how to open geotiff as base layer on openlayers

Unfortunately you cannot do this.

You can only display those images using OpenLayers which are spported by browsers. That is why using OpenLayers.Layer.Image you can only display JPEG, PNG & GIF images.

Other Image formats like Erdas Imagine, Tiff, GeoTiff ASCII etc need to be converted to a format which browsers understand, before they can be shown in the browser.