On 29 March 1996, D. Mckenna collected four images with STIS 2 with the new Fe55 source mounted in front of the CCD and viewed through a beryllium window. Fe55 undergoes inverse beta decay producing X-ray photons of such an energy that generate 1260 electrons upon interaction with the CCD crystal lattice. The beryllium window is transparent to these photons. The result has the appearance of a cosmic ray in the CCD image. These events can then be used to determine the CTE of the CCD by measuring the change in amplitude from the bottom left to the upper right of the CCD image.
Unfortunately, the mount for the Fe55 sources is such that only a small circular region of the CCD was illuminated by the beta decay.
The four images were added together and a single bias subtracted. The resulting image is shown below:
The pixels in the central region of the image containing counts
between 329 and 380 ADU (to remove the effect of partial
and multiple pixel events) were identified and used to fit a polynomial
surface using the IRAF routine utilities/surfit (a polynomial
of 1st order and with no cross terms was used - the
zeroth order term is therefore the average detected amplitude
of the beta events and the coefficients of the 1st
order terms in x and y are the fraction of signal lost per transfer
in the horizontal and vertical directions respectively. A similar result
is obtained by one-dimensional fits in the two directions). The resulting
fit had a d.c. term of 355.5 +/- 1.2 ADU, yielding a CCD gain
of 4.56 electrons/ADU, matching exactly with that found
in Noise Test. Since only a small
area of the CCD was illuminated by the source, the quality
of the fit is sufficient to state only that the horizontal
and vertical CTE are better than 0.999. An effort is under
way to fabricate a mount for the Fe55 source which will allow
the illumination of the entire CCD with X-rays.