WIYN / Bench Spectrograph

Off-Axis Collimator

The current collimator is a parabola with a focal length of 1021 mm, and its optical axis aligned with the central fiber in the slit. Because the fibers are aligned parallel to each other, the fibers at the end of the slit use the parabolic collimator at an off-axis angle of 2.1 deg in the current configuration. (It is unclear to me, at this time, whether the off-axis aberrations are corrected in the camera.)

One possibility worth considering is to use the collimator further off-axis in order to avoid the obstruction of the feed. With the current slit-length of 75mm, this would require up to 6.4 deg off-axis angles at the slit ends. Are the aberrations so far off axis acceptable?

			Current EE60		Current EE90		Proposed EE90
s		slit-length		75		75		75		[mm]
f.l.		collimator focal length		1021		1021		750		[mm]
Dbeam		beam diameter		150		200		150		[mm]
doff = Dbeam / 2		unobscured off-axis distance (slit center)		75		100		75		[mm]
doff-max = (doff2+(s/2)2)1/2		unobscured maximum off-axis distance (slit end)		84		107		84		[mm]
thetas = tan-1 [(s/2)/f.l.]		obscured off-axis angle for slit end (no offset)		2.1		2.1		2.9		[deg]
thetaoff = tan-1 (doff/f.l.)		unobscured off-axis angle (slit center)		4.2		5.6		5.7		[deg]
thetaoff-max = tan-1 (doff-max/f.l.)		unobscured off-axis angle (slit end)		4.7		6.0		6.4		[deg]