Adaptive optics solution applying image intensifier module

The image intensifier module, Quantum Leap, provides short gating time even at low light conditions. This makes it a perfect solution for adaptive optic applications.

The Large Binocular Telescope (LBT)

The Large Binocular Telescope (LBT) on top of Mount Graham in Arizona/USA. More information to the LBT are provided by the german Max Planck Institute for Astronomy.

The adaptive optics solution enhances the performance of the LBT

The image intensifier module, Quantum Leap, from Stanford Computer Optics is used to enhance the Scintillation Detection And Ranging (SCIDAR) system of the Large Binocular Telescope (LBT) on top of Mount Graham in Arizona/USA. The applied adaptive optics technique requires a imaging detector which provides short gating time at extremely low light conditions. For this combination the Quantum Leap image intensifier module supplies as perfect solution.

Limits of ground based telescopes

The optical resolution of ground based telescopes, like the LBT, is limited. The major limitations are scintillations introduced by atmospheric turbulences. Scintillations are rapid variations in apparent brightness or color of an observed objects. These variations cause a reduction of the image contrast due to the typical very long exposure times applied at the skyobservation with ground based telescopes. There are different approaches to exceed the limits of the observation through the atmosphere, so-called atmospheric seeing. Speckle imaging, aperture synthesis or lucky imaging are efforts to overcome these limitation. A further approach is the adaptive optics.

Adaptiv optics at Large Binocular Telescope

Adaptive optics repeatedly measures the image distortion introduced by the atmosphere and corrected it with changes in the optical detection system. At LBT these corrections were done by an adaptive secondary mirror. The determination of the scintillation is done by evaluating a image of a known object. The motion and evolution of the scintillation requires that the pattern be sampled every 10 to 20 milliseconds with an exposure time of 1 millisecond. The combination of the short exposure time, high repetition rates and extreme low light conditions is a delicate problem. Image intensifier in general meet both requirements of signal gain and short gating time. However, operation of image intensifiers at high signal gain involve a increased ion feedback which causes additional artificial signal.

Reduction of ion feedback

When the accelerated electron avalanche gets bigger and bigger on its way though the MCP-channel it might happened that atoms from the rest gas or adsorbed atoms on the channel surface get ionized. These ions then are accelerated by the MCP bias voltage towards the MCP input side. The so-called ion feedback phenomenon causes a second electron avalanche if the ion can achieve enough energy till it hit the wall of the MCP channel.

The "false" pulses are disturbing the measurement and adding an "ionic"-noise to the image. To prevent the ion feedback contribution to the image the dual stage MCP has the so called "Chevron" configuration. This ensures with a maximum angle between the channels of the MCPs that free ions can not gain enough energy to overcome the ionization energy. However, dual stage MCP require increased bias voltage and hence the power supply unit is supposed to meet these technical requirements.

Image intensifier improve the adaptive optics system

McKenna, et al. at LBT used an image intensifier module, Quantum Leap, with a dual-stage Micro Channel Plate (MCP). This configuration provides short gating times down to 200 ps, with a signal gain of up to 106 at and an excellent signal to noise ratio. Furthermore, it can be operated at repetition rates of up to 200kHz continuous and 3.3MHz in burst mode. Therefore the image intensifier module is a solution without a fault-prone mechanical shutter. Quantum Leap, from Stanford Computer Optics, solves the short exposure time at low light conditions contradiction for adaptive optics application in astronomy.

 


Title:       The LBT Facility SCIDAR: Recent Results
Author:    Daniel L. McKenna, et al.
Institute:  University of Arizona, Steward Observatory, Tucson, United States