Physical Parameters of UKIRT

Physical Parameters of UKIRT

  • Telescope design: classical Cassegrain 
  • longitude1 : -155d 28′ 23.”6±0.”2 
  • latitude : +19d 49′ 32.”2±0.”2 
  • altitude 2 : 4194±6.0 m
  • Diameter of primary mirror : 3802.5±1.0 mm 
  • Focal length of primary (PFL) 3 : 9516±10 mm 
  • Radius of curvature of secondary : 1725±2 mm 
  • Diameter of secondary mirror : 313.4±0.1 mm 
  • Focal plane scale at Cass. focus4 (FPS): 1.525±0.001 arcsec/mm 
  • Primary pole to secondary pole : 8715 ±2 mm 
  • Secondary pole to Cassegrain focus : 11384 ±3 mm 
  • Beam size on primary : 3725 ±10 mm 
  • Effective focal length = 206265″/FPS : 135256 ±87mm 
  • Amplification ratio = EFL/PFL : 14.199 ±0.009 

1. The coordinate system is believed to be Geodetic, Old Hawaiian Datum. This differs from the standard Geodetic, North American Datum (1927) updated to 1983, by a predicted amount 341m N, 291 m W (translation of coordinate grid from OH to NA’83).

2. The altitude refers to the dome floor, interpolating from the source map contours. The reference altitude is not stated: this is conventionally the local mean sea level. Contours were derived by (revised) photogrammetry in 1978. 

3. The primary focal length has often been reported to be 9526mm – for example on this Web page prior to Sept 4, 2001. This is due to some confusion between the various measurements. When the mirror was figured the PFL was measured as 9506mm, but in 1989 it was measured on the telescope as 9526mm. Both measurements should have been accurate to a few millimetres, at most. The number we have adopted here is the best estimate we can give because of the clear discrepancy.

4. The focal plane scale at the crosshead is smaller than this because of the presence of a large pupil imaging lens just above it, to prevent vignetting when the guiders are used off axis. (This change in focal plane scale is irrelevant to the instruments.)