How to use the seeing and r0 meter

This webpage is intended to teach you how to use the r0 and seeing meter. The theory behind the r0 meter is described in a paper by Schoeck et al, titled Atmospheric characterization with the Keck adaptive optics system I: open-loop data. The method was extended to use closed loop data. It works by looking at the deformable mirror voltages when the AO system is in closed loop and deducing the strength of the turbulence. It appears to give very consistent r0 and seeing estimates in NGS AO with relatively bright guide stars; its reliability for LGS AO observing, IF observing or NGS observing using faint guide stars is in the process of being verified.

WARNING:

The seeing estimates are based on the shape of the deformable mirror. If there is an AO problem, such as the focus is running away, or the tip-tilt is not offloading properly, or the LBWFS shows poor images, then the shape on the DM will not be representative of the seeing and the seeing estimates will be wrong! An example of this is the 2.9'' seeing estimate in the plot below, caused by an LBWFS problem.

This could result in a chicken and the egg problem: is it bad seeing that is causing AO problems or are the AO problems causing the bad seeing estimates?


Monitoring the seeing

There is an IDL script that takes seeing measurements every minute and writes them to a file in the nightly/tonight directory called seeing.arT. A web-based tool then displays the data in real time, as can be seen in the figure below.

To start the tool, select Log Seeing from the Menu in MAORI. You will not see anything happening, but the script is running in the background. If suspect that the tool is still not running, then look at the file nightly/tonight/seeing.arT, and check that it is being written to. You also need to reload the webpage to see the plot update.

Using the GUI

The GUI can be brought up from the MAORI pulldown (under Tools). To obtain a seeing estimate, make sure that the loops are closed, press Time=Now, press Estimate Seeing and wait a few seconds. If the DM loop was not closed during the entire interval over which the seeing is being estimated, then the tool will return that the DM loop was open. Try again!

You can also estimate the seeing at any time in the past by entering the UT date and time manually and pressing Estimate Seeing.

Obtaining seeing estimates for an image or directory

You can obtain seeing estimates for the start of a given image or set of images in a given directory by typing in one IDL command in k2aoserver (or k1aoserver):

  1. r0image finds the seeing for every image in the current NIRC2 directory
  2. r0image, filename='/s/sdata901/nirc2eng/2007sep28/n0057.fits' finds the seeing for that particular image
  3. r0image, directory='/s/sdata901/nirc2eng/2007sep28' finds the seeing for every image in that particular directory

Note that these commands have to be run before TRS is cleared; it is recommended that you run them at the end of the night. You should redirect the output to a file when running a whole directory: e.g.,

journal, 'seeing2007sep28.txt'
r0image, directory='/s/sdata901/nirc2eng/2007sep28'
journal

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