Available Foci

	Contents:
	f/2.87 Prime Focus
	f/8 Ritchey-Chretien (RC) Focus
	RETIRED Foci

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.

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.

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.