can be determined from two images with different
``apparent'' exposure time t1 and t2, as follows:
Since the CCD is a linear detector, when it is exposed to the
intensity I in the time treal, the level we read out N for a
given pixel is proportional to the product of the two,
One can use more images to get a better determination.
We used the temporal sequence of one 10 sec, ten 1 sec, and one 10 sec
If we let N1' denote the sum of the levels in the ten 1 sec exposures,
and N2' the sum of the levels of the two 10 sec exposures,
For the determination of the shutter correction, a number of dome flats
were taken, using the same lamp with constant voltage, thereby
hoping to get a constant intensity.
The images are listed in Table .
The images in Table were corrected for bias and dark.
The delta image was then computed using equation for
each of the four (1
10 sec, 10
1 sec, 1
sequences. The mean of the four was also calculated.
The results are in Table .
A visual inspection of the four delta images showed, that they were all almost flat, as the low standard deviations also suggest. There was, however, a small gradient in the x direction of about 0.001 sec, but since it was much lower than our estimated uncertainty of 0.02 sec (see below), we decided to neglect it.
Therefore, we decided to use a scalar delta value instead of a delta image. We set this delta value to the (scalar) mean of the mean delta image, which amounts to 0.41 sec. We estimate the uncertainty on this value to be half of the maximum difference between the four individual determinations, i.e. 0.02 sec.
The differences between the four is probably due to the light not being constant. The symmetric sequence (1 10 sec, 10 1 sec, 1 10 sec) makes the determination insensitive to a light variation which is linear in time, so the light variations was probably more ill-behaved than that.
A cl-script b_dcorr was written to implement the scalar shutter correction.
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)