# When using adaptive optics, what is the shortest timescale of atmospheric changes in refractive index that astronomers have to deal with?

I am interested in the astronomer's need for speed in wavefront correction, especially when measuring in NIR and visible domain.

• What is the fastest abberation coming from atmospheric turbulence in any measurement setting (sun observation during day, night-time observations)? Does any astronomer need correction in the visible spectrum beyond e.g. 100 Hz?
• Are fast systems practically limited by spatial resolution of the sensor?
• Is there any way to get a quantitative overview how widespread adaptive optics is in astronomy today and which setups are currently in use?

• I vaguely remember seeing 1 kHz mentioned for some wavefront correction system, but I don't know if that's a 3 dB point or not. It's not that the major frequency components are that high, but for instantaneous tracking accuracy (low instantaneous error) you'd like a cut-off much higher than the frequencies of what you are tracking. – uhoh Jan 2 at 2:12
• Both wavefront sensors and deformable mirrors can work at around 1kHz. See en.wikipedia.org/wiki/Astronomical_seeing might help to understand the speed of refractive index fluctuations. – WDC Jan 2 at 5:50
• Thanks for these first comments. Are these fast sensors then practically limited in their spatial resolution? And would this limit be the bottleneck in a complete sensor-actuator-system or is the actuator resolution limiting? I added this aspect to the original question. – Damian Jan 2 at 10:04
• I found astronomytechnologytoday.com/2017/07/04/10372 very insightful, where one of the main statements is that the adaptive optics has to match the framerate of the camera - which esentially means that every frame will be corrected. – B--rian Jan 2 at 21:03
• In the wikipedia article on Speckle Imaging I found the following paragraph: "The key to the technique, found by the American astronomer David L. Fried in 1966, was to take very fast images in which case the atmosphere is effectively "frozen" in place.[1] For infrared images, exposure times are on the order of 100 ms, but for the visible region they drop to as little as 10 ms. In images at this time scale, or smaller, the movement of the atmosphere is too sluggish to have an effect; ..." The publication by Fried should then hopefully have more quantitative observations. – Damian Jan 3 at 21:06

I apologise, I'm posting an answer because I can't comment.

What is the fastest abberation coming from atmospheric turbulence in any measurement setting (sun observation during day, night-time observations)?

It depends on the astronomical seeing conditions at the location of the telescope, and also varies quite a lot. It could be anywhere from $$1-100~ms$$.

Does any astronomer need correction in the visible spectrum beyond e.g. 100 Hz?

It depends on the what you're after. In solar physics, for example, observations are generally interested in resolving features on the Sun. You can also find polarimeters that operate beyond $$100~Hz$$, so an Adaptive Optics system that operates in the $$kHz$$ is a fairly standard requirement for most modern Solar telescopes. However, if you're only interested in collecting unresolved spectra (say, a star), you may not really benefit from having a really fast Adaptive Optics system.

Are fast systems practically limited by spatial resolution of the sensor?

I'm not sure what you mean by limited and sensor. Are you talking about the sampling of the wavefront by the wavefront sensor?

Is there any way to get a quantitative overview how widespread adaptive optics is in astronomy today and which setups are currently in use?