MAlign variously refers to a technique, a data-taking procedure, and an
analysis tool. Sometimes these have different names: malacq, align, etc.
The Technique
The MAlign technique involves spreading the 36 mirror segments out by a known
amount, taking one or more images of the resulting pattern, and analyzing that
pattern for the proper movement of the individual segment images. This
is very similar to a Shack-Hartmann test, in that variations of
the pattern from the ideal pattern yield a measure of the errors in
various terms of the telescope configuration. In particular, it is used
to measure errors in piston (i.e. focus), secondary tilt (coma), and
stack. It can also measure other aberrations, such as astigmatism and
higher order terms, but other than changing the stack we have no way
of using that information.
The Data-Taking Procedure
Most often called "malacq," this is sometimes called "malign" (e.g. NIRC).
In most cases (again, NIRC is an exception) the appropriate tool is run
by the OAs to take the sequence of images. In broad outline the procedure
must:
- store the current (observer's) instrument configuration,
- configure the instrument appropriately for the MAlign images,
- set the ACS "focus mode" and wait for it to settle,
- take images,
- optionally set to the focus mode of opposite sign and take more images,
- reset the instrument to the observer's parameters.
The Analysis Tool
What the OAs generally call "malign," but is called "align" from the
command line. This tool reads in each focus mode image and displays
it. The OA can then mark the segments, usually in an automatic mode
where (s)he only need mark the center of the hexagonal segment pattern.
The software then analyzes a set of image measurements, reporting
qualitative blur values for each of piston, tilt, and stack errors.
The OA can send any or all of the suggested corrections.
More information