ESO Faint Object Spectrograph and Camera
Helium-Argon Arc Lamp -- Wavelength Calibration

Long slit spectroscopy versus SpectroPolarimetry
Helium-Argon exposure times
Helium-Argon line identification plots
Spectral line shift due to CCD distortion
Internal Reflection
A sample observing block

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

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:

The FITS image output by the template is named EFOSC_spec_HeAr.#.fits

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:

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Last modified: Sat Apr 9 18:44:11 CLT 2005

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