YuefanGuo - 23:25, Friday 14 April 2017 (454)
Check the reason of astigmatism
We finished the picomotor pins of the PR chamber in the morning and checked all of them work well. So this afternoon we tried to move the beam outside the dichroic mirror.
At first we tried to move the beam in the horizontal direction out of dichroic mirror. But while we were moving, we thought that if the beam is elongated in the horizontal direction because of the mirror's wedge, when we scan the mirror maybe the beam shape will change. We scanned it,but the beam is too small,it was hard to say if it has some change or not, then we thought maybe we can put a lens with very short focal length on the BS bench to make the beam bigger in the short distance, but it was also hard to say since we scanned the beam in very small step, then on the BS bench the beam moved a lot and out of the lens, then we cannot exactly sure every time we put the lens in a good position to let the beam cross its center, if we did not cross center, the beam will also effected by the lens.
Then we followed our original plan to move the beam out of the dichroic in the horizontal direction, but it stopped by the mount of the mirror, so we tried in the vertical way, when we reached the pitch limitation of turning mirror, the beam was on the top edge of the dichroic, we thought it will be better if we move the beam totally outside the dichroic, so we decided to change a little bit the degree of last mirror on the bench. Before we changed the last mirror, we use the target to set the position of the green beam after the window for easier recover the beam. Then we let the beam go outside and far away from the BS chamber, to check the situation of it.
There were some strange situation that in the near field of the BS chamber, the beam elongation is much serious than the far field. We guessed that maybe because there are some overlap in the near field to make us feel the beam is longer, then in the far field, the other small beams separated from the main one. Actually, we did see that in the far field there were more small beams. But at least, the beam got better in this test.
The other situation I want to mention is that, I am not sure that the beam got better is only because of moving out from dichroic, we also have other changes in this test. Like I showed the simple drawing in the first picture, the circle 1,2,3 is the changed point.(It maybe a little bit hard to understand since I change the 3D situation into a 2D drawing, the red line is after we made the beam path goes up, so the red beam reached the higher position of the turning mirror and pass the dichroic from top of it)
1. The beam position on the window
2. The beam position on the turning mirror
3. The beam did not cross the dichroic
Also I showed the beam shape in two situation and in the near field and far field.
The second picture is I showed yesterday on the meeting for better understanding of the situation.
At first we tried to move the beam in the horizontal direction out of dichroic mirror. But while we were moving, we thought that if the beam is elongated in the horizontal direction because of the mirror's wedge, when we scan the mirror maybe the beam shape will change. We scanned it,but the beam is too small,it was hard to say if it has some change or not, then we thought maybe we can put a lens with very short focal length on the BS bench to make the beam bigger in the short distance, but it was also hard to say since we scanned the beam in very small step, then on the BS bench the beam moved a lot and out of the lens, then we cannot exactly sure every time we put the lens in a good position to let the beam cross its center, if we did not cross center, the beam will also effected by the lens.
Then we followed our original plan to move the beam out of the dichroic in the horizontal direction, but it stopped by the mount of the mirror, so we tried in the vertical way, when we reached the pitch limitation of turning mirror, the beam was on the top edge of the dichroic, we thought it will be better if we move the beam totally outside the dichroic, so we decided to change a little bit the degree of last mirror on the bench. Before we changed the last mirror, we use the target to set the position of the green beam after the window for easier recover the beam. Then we let the beam go outside and far away from the BS chamber, to check the situation of it.
There were some strange situation that in the near field of the BS chamber, the beam elongation is much serious than the far field. We guessed that maybe because there are some overlap in the near field to make us feel the beam is longer, then in the far field, the other small beams separated from the main one. Actually, we did see that in the far field there were more small beams. But at least, the beam got better in this test.
The other situation I want to mention is that, I am not sure that the beam got better is only because of moving out from dichroic, we also have other changes in this test. Like I showed the simple drawing in the first picture, the circle 1,2,3 is the changed point.(It maybe a little bit hard to understand since I change the 3D situation into a 2D drawing, the red line is after we made the beam path goes up, so the red beam reached the higher position of the turning mirror and pass the dichroic from top of it)
1. The beam position on the window
2. The beam position on the turning mirror
3. The beam did not cross the dichroic
Also I showed the beam shape in two situation and in the near field and far field.
The second picture is I showed yesterday on the meeting for better understanding of the situation.
Images attached to this report
Comments related to this report
We forgot to check the beam under the red line situation after the window and after the turning mirror, also the reflection beam of the window. So maybe we can get rid some of the reasons after these three checking.