Patent Number: 
Section: claims

1. An apparatus for projection lithography comprising: a lens system comprising at least one magnetic doublet lens and a back focal plane filter positioned in the back focal plane or some equivalent conjugate plane of the magnetic doublet lens in the lens system, the back focal plane filter having a first aperture which is adapted to transmit insignificantly scattered radiation from a radiation source therethrough wherein the magnetic doublet lens system has a pair of ferromagnetic clamps, a first clamp associated with the first lens in the magnetic doublet lens system and a second clamp associated with the second lens in the magnetic lens, wherein the first clamp and the second clamp are configured to substantially separate the magnetic field of the first lens from the magnetic field of the second lens. 2. The apparatus of  claim 1  wherein the magnetic clamps are made of a magnetic material. claim 1 3. The apparatus of  claim 1  wherein the back focal plane filter is placed between the two lenses of the magnetic doublet lens and wherein the clamps are placed and configured to prevent the magnetic fields of the first and second lens from extending to the back focal plane filter. claim 1 4. The apparatus of  claim 2  wherein the magnetic material is a ferromagnetic material. claim 2 5. The apparatus of  claim 1  wherein the magnetic doublet lens has a symmetry for image magnification or demagnification and wherein the first and second clamps have a geometry and dimensions that are selected to preserve the symmetry of the magnetic doublet lens. claim 1 6. The apparatus of  claim 5  wherein the magnetic doublet lens provides 4:1 image demagnification and the first clamp and the second clamp have a dimensional relationship such that the dimensions of the first clamp are four times the dimensions of the second clamp. claim 5