4. Test of the program with artificial faint moving objects
Before using the program on the observational data some tests were performed with artificial faint moving objects. These artifical faint moving objects were added to the images corresponding to the CCD 1 of the field number 3. They were obtained by using the image of a selected star found in each image ( 20) and normalized to different magnitudes before inserting it at known positions corresponding to a velocity motion of 3 arcsec/hr. The following magnitude were used: 21.0, 22.0, 23.0, 23.6, 23.8, 24.0, 24.2, 24.4, 24.6, 24.8 and 25.0.
In testing the program the key parameter appeared to be the threshold of detection. This can be expressed by comparison with the standard deviation of the background intensity due to the sky background noise. For a threshold equal to this standard deviation, the limiting magnitude appeared to be surprisingly high. Indeed all the artificial objects up to magnitude 24.8 were successfuly detected. Nevertheless the data processing appeared to be long and, above all, it appeared that it was nearly impossible to check visually the detections on the computer screen (because of the faintness of the objects). Given that several false detections also occured, such a threshold of detection did not appear practically useful.
Finally, after different tests, a good value for the threshold of detection appeared to be about 1.5 times the standard deviation due to the sky background noise. The limiting magnitude ( detection limit) appeared to be about 24.2 on the set of images used for the tests and the time of calculation became more acceptable 1. Since the mosaic camera consists of eight different CCD arrays, each of them having a slightly different quantum efficiency, the average limiting magnitude can vary slightly around 24.2 depending upon the CCD considered. Nevertheless the examination of the sky level in the different images shows that the difference of sensibilities are very limited, i.e. the variation in the limiting magnitude does not exceed about 0.2 magnitude for a given CCD.
© European Southern Observatory (ESO) 1999
Online publication: August 13, 199