A method of placing a grating material at an optimum distance from a sensor that will produce a reference spectrum on that sensor. In a preferred embodiment of the present invention, a device with a diffraction grating end will protrude into the empty lens well of a camera and with the camera inserted into a telescope eyepiece holder will focus both a image of an object, that the telescope is pointed to, and its spectrum onto the light sensor in the camera. This dual image, called a reference spectrum, allows the separation between the image of the object and a point in the image of its spectrum to be determined quickly. This separation distance to the spectral point within the spectrum is then a referenced distance. Once a reference distance is known it is then used to determine the wavelength of any other point or spectral line within the spectrum.