Sky Subtraction

Contribution of background radiation must be removed from the spectrum of the target source . This is achieved by subtracting from the object frame the sky frame taken nearest to it in time. Since a typical observational sequence produces object- sky- sky- object one obtains a pair of object minus sky frames from it. These are then co-added to produce the final spectral image of the target source. When observing a point source one can nod the telescope so that one keeps the source in the slit but at a different position (a 10 pixel offset relative to the original position was used to obtain the data presented here). This means that the sky frame will have the background contribution where the spectrum of the target was located in the object frame and the target spectrum at a position where the background was recorded in the object frame. The result of subtracting sky from object is to have the spectrum of the target source (positive beam) and its negative (negative beam) on the same frame (see Figure 3.2). This is the final reduced spectral image from which the spectrum of the target source is to be extracted.

If the atmosphere were to change its transmission between the object and the sky frames the subtraction object minus sky would leave a residual contribution from the atmosphere. Such a contribution can be eliminated by 'poly-fitting' the object minus sky frame. 'Poly-fitting' consists of choosing areas in the frame, either side of the rows where the spectrum of the source is, only with the contribution of the atmosphere. For each column, a polynomial is then fitted through the corresponding areas. The result is an image that should contain the remaining atmospheric contribution. Subtracting this from the original image results in a frame with the residual sky contribution removed.

The frequency with which one has to change between object and sky frames depends on the observing conditions and on the wavelength where the observations are being carried out. A rapidly changing background implies a higher nodding frequency. At wavelengths that correspond to the thermal infrared () variations in the background emission occur so quickly that chopping must be performed. That is, one must change from object to sky with frequencies of the order of a few Hz.