New Camera
Simulations of the optics of solar 2 revealed an interesting possibility for a redesign of our camera. In the past, Solar Two was designed such that one photomultiplier tube (PMT) was located at the image of one heliostat in the focal plane of the secondary mirror. Thus, in Solar Two's first phase, 32 heliostats mapped directly to 32 PMTs. This was done so that if a signal was detected from a PMT, it was obvious where the photon originated in the field.
This is important to reconstruct the shape and photon distribution of the original cherenkov light pool. However, simulations showed that if a PMT was placed in the focal plane such that more than one heliostat image fell on its face, they would be from different rows in the field. Because the rows are at varying distances from the tower, the signals from multiple heliostats would be delayed relative to each other.
Blue dots are photons originating from heliostats in even rows, red dots from odd rows.
Simulated image of the field in the focal plane of the camera with location of winston cone faces (attached to front of the PMTs) for the new 80-channel camera superimposed. Red arrow is indicating that two or more heliostat images fall on the face of one cone.
The delay is anywhere between 35 - 55 nanoseconds depending on where in the field the heliostats are located. Because the cherenkov pulses only last 10 - 20 nanoseconds, Solar Two's fast electronics (0.5 nanosecond resolution) can differentiate between them.
Using this technique we have increased the size of our experiment from 32 heliostats with a total surface area of ~1300 square meters
to 165 heliostats with a total surface area of ~6800 square meters this insures that we are sampling a significant portion of the total cherenkov light pool (light pool is approximately 250 meters in diameter as demonstrated in the figure)
The geometry of the field tells us where in time we can look for signals from heliostats in nearby rows. Each channel is assigned a central heliostat and all signals are delayed according to the time of flight from the camera to that heliostat. Then hits are looked for in windows centered at times appropriate for the time of flight for nearby rows. Doing this, we are able to detect signals from as many as 5 heliostats in one PMT! Conversely, we are able to detect as many as 5 hits from one heliostat that can be arbitrarily close together in time because they came from different pmts!!! This technique theoretically allows us to detect up to 5 times more cherenkov photons than normally possible with a conventional 80 channel (1 channel to 1 heliostat) camera.
