Turn up the TV integration time to confirm that there really are no stars visible, and that the telescope is not simply looking at a relatively blank patch of sky. If you see even faint stars (you can make sure they're stars by moving the telescope a few arcseconds between TV integrations) the problem may have to do with pointing the telescope--see `Cannot find targets', below.
If, however, still no stars are visible, something may be blocking the lightpath..i.lightpath, blocking of; Are the mirror cover, rainscreen, and dome open? Is the telescope looking out the slit (at the 1-meter, check that the `autodome' switch at the bottom of the Telco rack is on and that the `dome in way' warning light is not lit). Have clouds or, heaven forbid, fog come in?
Check the camera selection switch to make sure you're using the camera you think you're using. If it's the TUB camera, make sure that you're in mirror position #3 at the 3-meter, or that the TUB diagonal switch is in the `in' position at the 1-meter. Check that the neutral density filter (red switch on the TV camera control panel at the 1-meter) is in the `out' position.
If you're using the boresite camera, make sure that the cover has been taken off the dichroic beamsplitter. Check that the TUB diagonal is out: mirror position #4 at the 3-meter, TUB diagonal switch is in the `out' position at the 1-meter.
Finally, the telescope or the TV cameras may be so out of focus that images are just not visible. Check that the telescope is at, or near, its nominal focus for LIRC-II (060.00 at the 3-meter, 295 at the 1-meter). At the 1-meter, if using the TUB camera, check that the folding flats, located in the TUB, are near their nominal position: 7.6 on the metric scale (see the Nickel Telescope User's Manual for an explanation of the folding flats and their adjustment).
Check that the filter and lens wheels are in their correct positions, and that none is in its `dark' position. Try resetting them to their `home' positions to reinitialize the encoders, then set them to the appropriate positions for your observation. Check that the dewar window cover has been removed and that the window is free from condensation. Make sure that the TUB diagonal mirror is in the proper position (at the 3-meter the mirror should be in position #3 for the TUB camera, and position #4 for the boresite camera; at the 1-meter, it should be `in' for use with the TUB camera, and `out' for use with the boresite camera). If using the TUB camera, check that the beamsplitter cover has been removed.
If using the TUB camera at the 1-meter, did you remember to offset the telescope? Are you sure that the offsets are accurate and that you moved the telescope in the correct sense? Is your integration time, or number of coadds, sufficient to reveal faint sources on the array? Look at the K-prime sky. Does the flux increase with integration time at about the expected rate (levels should reach the linearity threshold of about 20,000 DN in about 5 to 30 seconds in the intermediate field of view, depending on temperature and sky conditions)? Try moving to a very bright star, offset the telescope if using the TUB camera, and take an exposure.
A failure of the IR array itself would most likely be accompanied by obviously abnormal readouts. If this is observed, or, if after trying all of the above, you still see no star on the array, call a support scientist or technician for help.
If stars are visible on the TV, but you are unable to locate your target or recognize its field, the telescope may be incorrctly pointed. This can be due to poorly calibrated encoders or operator error. Before checking the encoder calibration, double check your coordinates and epoch. If you're at the 1-meter, are you using Setel, the precession and pointing correction program? Are you sure that your target is not too faint or diffuse for the TV camera you are using (the boresite camera has lower sensitivity than the TUB camera). Are you sure that your finder chart corresponds to the TV? Check the directions on the TV camera by finding a star and moving the telescope slightly--remember that moving the telescope one direction causes the star to move in the opposite sense.
At the 1-meter, the telescope's coordinates may need resetting. This is especially true on the first night of a run following an instrument change, or if the last TV camera to be used was other than the one you're using. The TUB and boresite cameras look at slightly different positions in the sky, and require that the coordinate readouts be appropriately calibrated for their respective fields. Changing the TV Camera describes two methods for resetting the coordinates.
In the rare worst case, the telescope's position encoders may be completely lost, due to a power glitch or a Telco failure, and you will be unable even to find a bright star in the finder telescope. At the 1-meter, you may reset the telescope from first principles by following the instructions posted on the panel in the left-hand rack, labeled `Nickel Telescope Fiducial'.
Exit the data-taker (control-y), and restart by typing `lirc2'. Watch the window during program initialization. If you see the error message `NO RESPONSE FROM CCD', exit the program at that point (control-c). Try cycling the power to the array controller. At the 1-meter, the controller is located on the telescope, above and to the west of the dewar. Its power switch is on the front panel. Turn it off, wait a few moments, and turn it back on. Try running the data-taker again. If the error message recurs, call a support scientist or technician.
At any wavelength, baseline counts can give the illusion of high background. Are you taking the baseline into account? Is the baseline set too high? Take a zero-second dark to confirm that the baseline is set near its nomnal level of about 2000 DN.
Verify that the detector is operating properly by testing the thermal background. If the test reveals significantly higher than average dark current, check that the array temperature is near its nominal value of -195; the temperature is displayed on the array controller, mounted on the telescope.
Note that K or K-prime skies can, at times, get high enough that, when using the wide field of view, saturation occurs in less than the minimum exposure time, precluding the use of that plate scale with those bandpasses.
Check for clouds. Check for vignetting by the dome (at the 1-meter, the telescope is partially vignetted whenever it is within six degrees of zenith-- look for a zenith warning light on Telco).
Check that the dewar window is free from condensation. Is the captive air cover installed and its power supply plugged in? Condensation typically begins forming at the center of the window. It may appear as a uniform or blotchy circle of reduced sensitivity. If condensation is present, do not attempt to dry the window--call a staff member for help.
Remember that the large sky contribution in the near infrared can make targets seem faint, or even invisible, on single frames before sky subtraction. A quick sky subtraction will allow a more accurate assessment of the camera's performance.
Recheck the focus. Both the size and symmetry of star images can be measured using the stellar statistics routines under `itv' in the data-takers image control window.
Elongated star images are usually indicative of inaccurate tracking or guiding. If you are using the autoguider, be sure that you have entered the appropriate guide matrix for the TV camera you are using, and that the autoguider's settings are appropriately adjusted (see the Nickel Telescope User's Manual for a thorough introduction to autoguiding).
Whether or not you are autoguiding, a poor track rate can cause images to appear elongated. Check that the manually entered rates (thumbwheels on the Telco panel) are at there nominal settings of -0.04 in right ascension, 0.0 in declintaion. If wind is a factor, use the windscreen and avoid working into the wind, if possible. Poor collimation of the IR camera can cause distorted images. Check the collimation.