|Westerlund telescope flatfield system|
"This thesis considers flatfield calibration for a digital imaging system. The new Å2 90 cm telescope has been commissioned in February 2004. Currently the telescope is equipped with a CCD camera for direct imaging. In a CCD there are 3 important sources of noise: bias level, dark current and difference in sensitivity of the optical system. Bias level and dark current strongly depend on the temperature of the CCD, they can be removed by taking Bias and Dark exposures.
The optical system consists of a telescope, illumination of the field of view, lenses, filter and cover glass of the CCD. By exposing the CCD to a stable light source with uniform light, the differences in pixel sensitivity can be measured. This is called flatfielding. A flatfield frame gives the response for the whole system. To reduce random noise in calibrations, several flatfield frames are used to create a MasterFlat. To create a MasterFlat following steps have been made: exposing bias and creating a MasterBias, exposing darks and creating a bias reduced MasterDark and exposing flatfields and creating a MasterFlat. MasterFlat is normalized, bias and dark reduced.
First, a flatfield source was built. It consists of a diffusely reflecting white screen and is illuminated by two halogen lamps. The lamps are not aimed directly at the flatfield screen, both lamps shine on two reflector screens. This is done to create uniform light at the flatfield screen.
As software routines, a program called CreateFlatField has been made. In this program, all calibration procedures are being made. Finally, collecting test calibration data and characterizing the system has been made."
The complete flatfield system report may be downloaded as a pdf (local access only).
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