MDI Flat Fields
SOI TN 96-134
R. S. Bogart, L. A. Discher de Sá, J. R. Kuhn, X. Scheick,
T. D. Tarbell, & I. Zayer
revised 18 March 1996
Table of Contents
There are two distinct kinds of observations that can be used to determine
the MDI instrumental flat field:
The following key sets of observations to obtain data for determining
the MDI instrumental flat field have been obtained:
- PZT Flats, in which the piezoelectric transducers (PZT's)
of the Image Stabilization System are used to move the target field within
their limited range. The advantage of these observations is that the field
can be moved very rapidly relative to the 3-second integration time, so that
the changes in field illumination from the non-uniform Sun due to
oscillations and evolution of the granulation pattern are minimized.
The disadvantage is that the optical axis is not aligned when the PZT's
are moved from their nominal position, so that the large-scale transmission
gradients due to different transmission profiles of the Michelson
interferometers are altered at the different positions.
- Leg Flats, in which the legs are used to physically repoint the
MDI to different directions. Leg flats have the advantage that they preserve
the optical path, but suffer from the disadvantage that because it takes a
long time (several minutes) to move the legs, the solar illumination pattern
changes between exposures. Also, because the observations sequence takes a
long time to complete and would disrupt normal continuous helioseismic
observations, and has the potential for disastrous failure if the legs stick
at the wrong position, these observations cannot be undertaken frequently.
Data analyzed to date are actually from the quick-look telemetry obtained
as part of program:mdi_eof. The data are reasonably complete and there
is no reason to believe that they will differ from the definitive data in
The current "candidate" flat fields are on the SOI Configuration Managed
tables directory, ~CM/tables, and are named flat_NNN.fits,
where NNN is a unique identification string.
- Jan 19, 1996: a set of full-disc continuum PZT flats.
- Feb 5-6, 1996: a set of full-disc continuum Leg flats.
- Mar 8, 1996: a second set of full-disc continuum PZT flats.
flat_001 is the result after 1000 iterations, but is not converged. Both
flat_002 and flat_003 are fully converged after 200 iterations.
- 000: based on pre-flight observations using a lamp as the source
of illumination and leg motions.
- 001: based on PZT full-disc observations of Jan 19, 1996.
- 002: based on selected leg full-disc observations of Feb 5, 1996
at nominal anti-alias focus.
- 003: based on selected leg full-disc observations of Feb 5, 1996
at "out-of-focus" focus.
- 004: based on PZT full-disc observations of Mar 8, 1996. flat_004
is the result of 3500 iterations; flat_004a is the result after 1000 iterations.
Please address comments and questions to the author(s).
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