FORS2 CCD upgrade
Table of Content
In GeneralSince March 22, 2002 we are operating FORS2 with new red/IR-optimized MIT CCDs. There were some very late changes done which are not yet documented in user manuals and on these WEB pages. Please take the following supplementary informations as update to the existing documents.
The older version of the MIT CCD upgrade page is still available.
Gain factorsThe Video board and CCD configuration parameters were modified. As a consequence the gain factors have changed:
|200kHz low gain||1.25|
|100kHz high gain||0.7|
Another brick in the wall
|Not only the FORS2 CCDs look like that! The UV and Blue flat fields show some brick wall like pattern which has it's origin in the thinning procedure.|
|The 3 frames are taken in the lab at 400nm, 600nm and 900nm. This pattern can be easily removed with twilight flats.|
FIMS - Mask Preparation Software - patch 126.96.36.199Bad news! The mask preparation software had to be patched. For the released version 188.8.131.52 we have mistaken keyword DET.NAME with DET.ID to check for old SITE images for which we overwrite the astrometry in the fits header with improved new distortion coefficients. The users have to reload the masks prepared on FORS2 SITE CCD images with fims version 184.108.40.206 and save it then without any further changes in the mask design to correct the astrometry. Masks based on MIT pre-images are not affected by the bug. Users affected by the bug will be informed in the next days by the user support group (USG).
Zero pointsExpected and measured changes of the photometric zero points. The November 2001 measurements were taken with a dirty M1/M3.
| ||MIT/ETC||MIT - Nov. 6, 2001||SITE/ETC||SITE - Aug.18, 2001|
|z||27.15|| ||26.31|| |
Compare the latest FORS1 and FORS2 values (zero points in e-/s):
Sky brightness On FORS2, due to the higher NIR throughput of the MIT CCDs the effective wavelength of the filters have been shifted toward the red and therefore the night sky background is over-proportionally increased after the CCD upgrade. The following sky brightness measurements have been performed:
For narrow and intermediate band filters as well as for spectroscopy there is of course no problem with this feature. One should be aware that imaging exposure times are now restricted to 240 seconds with I_BESS and 120 seconds with z_GUNN and about 3000 seconds in spectroscopy to avoid saturation.
Fringes The detector cosmetics and fringe amplitudes are excellent! Please take the following z_GUNN image as an example: 6x100s imaging (combined) with the z_GUNN filter of an empty field arround quasar Q0044-273. The raw data was flatfielded with twilight flats (no de-fringing was applied, no sky subtracted). Only about a section of 1' x 1' of the image is displayed.