Optique théorique et appliquée
The Homer test bench
The elements of Homer
The Wave Front Sensor (WFS)
The WFS is a Shack-Hartmann type analyzer comprising 7 x 7 sub-pupils. Wide field AO systems provide an analysis of atmospheric turbulence in several directions. So we have to use a WFS for each of the directions that we wish to analyze. For the sake of bulk and adjustment, we decided to use a single wide field WFS for the wave front analysis. Each sub-pupil of the WFS sees the whole field and so all the guide stars. The analysis zone is selected by the
RTC around one or several well defined sources.
The WFS module is therefore made up of:
- an ANDOR iXon DU895LC camera, front illuminated EMCCD type, with 1004 x 1002 8 micron pixels, supplied by ANDOR. It operates at a maximum speed of 24 Hz,
- a matrix of 7 x 7 micro-lenses (figure 3).
Figure 1: Photo of the wide field WFS of HOMER: the camera can be seen in the bakground and in front of it are the micro-lenses in a metal support. The lens in the foreground is used to collimate light on the micro-lenses. The presence of its out-of-field stars does not bias the analysis since they are outside the zone.
The WFS camera
Characteristics of the camera:
- Size of the camera pixels: 8 microns
- Total number of camera pixels: 1004 x 1002 pixels
- Number of useful pixels: 994 x 994 pixels
The WFS camera is an EMCCD type camera with very low noise levels. There are different modes for this camera: a pre-amplification mode with 3 possible levels of gain and a mode with EMCCD gain.
For the pre-amplification gain, in the absence of EMCCD gain, we have the following noise and gain values:
|
Position of the gain
|
Effective gain
|
Reading noise
|
|
G = 1x
|
4.8
|
32.6 e-
|
|
G = 2x
|
2.9
|
33.3 e-
|
|
G = 4x
|
1.26
|
28.4 e-
|
Wide field measurement with the camera
Wave front analysis is based on the
Shack-Hartmann principle. The particularity of this wide field system comes from the fact that a micro-lens sees the whole field of analysis and so several guide stars. We use a single WFS which means the adjustment and calibration procedure for the system is simplified. So we can easily change the configuration of the guide stars as well as the number of stars for the analysis.
Using the RTC, we create digital WFSs in each direction of analysis.
Figure 3: Top: example of an image seen by the wide field WFS. We can distinguish the 7x7 sub-pupils that see the whole field. Bottom: Zoom on a sub-pupil. 5 stars have been illuminated but the analysis is only done on 3 of these stars (analysis zone surrounded by a red dotted square).
The micro-lenses
The matrix of micro-lenses has been designed to have 7 x 7 sub-pupils with a focal length providing Shannon sampled images, at the wavelength of the source. These characteristics were achieved using ONERA technology that exploits the index difference between two molded resins.
Characteristics:
- Number of pixels per sub-pupil: 142
- Size of the sub-pupils: 1136 microns
- Theoretical focal length to obtain the imagettes at Shannon at 635 nm: 28.6 mm
- Real focal length of the micro-lenses: 30 mm
- Size of the pupil on the micro-lenses: 7.9 mm
- Shape of the micro-lenses: square
Figure 3: Photo of the micro-lenses manufactured for HOMER
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