Wavelength Calibration

Wavelength calibration can take two forms within Portable--CGS4DR i.e. viz. estimated or calibrated. The estimated method generates a rough scale from the grating wavelength and dispersion and is usually accurate to better than 20% of the resolution. The X axis label will be set to `estimated wavelength' in microns.

If the wavelength calibration option is set to calibrated, Portable--CGS4DR searches the index file for a suitable CALIBR frame of the form cayymmdd_oooo and having the same instrument configuration and oversampling factor as the observation to be calibrated and uses that. Note that when such a CALIBR observation is used, the X axis is simply copied over and no attempt is made to re-bin the data. This means that if anything other than a straight line fit is used in the ARC function, the data will end up with an X axis whose values are not equally spaced.

Wavelength calibration by either method takes place when the reduced observation file is created and before any integrations are reduced so if you change your mind about wavelength calibration and the reduced observation file already exists, you will have to delete it before new instructions take effect.

A CALIBR frame is created by observing a wavelength calibration source such as a lamp or the sky (OH-lines) and this is normally called an ARC frame. This data is reduced in the usual way (and may be wavelength calibrated!). CGS4 has argon, xenon and krypton lamps for this purpose and the in-vacuum wavelengths for most infra-red emission lines from these lamps are included with the software (as indeed are the OH lines). They are recorded in Å but may be converted to m by the calibrate procedure.

Arc-lines are identified using the mouse and selected by depressing the space bar. Note that if you know the exact value of a spectral line, you can over-ride that chosen by the software by specifying the the `E' suffix when inputting the wavelength value to the prompt. For example, the wavelength 16952.3 Å may be specified:

? Enter wavelength (null to delete) - /''/ > 16952.3 E

After this procedure is complete, the frame is automaticall filed as a CALIBR frame. By invoking file_calib manually, the x-axis label and units may be altered by the user but default to those contained in the original frame.

Observations such as BIAS, DARK, FLAT, OBJECT or SKY may be used for wavelength calibration but once filed as a CALIBR frame it cannot be used for any other purpose.