Analysis based on CCD8 data, but timings still valid for new CCDs
ALFOSC shutter is "butterfly"-type and is located between the filter and
The minimum time to open and close the shutter is about 0.3-0.4 seconds
(Figure 1). So, one ***can _not_ take*** an exposure shorter
than ~0.4 sec.
In addition, the actual shutter opening time is about 0.1 seconds longer
than the requested time.
The shutter pattern is virtually non existent. Several 0.2 and 5 seconds
overscan subtracted images were coadded and normalised to one second. The
ratio of the long and short images indicates that there is a slight
gradient from bottom left to top right hand corner. However, the
difference between the corners is less than 1 part in 1000 for a 0.2 sec
Click here to see the image only.
Top panel, integration time versus measured count level (using Beta light)
from overscan subtracted image.
Below, linearity residual versus integration time using the requested
integration time (blue) and with 86 msec correction (green).
The dotted and dashed lines show 0.5% and 1% lines, respectively.