The Tip-Tilt sensor is mounted on the IMB X-Y stage behind a focal
reducer that provides an image scale of 0.6''/pixel (see Fig.
23). The stage has a travel of 
31.3 mm in the f/18
(5''/mm) telescope beam, so an offset guide star is needed within a
square field of 
extent centered on the object. The
Tip-Tilt CCD has 
pixels, giving a full field of 
.
Operation of the Tip-Tilt system is fully described in the Tip-Tilt manual. Factors relevant to its operation with CASPIR are summarised below.
Give the Tip-Tilt system access to the telescope by typing:
ENLIST TT CONFIGURE EFFECTIVE_WAVELENGTH 2000 CONFIGURE GUIDER_WAVELENGTH 700
into the telescope console terminal.
The Tip-Tilt software is run on MISTY under X Windows from any user account (your own or ``user23''). Start the system by typing:
tiptilt &
After the initialization sequence completes, click on the ``operate'' button to select acquire mode. Correct and acquire integration times should be set to something less than 100 mS. When a suitable guide star is in the image display, select it by pressing ``Shift'' and clicking on the star with the left mouse button. The correct box is moved to this object, and tip-tilt correction is commenced by clicking on the ``correct'' button.
After the telescope and CASPIR have been properly configured, type:
CALIBRATE STAGE
into the telescope console terminal and follow the instructions. This procedure defines the IMB X-Y stage zero point and Tip-Tilt correct subframe to the telescope system.
Now enable use of the Tip-Tilt system and IMB X-Y stage motion from CASPIR DO files and in NOD mode by typing:
CASPIR/TIPTILT/STAGE_OFFSET
into the CASPIR command terminal.
Normally, the CASPIR array and the Tip-Tilt CCD will remain cofocal and
no adjustment of the Tip-Tilt focus should be required. If
focus adjustment is required, focus the telescope on the infrared
array, then adjust the focus of the optical image in the Tip-Tilt
display using the IMB/FOCUS command. Use the up and down arrows
to change the focus of the acquistion unit focal reducer, and type Q
to quit. Do not move the telescope focus to focus the optical image.
The Tip-Tilt sensor can be used in acquire mode for normal object
acquisition. With the instrument rotator at a position angle of
180 
, North is up and East to the right on the Tip-Tilt display,
as shown schematically in Figure 1
To guide on an offset guide star, it is first necessary to offset the IMB X-Y stage to the location of a suitable star. The small size of the Tip-Tilt CCD field of view relative to the offset guide field makes it essential to pre-select suitable reference stars from charts of your fields. Procedures for doing this is described in §3.14. The Tip-Tilt systems allows the observer to specify reference stars to the telescope system by their coordinates, e.g.:
TIPTILT/COORD 12 34 46.5 -15 34 23 J2000
or from a telescope coordinate file, e.g., :
TIPTILT BS2015
or by their offsets in arcseconds on the sky, e.g.:
TIPTILT/OFFSET/SCALE 12.0 -3.4
or by typing:
TIPTILT/TRACK
if the object at the tracking coordinate (the infrared object) can be used as the reference star, or by typing:
TIPTILT/HERE
if there is a suitable reference star in the Tip-Tilt acquire frame and you wish to drive the reference star to the correct subframe, or
TIPTILT/FIND
if there is a suitable reference star in the Tip-Tilt acquire frame and you wish to move the correct subframe to the pixel location of the reference star.
When the Tip-Tilt system is correcting, the position of an object on
the CASPIR array can be adjusted by translating the IMB X-Y stage
using telescope TIPTILT/OFFSET command so that the object is
dragged, by the Tip-Tilt system, to the desired infrared position.
Measure the desired offset in arcseconds from pixel locations on the
CASPIR array and apply this correction to the specified reference star
offsets.
The Tip-Tilt system can also be controlled through the DO file used to acquire CASPIR data. The DO file commands controlling Tip-Tilt are:
tiptilt - Turns on Tip-Tilt operation. notiptilt - Turns off Tip-Tilt operation. stage_offset - Enables IMB X-Y stage motion. nostage_offset - Disables IMB X-Y stage motion. track_coord - Track on a new object coordinate (i.e., RA, Dec.). guide_coord - Defines a new guide star coordinate (i.e., RA, Dec.). gra_offset - Defines guide star RA offset in arcsec on sky. gdec_offset - Defines guide star Dec. offset in arcsec on sky. ttx - X coordinate of Tip-Tilt correct subframe center. tty - Y coordinate of Tip-Tilt correct subframe center. ttdx - X size of Tip-Tilt correct subframe. ttdy - Y size of Tip-Tilt correct subframe. acqx - X coordinate of Tip-Tilt acquire subframe center. acqy - Y coordinate of Tip-Tilt acquire subframe center. acqdx - X size of Tip-Tilt acquire subframe. acqdy - Y size of Tip-Tilt acquire subframe. tt_mode=correct - Operates Tip-Tilt in correct mode. tt_mode=guide - Operates Tip-Tilt in guide mode. tt_mode=acquire - Operates Tip-Tilt in acquire mode. tt_mode=recalibrate - Operates Tip-Tilt in recalibrate mode. tt_atime - Sets acquire mode integration time (ms). tt_gtime - Sets guide mode integration time (ms). tt_ctime - Sets correct mode integration time (ms). tt_find - Enables Auto-Acquire mode. nott_find - Disables Auto-Acquire mode. tt_error - Do not abort on Tip-Tilt error. nott_error - Abort DO file if Tip-Tilt errors encountered.