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 is provided by Robert Laing and
Janet Sinclair in
La Palma technical note 122.
Observations in late 1997 and early 1998
show that blind-offsetting is accurate to < 0.2 arcsec rms in each coordinate
over offsets up to 20 arcmin.
In 2008, Dan Bramich made repeated observations of 3 star pairs
with separations 9, 15 and 15 arcmin, and found rms ~ 0.3 arcsec in
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).