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Problems you may encounter during data collection

Note well: this list includes some of the problems you may encounter during data collection, but it does not claim to be exhaustive! In case of doubt, call the CHESS operator, who will contact MacCHESS personnel if necessary. If you find errors, omissions, or confusing instructions in this guide, please inform Marian Szebenyi.

  • Beam problems
    • No counts through collimator:
      • get CHESS operator.
    • Low counts through collimator:
      • do table alignment (if counts come back up - continue with experiment, if not get CHESS operator to check X-ray optics.
  • Cryosystem problems
    • Can't get good crystal freeze:
      • check whether liquid nitrogen dewar is empty;
      • check positioning of cooling nozzle - adjust if necessary;
      • get help from MacCHESS;
    • Crystal icing up:
      • adjust the "shield" gas flow (regulator for red hose up at the wall) carefully between 5 and 10 L/min. Best be done with the overhead lights switched off, just the LED lighting on, watching the gas flow against a dark background;
      • brush frost off crystal periodically;
      • come back in the winter when the humidity is lower!?
  • CCD Distance problems
    • Digital distance readout for the CCD is wrong:
      • ADX loads the current distance into all the "distance" fields when it starts up. When the distance is changed, use the "Get Current" option for distance in the Manual Control window to update the distance fields.
      • To get the current distance, hit Enter in a blank distance field in the Snapshot or Run(s) window. When a snap or run starts, a warning will be posted if the requested position does not agree with the current one.
  • Diffraction problems
    • Completely blank image - could be due to:
      • no beam - wait for new fill, or get CHESS operator to fix problem;
      • safety shield in front of CCD did not go up - press "garage door open" button on the control panel, get help from MacCHESS if shield fails completely;
      • software problem:
        • has adxv loaded an image? if it has, the image file name is given in the title bar, check whether the image file is the right size;
        • check display parameters, particularly upper and lower limits for grayscale;
        • for ADSC software, check that the "ccd_xform_new_api" process is running;
        • if not, stop software with "EXIT" and "Stop All";
        • restart software by clicking "Collect Data" icon.
      • detector problem:
        • check for error messages on PC;
        • get help from MacCHESS.
    • Beam stop shadow visible but no diffraction spots - could be:
      • diffraction is very weak - try a longer exposure;
      • crystal not in the beam - try rotating it 90 degrees and taking another shot. If you see diffraction then, try to center the crystal better. If still nothing, or centering looks good but crystal only diffracts at some positions, could be a problem with video camera cross-hairs or spindle height - get MacCHESS help to fix;
      • crystal is no good (sorry!) - try another.
    • Oscillation image looks like a still:
      • check the oscillation range that you entered;
      • get CHESS operator to check spindle motor.
    • Shadows on image:
      • check for cooling nozzle too close;
      • other equipment blocking part of the X-rays.
    • Areas of high background on image:
      • check position of beam stop, adjust if necessary;
      • beam stop is too small, move it forward or get operator to put on a bigger one;
      • have CHESS operator add shielding.
    • Strong discrete spots or rings on image - could be due to:
      • ice or salt in or around crystal - change crystal;
      • if problem persists with a new crystal, is probably due to one of the following:
        • crooked collimator ion chamber - straighten it;
        • particles of dust, etc. in collimator - blow out collimator with air;
        • switch to another collimator.

Marian Szebenyi