RECONSTRUCTION OF FLAT-FIELD IMAGES
The command FLAT permit reconstruction CCD flat-fields by using non-uniform background illumination sources (i.e. deep-sky images, Sun ou planetary surface images, and so on). This is a very powerful tools in situations where twiligth flat-field are not disponible or impossible to obtain.
The key principle is the precise know of the value of small displacements of the telescope between exposures. Because the linear translation of the image on the CCD with respect to the flat-field function, a formulation relates the flat-field to these shifted data sets.
For example, look on the figure 1 the first images of a sequence of 7 images of planet Jupiter.
Figure 1. Raw images of Jupiter (no flat-field correction, no contrast enhancement). Note the random shift between CCD frame.
First we compute the precise shift between images by using PREGISTER command (remainber to offset the background to zero before apply this command). The file SHIFT.LST is generated in the working directory:
PREGISTER JUP I 256 7
Now execute the FLAT command:
FLAT JUP 32767 0 10000 2 7
Figure 2 show the result:
Figure 2. Flat-field synthetised by FLAT command. The result is very instructive: numerous dust are visible (the flat-field is only valid in the region of Jupiter images).
The last operation is to correct the raw images:
DIV FL 10000
DIV FL 10000
or, more efficient:
DIV2 JUP FL R 10000 7
Click here for another example.