DX AO - new and improved

    Just now getting around to writing a post about our AO recalibration from last week, so these posts will be a little out of order.

    With the new adaptive secondary mirror (ASM) installed on the right side (DX side), we're nearly ready for binocular (dual-aperture) operation again (see the LBTO ASM blog for all the ASM re-commissioning details). Our complete adaptive optics system has two main components: the ASM and the wavefront sensor (WFS). The ASM can flex to correct for image distortions injected by the atmosphere, but only if it is provided with a measurement of those distortions. These measurements come from the wavefront sensor, so-called because the technical term for these types of image distortions is "wavefront distortions," or distortions in the light waves that reach the telescope. The WFS is essentially a camera which uses a clever way of imaging a star to sense the distortions to that star's light.

    Over the last month, Guido, Phil, and I have been preparing the wavefront sensor hardware and software for the calibration process, ironing out the kinks that were preventing us from reaching the best performance in the past. The images on the wavefront sensor must have a precise magnification and geometry in order to provide accurate wavefront measurements, so we've been playing with some of the hardware within the WFS unit to improve our images.

    The WFS and ASM must be calibrated together in order to form a working system, which means that because we have a new mirror and a re-aligned WFS, we need a new calibration.

    Jared, Guido, and I arrived at LBT on Thursday to finish up the process. Alas, Jared had to leave his nunchuck collection at home.

    We were greeted by howling winds at the summit - 90mph gusts the night before had shattered one of the windows in the common area (luckily only the outer pane of a two pane window).

    All the wind and fog iced over the dome, unfortunately for the observers. Thankfully, it had all melted off by the end of the next day.

    Inside, the dome was much calmer, and we got to work aligning the calibration unit.

    During the calibration process, we put known shapes (called "modes") on the mirror and measure the resulting image distortions with the wavefront sensor. The more modes you can sense and correct, the better your final image will be. Thankfully the Arcetri team had laid the framework for us, providing a validated set of 500 modes for us to use for calibration.

    Over the course of the next several days, we took closed-dome (off-sky) calibrations with up to 500 modes - the highest number of modes we've ever been able to calibrate on either side!

    While we don't yet know if the 500 mode calibration can be stable on a real star (the seeing was too poor to use it last night), we our new 400 mode calibration performed well on-sky. For the first time, both the right and left sides have 400 modes of correction available, which should make for some excellent high-resolution images this fall.

    quote of the day(s): "That's a great modal plot!" - Alfio, approving of the performance of our 500 mode calibration. We breathed a sigh of relief after that!
    [He was referring to the plot below, created closed-dome using simulated turbulence. The x axis is the mode number (low spatial frequencies on the left, high on the right) and the y axis is the wavefront error in each mode. The red line is the raw wavefront, and the white line is after AO correction]


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    Nov 12, 2013

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