Short instructions for normal use (obsolete?)

This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.

Active optics System

Short instructions for normal use

 J.Baldwin, 15 October 1998



Corrections should normally be ON for all foci (prime, f/8).
The present table causes the primary mirror to be bent to correct for a systematic astigmatism effect which is due to problems with the primary mirror support system.


Should normally be OFF whenever f/14 is in use.


3.1 Before start of night,

  • If the IMAN software is running, stop it by typing quit<return>
  • Reboot the IMAN PC. It is best to turn its power off and on. If there is an error message, reboot it again.
  • Once the IMAN PC is rebooted with no error messages, start the NFS link by typing: gonfs<return>. Answer "i" (ignore) to the error message about Authentication Failure.
  • Go to the cass cage and check that the power switch for the iman camera's electronics box (which is mounted on the side of the offset guider module) is set to "ON (NO TEC)".This setting disables the ThermoElectric Cooler, which is what is wanted on all but the warmest of nights. The camera power switch should be left in this setting at all times.. The power can then be switched on and off remotely from the console room.

3.2 Just before using IMAN

  • Turn the camera ON using the TCP menu commands

  • Only after the camera is ON, start the IMAN software in the IMAN PC by typing

    • iman<return>
    • Software should come up in IDLE mode. If not, type s<return>
    • On startup, the gain is automatically set to 2 and the offset set to the corresponding value of 231. These are the nominal values which should be used for IMAN measurements. (Modified slightly 18Dec00 r.c.and b.g.)


3.3 Measuring a star.

  • Move rotator mirror to position 1or 2
  • Move guide probe to optical axis
  • Select FLAT MIRROR OUT and then PELLICLE IN (and vice versa if you are going to use the flat mirror). Now 10% of the starlight will go to the guider TV, and 90% to IMAN. The flat mirror in the guider optics will be left out of the beam for all remaining steps.
  • Select SMALL APERTURE from the IMAGE ANALYZER menu. A 13 arcsec dia. aperture will be put into the focal plane ahead of the IMAN optics, but will not affect the field of view of the guider camera.
  • Put the Shec Guider cursor at the following position: x=257, y=106 if you are using the pellicle, and x=200, y=153 if you are using the flat mirror (LAST UPDATED: 3 Feb 2001). Move the telescope to center a 11 mag star (10.5 to 11.5 should work) on the guide probe cursor. Guide the telescope either with the autoguider to keep the star at this position.
  • Ensure that the star image on the guider is in focus. Adjust the guide probe focus if it is not.
  • If you are measuring the TWEAK correction in a new part of the sky, use the RESET option in the TWEAK ADJUST command (from the PRIMARY MIRROR CONTROL menu).
  • Select STAR SEQUENCE (enter * on the keyboard). This will cause a series of three 30 second exposures to be taken and analyzed. A blue window will open on the Telescope Operator's Sun and the results will start to appear.
  • Use the CTRL-F2 keys to get rid of the blue windows which show IMAN results and messages.
  • Select OBSERVE POSITION. This will turn off camera power, move the pellicle out of the way, and move the flat mirror to the GDR position. The guider is now ready for normal use.
  • Move the guide probe out of the way of the instrument.
  • On the IMAN PC's keyboard, type quit<return> to stop the IMAN software.
  • To start another measurement later in the night, return to step 3.02.



  • At the end of the STAR SEQUENCE, a red box will appear on the TCP screen giving you the options of tilting the secondary, bending the primary, or quitting.
  • If you elect to tilt the secondary or bend the primary, you will then be asked whether you want to accept the recommendations made by the IMAN program. These are the "Y" and "N" entries in the Tweak? line in the IMAN output. Normally, you should not change anything unless a change is recommended ("Y").
  • If at some later time you wish to tilt the secondary to correct for the coma, measured here, use either LAST LOG ENTRY or OLD LOG ENTRY from the TILT SECONDARY or PRIMARY MIRROR CONTROL menus, as appropriate.
  • If you apply a tweak to the primary mirror, you must then make sure that the ENABLE option in the TWEAK ADJUST command has been selected (the TCS Status Display will show a flashing "TWEAK ON" message).
  • If the tweak correction was larger than 2 microns, it is worth taking another IMAN measurement and repeating the tweak if necessary.



  • Some common IMAN error messages:

    • ERROR -- STAR IS TOO BRIGHT. Too few spots have been found and more than 1000 pixels (average of about 5 per spot) have signal levels of 4095 (CCD saturation). Find a fainter star.
    • ERROR -- STAR TOO FAINT. Too few spots have been found and less than 500 pixels are more than 150 ADU above the background. Find a brighter star.
    • ERROR -- BACKGROUND TOO BRIGHT? Too few spots have been found and the average ADU/pixel is more than half the saturation value. Find a darker sky.
    • ERROR -- FOUND TOO FEW SPOTS. Too few spots have been found and none of the 3 previous errors have been detected. take another star sequence and watch the IMAN image display monitor as the 30 second exposures are read out. Is the star way off center? Does the image turn to noise half-way through the picture?
    • ...and see "IMAN Image Analyzer" Sect. 4.7 for further exciting possible disasters.
  • Image displayed on IMAN monitor is full of garbage. A noise spike has gotten into the data stream. The reduction program probably will skip over the bad image and not include it in the average values for the aberrations. If so, and if you get two good images out of a star sequence of three images, that probably is good enough. If not, take another star sequence.
  • Images continue to be full of garbage. Stop the program on the Iman PC. Turn camera power off, then back on. Restart the IMAN PC starting at step 3.01 above.
  • During star sequence, the message in the blue box on the TCP screen freezes. You should stop and restart the TCP program... it has lost communication with the Sun. The message in the blue box initially should read "Starting file deletion, waiting for first image", but should then quite quickly change to say "image size = 0", then larger sizes as the first 30 second exposure starts to be transfered, etc. Then the box should show the reduction results as they are built up. To restart the TCP program:

    • go to the highest-level TCP menu and select "Exit TCP". The window with the TCP menu should disappear.
    • Then open the console window and type "tcp4m <return>". The window with the menu should reopen.
    • Now try running IMAN again.
    • Aperture wheel or flat mirror do not complete their motions. Try the "iman stop" command in the TCP command window, then try again to move the device. You must turn the camera power back on after using "iman stop".
    • Timeout messages while moving a device. Probably a fatal hardware failure. It's time to call for help.