WHT blind offsetting

The accuracy of blind-offsetting at the WHT, for offsets of up to 20 arcmin, is believed to be <~ 0.2 arcsec rms in each coordinate. I.e. if a star is acquired onto a spectrograph slit by blind offset, the rms error in positioning, perpendicular to the slit, is <~ 0.2 arcsec.

The accuracy is typically measured by centring a star of accurately-known coordinates at a reference position on an acquisition camera, blind-offsetting to a second star and measuring the telescope movement required to re-centre on the reference position. A list of blind-offset star pairs was provided by Robert Laing and Janet Sinclair in La Palma technical note 122.

Observations in late 1997 and early 1998 showed that blind-offsetting is accurate to < 0.2 arcsec rms in each coordinate over offsets up to 20 arcmin.

Tests in 2008 (Dan Bramich, repeated observations of 3 star pairs with separations 9, 15 and 15 arcmin) and 2009 (Andrew Cardwell) confirmed that the blind-offset accuracy is <~ 0.3 arcsec in each coordinate.

Blind-offsetting accuracy is < 1 arcsec (better than pointing rms) over 20 deg (Jul 1999). This is useful for checking which of two similar objects on the TV is the target, by blind-offsetting from the nearest pointing-grid star (tip from Peter Sorensen).