Annual Report 2011 Max Planck Institute for Astronomy

82 IV. Instrumental Developments and Projects
More recently, engineers and astronomers have determined
that measurement noise can be reduced using
standard statistical techniques. By alternating between
each shape and its negative, and taking multiple measurements
of each state, these data can be combined to
get a more accurate value. This technique is called the
“push-pull” method.
For example, one of the fundamental shapes that is
applied in the sequence is “pure focus.” This is a parabolic
shape that appears somewhat like the shape of a
cereal bowl. With the push-pull method, both the rightside-up
cereal bowl and the upside-down cereal bowl
shape are measured, and these shapes are measured several
times each. All data is then combined to produce a
single measurement.
But if there is time to take only 8 measurements per
mode, which is better: To take 4 positives in sequence
and then 4 negatives; or to alternate, positive-then-negative,
4 times? Or is a compromise best: 2 positives and
2 negatives repeated twice?
Prior to this study, engineers and astronomers based
this decision on intuition. But from the study performed
at MPIA, it has been determined that it is the second option
listed above, alternating positive and then negative,
one frame at a time, which gives the best result. This is
shown in Figure IV.5.3 taken from the publication. The
purple column on the far right of the figure is shorter
than the blue column on the far left (and also shorter
than the orange and green columns in between). Since
a smaller value indicates a more well-determined interaction
matrix in this figure, it is clear that 8 push-pulls,
with one frame per dwell, is the preferred method.
Albert Conrad, Thomas Bertram,
Florian Briegel, Frank Kittmann,
Daniel Meschke, Fulvio De Bonis,
Jürgen Berwein