Читать книгу The Doppler Method for the Detection of Exoplanets - Professor Artie Hatzes - Страница 35

CCDs have slight variations in the QE from pixel to pixel. The recorded CCD frame may also have ghost images, reflections, or other artifacts from the spectrograph. If these variations are not properly removed, this will affect the quality of your spectrum and ultimately, your RV precision.

The process of “flat fielding” consists of taking an observation (spectrum or image) of a white-light source commonly referred to as the “flat lamp.” Divide your observation with your flat-field exposure. Flat-field errors can arise when the variations are not taken out completely. This can be the case because flat lamps are usually inserted in the light path just before the entrance slit of the spectrograph and thus do not follow the same optical path as your observation of the star. For this reason, it is a common practice to take a “dome flat.” The telescope is pointed to a white screen mounted on the interior of the dome, which is then illuminated by the white-light source. The light from the flat field thus follows, more or less, the same optical path through the telescope as the starlight.

Figure 2.19 shows an example of the flat-fielding process as applied to imaging observations where you can better see the artifacts and their removal. The top-left image is a raw frame taken with the Schmidt Camera of the Tautenburg 2 m telescope. The top-right image shows an observation of the flat lamp, where one can see the structure of the CCD, as well as an image of the telescope pupil caused by reflections. The lower image shows the observed image after dividing by the flat lamp observation. Most of the artifacts and intensity variations have been removed by the division.

Figure 2.19. The flat-fielding process for CCD reductions. (Left, top) A raw image taken with a CCD detector. (Right, top) An image taken of a white-light source (flat field) that shows the CCD structure and optical artifacts. (Bottom) The original image after dividing by the flat field.