Primary ("M1"):

 
F/7.8 secondary ("M2"):

 
F/14.5 secondary: Retired!

F/30 secondary: Retired!

Prime Focus f/2.87

• Plate scale = 18.0 arcsec/mm
• Focus drift with temperature
• Daily procedures
• More details on the PFADC
• Instrument available at f/2.87:
• Mosaic II Imager [1] (through 2011)
• DECam (starting 2012)

The only instrument currently available (2011) at Prime Focus is the 8192x8192 pixel Mosaic II Imager which uses 8 2kx4k SiTe CCDs with 15 micron pixels. Mosaic II will be replaced by DECam starting in 2012.

This focus is equipped with a six-element corrector (PFADC) which corrects the wavefront from the hyperbolic mirror and includes Risley prisms for compensation of dispersion due to atmospheric refraction. The PFADC changes the native f/2.66 f/ratio of the primary to f/2.87.

The PFADC is capable of producing excellent images at all wavelengths from 3500-10000 Angstroms over a field of more than 50 arc-minutes diameter. Under perfect seeing conditions, the FWHM would be less than 0.25 arcsec over the center 30 arc minutes and less than 0.50 arcsec at all wavelengths over the entire field. Images with FWHM of about 0.7 arcsec are seen under best conditions. Note: The Plate Camera is no longer offered at PF.

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Ritchey-Chretien (RC) Focus f/8

• Plate scale = 6.56 arcsec/mm
• Focus drift with temperature
• Collimation
• Daily procedures
• RC Focus with Atmospheric Dispersion Compensator (RCADC)
• Acquisition & Guiding at the f/8 focus
• Instruments available at f/8:
  • Hydra multi-object fiber-fed spectrograph [2]
  • Infrared Side Port Imager (ISPI) [3]

This focus is surrounded by a roughly hemispherical Cassegrain Cage. It is equipped with an instrument rotator and offset guider as standard equipment. (WARNING: The rotator may not be functional with all instruments due to interference between the instrument and the surroundings in the Cassegrain Cage.) User supplied instruments may be attached on either the rotator directly or the guider. The nominal focal plane is at a position 6" (15.24 cm) below the Guider mounting surface. The Cass. Cage is entered by means of a walkway that is normally used when the telescope is at the zenith position. Power (110v, 60Hz) is available from an uninterruptible source.

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Retired Foci

Tip-tilt Focus f/14.5 Retired! (3.56 arcsec/mm)

A tip-tilt secondary is mounted in the Prime Focus Cage that can be flipped in below the PF pedestal (where the Mosaic is mounted). This secondary is supported on three piezo-electric actuators which permit fast tip-tilt compensations for guiding errors and atmospheric wavefront tilt. The focus is produced in the Cassegrain cage, 30" (762 mm) below the mounting surface of the offset guider. A Tip-tilt Box is installed on the offset-guider mounting surface. The Tip-tilt Box contains a fold mirror, a dichroic and a fast guide camera for measuring the visible image centroid of a guide star. The near-IR instrument mounts on the bottom of the Tip-tilt Box, which is a distance of 11" (279.4 mm) to the focal plane.

Chopping Secondary Focus f/30 Retired! (1.33 arcsec/mm)

The chopping secondary has been used exclusively for mid-IR instrumentation in recent years. Its support has now been discontinued and is no longer offered for visitor instrumentation.

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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 [4]

The detector system for the guider is another CCD-TV running with special software provided by Steve Shectman (Carnegie Institute [5]). 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 [6]. This is the direct link to STScI...a bit slow from Chile.