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LIRIS Instrumental Issues

This is a collection of known issues and problems with LIRIS at WHT. All observers should be aware of these when observing with LIRIS and / or reducing the data.

  • MODERATE: Very bright sources and reflected ghost image. When doing spectroscopy of very bright sources (and sometimes in flat fields), a ghost image of the source (or slitlet) appears in the detector, reflected around the centre of the chip (i.e. optical axis). This has been noticed mainly in MOS spectroscopy mode, but also in long-slit mode.
    Hopefully, due to these ghosts being reflected around the center of the detector, their location can be predicted by a script located here, and accounted for during data reduction.
  • MODERATE: Jump in the sky background between the upper and lower two readout quadrants. It is observed that the sky background increases towards the lower edge of the readout quadrants. This leads to a discrete jump in the sky background when passing from the lower to the upper half of the image. When a series of images at a given dither position are taken without moving the telescope, then this profile changes with the exposure number, stabilisising from the third image onwards. If the telescope is moved after such an image sequence, the detector undergoes the same equilibrium process with the same sky background profile.
    Example: If a 5-dither observation is done with 12 subsequent images at each dither point, then the first images of each dither position should be grouped together for the data reduction (sky background modelling). The same holds for the each second image of the sequences. Numbers 3 to 12 can be processed together.
    Remaining residuals can be corrected by collapsing the images along their x-axis, using outlier rejection, and subtracting the average profile from each column.
  • MODERATE: Telescope control. Sometimes the connection between the instrument control and the telescope control suffers from hickups. If this happens especially during long spectroscopic exposures, the scripts will likely fail due to a timeout error.
  • MINOR: Detector resistancy. The LIRIS detector shows a significant amount of resistancy when bright objects are present in the field. When applying a dither offset, bright objects leave a streak in the image connecting the old dither position with the new one. In order to suppress this effect, the detector is by default read three times before the telescope moves to a new dither position. If you have an exceptionally bright source in the field, you still might notice this effect on a low level. In this case increase the number of flushes in the call of the dither script.
  • MINOR: Detector flaws. The LIRIS science grade detector has a number of cosmetic features. Most noticeably are:
    • Two small scratches at the outer edge of the upper left quadrant, and a bright spot surrounded by dark pixels in the lower left quadrant. These should be masked during coaddition.
    • A number of extended darker spots in the two left quadrants. This is a pure flatfield effect and is easily corrected. It is not a point of concern if your target ends up in this area during a dither move, for example.
    • An extended and shallow, fan-like feature in the lower right quadrant. Contrary to the spots in the lower left quadrant this is not a flatfield effect. It can be removed by sky subtraction. The feature could be due to electronics behind the sensitive surface.



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Contact:  (LIRIS Instrument Specialist)
Last modified: 23 August 2013