The DGM is a disk placed over the front of the telescope tube. The slit width should equal the bar width. The size of the slits should vary with the average wavelength of the light from the star being measured. A protractor ring is placed around the tube end and a pointer is placed on the grating. The grating is rotated and the angle read. A sketch and some geometry is needed to compute the separation. Calibration is required ! Also, one-half of the light falling on the tube is blocked by the grating, limiting which pairs you can measure and limiting DGM use in light polluted areas.

I would suggest trying the Chronometric or Transit method instead. No calibration is required. All you need is a PA Dial, a stopwatch and an eyepiece with crosshairs.

If you want more info, contact me at :

izar@juno.com