CCD StageThe stage carries the chip in its dewar, the LN2 reservoir, and some of the readout electronics. Its purpose is to permit the user to move the chip with respect to the fixed spectrum so as to allow selection of the desired central wavelength (motion in x), and to move the chip perpendicular to the dispersion so as to place the spectrum on the cosmetically cleanest part of the chip (motion in y). The standard setup for the dewar puts the dispersion along the CCD's x-axis (with wavelength increasing with increasing column number). The grisms cannot be tilted, so to change the central wavelength, the CCD is moved in the x direction. The stage's x motion is controlled through the Motor Control GUI, provided the stage's E-W locking screws are not tightened (see below). A numerical increase in the x setting moves a given wavelength to the left on the CCD, toward lower numbered columns, decreasing the central wavelength. A change of 5.4 units moves the spectrum by one pixel for the current detector (27 micron pixels; 0.1875 pixels/unit). A move in the y direction is perpendicular to the dispersion. Most users agree on which part of a given chip is cosmetically best for the spectrum, so the y setting is rarely changed. Increasing y moves the spectrum to lower row numbers. The y stage can only be moved manually. To do so, unlock the clamp on the west side of the stage, then move the stage with the knurled shaft near the clamp while reading the position on the micrometer dial. Do not loosen the small knurled screw on the micrometer itself, or the zero point for the micrometer may be lost. The position is read as a.bb, where a comes from the very small dial in the micrometer and bb is read from the larger dial. The scale is mm per full turn of the big dial, or 0.92 pixels per dial unit (for 27 micron pixels). There may be backlash in the screw, but there should not be any in the spring loaded micrometer readout. Be sure to relock the stage firmly. As described under flexure, there are two locking screws for the CCD stage which somewhat reduce, but do not eliminate, flexure in the stage assembly. These are two long, light grey thumbscrews which press against the stage from the east and west sides. If you decide to use them, the cost is that you cannot conveniently change stage settings during the night from the computer. There are two important cautions regarding their use; first, set up your central wavelength carefully before locking the stage in place, and once it's locked set the CCD Stage in the Motor Control GUI to Disabled so you won't inadvertently ask the computer to move the locked stage; and second, be certain to loosen the screws all the way when you're through with them so the next observer won't encounter them accidentally during an x-stage move.
Support Astronomers (firstname.lastname@example.org) Last modified: Tue Jul 17 18:37:09 PDT 2012