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1/ Last Friday @ESO released this awesome image of the surface of Betelgeuse, taken with the SPHERE instrument. You may have read that SPHERE uses a deformable mirror to correct atmospheric turbulence. But how do we know how to deform the mirror? Keep reading!
2/ This technique, called adaptive optics, uses a device called wavefront sensor, which measures whether the incoming light-rays are parallel to each other or not. The most common design is called Shack-Hartmann, and it consists of an array of tiny lenses.
3/ When you point this device to an astronomical target, each microlens creates a small image of the target. If there’s no turbulence, the wavefront is flat, meaning that the incoming light-rays are parallel to each other, and these images will appear at the center of the lenses.
4/ But our atmosphere does create turbulence, and light-rays don’t arrive to the sensor parallel to each other. The image created by each microlens will be offset from the center, and by measuring these offsets we can reconstruct the shape of the incoming wavefront.
5/ We can then use this information to deform a small flexible mirror inside SPHERE and cancel out the turbulence. Turbulence varies very rapidly: the shape of the wavefront changes 100s or even 1000s of times per second! But SPHERE can deal with that.
6/ Besides adaptive optics, S-H sensors are also used in *active* optics. Large mirrors are so heavy that they bend under their own weight, degrading image quality. Active optics deforms the primary mirror to correct for this, as shown in this video I recorded at Paranal:
7/ The names can be confusing, so remember:
- active optics: very slow corrections of the primary mirror of a telescope to counteract mechanical (& thermal) stress
- adaptive optics: very fast corrections of a small mirror to counteract atmospheric turbulence
- active optics: very slow corrections of the primary mirror of a telescope to counteract mechanical (& thermal) stress
- adaptive optics: very fast corrections of a small mirror to counteract atmospheric turbulence
8/ If you’re an amateur astronomer, the name Hartmann may sound familiar, as you might have used a Hartmann mask to focus your telescope. The German astronomer Johannes Franz Hartmann used this method to test optical instruments a century ago. (Img source https://www.cloudynights.com/topic/610955-the-original-hartmann-mask/)
9/ The idea goes all the way back to the early XVII century, when the physicist and astronomer Christoph Scheiner used a similar mask with holes to study the focusing behaviour of lenses and the human eye (Fig. from Biedermann 2002). http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.576.5870&rep=rep1&type=pdf
10/ Then in the sixties Roland Shack improved upon Hartmann’s design by replacing the holes with lenses, leading to the Shack-Hartmann wavefront sensors we use today.
11/ Fun fact: Shack-Hartmann sensors have spinoff applications in ophthalmology! If you illuminate the retina and analyse the emerging light rays with a S-H sensor, you can measure and correct ocular aberrations, thus obtaining sharp images of individual retinal cells.
12/ I hope this thread shows you a glimpse into the amazing technology that enables astronomical discoveries, and how that technology is also present in other areas of our daily lives.
