EFOSC2ESO Faint Object Spectrograph and Camera
Helium-Argon Arc Lamp -- Wavelength Calibration
Helium and Argon lamps are used to obtain the wavelength calibration of the CCD. The observing template takes an exposure of a Helium lamp followed by an Argon lamp resulting in an image with a combined Helium and Argon spectrum.
Note One needs to observe He-Ar arcs for every grism used during observations. In general 3-5 exposures at the beginning of the night and an equal number at the end of the night are sufficient. Observers who wish to calibrate "on target" may want to have a separate OB with just 1 exposure.
The y-position of slits vary from one instrumental set-up to another
and from one slit to another by a few pixels. So observers
must take the calibration frame during their run or risk wavelength uncertainty
by a few pixels. However unless the slits are re-inserted the calibration
is stable from day to day.
|Long Slit versus SpectroPolarimetry|
The wavelength calibration for both modes uses the same template. The differences are (see the sample OB below):
|Helium-Argon Exposure Times|
The exposure times for the Helium and Argon lamps are different for each grism and have to be specified in the observing template. Important: Always check that your favourite lines are not saturated as the observations are being made.
|Spectral line shift due to CCD distortion with telescope pointing|
Spectral line shift due to CCD distortion
with telescope pointing (click on link for image). For most
observing programmes such shifts are small enough not to be of concern
and so arc lamp observations at the beginning and the end of the night
are sufficient for calibration purposes. However programmes with more stringent
wavelength calibration requirement will require "on target" He-Ar arc lamp
images during the night.
Internal reflection (click
on link for image). For a few grisms, ghost lines due to internal reflections
can be see in the He Ar image. The ghost lines tend to be quite faint and
have a different curvature to the real lines. They can be seen at least
for grism 12. Nothing much can be done about these, and care must be taken
not to misidentify these lines when doing wavelength calibrations.
|A sample observing block|
jP2PP Panel with Typical Parameters
The above example shows an OB comprising a sequence of:
As an example the following image is given, which shows a Long slit spectroscopy He-Ar arc frame with Grism #6, Slit 1.0", normal readout and 2x2 binning. Click on the image to get a larger version:
Parameter Values: One does not have to (in fact, should not) change the value of any parameter other than:
|Send comments to : ls-spectro
Last modified: Sat Apr 9 18:44:11 CLT 2005