A project to upgrade PUEO, the CFHT adaptive optics (AO) system, was firstly proposed in 2002. As part of the upgrade effort, a technology project named FlyEyes was conceived to evaluate and characterize the back-side illuminated CCID-35 detector as suitable a replacement for the array of avalanche photo diode modules (APDs) in the existent curvature wavefront sensor. IAA participated in FlyEyes since Oct 2005, mainly contributed the effort of hardware interface definition, detector controller system software design, CCD characterization, and AO system simulation.
Based on curvature wavefront sensing with a 19-element bimorph deformable mirror (DM) and 19 passively quenched APDs, PEUO has been in service since first light in 1996 and continues to see significant usage. For the purpose of upgrading with a cost-effective means, FlyEyes replaces the extremely expensive APDs with a CCID-35 detector and the system is expected to possess 36 degree of freedom. Figure 1 shows the concept of the replacement while Figure 2 gives the big picture of the project.
The optical fibers utilized to conduct light from wavefront will be removed from the APDs and rerouted to the CCID-35s in the form of a bundle. Figure 3 and figure 4 shows the appearance of Fiber bundle.
Since CCID-35, which is designed in Lincoln Laboratory, MIT, is dedicated to curvature wavefront sensing, the number of photoelectron integrated during half a period (125 μs) of deformable mirror is very small. The operation and readout scheme of CCID35 must be considered deliberately to achieve ultra low readout noise to benefit the low signal level. SDSU3 system was employed as the detector controller and all the critical timing control routines were written in DSP language. A normal readout mode to acquire one CDS frame had been implemented in the software to characterize the performance of whole camera system. Figure 6 is the image of illuminated fibers. Since flat-filed was not available, a non-standard transfer curve method was developed and the result of measurement is shown in the Figure 7. The readout noise varied from a low of 1.8 e- to 2.6 e-, which was slightly higher than the goal of 2.0 e-. R. Dorn achieved < 1.5 e- noise performance with a front-side illuminated version of the CCID-35. A slightly higher noise was expected since the FlyEyes CCID-35 is a back-side illuminated device.
The software for operating in AO mode was written with ultra care to carry out charge movement, accumulate, binning, serial readout, and AD conversion in a very short duration as mentioned above. The overhead of CPU’s execution, even the operation of CPU is thought in the level of assembly, must be reduced as much as possible. Furthermore, the synchronization between membrane mirror and the SDSU3 controller makes the timing even tighter. The developers have put much effort on this part and made things working with the existed components.
The first close-loop, on-sky test was happened in the middle of April, 2007. The data presented in the Figure 8, figure 9, and figure 10 shows a good result. The first shows open and close loop images of an 8.1 mag star at a 1kHz sampling rate. Diffraction rings are quite noticeable in the close loop image. The second are cuts of thru the images. FWHM of the corrected image is ~ .13 arcsec. The last is a composite image of Saturn with the loop locked on Dione.
As the result obtained from the engineering run, AO has been proved helpful for astronomical imaging again. The FleyEyes of CFHT is the first instrumentation brings CCID-35 to the sky. The project demonstrates a potential cheap solution of an AO system, also, constructs the foundation stone for further upgrades.
Fig.1 New element implemented in FlyEyes.
Fig.2 The big picture of FlyEyes.
Fig.3 Fibers during assembly.
Fig.4 End view of fiber bundle.
Fig.5 CCID-35 die.
Fig.6 Fiber spots on CCID-35.
Fig.7 Noise vs drain voltage for 8 amplifier.
Fig.8 Image of a Mag 8.1 star. The left is open loop while the right one is close loop.
Fig.9 The cut of the Figure 8.
Fig.10 A close loop image of Saturn.