Transcript Slide 1

Helioseismic & Magnetic Imager – Calibration and Expected Performance
SP51B-14
Jesper Schou and the HMI Calibration Team, Stanford University and Other Places
IMAGE QUALITY
ABSTRACT
The HMI instrument was delivered in November 2007 and has
been integrated onto the SDO spacecraft with a launch
readiness date of December 2008.
During the integration and testing of the instrument an
extensive series of calibrations were performed.
Here we describe some of the calibrations performed and show
selected results.
We will also discuss remaining efforts, such as on-orbit
calibrations as well as describe the expected performance of
the instrument.
Late in the development the original flight front window was replaced with a
spare window that had been polished to take out wavefront errors introduced by
internal defects.
The MTF with the new window is excellent as shown in Figure 1. The MTF still
has to be corrected for the measured aberrations in the stimulus telescope,
which have been independently measured. We expect that phase diversity
analysis performed on-orbit will be able to constrain the MTF.
Wavelength & Filter Transmission
Another critical area is to determine the wavelength response. This is done using
sunlight, a tunable laser and white light. Selected results are shown in Figures 8-11.
The phase maps and contrasts are excellent and should not cause significant
degradation of the quality of the calibrated data. Among the interesting features
seen are the weak fringes in the front window seen in calmode. The transmission
profile in Figure 10 as well as the Doppler performance in Figure 11 are excellent.
POLARIZATION
To measure the polarization properties the PCU (from HAO) is
placed between the stimulus telescope and the instrument. The PCU
consists of a polarizer followed by a (~ 1/4λ ) waveplate, both on
linear and rotary stages allowing them to be moved in and out of the
beam and rotated. Using various PCU and internal mechanism
settings it is possible to infer the retardance of the telescope and
internal waveplates as well as their orientations and a number of
other polarization properties.
Figure 12 shows an estimate of the repeatability of the polarization
calibration. As an illustration of the kind of features we need to take
into account in the calibration,
Figure 13 shows the temperature dependence of the retardance of
the three internal waveplates.
Similarly the field curvature, as determined by offsetting the pointing of HMI
relative to that of the stimulus telescope, is quite acceptable, as illustrated in
Figure 2. This measurement will also benefit from being done on orbit.
The only outstanding issue here is that of temperature induced
stress birefringence in the front window. When the window rim is
heated up a significant effect is seen. A measurement of a spare
window has been made and the effect appears to be due to a center
to edge temperature gradient (fairly harmless) rather than a bulk
temperature change (more unpleasant).
Substantial image motions are consistently seen when wavelength selector
waveplates number 2 and 4 are rotated, as illustrated in Figure 3. That the image
motion is only vertical (note that there is no corresponding shift in horizontal
direction) is hard to explain by waveplate deformation and may indicate that it is
related to the test setup, but it has been consistently observed throughout the
calibration process. Any residual image motions from this or any other causes
will be taken out in the level 1 data processing.
HMI calibration activities were divided into two main phases:
The rotation between the cameras is stable around 0.08o The shift shown in the
table below varies by a few pixels when the environment of the instrument (e.g.
thermal/mechanical stresses) changes and changed by about one pixel after the
instrument was shipped (Nov 11) and integrated into SDO, presumably due to
gravity effects or a different temperature.
•In-air calibration
Some calibrations can only be performed in air. In particular the measurement
of the telescope polarization would be compromised by the presence of the
vacuum tank window, as might aspects of the image quality.
•Vacuum calibration:
Most calibrations were repeated with the instrument in a vacuum tank, resulting
in a dramatically lower noise level.
In both cases light can either come from a heliostat or from a
stimulus telescope. The latter is able to project known images into
the instrument or use laser light to calibrate the wavelength dependence.
In addition a Polarization Calibration Unit (PCU), described later, can
be inserted in the beam in front of the instrument.
We have determined the flat fields by offpointing HMI relative to the stimulus
telescope. Results are shown in Figures 4 and 5. The most prominent largescale flat field feature is the different gains of the four quadrants and the
vignetting towards the edge of the field. (The vignetting radius is always larger
than the radius of the Sun.) On small scales, we see bubble-like structures
which are traces of dust in the optics, and finger-like structures which are only
visible when the CCD is cold. We suspect that this is due to condensation on
the CCD.
Fig. 8: Calmode phase maps and line
parameters obtained with sunlight.
Miscellaneous
In addition to the calibration topics listed in the previous sections,
several other properties have been or will be measured. These include
items such as properties of the CCDs and cameras, checking for
ghost images and contamination, thermal effects, ISS performance
and alignment leg repeatability.
As can be seen from Figure 6 the image scale is very close to the desired
0.5"/pixel.
Once on orbit a comprehensive set of calibrations will be run as soon
as the instrument is operational followed by periodic measurements
of a variety of properties.
In the following sections a few of the results of the calibrations will be shown
ordered by subject:
• Image quality
• Filter transmission
• Polarization
• Miscellaneous.
The latter covers items such as CCD/camera performance, contamination, thermal
effects, ISS performance etc.
Figure 7 shows a distortion map derived by offpointing the instrument relative
to the stimulus telescope using the alignment legs. While the distortions are
significant in some places, this is easily correctable since the images will be
remapped as part of the ground processing.
Finally it is possible to obtain real observing sequences and to
calculate observables. However, due to the poor seeing, limitations of
the instrument, lack of solar activity and the weather during the first
Sun test few of these were taken.
Camera Shift Table
Date
Δx
Δy
Feb 18
-6.6
5.5
Feb 14
-6.4
Jan 30
-6.2
Date
Δx
Δy
Nov 03
-6.1
4.3
5.7
Nov 02
-6.4
4.2
5.3
Oct 28
-6.7
3.5
Fig. 11: Expected Doppler errors as a
function of input velocity for a number of
different LOS field strengths.
Fig. 10: Non-tunable element (Lyot elements) transmission
profile averaged over the HMI aperture. This preliminary profile
was obtained by using the dye laser at different wavelengths.
Fig. 1: The MTF as measured with the help of the Stimulus telescope.
The plot shows the theoretical MTF of the combined HMI and Stimulus
telescope (dashed line), and the actual MTF of this system for front and
side camera at best focus. The MTF for each camera is represented by
two lines representing the best and worst azimuths. The difference of
the two lines represents the residual astigmatism. The Strehl ratio is
0.79 and 0.81 for front and side camera, respectively.
On-Orbit Calibrations
In addition to the calibrations described in the previous sections we
will also be performing various calibrations on-orbit. Some of these,
such as final focus, focus gradients and parts of the polarization are
best done in space. Others, such as the focus, Michelson drifts and
flat field changes, will have to be tracked during the mission.
Fig. 13: Temperature dependence of waveplate retardances.
Fig. 4: Flat field for front and side camera.
The HMI calibration team members include Sebastien Couvidat, Cristina
Rabello-Soares, Richard Wachter, Tom Duvall, Juan Manuel Borrero and Jesper
Schou. Other people providing significant help include: Steve Tomczyk, Aimee
Norton, David Elmore, Greg Card, Jack Harvey, Phil Scherrer, Jim Aloise,
Jeneen Sommers, Todd Hoeksema, Keh-Cheng Chu, Hao Thai, Karen Tian,
Rock Bush, Yang Liu, Rick Bogart, Ted Tarbell, Dick Shine, Barbara Fischer,
Brett Allard, Brett Pugh, Carl Yanari, Claude Kam, Dave Kirkpatrick, Dave
Sabolish, Gary Linford, Gil Mendelilla, Glenn Gradwohl, Hank Hancock, Jerry
Drake, John Miles, JP Riley, Keith Mitchell, Louis Tavarez, Roger Chevalier,
Ron Baraze, Rose Navarro, Tom Cruz, Tom Nichols and Tracy Niles.
For entertainment “first light” Doppler and line-of-sight
magnetograms are shown in Figure 14.
The on board calibrations will be performed on a variety of timescales. Flat fields, in particular bad pixels, will be continuously
monitored using the regular images. Other items, like calmode
images, full line scans and detunes for wavelength calibrations may
happen on a daily or weekly basis. Finally some items require
significant disruptions to the observing sequence or have other
constraints. This includes items such as spacecraft offpoints to make
flatfields and observations of eclipses and transits to determine the
absolute roll angle.
Fig. 7: The optical distortion derived
from offset images.
Acknowledgements: The first author is grateful to the large number of people
who have helped build HMI or worked on the calibrations.
Fig. 12: The error in an observation based on the average of
several calibrations demodulated by the results of one of them.
The requirement is 1%
Fig. 9: Obsmode phase maps and contrast
measurements made with the tunable laser.
Fig. 2: Field dependence of best focus for
front (upper panel) and side (lower panel)
camera. The linear term (field tilt) has
turned out to be difficult to measure
consistently, as temporal focus drifts due
to temperature changes are
indistinguishable from field tilts. However,
the focus tilt from side to side is always
below one focus step.
Fig. 14: First light (almost) Doppler and line of sight magnetograms
obtained with HMI.
Fig. 5: Highpassfiltered flatfield of
the front camera.
The circular
structures are
caused by
contamination of
the optics. The
horizontal ``finger''
in the upper right
quadrant is likely
caused by
condensation when
the CCD is cold.
Details about HMI can be found at http://hmi.stanford.edu The HMI project is
supported by the National Aeronautics and Space Administration under
contract NAS5-02139 to Stanford University.
Fig. 3: Image motion for rotating waveplates. Each of
the twelve minor ticks represents a 30o rotation of the
respective waveplate.
Fig 6: The image scale as a function of focus position has been
measured by evaluating the radius of the solar disk. Note that
atmospheric perturbations introduce noise in the measurements.
Conclusion
We have by now calibrated
all parts of the assembled
instrument. As far as we
can tell the performance is
excellent. However, we still
have to complete some of
the detailed calculations
and write the code needed
to apply the calibrations to
the data.