The Blanco 4m Telescope


The Blanco 4m telescope was commissioned in 1974. It is a near twin of the Mayall 4m telescope on Kitt Peak. With the advent of the SOAR 4.2 m telescope, under construction on Cerro Pachón, it is the policy of the Observatory to develop the two telescopes to complement each other: wide field applications on the Blanco 4m and high resolution applications, which exploit the better image quality available over a more restricted field, on SOAR.

Basic Optical Parameters:

F/2/7 primary:
Radius of curvature:
21311.6 mm concave
Conic constant:
Mirror diameter:
4022.9 mm (see CH2150.260-A001)
Bare mirror clear aperture:
diam 3965.4 mm
Central hole:
diam 1317.8 mm
Used clear aperture:
diam 3934 mm (August 00 after installation of seal CH2922-A001)
Central obscuration:
diam 1651 mm (PF cage baffle for f/8 M2)
Light-collecting area:
10.014 m^2

F/7.8 secondary:
Distance from primary:
7494.25 mm
Radius of curvature:
9569.6 mm convex 
Conic constant:
Clear aperture:
diam 1295.4 mm
F/14.5 secondary: Retired!
Distance from primary:
8.6212e03 mm
Radius of curvature:
4.9865e03 convex 
Conic constant:
Clear aperture:
F/30 secondary: Retired!
Distance from primary:
9.6871e03 mm
Radius of curvature:
2.1153e03 convex 
Conic constant:
Clear aperture:


Prime Focus with ADC f/2.87 (18.0 arcsec/mm)

The only instrument currently available at Prime Focus is the 8192x8192 pixel Mosaic II Imager which uses 8 2kx4k SiTe CCDs with 15 micron pixels.

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.

More details on the PFADC can be found in an unpublished paper by Tom Ingerson

The Plate Camera is no longer offered at PF.

Ritchey-Chretien (RC) Focus f/8 (6.56 arcsec/mm)

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. The standard instrumentation used at this focus is:

  • Hydra fiber feed (with Bench Spectrograph)
  • RC Spectrograph
  • ISPI infrared imager
  • As SOAR instrumentation becomes available, the only instrumentation normally offered at this focus will be Hydra and ISPI. The intention is that these two instruments will be permanently and continuously available.

    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.

    22 July 2008

    bgregory at, or tabbott at