 EXPERIMENTAL SCHEDULE The aim of this study is to characterize ocular blood flow choroid and optic nerve regulation in patients in 4 populations: 1 glaucoma patients 2 OSA patients without clinical evidence of optic nerve disease, 3 OSA patients with NAION, 4 matched healthy subjects . OSA patients of group 2 and 3 will be studied 3-6 months after nCPAP treatment. 