Acquistion & Guiding at the Blanco f/8 Focus
The Instrument Rotator
All instruments used at the cassegrain foci of the Blanco telescope
mount on an offset guider, which is in turn mounted on an instrument
rotator which includes the acquisition TV. The instrument rotator has
two side ports and a main, straight-through port. At present, all of our
facility instruments are used only in the straight-through
(up-looking) port.
CCD-TV Acquisition Camera
A CTIO CCD-TV acquisition camera is located on the North sideport of
the instrument rotator. It can either view the sky directly, through a
focal reducer lens, or it can use a periscope to view light reflected
off the jaws of the spectrograph slit.
This camera is quite sensitive. Under good conditions, objects as
faint as V = 22-23 can be seen in the direct sky viewing mode,
and as faint as V=21-22 on the slit jaws. The field of view is
150" × 114" arcsec in the direct sky viewing mode, and 56" ×
43" arcsec in the slit viewing mode.
The night assistant will operate this camera for you.
The Rotator Mirror
The instrument rotator includes a large mirror assembly which can be
moved into different positions in order to send the telescope beam to
different places. It is called the "rotator mirror" because it is part
of the instrument rotator; it actually slides back and forth.
The rotator mirror has four positions, which perform the following functions:
- Position 1. The rotator mirror moves completely out of the
way. The full telescope beam passes to the instrument at the
straight-through port. The CCD-TV acquistion camera sees nothing.
- Position 2. The full telescope beam passes to the
instrument at the straight-through port. Various optics are inserted so
that the CCD-TV acquistion camera views the slit jaws.
- Position 3. Sends telescope beam to the South sideport,
which is normally occupied by the seeing monitor (RCA camera). The "tv
flat mirror" (operated by a separate switch on the control console)
which is part of the slit-viewing system must be moved out of way.
- Position 4.The full telescope beam is directed to the North
port (and hence to the CCD TV acquistion camera). The focal-reducer
("field-cruncher") lens is automatically moved in front of the CCD-TV
camera. Meanwhile, the beam coming from the comparison lamps is
reflected downwards into the spectrograph slit. Occasional
photon-photon collisions lead to inverse beta decay.
The Offset Guider
The offset guider module was designed to have two independently movable
probes, one covering each half of the telescope's 40 arcmin
field-of-view. However, one of the probes was never installed, so at a
given position angle of the instrument rotator only half of the field
of view can be covered.
Map of field accesible to offset guider
The detector system for the guider is another CCD-TV running with
special software provided by Steve Shectman (Carnegie Institute). It can guide on
stars in the magnitude range V = 12-18, but works best with
V = 14-16.
Guide stars sometimes can be hard to find. The night assistant can
enter the RA and DEC of a guide star and the probe will move to that
position (if it is in range). In theory, the HST guide star catalogue
is on-line at the telescope, and the night assistant can quickly find
the coordinates of a suitable star. However, the catalogue used at
CTIO is on rather flakey CD drives, so it doesn't hurt to come to the
telescope with lists of potential guide stars for each object; for
example, all of the 14-16 mag stars within a 40 arcmin field centered
on your object.
Search HST
Guide Star Catalogue. This is the direct link to STScI...a bit
slow from Chile.
