During reduction of our data it has become evident that the VE2 filter gives us certain problems. A new problem is that VE2 exposures tend to have a ‘sky component’ that is much larger than in the B,V,VE1 or IRCUT filters. Here is a plot of the relationship between exposure time and the ‘pedestal’ – i.e. the offset that has to be subtracted from the VE2 image to bring the sky level down near to 0 (in bias-subtracted images, of course).
It does seem like the pedestal height is a decreasing function of the exposure time. Why is it any function of exposure time, when the other filters do not show this behaviour?
If there was a source of light in he image frame, then its level should INcrease with exposure time, surely? Here is the plot for the VE1 pedestal:
The level is much smaller than in the VE2 case – and it does not appreciably depend on exposure time.
What is going on?