Phase Camera 1: Meaurement with Central Spot Heating WI 14 Jan 09

we used the phase camera 1 to observe the thermal distorsion of the west input mirror heated by the central spot.

The under figure show the initial state:



With the
previous telescope the phase image CAR-USB was a tilted plane changing
by 1.2 rad.
We set a phase image baseline using gps 915880307.
We then switched on the central heating to full power and measured it with the power meter to be 180 mW (see calibration curve).
Initially we saw nothing as the motorized mirror inside the tower was not aligned and so the central spot was not centred.
We aligned the mirror to position:
Up_Left = 6200, Bottom_Left = -300, Bottom_Right = 300.
We see clearly a phase distortion due to the heating of the central
spot in the centre of the mirror. The phase distorsion is around 1 rad.
Preliminary calculations indicate this to be about a factor 2 lower
than expected.
The phase image reacts very quickly to central spot changes and seems
to agree well with the expected time constant of 5 minutes.



We then looked at the phase images with 40mW



and 80mW of central spot power.



The predicted YAG absorption for WI is 40mW so with the current
configuration it will be difficult to measure directly the YAG heating
with the phase camera.
We then switched off the central heating. The final state is:



From Channel PC1_PHIDIFFCARPHI0_USB_Image it is calculated the Phase.



The minimum phase of this image is linearly proportional to the power
absorbed by the mirror. The simulation gives the following calibration:
1W absorbed corresponds to -3.3rad. The minimum phase was around
0.6rad for 180mW CO2 central spot power which is exactly what was
expected from the simulation.