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The calibration unit is mounted at the broken Cassegrain focus and contains one slide with lamps and two filter slides. The unit is operated from the AF2 control system.
As the lamps are on a slide, only one lamp can be used at a time.
The default set of lamps mounted in the WYFFOS calibration unit (and their position number) are:
Also available are:
Any of these lamps can be mounted in the calibration unit if requested by the observers, who need to let their support astronomer know at least two weeks in advance.
The filters currently mounted in the filter slides A and B are:
Filter slide A:
Filter slide B:
The system works by shining the light onto the tertiary mirror which is then relayed to the fibres at prime. Consequently, light from the arc lamps can be superimposed on the starlight. Hence, primary mirrors petals should be closed when taking arc or flat frames.
The ThAr hollow-cathode arc lamp, with its rich line spectrum, is useful for those science programs that aim for high radial-velocity precision. In the red end the arc lines are much more intense than in the blue wavelenghts. This lamp can be used in the echelle mode, and in the normal mode with the high resolution gratings R1200R, R1200B, H2400B. It is not recommended with lower resolution gratings because the lines are mostly blended.
Cd and Zn arc lamps are complementary lamps.
Cd and Hg lamps need long warming times, as the relative flux of the arc lines can change drastically in the first 1-2 mininutes after switching on.
The other lamps need just a few seconds of warming, ~20 sec.
More details on the calibration lamps and the arc maps for the He, Ne, Hg, ThAr, and Cd lamps when used with WYFFOS/Red+4 can be found at the ING Technical Note TN136. In there, it is showed the arc spectra for each grating and spectral range, hence the user can select the set of lamps that better suits their setup.
NOTE: A new ThAr lamp has been recently purchased by the ING, which is 3-4 times more intense than the old one, and contains also Chromium lines (mainly in the blue end of the spectra).
The arc maps in TN136 refers to the old ThAr lamp. The arc maps for the new ThAr lamps can be found at ThArCr_arcmaps. These arc maps were taken with ISIS, with a resolution R=13000-18000, much higher than the resolution we get with WYFFOS.
Flat-fielding and fibre trace
The flat frames can be either QTH lamp frames or twilight sky exposures. The QTH flats have the advantage of a smooth spectrum that can be easily normalized with a low-order function. However, due to the difference in the optical path with respect to the science exposure, this is not optimum for flat-fielding correction, e.g. the FWHM in the spatial direction is narrower than in the science spectra. Sky flats are better for this purpose, but have the disadvantage of showing some absorption lines, which make normalization of the spectra difficult, a high order function being needed.
For tracing the fibres, both QTH lamp and sky flats are equally good. However, the QTH lamp has a very red spectrum, showing low flux at the blue end, and this can be a problem when using a low resolution grating, as the red part can be well traced while there are not enough counts in the blue part to find the trace. In these cases the sky flats are recommended.
Sky subtraction with fibres is never as good as with a slit spectrograph, since one is limited by the fibre diameter. Achieving the best possible sky subtraction with fibres requires careful planning and observing strategies, depending on the type and magnitude of the science targets, and inevitably involves tradeoffs.
For instance, for bright objects it should be enough to assign a 'sky' fibre next to each target fibre. This technique allows measurements of the sky simultaneously with the science spectra, which is an advantage, but it reduces the fibre multiplexing advantage by a factor of ~2.
When the targets are faint this is not the best strategy, as the relative throughput and the small differences in the spectral resolution of the fibres prevent an accurate sky substraction that can pollute the science target spectra.
In the case of faint targets, a better strategy is to 'beam-switch' (move the telescope back and forth between objects and neighbouring sky), so that the sky spectra are measured on the same fibres as the science objects.
The disadvantage of this method is that it reduces the integration time on source, and also that the sky is not measured simultaneously, hence the sky conditions need to be very stable in the spectral range of interest to get a good sky substraction. E.g. it might not be the best option when observing at the red end, where the sky lines are strong and very variable.
Flux calibration can be done by placing a spectrophotometric standard star in a fibre used in any configuration. It is recommended to take spectra of the standard through 2 or 3 different fibres, to check that the derived flux calibration is not sensitive to possible errors in the measured relative fibre througput.
Note that it is difficult to get absolute flux calibration with fibres, because of the flux fraction lost outside the fibre. An aperture correction, depending on the seeing, need to be applied.
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