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LIRIS Observing Techniques

Near-IR imaging and spectroscopy is much more difficult / different than in the optical.

  • The sky background is dominated by night sky emission lines, and especially in the K-bands by the temperature of the atmosphere. The IR detectors usually get saturated after 10-20 seconds of exposure time in K, which leads to a very large number of images with rather short exposure times. By means of automatic averaging of images in the software the number of files can be kept within an reasonable amount. An exception is spectroscopy, since there the little light that passes the slit is distributed over the whole detector.
  • Although IR instruments are usually cooled down to about 60-70 K in order to suppress heat radiation, the telescope and surrounding dome remain at ambient temperature. Their heat radiation has to be carefully kept out of the instrument in order to remain sky background limited. Cold stops (various pupil masks) in LIRIS do this job efficiently.
  • The night sky emission in the IR varies on time scales of a few minutes. Proper calibration of the data is therefore not straight forward. (Spectro)photometric standard stars have to be chosen reasonably close to the observed target. The best flat fielding and sky subtraction is usually obtained by wide dither patterns (point like targets), or by subsequent on-target and off-target imaging (extended objects, spectroscopy).
  • IR detectors are very different from CCDs used in optical astronomy. For example, they show no blooming effects, and the same image can be read out several times (multiple non-destructive read mode, MNDR) in order to reduce the readout noise (useful for spectroscopy).
  • Data reduction is in principle similar to the optical, however, differences exist. Flat fielding of the data might work better with median-combined empty (off-target) fields than with sky flats gained at the beginning of the night. Besides, the number density of objects in a single IR exposure is usually much lower than in the optical, due to the short exposure times. Often, they become visible only after a proper sky subtraction. This might limit the accuracy of the astrometric solution or other tasks requiring a reasonable number of sources. Often, looping over some reduction steps is necessary.
    IRAF has a variety of built-in tasks that can do the job, however, these might not give optimal results, depending on observing conditions, the site, the imager and the telescope. For LIRIS, a dedicated IRAF package has been developed that yields good results.

1. Flat fields

It is a common practice among IR astronomers to obtain flatfield images by median filtering of dithered frames since images are usually background-noise limited with a high level of counts (thousands of counts).

For the K band (K, Ks, Kcont, brg..) it is however recommended to take twilight or dome flats with the same exposure time but with different levels of illumination. For the dome flats this can be obtained by taking flats with all lights off and then with the flat field lamp on. The difference of the two will give a flatfield free of telescope thermal emission. Take a high number of flatfields of each kind in order to decrease the noise.
However, first experience with twilight or dome flats show that they may not work very well. Instead, flat fields determined from the data itself work better. More experience is needed in order to find the optimum way.

IMPORTANT: If you observe extended targets, i.e. your object is half the size of the chosen dither pattern or even larger, then we strongly recommend to take a similar number of off-target images for proper flat fielding and sky subtraction. Otherwise you will not be able to to calibrate your data properly.

2. Darks

You should take dark frames always with the same exposure time as the images from which you want to subtract them. If you scale them to a different exposure time, then you also scale the readout noise, which is independent on the exposure time. Besides, even though they are small, non-linearities exist in the images.
However, note that the BIAS and DARK is usually not stable. If you require dark subtraction, take the darks as close as possible to your observations.

3. Point sources, extended sources, sky frames

Observation of point sources saves you exposure time, since you can apply a wide dither pattern and calculate flat fields and sky subtraction models from the data itself by proper median combination. Extended sources, however, require a similar amount of images taken on a blank field of sky a few arcminutes to the side of the target, thus doubling the exposure time.
For spectroscopy, you can apply the nodding technique, i.e. move the object along the slit to one or two other positions in the slit and take another spectrum, then move back (
slit parallelity). Like that you don't have to spend time on off-target imaging. If your spectroscopic target is as extended as the slit, however, then you can't avoid to take off-target spectra of a blank sky position.
The nodding technique is available as a script, and we find that the object stays well within the slit during several nods. However, you might want to check the object position with respect to the slit after a number of nods.

4. Spectroscopy with LIRIS

The long-slit spectroscopic mode of LIRIS is basically identical with the imaging mode. The only difference is that you do not leave the grism wheel in its blank position, but select a grism. Besides, you choose from the six available long slits (0.65", 0.75", 1.0", 2.5", 5.0" and 10"), which you can overlay in the Real Time Display (RTD) during target acquisition. Ask the telescope operator to rotate the instrument if you require a certain slit orientation with respect to your target.

Multislit spectroscopy is also available with LIRIS, however, the number of MOS mask positions is limited to about 6, depending on the current slit mask wheel setup. MOS mode is offered only in collaboration with the LIRIS instrumental team due to the potential long lead time, once per semester oportunity to insert the masks in the cryostat, and the limited number of mask positions.
If you plan to submit a MOS proposal, please do not hesitate to contact the LIRIS instrumental team ( and ) and ING (LIRIS MOS Specialist) well in advance of your proposal preparation for further discussion and advice. However, please be not disappointed if we can not allocate observing time to your MOS programme. LIRIS is not mounted permanently at the WHT, and the available time for MS mode is very limited.

5. Coronography

Currently not available in this semester.

6. Polarimetry

For imaging polarimetry the same rules as for normal imaging apply. One has to put the Wollaston into the optical beam. A major difference you have to be aware of, is that the twilight sky is highly polarised. Therefore, when taking polarimetric flat fields, you should point the telescope as low to the horizon as possible in order to get comparable illumination for all four polarimetry vectors.

Spectropolarimetry this mode is not fully commissioned yet. Though it is possible with LIRIS, just move the according Wollaston prism into the optical beam in addition to the slits and grisms as mentioned above. Do this before moving the target onto the slit.

Sample of polarimetric calibration targets.

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