Acquisition & Guiding at the f/8 focus


The information here is old, it is better to review the documentation for the instruments themselves for information on guiding.


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.