Astigmatism due to de-collimation of the beam incident on the grating is less than the intrinsic spatial PSF of the telescope and instrument (~0.3") provided the instrument is focused on the slit with the collimators positioned within ~±2500µ of their autocollimation positions, tabulated below. For general runs it's strongly recommended that the instrument is focused with the collimators within this range to ensure that the spatial profile isn't degraded significantly by astigmatism, even in very good long-exposure seeing. The exception to this is for lucky-imaging programmes, which attempt to resolve spatially down to the intrinsic telescope/instrument PSF, where it's recommended the instrument should be focused with the collimators within ~±1250µ of their autocollimation positions to ensure the spatial profile is dominated by the intrinsic PSF.

Following the switch from the 4MS to PLC controllers in early 2012 the autocollimation positions were measured as 5400µ for the blue arm, and 9300µ for the red arm. In the red arm, the focus offsets of the GG495 blocking filter and the standard D5300 dichroic are +800µ and +860µ respectively. These focus offsets translate the autocollimation positions to preserve the focal length between slit and collimator, and so the optimal range of the collimator changes. The tolerances of the collimator positions for each arm, centred on the autocollimation positions, are given in the table below.

The focus offsets of the other dichroics are listed here. For example, if using the D6100 dichroic in the red arm (offset +1340µ) with the GG495 blocking filter deployed, the autocollimation position would be 11440µ and the anastigmatic collimator range would be 8940-13940µ.

ISIS red arm anastigmatic collimator range
GG495	D5300	Optimal range (microns)	Autocollimation (microns)
Out	Out	6800 - 11800	9300
Out	In	7660 - 12660	10160
In	Out	7600 - 12600	10100
In	In	8460 - 13460	10960

ISIS blue arm anastigmatic collimator range
D5300	Optimal range (microns)	Autocollimation (microns)
Dichroic	2900 - 7900	5400
Mirror	2600-7600+	5100+

⁺ The exact focus offset in the blue arm on changing from dichroic to mirror depends on the grating, and has a small range of ~-100µ to -500µ. The mid-point of this range is adopted in the table.