Sky Subtraction

Background emission from the sky have to be subtracted from the spectra of objects and standard stars. This is achieved by subtracting the sky spectrum from the object or standard star frames. The sky frames are usually taken during observational sequence, object - sky - sky - object. The sky frames should be taken near the object frames in time because the sky background emission varies with time. Sky subtraction would leave a residual if the spectra of sky and object frames have different background levels. Such a residual can be eliminated by subtracting a scaled sky frame which is an original sky frame multiflied by a factor to scale its intensity level matching the background level of the object frames. Each aperture has a different scaling factor.

If the object is small compared to the slit length, then the sky can be removed by using either side or both sides of the slit image free of object emission. The sky level is used a median value calculated the either or both sides, or straight line connected between the both sides. This can be achieved by BACKGROUND task in the noao.twodspec.longslit package. Figure 44 shows an example of sky subtraction from an object frame with the BACKGROUND task. The distortion of the slit image must be corrected before subtraction.

Figure 44: Sky subtraction with the BACKGROUND task. An aperture corrected for its geometrical distortion before (upper image) and after (middle image) the sky subtraction. The lower image in the box is a sky subtracted frame without any distortion correction applied.