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Home > Astronomy > LIRIS > Cookbook - Chapter 5

The LIRIS users guide
  1. Starting the system
  2. LIRIS real time display (RTD)
  3. LIRIS mechanisms
  4. Focusing
  5. LIRIS observing commands
  6. End of night
  7. Copying data
  8. Important notes

5. LIRIS observing commands

A complete list of all available LIRIS user commands is available.
A complete list of the UltraDAS commands can be found in the UltraDAS command dictionary.
A separate guide to take calibration frames for LIRIS is available.

In the following we summarise the most often used commands. They should keep you running throughout the night.

  • Selecting the desired LIRIS configuration
    For the options see the predefined configurations page and the current LIRIS configuration.
    • Imaging mode. Type:
      SYS> limage <option>     options+details
    • Spectroscopy mode. Type:
      SYS> lspec <grism> <slitmask> grisms and slit masks
    • Coronography mode. Type:
      SYS> lcor <option>       options+details
    • Polarimetry mode. Type:
      SYS> lipol <option>      options+details
    • Calibration mode (darks). Type:
      SYS> lblanks             details
  • Getting exposures
    • SYS> run liris <int time> ["title"]
      SYS> mult
      run liris <N> <int time> ["title"]
      The run command takes an exposure and saves it to disk. With multrun N exposures are taken.
      There are the following variants of the run command, which essentially do the same but just set the OBSTYPE FITS header keyword to a different value. Also, in the nightlog this image type is then highlighted. These commands are:
    • bias liris
    • arc|dark|flat liris <int time> ["title"]
    • multarc|multdark|multflat liris <N> <int time> ["title"]
      Before taking darks you have to call lblanks in order to close the light path.
      The following commands take temporary data which is overwritten the next time the same command is invoked. These are useful for checking and finetuning target positions.
    • SYS> glance liris <int time>
      SYS> multglance liris <N> <int time>
      The eposure is saved as
    • SYS> scratch liris <k> <int time>
      Takes an image and saves it in: k is an integer within the range 1-99. is overwritten when a new scratch <k> is taken.
    • SYS> multscratch liris <N> <int time>
      Same as run but repeats the run sequence N times and generates m output files.
  • Guide stars
    The Observing Support Assistant (OSA) will select a suitable guide star for you. Keep in touch with him and let him know if your exposures require guiding.
    When taking spectra, make sure that the telescope is guiding and that your target is on-slit before starting the exposure.
    Let the OSA also know the extent of the dither offsets you will make so that he can choose an appropriately sized guiding window to avoid the loss of the guide star.
    Choose the I-band filter for the autoguider in order to minimise effects due to differential atmospheric dispersion. This will only affect you for spectroscopy and in higher airmass. For imaging you can safely go with the clear filter if no suitably bright guide star is found.
  • Data storage types and read modes
    • SYS> rmode liris mndr <Nr> <Nav>
      Sets the number of multiple non-destructive reads (MNDR) to Nr and the number of coaverages to Nav. Nr is the number of times that the array is read non-destructively prior and after an integration period. The array data from the Nr pre- and post-integration reads, respectively, are averaged independently. Nr is an integer between 1 and 16 (default is 1; i.e. double correlated sampling). Nav is the number of accumulated exposures that are averaged and written as a single output file.
      We recommend to set Nr = 1 for imaging and Nr = 4 for spectroscopy, since in the latter case the readout noise becomes dominant over the sky background noise.
    • SYS> storemode liris diff
      Saves FITS file with one extension. It contains the difference between the post-read and the pre-read. This is the recommended storemode.
    • SYS> storemode liris diff_pre
      Saves FITS file with two extensions. The first contains the difference between the post-read and the pre-read, the second one the pre-read itself.
  • Frequently used imaging scripts
    • SYS> [ag_]mdither <ndit> <int time> ["title"]
           [-ncyc=int] [-nruns=int]
           [-xoffset=float] [-yoffset=float]
           [-jitter=float] [-clean=int] [-mndr=int] [coave=int]

      Moves the telescope to ndit (valid: 2,3,4,5,8,9) dither positions (see here, scroll half way down). Prepend "ag_" at the beginning of the script name if you are using the autoguider. At each dither position nruns images are taken with the specified integration time. Every subsequent coave (default 1) images are coaveraged. The dither pattern is repeated n_cycles times. The size of the pattern is given by the xoffset and yoffset parameters (in arcsec, defaulted to 12"). When the script has finished, the telescope returns to the starting point, which is the centre of the dither pattern.
      You can set the jitter parameter to a value between [0.0, max]. The upper limit, max, must be less than 30% of the x/y-offset parameter. If the x/y offsets are different, then the smaller one is taken as the reference. Each dither point will then be offset from its nominal position by a random vector of maximal length max.
      In order to remove remanence of very bright sources in the field, you can set the -clean parameter, the number of clearing reads before the actual exposure, to a large value (e.g. 6; the default is 3).
      You can also set the number of multiple non-destructive reads (see above) with the -mndr option. This will temporarily override any mndr Nr previously set by the SYS> rmode liris mndr <Nr> <Nav> command. The default is 1 for imaging.
    • SYS> rdither <npts> <int time> ["title"]
           [-jitter=float] [-clean=int] [-mndr=int] [coave=int]

      Takes dithered images following a random pattern of a specified number of points npts. At present this script can only be used without autoguiding.
    • SYS> [ag_]mdither_ext <ndit> <int time>
           <sky_offset> <pa_offset> ["title"]
           [-ncyc=int] [-nruns=int]
           [-xoffset=float] [-yoffset=float]
           [-jitter=float] [-clean=int] [-mndr=int] [-coave=int]

      Same as [ag_]mdither, but goes to a blank sky position after one cycle. The offset of the blank position with respect to the target is specified by sky_offset in arcsec and the position angle (pa_offset).
  • Frequently used spectroscopy scripts
    For each spectroscopic script the multiple non-destructive read is set to 4 (mndr = 4) by default, and back to 1 (mndr = 1) once the script has finished. You can change this parameter by explicitly setting the -mndr option to a different value.
    • SYS> [ag_]spec_nod <int time> ["title"]
           [-ncyc=int] [-nruns=int] [-offset=int] [-start_center]
           [-mndr=int] [-jitter=float] [-clean=int] [-coave=int]

      Moves the object between two positions (A,B) along the slit. Prepend "ag_" to the script name when using the autoguider. Positions are separated "offset" arc seconds. Assuming the object is centred on the first position A = (0,0) , the first exposure is taken there and the second one at B = (0, offset). Alternatively, if start_center is specified, position A will be (0, -offset/2) and position B will be (0, +offset/2).
      The nruns parameter determines how many images are taken per nod position.
      Every subsequent coave (default 1) images are coaveraged.
      If ncyc is larger than 1, then the pattern is repeated as follows (example: ncyc = 5): AB BA AB BA AB.
      The jitter parameter in the range [0.0, max] triggers that the A and B positions are randomly generated to lie within a segment that is centred at each position (for ncyc>1). The dimension of the segment is given by the jitter argument. The parameter "max" must not be larger than 30% of the offset specified.
      Use -clean to wipe the detector the desired number of times before the start of the exposures (default 3).
      The -mndr sets the number of multiple non destructive reads. The default for spectroscopy is 4.
    • SYS> ag_spec_nod3
      Same as ag_spec_nod, but uses three dither positions. Assuming the object is centred on the first position A = (0,0) , the first exposure is taken there, the second one at B = (0, offset) and the third one at C=(0,-offset).
    • SYS> ag_spec_ext <int time> <sky offset>
           ["title"] [-ncyc=int] [-nruns=int]
           [-mndr=int] [-jitter=float] [-clean=int]

      Switches the telescope between the OBJECT and the SKY.  Exposures at OBJECT positions are done with the autoguider ON. If the pattern is to be repeated (ncyc > 1), the SKY positions are randomly generated to lie on a 15 arcsec segment perpendicular to the slit centred at (sky_offset, 0). At each position nruns images are taken. Use invert to reverse the sky offset.
      If -jitter is set, the OBJECT positions will be randomly generated to lie within a segment of "jitter" value size along the direction of the slit. The jitter size must be equal or less than 30% of the offset specified.
      For ncyc = 4,  the acquisition pattern is as follows:
      Obj Sky1   Obj Sky2  Obj Sky3   Obj Sky4
    • SYS> ag_spec_map <int time> <npts> <step> <sky offset>
           ["title"] [-cycle] [-nruns=int]
           [-mndr=int] [-clean] [-invert]

      Makes a spectroscopic map of an extended target. First a spectrum of the object is obtained at position (0,0), then from the sky at (sky_offset, 0). Then the telescope moves back to the object and along the slit, to position (0, step). Again, one pair of object and sky spectrum is obtained. This is repeated npts times. If the -cycle option is set, then each object-sky pair is taken twice.
      For e.g. npts=4,  the acquisition pattern is as follows:
      Obj1 Sky1   Obj2 Sky2  Obj3 Sky3   Obj4 Sky4
    • SYS> lobject_inslit <slit mask> <x> <y> [-noyoff]
      Moves an object at image position (x | y) onto the slit. If you want to move the object only in x-direction (preferred), then make sure to provide the -noyoff option. If you want a certain y position, you have to move the object there separately. Do not use the offset arc command for this purpose but let the OSA move the telescope. This is because the nodding scripts "ignore" the offset command, and your source will end up at a different y position.
      Tell the OSA to move the source reasonably close to the slit (so that the autoguider does not loose the guide star). Make also sure with the OSA that the current offset arc is zero and GUIDING ON, then run the script.
      In addition, from the real time display select the according mask and overlay it over the data shown. You do this by selecting the according slit mask from the RTD control window. If the mask does not show up, try zooming in once or twice in the RTD.

      Example 1
      Example 2
      Remark: The overlaid polygon which is shown then on top of the exposure has been obtained directly from a through-slit exposure. You do not need to take a separate through-slit exposure, but of course you are free to cross-check the positioning by inserting the slit mask into the beam with lslit <slit mask name>. Then take another image and look for the object in the slit.
      The same procedure holds for multislit masks, but there you should use the reference holes and the according stars. 
  • Imaging polarimetry
    • SYS> lipol <option>
      If you want to acquire with full field of view give this command after lobject_impol.
    • SYS> lobject_impol <x> <y> [-prepol]
      As you will see the target in all for beams, it is important to take the x and y values from the second beam from the top.
      Use -prepol if you want to acquire with full field of view. In this case the command lipol <option> comes after lobject_impol. The script asks if you want to move the telescope, answer yes.
    • SYS> nod5_pol <int time> ["title"] [-ncyc=int] [-nruns=int]
           [-xoffset=float] [-jitter=float]
  • LIRIS diagnostic commands
    • SYS> detstat Shows detector status and temperature of the monitoring points.
    • SYS> lmchstat Shows mechanisms and LIRIS controller status.
    • TCS commands (usually done by the telescope operator)
      • user "ENTER APERTURE <number> <ra offset> <dec offset>"
        Defines the aperture < number> characterized by <ra offset> and <dec offset> arcsec offsets in RA and DEC respectively (e.g.
        user "ENTER APERTURE 1 -400 0")
      • aperture <number>
        Offsets the telescope according to the previously defined aperture.
      • tcsslowoff <ra offset> <dec offset> <rate>
        Offsets the telescope by <ra offset> arcsec and <dec offset> arcsec in RA and DEC respectively from the nominal position. The rate is the slewing speed in arcsec/sec and can be up to 20"/s.
      • gocat <object_name>
        Moves the telescope to the object <object_name> if this has previously been loaded into the TCS.
      • object <object_name> <ra(hh mm> < dec(dd mm> <equinox>

        Adds the object <object_name> to the catalogue.

    Previous: 4. Focusing
    Next:       6. End of night

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    Contact:  (LIRIS Instrument Specialist)
    Last modified: 12 December 2013