CCD Stage
The stage carries the chip in its dewar, the LN
2 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.