Operating the NGWFC

This webpage is designed to help the Observing Assistants and Support Astronomers with the transition to using the NGWFC system. The NGWFC is now implemented on both telescopes, and they both have identical software and hardware (except that LGS is only implemented on Keck II). Marcos will try to update it as often as he is able to: unfortunately, the AO system is evolving rapidly, so some of the GUIs might look a little differently to what is displayed on this webpage. This page contains the information needed to run the system in NGS AO. The information here is also valid for LGS AO, but I am ignoring other aspects of the LGS AO system, such as the LBWFS, since they have not changed from the user perspective. Please feel free to send me suggestions on how this documentation might be improved or to edit the webpage yourself. Happy stargazing!

The following AO tools should be up:

  • MAORI AO Control GUI
  • Wavefront sensor intensity display
  • Fault detector running in an iconified IDL session
  • Reconstructor running in an iconified IDL session
  • AO acquisition Tool

All of these tools can be brought up from MAORI.
There is also a new AO acquisition tool which can be used for both NGS and LGS AO.

What's new?

Below is a summary of the latest changes.

The new AO acquisition tool can be used for both NGS and LGS AO. It replaces both the old NGS AO acquisition tool and LGS AO acquisition tool and has a few additional nice features.

There is a new tool to measure r0. This 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; its reliability for LGS AO and IF observing are in the process of being verified.

Failure modes

There are two failure modes of the wavefront controller that you are likely to experience. One of them will autorecover, and the other can be recovered from at the push of a button. If any of these failures does occur, please enter the failure number or name in the nightlog (do NOT simply report it as a WFC CRASH!).

Failure Mode 1: The symptoms of the first type are that you have a fault state in either the TT or DM LOOP STATE. This is illustrated in the figure below.



This fault will automatically recover, followed by an audible alarm saying "The AO loops have faulted" and a pop-up will inform you that the TT or DM loop went into a faulted state. In that case, reclose the loops using MAORI and continue observing.

If this fails to recover, you should hit INITIALIZE WFC from the Menu pulldown on the MAORI GUI. An IDL session will pop-up temporarily and inform you of progress in re-initializing the WFC. If this doesn't work or you have another kind of problem, we have to reboot the WFC. If this problem occurs three times in a row, then we have to reboot the WFC.

Failure Mode 3 ("Freeze"): The third type of failure mode is when everything appears normal but telemetry is not updating. Note that there are other times when the WFC intensity display might not update, such as when a background is being taken or the frame rate is being changed. However, if telemetry is not updating for 20s when you are not doing anything, then it is likely we have a freeze problem. Usually, the fault detector will detect the freeze and warn you that the WFC may have crashed. To verify that indeed, there is a freeze, do the following:

  1. Check that the intensity display and the tip-tilt graphs are not updating.
  2. In any IDL session, type checkwfc. This will run a test to determine whether the system is OK. If it reports that it is OK, try to INITIALIZE WFC from MAORI. If not, you will have to reboot. Record the failure mode in the nightlog.

To reboot the WFC, select REBOOT WFC from the Menu pulldown on the MAORI GUI. Watch the IDL session to determine when the system is back up and ready to go. Then reacquire the star by clicking SETUP BENCH and then ACQUIRE GUIDE STAR on the AO Acquisition GUI.

Changing the frame rate

You can change the frame rate using the MAORI GUI. Enter the new frame rate (between 30 and 2000 Hz) and then press ENTER on your keyboard. An iconified IDL window will come up, change the frame rate and then disappear. You have to take a new background by clicking Offset 30'' E in the intensity display GUI.

You can also change the frame rate using the WFS Configuration GUI. This allows more flexibility in that the binning and the attenuation can also be changed.

  1. Select the binning.
  2. Enter the frame rate.
  3. Select the attenuation.
  4. Press Calculate.
  5. Press Set.
  6. Take a new background by clicking Offset 30'' E on the intensity display GUI.

There is usually no need to change the binning mode; the best binning mode is 2x2 for every observation done so far. As we gain more experience with the system, we will learn which mode to use for a given observation.

The frame rate needs to be changed if, for example, clouds are causing the counts on the wavefront sensor to drop. In general, the frame rates selected by the auto AO setting script will be very close to optimal.

The attenuation, also known as the camera gain, is used to reduce the gain of the camera when observing very bright objects.

You may dismiss the WFS Configuration GUI and bring it up as needed. The easiest way to bring it up is from the Menu pulldown in the MAORI AO Control GUI.



In the old system, there used to be a fault detector running as part of the AO acquisition widget. This fault detector is now run in a stand-alone IDL session. There should always be a fault detector running. If you are not sure that you have one up, you can start one by selecting START FAULT DETECTOR from the MAORI Menu. This will kill any existing fault detector and start another one.


Reconstructor

The reconstructor does not have a GUI any more. The functionality that was incorporated in the GUI (e.g., manually triggering a new reconstructor) is incorporated in the MAORI GUI. There is still an IDL session running. You should keep it iconified. The reconstructor can be started or restarted by selecting START RECONSTRUCTOR from the MAORI Menu. This will kill any existing reconstructor and start another one.

Wavefront sensor intensity display

There are three things to be aware about the intensity display.

The first is that, even in the absence of light, the intensity does not read zero. This is because the background is subtracted from each pixel and the result is thresholded to zero. Hence, all values less than zero are set to zero and all positive values remain positive. The sum of all the pixel values is hence always a positive number! We might eventually remove this feature, even though it makes the wavefront sensor work better, in order to make operations easier. The best way to tell whether there is light on the wavefront sensor is to look for the hexagonal intensity pattern on the display. In addition, if there is no light on the wavefront sensor the mean centroid will always be close to zero even when the loop is open.

The second difference is that the units of intensity are different. The new CCD has four different camera gains (the old system had two), which allows us to use very bright stars without using neutral density filters in fron of the wavefront sensor. Any old rules of thumb that you used to understand the intensity display are probably no longer valid.

Finally, the pixel saturation occurs at a much higher intensity. The maximum pixel intensity is 16 384, so we are in danger of saturating when the number of counts per subaperture exceeds about 40 000.

IDL

The NGWFC has transitioned from IDL5 to IDL6. This means that if you want to bring up an IDL session, you need to do the following:
  1. Bring up a k1aoserver window as k1obsao.
  2. Type idl.
If any of the IDL sessions crash for any reason, please do the following:
  1. Start up the tool again from the pulldown menu.
  2. Copy the complete error message and email it to . If possible, please include a detailed description of what happened that might have caused the tool to crash.



For more information, please contact
.