The above graph shows the mathematically calculated diurnal velocity change for Arcturus. The area we are concerned with is outlined by the box. This data confirms the system's ability to consistently detect very small shifts in velocity for bright stars. The graphs above show the cross-correlation measurements of the radial velocity of the star as red squares. This first step in the process corrects for most of the mechanical drift of the system.
All spectrographs experience mechanical flexure from thermal and other changes.
This can be quantified by monitoring a reference lamp during the course of the observations - for the data presented here, this monitoring takes place at the same time as the star is being observed. There are three fiber beams, one for the star and two for the reference lamp. Some of this effect can be corrected by noting the difference in the drift as measured by fiber 1 and fiber 2. The blue dotted line represents the mathematical calculation for the velocity shift, as shown in the Arcturus diurnal motion graph. The lines have been arbitrarily shifted vertically so they cross at the point during the night when the star was on the local meridian, and hence shows zero diurnal velocity shift.