Preliminary raytrace studies indicated that, while evaluating the filter clear aperture, vignetting was occurring in the collimator section.

The mechanical clear apertures of the lens mounts in the collimator were used. From the mechanical drawings these are:
Field lens 230 mm
CL1 lens 150mm and 140 mm
CL2 and CL3 group 220mm and 230 mm

A field set was defined such that the extreme position in the field is chosen such that, any wavelength in the range 495 - 507 nm reaches the detector (2048 x 2048, 0.015 mm pixel size). This is a wider definition than the fields use in earlier work, where edge field position were chosen, whose whole spectrum (495 - 507 nm) fitted onto the 2k square detector.

The PNS system was raytraced, in the TNG telescope, with this field criterion and thru-put was measured, and the critical surface responsible for the vignetting identified. This was done for the 600 line grating concave arrangement and for the full sized 1200 line grating.

Plots were made of the results, and copies faxed to ND, and are kept on file.

The conclusions were that in the 600 line grating case, some minimal vignetting occurred, from the field lens mount diameter, on one side of the field, for the "top camera", and on the other side for the "bottom camera". The thru-put drops approx 10 % for each camera, near the edge of the detector between 14.8mm and 15.35 mm for the 507 nm case.

In the case of the 1200 line full sized single grating case, the plots show the vignetting is more severe . Again for the TNG case traced, the vignetting occurred at the field lens first, and then just cleared the other elements. The plots show for the 507 nm WL, the edge of the detector has 0% thru-put (100% vignetted) at 15.35 mm., and on the other edge of the detector, for 495 nm, the thru-put is 39% at -15.35 mm. The detector position values for 0% vignetting in the collimator are at 13 mm on the 495 nm side, and at 12.2 mm on the 507 nm side.

The vignetting plots are shown below. (six graphs and two Zemax footprint diagrams.) Explanation. The graphs show the thru-put for that position on the detector. The x-axis on the graphs, is the y position on the detector, and for all cases the x position on the chip is fixed at 15 mm ie along the edge. The one sample zemax footprint of the detector plane shows this. It is for wavelength of 507 nm only, and shows a series of positions on the detector. The 495 nm position for this field position is well off the top of the detector. The other sample zemax footprint, show these same series of field position on the aperture on the field lens of the collimator. In this example both these footprints are for the 1200 line grating, single camera mode, on the TNG, and they correspond to the graph labelled "1200 grating one camera 507nm". The vignetting can be seen at the field lens, and the corresponding drop in thru-put is seen on the graph.

note: the graphs referred to here are included in the .doc file which can be downloaded

note: following to be checked

Gabe Bloxham has done raytracing and shown that 182 mm is enough if one takes into account that the field is shifted (by about 50 pixels) between PA1 and PA2=PA1+180 and that the only "useful" area is common to both. This common area is unvignetted. He has offered to fax a copy of the filter "footprint" to anyone interested.

Document Date: Nov 29, 1999