Gnomonic

The gnomonic (or tangent plane) projection is a geometric projection from the centre of the celestial sphere onto a plane which is tangent to the celestial sphere. The area of sky covered by a pixel depends on how far it is from the tangent point. For this reason this projection is not a good one to use if large fields are being mapped. The gnomonic projection is used (for example) for the IRAS 16.5 degree extended emission SKYFLUX images, which suffer from a 6% change in pixel area across an image.