The purpose of the visit was to see if the presence of visible spots on the surface could be compliant with the observations gathered through the thermocamera.
The assumption was that actual PA embedded in the coating could not be seen.
Indeed, thanks to the laser green light, well aligned at the center of the mirror, at a glance we suddenly noticed at least three spots.
1) One is compatible with the position X~40mm, Y~40 mm circled in the map provided by LMA concerning EM04 (Fig.1, file WE4.png), this spot is the brightest seen by the thermocamera and its position is also compatible with a reconstruction done by Cagnoli-Melo. Notice that the spot at (40,40) has a companion.
2) another, in situ even brighter, is at the center. This is compatible with the green pixel at the center of the map. Remarkably, this is not seen by the Interferometer through the beam centering on the mirror, but we must consider that it is quite centered and this fact may play a role.
3) other smaller dots are present.
We performed the inspection while the payload was not blocked and we never stood inside the chamber trying not to spoil the environment. Normally only one person is inside with the naked mirror.
Then we tried to take pictures. Indeed this is very difficult. Even though the center spot was evident by eye we did not succeed in taking decent pictures as the observation critically depends on the incidence. Moreover it is definitely clear that these spots are not spheres, so the result depends on their angles of position.
Some dust spots were present, mostly on the circular area around the first contact surface. We observed that they could be removed with the standard clean operation soft sticks.
Since we worked around the mirror for one hour we could have produced new ones. During all the operations we used continuously the ionized N2 gun.
Since we saw the above mentioned spots we tried to manually remove them. In several ways, but unsuccessfully. At least for a couple of cases, by hand, one can feel the friction of the dry clean stick stroke on the spot (of course it is a very gentle grasp). We pure water and isopropyl alcohol. The we wiped the overall surface.
There was no reason to apply the first contact as we observed that these dots are still there and no improvement could be achieved in order to remove them.
Just as a check of the major spot presence after these very hard manual actions it was decided to evacuate the chamber.
In fig 2 (file WE.png). The same spot (the brightest with the thermo-camera) is seen on the HR and through the AR. In fig. 3 (file WE2.png) you see how faint and difficult is the picturing. The spot appears and disappears as the mirror oscillates and the incidence is slightly changed. In Fig 4 (file WE3.png) some spots (all of them fixed on the surface) taken from the HR side.
Conclusion:
- The scattering image produced by LMA produces rough but potentially valid information about PA
- The effect of PA is strongly dependent on the orientation
- Bright dots seen through inspection were not removable by cleaning
Here attached the reconstruction done. Geppo performed the calculation of the rotation matrix using the Euler angles and plotted the result considering the angles of the viewport w.r.t. the center of the payload that we used to take pictures with the thermocamera. Then we superposed the result with an image of the thermocamera in order to estimate the position of the point absorber. Looking at the figure, the orange sign is at the center of the mirror and the ruler steps (spacing between green lines) measure 2 cm. As a result, we have the estimation that the brighter point seen by the thermocamera is roughly 4 cm to the right and 4 cm upwards with respect to the center of the mirror. The other bright points are distributed downwards tooking as reference the main (brighter) one.
