide star, so it's not lost later during nodding along the slit, and start guiding as soon as possible to prevent the acqusition being lost. The procedure can be repeated as many times as desired, taking images and calculating small shifts to perfect the acquisition. N.B> The TWEAKs can be done while guiding.

2) You can ask the observer to tell you an approximate offset which you can do with the TCS handset (the advantage is that you can do this in steps of 20 arcsec to see which way the guide star moves so that you don't lose it) and you can start guiding at that point. Then they can do another acquisition image, calculate the offset using the script and these final small adjustments can be very accurately achieved using the TCS handset (use (A)POFF if guiding already).

In summary, both the initial large movement and final small adjustments can both be done with either the script or using the TCS handset (or a mixture of the 2 methods), but I prefer doing it all with the TCS handset.

Dithers of up to about 40 arcsec along the slit without losing guiding can be achieved by putting the guide star in one corner/edge of the guide field and using a large guide box/window. You have to first check which way the dither will move the guide star to decide which corner/edge to put the star in. Normally long slit dithering is upward (HANDSET -Y) along the slit to position B, then back down to position A.

Sometimes the observer has a favourite place on the slit and this could be far from the default pointing position. In this case it can speed up acquisition to to set an APERTURE 0 to bring the star much closer to the desired position. The amount of the aperture offset can be determined by looking at the size of the first few tweaks made by lobject_inslit. In Sep 06 a good value was X=+16, Y=+42 arcsec (aprox.) the value of the tweak depends of the rotator angle and the pointing of the telescope . It's important to use an aperture or a tweak to make this movement, not OFFSET because it will be cancelled out by the SLOWOFF of the dithering.

Multi-Object Spectroscopy (MOS)

Generally each mask is observed for a long time, maybe even a few hours, so there's not much to do after the beginning, except if the S/N has fallen and you need to check the acquisition.

SYS> gocat maskname
SYS> rot sky XXX (they will have a fixed sky PA which needs to be used for each mask)

Acquisition is done by the observer. They take an image of the field and note accurately the positions of several "fiducial" stars. These stars need to be perfectly aligned with the positions of round holes in the multi-object slit overlay. They will use some LIRIS software which calculates an offset in X and Y and a rotation which are sent directly to the TCS, in order to align the stars with the holes. After this first movement has been made, start guiding. The observer then checks the mask acquisition again and will probably ask you to do some small t/s movements with the handset to perfect it. Remember to use (A)POFF mode for handset movements when you're guiding. Finally the observer may take an image through the multi-object slit mask to check the alignment.

Often the same field is observed several nights in succession, so it saves time acquiring on subsequent nights if you write down the following data:

  • autoguider reference position on the TCS (x and y pixels)
  • autoguider probe radial and theta positions
  • the exact final rotator position

When you want to acquire the same mask another time, set the sky PA to this value, send the probe to this position and use the command

Alternatively do: GUIDE ON pixel on the Autoguider GUI, and enter x and y here instead.

Both these methods will pull the telescope into the correct position (this could take about a minute for the guiding errors to decrease to normal). After this step, the observer can check the acquisition, but only a minimal correction in x,y is expected.

Note: this procedure also helps to save time when re-acquiring in long-slit spectroscopy, and it works for other instruments too.

Imaging Polarimetry

Sky flats are done early, around sunset. The best place to do them is far away from the sun at low elevation (and with tracking off is fine):
  • USER> az 90
  • USER> dome 90
  • USER> el 20
Observing is just like normal imaging: usually at sky PA 0, but sometimes observers might ask for PA 90 instead. Normally exposures are short so no guiding is required. The handset works in the normal sense.


You need to make an aperture offset to put the star near the centre of the Coronograph mask. In June 2004 the following was calculated and sent by the LIRIS observing software:

ENTER APERTURE 0 -29.5 -59.5

Dome Flats

Dome flats require the use of the top ring lamps. Detailed instructions for dome flats are in the observing guide that your observer will be using. To switch the lamps on, use the 'fflamp' command. Any combination of 5 lamps can be used depending on what the observer requires. There is a lookup table in the observing guide. Move the dome to position 30. Park zenith and then 'el 45' on the telescope. Make sure the mirror petals are open.


If the LIRIS displays disappear from the observer's system, restart with:
  • start_liris_info &

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Last modified: 10 November 2011