Theoretical and Applied Optics
The HOMER test bench
The elements of Homer
the source module | the turbulent module | deformable mirrors
| the principle of the constant envelope system | the imaging camera
| the wave front sensor | the real time computer (RTC)
The turbulent module
In order to test AO systems under realistic conditions, a generator of turbulence is necessary. The turbulent module was manufactured by the
GEPI of the Paris Observatory. It is composed of three motors, each one driving in rotation a screen in transmission which simulates a Kolmogorov type turbulence. It is placed in a collimated space, where the pupil is defined and has a diameter of 4 mm.
The turbulent module allows to simulate a turbulence distributed on three layers with typical equivalent altitudes of 0, 5±3 and 11±3 km, which corresponds on the bench to place screens in the pupil, at 15 mm and 33 mm of the pupil with the possibility of translating them on ±6 mm.
The turbulent module on which two of the three turbulent screens in transmission are installed
Temporal characteristics
The speeds of screens allow to represent similar effects to those of a 8m telescope with an OA working to F
vltspl = 500 Hz. The reduced parameter v
vlt/(D
vlt × F
vltspl) is preserved. Thus the speed of rotation of the screens v
homer on HOMER represents speeds of winds v
vlt seen by a VLT-type telescope . One of the major difficulties of the manufacturing of this module was to find motors whose speeds v
homer are so slow i.e ranging between 19 and 79 min/turn. These low speeds are related with the HOMER design: the pupil is small in turbulent space since it measures 4 mm and the bench generally works at a frequency of 10 Hz because of data processing of the WFS camera. Stepper motors are used coupled with speed-reduction gears. The following table shows minimal speeds of the screens.
| vvlt (m/s) |
5 |
9 |
20 |
| vvlt/(Dvlt × Fvltspl) |
1.25 x 10-3 |
2.25 x 10-3 |
5 x 10-3 |
| vhomer (min/tour) |
79 |
43 |
19 |
Phase screens
In order to carry out tests under favorable conditions, low D/r0 conditions are considered in order to limit the moving of the spot on the zone of analysis of the WFS, i.e a total D/r0 of 7 at 635 nm.
We wish thereafter to have two relative distributions of C
2n : 0,5 - 0,1 - 0,4 and 0,5 - 0,2 - 0,3 respectively for the three turbulent layers. The choice of the technology of these screens was made on phase screens provided by the Observatory of Paris, under development. They consist of a substrate of glass on which is deposited a photosensitive resin. This resin is insolated through a mask in order to generate a variable thickness profile. In addition, we have 2 turbulent screens manufactured by the company SILIOS whose D/r0 are respectively of 4 and 2.7 under our conditions of use. The thickness of these screens is of 1.5 mm what is equivalent to turbulent layers 500m thickness.
