Field-of-view Alignment Tips
The algorithm we usually use for the FOV alignment is:
- Start with the interferometer as-is, or fairly level
- Adjust the mirror angles for each view to center the FOVs in the mirror direction for each view respectively
- Adjust the screw by the rear kinematic mount up or down to to center the FOVs in the perpendicular direction
- If there is skew between the ABB and HBB views, then adjust the screws by the side kinematic mounts. There are two screws on either side, then can go up or down, so try out each of the 4 variations to see which removes the skew the most. The goal is remove the skew and have the HBB & ABB patterns both fall around the same area of the y-axis. At this stage it doesn’t matter if they are centered in y: that can be adjusted afterwards with the rear screw (which has the effect of moving all 3 views up/down the y-axis). Also the Sky FOV map doesn’t matter at this stage, because if the ABB & HBB views get de-skewed then the Sky view should automatically be fine.
- There’s no need to do a full FOV map after each adjustment. Just map out the edges of the pattern along the centerline, along the direction of interest. Once the FOV is satisfactorily aligned, you can do a full FOV map for your records.
- In general I wouldn’t be too concerned about the max amplitude of the FOV map. It could be dependent on the intensity/angle of the light pen, the exact spectral range used, or the detector sensitivity. The best metric is to look at the instrument responsivity before and after the repair to determine if it’s reasonably consistent.
- It’s more important to center the edges of the pattern than the peak, in order to avoid any potential clipping at the aperture edge.
- I usually do these adjustments by varying the mirror angles in FTSW, and then just quickly comparing the values of the F2/F10 and F3/F9 grid boxes and try to roughly match up their amplitudes to center the pattern. First centering the pattern along the scene mirror motion axis will make centering along the perpendicular axis easier.
- The patterns being a bit oblong are not a major concern. It might be worth comparing the patterns to the original FOV maps that were done at ABB prior to delivery, that should be included in the documentation package first delivered with the instrument. If the shapes are substantially different, then it’s possible that the aft optics or the detector could have been perturbed.
- With our FOV jig, the pen angle w.r.t the jig does have a small effect of changing the recorded intensity. However this effect is fairly small. We make a reasonable conscious effort to hold the pen at the same angle for the test, and have the same pen operator do the whole test, and we’ve never had issues.