Instrument Science Report NICMOS 99-011
Long-term trends in the NICMOS Camera 2
obscuration pattern and aberrations
John Krist
November 15, 1999
ABSTRACT
Phase retrieval measurements of well-exposed NICMOS Camera 2 star images show that
long-term changes occurred in coma and the cold mask position over the 1997-1998
lifespan of the instrument. These caused time-dependent variations in the PSF diffraction
pattern, notably the shape of the Airy rings and the positions of bands in the diffraction
spikes. These effects are evident in both direct and coronagraphic images when two
observed PSFs obtained at different times are subtracted from each other. The derived
trends have been implemented in Version 5 of Tiny Tim. PSF monitoring programs for
NICMOS after the cryocooler is in place should include similar exposures so that any
further cold mask shifting can be tracked.
Introduction
The NICMOS dewar anomaly caused significant changes to the optical alignment of the
instrument. The most obvious were variations in focus and plate scale as the dewar
expanded and then contracted. These effects were detected soon after the instrument was
activated on-orbit. Later, short-term changes in the alignments of the cold masks were
identified from analysis of highly-exposed star images by Krist et al. (1998; hereafter
K98). A subsequent study of the NICMOS focus monitoring data by Suchkov et al.
(1999) showed that coma and astigmatism were varying almost linearly on long (multimonth) timescales.
All of these variations lead to an unstable PSF, especially when evaluated over periods of
months. This significantly complicated NICMOS PSF subtraction, which was often
required for high-contrast imaging programs. It is common practice to subtract one
observed star image from another in order to detect any faint objects that lie within the
wings of the PSF. However, if there are mismatches between the two images caused by
time-dependent variations in the optical system, then significant residuals may occur at
levels that obscure these objects.
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Such problems were the basis for the K98 report. Using NICMOS camera 2, a
SNAPSHOT survey of low-mass companions to nearby stars was undertaken (GO
program 7420 in cycle 7 and 7894 in cycle 7.5). Because any such companions would be
significantly fainter than the primary stars, the images had to be well exposed to provide
adequate signal for detection. This typically resulted in nearly overexposing the primary
star to the point that diffraction structure could be seen throughout the entire 20” x 20”
field. Subtraction of one star observed in the program from another revealed residuals in
the diffraction structure indicative of obscuration pattern changes. These were traced to
the apparently random motion of the cold mask between visits (termed wiggle). These
errors produced unexpected asymmetries in the point spread functions (PSFs), including
elliptical diffraction rings and asymmetric banding patterns in the diffraction spikes. K98
reported that camera 2 star images taken hours to months apart showed that the cold mask
was varying randomly in position by up to 0.5% of the pupil radius on short (orbital)
timescales.
Presented here is an expanded analysis of the SNAPSHOT data, with specific emphasis
on long-term changes to the NICMOS camera 2 PSF.
Long-Term PSF Variations
K98 did not detect any noticeable, long-term trend in the cold mask position, but they
only looked at data taken early in the survey. The SNAPSHOT program would
eventually have over 100 visits scattered between July 1997 and November 1998.
Comparisons of star images taken early in the program with those from towards the end
showed larger changes than those expected from mask wiggle. It was obvious that some
larger, longer-term variations in cold mask position had occurred.
Figure 1 shows the result of subtracting PSFs taken early and late in the program. The
residuals in the diffraction rings and spikes are characteristic of cold mask shifting.
Notably, the positions of the bands in the diffraction spikes do not match. These bands
are dependent on the distance between the cold mask spiders and the telescope’s, as
projected into the pupil plane. Figure 2 shows the banding patterns from data taken over
the NICMOS lifespan. The bands moved away from the PSF center with an
approximately linear relation with time.
PSF Analysis
Phase retrieval analysis of star images is required to quantify the changes in the optical
system, as described by Krist & Burrows (1995). A version of the Krist & Burrows
software was used for NICMOS focus monitoring, as described by Suchkov et al. (1998).
2
O bserved PS F
Pupil Patte rn
1997a - 1997b
1998 - 1997
Figure 1. Comparison of good and bad NICMOS PSF subtractions. An
F222M star image from the NICMOS snapshot program is shown in the
upper left (full 20” x 20” NICMOS camera 2 field). Its corresponding
pupil pattern is shown to the right. The thick spiders, rectangular tabs, and
larger central obscuration are the NICMOS cold mask; the thinner spiders,
circular pads, and smaller central obscuration are HST telescope
obscurations. In the bottom row are PSF subtractions using two star
images close together in time (1997a – 1997b) and two far apart (1998 –
1997). Note the increased residuals in the image on the right caused by
mask shifting. The PSF images are logarithmically scaled.
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Figure 2. Above are extracted and rotated images of one NICMOS
camera 2 diagonal diffraction spike. These images span from July 1997
(left) to October 1998 (right). The PSF core is at the center of each spike.
Above and below the core are bands caused by the HST and NICMOS
cold mask spiders. Notice that the band positions with respect to the
dotted horizontal reference lines change with time as the cold mask shifts.
The long-term variations in coma and astigmatism reported by Suchkov et al. (1999)
were derived from the focus monitoring data. These consisted of images of an open star
cluster taken at different focus positions. Unfortunately, the exposures had a limited
range of defocus and rather low signal levels. While adequate for measuring low-order
aberrations (focus, coma, and astigmatism), the data did not have the high-frequency
spatial structure in the wings that was necessary to derive the obscuration pattern. Thus,
the version of the phase retrieval software used on this data did not fit for obscuration
changes.
The camera 2 images from the companions survey had copious signal in the wings, and
were the only data available that are suitable for deriving obscuration pattern changes
over time. These images were processed using a version of the phase retrieval software
modified to fit for cold mask position, rotation, and size (spider width, outer radius, and
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inner radius were each fit separately). A set of 27 star images that sampled the lifespan
of the NICMOS instrument were selected. F222M images were used, since that filter’s
bandpass is fairly narrow and the PSF is more sensitive to obscuration pattern changes at
longer wavelengths (though it is less sensitive to aberration changes). Well-exposed,
unsaturated images were chosen, and the central 135 x 135 pixels (10.3” x 10.3”) were
extracted for analysis.
Each image was individually analyzed. In addition to cold mask changes, the following
parameters were included : focus, coma, astigmatism, spherical aberration, clover, pixel
size, uniform background level, and PSF centering. Clover (trefoil) exists in the HST
optics and was expected to stay constant with time, as was spherical aberration. The
blueprint-specified dimensions for the cold mask were used to define the initial mask
pattern, assuming a beam diameter at the mask for an ideally aligned system.
0° Astigmatism
45° Astigmatism
RMS Error (µm)
RMS Error (µm)
0.015
0.010
0.005
0.000
-0.005
-0.010
100 200 300 400 500 600 700
Days Since 1-Jan-1997
0.010
0.008
0.006
0.004
0.002
0.000
-0.002
100 200 300 400 500 600 700
Days Since 1-Jan-1997
X Coma
Y Coma
0.025
0.020
0.015
0.010
100 200 300 400 500 600 700
Days Since 1-Jan-1997
RMS Error (µm)
RMS Error (µm)
0.030
-0.010
-0.015
-0.020
-0.025
-0.030
-0.035
-0.040
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Figure 3. Astigmatism and coma measurements over time from phase
retrieval analysis of NICMOS 2 images. Aberrations are in the detector
coordinate system. The solid lines represent the trends used by Tiny Tim.
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Results
The phase retrieval measurements (Figures 3-5) show significant trends with respect to
time for coma and the cold mask position; there are weaker, compensating trends for the
pixel scaling and cold mask size.
The average position of the NICMOS camera 2 cold mask changed by 1.4% of the pupil
radius over 500 days, almost exclusively in the vertical direction as viewed in the
detector coordinate space. It had an average offset of 12% of the pupil radius. The level
of coma in the vertical direction increased from about –0.015 µm to beyond –0.04 µm
RMS, though the coma in the horizontal direction remained essentially constant at 0.022
µm, with some random scatter. Monitoring of the NICMOS coronagraph spot position
showed that it also followed a generally vertical path.
Mask X
Mask Y
8
7
11.0
% Rpupil
% Rpupil
11.5
10.5
5
10.0
100 200 300 400 500 600 700
Days Since 1-Jan-1997
4
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Pixel Scale
7.500•10-2
7.490•10-2
7.480•10-2
7.470•10-2
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Mask Radius
98.10
98.00
% Rpupil
Arcsec
7.520•10-2
7.510•10-2
6
97.90
97.80
97.70
97.60
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Figure 4. TOP : Camera 2 cold mask X and Y positions with respect to
time as a percentage of the pupil radius. Coordinates are in the detector
X-Y frame. BOTTOM : Pixel scale and cold mask outer radius as a
function of time. These compensate each other, resulting in a constant
Airy ring radius (neglecting cold mask shifting effects). The solid lines
represent the trends used by Tiny Tim.
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There is no definite trend in astigmatism with time, in contrast to the findings of Suchkov
et al. (1999). As noted previously, Suchkov et al. did not fit for the position of the cold
mask, and they also forced y-astigmatism to remain constant. Their astigmatism change
may have thus been compensating for cold mask shifting. Clover and spherical
aberration remained essentially constant over the timespan of the data. The average focus
was off by the equivalent of 4 µm of breathing.
The pixel scale and mask outer radius both changed by about 0.2%. The scale decreased
while the outer radius increased. For a given pixel scale, an increase in the mask outer
radius would cause the PSF to shrink. Conversely, for a fixed outer radius, a decrease in
pixel scale would cause the PSF to expand (relative to the detector array size). When
these two effects are combined, the net result is that the distance to a given Airy ring
remained the same throughout the timespan of the data, neglecting changes caused by
mask shifting. Why the measurements show a trend in scale and mask radius is unknown.
Perhaps the software needed to compensate for some mask property that was not in the
model (e.g. perhaps the mask is not coplanar to the pupil) and thus was forced to modify
some other parameter instead.
X Clover
Y Clover
RMS Error (µm)
-0.0050
-0.0060
-0.0070
100 200 300 400 500 600 700
Days Since 1-Jan-1997
0.0125
0.0120
0.0115
0.0110
0.0105
0.0100
0.0095
0.0090
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Focus
Spherical
0.00
-0.02
-0.04
-0.06
100 200 300 400 500 600 700
Days Since 1-Jan-1997
-0.008
RMS Error (µm)
0.02
RMS Error (µm)
RMS Error (µm)
-0.0040
-0.010
-0.012
-0.014
100 200 300 400 500 600 700
Days Since 1-Jan-1997
Figure 5. Measured clover, focus, and spherical aberration values. Note
that the scale of focus is in microns of wavefront error, not HST secondary
mirror (or breathing).
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Measurements of the separations of stars in the focus monitoring data were used to derive
plate scales for the NICMOS cameras, which were found to vary with time as the dewar
expanded and contracted. The scale that defines the diameter of a PSF diffraction ring
may be different from that which defines the separation on the detector of two objects at
different field positions (ie. one scale is defined by diffractive optics while the other is
geometrical). This may be a result of the complexity of the NICMOS system.
The mask spider width, outer radius, and inner radius were all greater than the input
estimates. This may be due to a different beam diameter at the mask than was assumed
for an ideal system.
The shifting cold mask also affected coronagraphic observations. Figure 6 shows the
subtraction of one simulated coronagraphic PSF from another, each with different cold
mask positions. Members of the NICMOS Science Team have indicated that the
residuals resemble those sometimes seen in actual subtractions.
Figure 6. Subtraction of a simulated coronagraphic PSF from another one
having a cold mask shifted by an additional 0.5% of the pupil radius.
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Recommendations
A focus monitoring program will likely be undertaken after the installation of the
NICMOS cryocooler. The program should retain the method of the previous one
(imaging of stars at various PAM focus positions), but with some modifications.
The through-focus sweep should use short wavelength filters, where the PSF is more
sensitive to focus and other aberrations. Phase retrieval analysis of WFPC2 F555W and
F814W PSF has shown that the shorter wavelength data provides better results, even if it
might be undersampled. This may be due to the sensitivity of astigmatism to focus (the
axis of ellipticity caused by astigmatism changes 90° through focus). An exception is
camera 3, where the subpixel response function and undersampled core could
significantly affect the measurements, so a longer-wavelength filter is more appropriate.
The phase retrieval software generates monochromatic models, so it is important to use
medium or narrow-band filters where polychromatic effects are limited.
The signal levels in many of the images from the previous program left something to be
desired. It is therefore suggested that another target be selected (a less dense, brighter
cluster or isolated star). If necessary, fewer PAM positions could be used if longer
exposures are needed.
The images obtained in the survey program have proven to be a source of considerable
insight to the NICMOS optical system. The focus monitoring data are not very sensitive
to obscuration pattern changes, while the survey images are. Thus, a new focus
monitoring program should include deep exposures of an isolated star, from which the
cold mask position may be derived. Then, this new position may be used in the phase
retrieval software to produce a better model for fitting the focus sweep data. A longwavelength, medium or narrow-band filter should be used. These data should be
obtained in each camera.
References
Krist, J. and Burrows, C. (1995) Applied Optics, 34, 4951
Krist, J., Golimowski, D., Schroeder, D., and Henry, T., (1998) P.A.S.P., 110, 1046
Suchkov, A. & Galas, G. (1999) “NICMOS optical aberrations : coma and astigmatism.”
NICMOS ISR 99-003 (STScI).
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