Blind-offset stars: in order to ensure that the observer can acquire the right object (specially when it is faint) in the slit under the requested observing conditions, we may require a blind-offset star, which will be brighter and easier to acquire than the main target. The observer will acquire this star and then the telescope will blindly slew to your target. The blind-offset star must be located at a distance of less than 20 arcmin from your target at the WHT, 8 arcmin if you are using the INT. Coordinates of both the blind-offset star and the target must be in the same astrometric system and both with precision 2 decimal seconds in RA and 1 decimal arcsec in Dec. Both should have low proper motions. The finding chart generator or the long-slit tools can help you to find suitable stars.

If your target is fainter than the limiting magnitudes in the representatives requested observing conditions given below, then provide a blind-offset star (brighter than the limiting magnitude) in the target list if you are using the long-slit spectroscopy mode of one of these instruments:

ACAM:

Seeing	Sky Brightness	R_lm
<1.0"	Any/bright	18.6
<1.0"	Grey	19.2
<1.0"	Dark	19.5
<1.5"	Any/bright	18.2
<1.5"	Grey	18.8
<1.5"	Dark	19.2

ISIS:

Seeing	Phot. condition	Sky Brightness	V_lm
Any	Any	Any/bright	~16.5
<1.5"	Any	Any/bright	~17.0
<1.2"	Photometric	Grey/dark	~18.0

LIRIS:

Seeing	Phot. condition	Sky Brightness	J_lm
Any	Any	Any/bright	~16.5
<1.5"	Any	Any/bright	~17.0
<1.2"	Photometric	Grey/dark	~18.0

IDS:

Sky brightness: Bright Moon illumination: ~90%, DIMM seeing: 0.2-0.5"
Moon separation	Airmass	V_lm
10°	1.8	~14.1
20°	1.4	~15.0
30°	1.2	~15.1
40°	1.1	~15.9
55°	1.0	~16.2
90°	1.6	~15.1
Sky brightness: Dark/Grey
Seeing		V_lm
~2 arcsec		~17.0