Another solution to anti newton glass which I use because my cameras have glass vacuum backs specially designed to avoid the double layer of glass in usual enlargers but not with the anti newton glass. . . A piece of mylar works great. Apply under behind negative with textured side toward the negative. The exposure will need a bit of increase as the mylar is not clear and will dampen the light a touch. I use this even when enlarging 3500% with no negative effects on focus or sharpness so long as there are no scratches on the mylar.