Multiple Arc Frames

Multiple arc lamp images are usually taken to monitor any possible shift in the object spectrum which may have occurred during the exposure.

When no shift is evident then the simplest course is to add the two arc frames together and provide the result to the ECHOMOP package whenever it prompts for the arc frame name.

When a shift can be discerned you must decide whether meaningful wavelength calibration can still be applied. If so, then there are two courses of action available:

• Add the two frames together as usual, providing a single image for calibration, albeit with degraded resolution. This permits an estimate of the resolution in the final spectrum because the arc frame is extracted in an identical manner to the object, and the resolution observed directly.

• Calibrate each arc frame separately and interpolate between the wavelengths (possibly taking into account time elapsed between arc1 - object - arc2). This technique allows the calibration to proceed even when a gross shift has taken place, but you should be cautious about the resulting wavelength scale unless independent confirmation can be obtained from features in the object spectrum itself.