WFPC2 GHOSTS, SCATTER, AND PSF FIELD DEPENDENCE
JOHN E. KRIST
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218
August 9, 1995
Abstract. Scattered light and ghosts in WFPC2 images are discussed, with emphasis on their eects
on imaging of faint objects near bright ones. Scattering within about two arcseconds is primarily
due to diraction eects from zonal errors in the HST optics. At larger angles, the main source of
light is scattering from the CCD surface.
Also, a number of ghosts are present in WFPC2. Some of these may be mistaken as stars, and
others may interfere with the imaging of nearby material.
The eld dependence of the WFPC2 PSF is also discussed. PSF structure can change appreciably
across a single chip due to obscuration shifting, focus, coma, and astigmatism changes. These changes
can aect PSF subtraction and prole tting photometry.
Key words: Scatter { Point Spread Function { Aberrations
1. Introduction
Instrumental artifacts signicantly aect the ability to detect faint objects or material
around bright ones (such as circumstellar disks or quasar host galaxies). Often, it is
necessary to subtract the PSF of the bright object to detect the fainter material. If
nonuniform scatter is present, signicant residuals may be occur in the subtraction,
reducing the scientic usefulness of the data. Also, any ghosts may make it dicult
to distinguish real objects from false. And if the PSF varies with position, both
subtractions and PSF tting photometry may suer from mismatches. All of these
occur in WFPC2.
2. Scatter
One of the goals for WFPC2 was the imaging of faint material around bright objects,
such as Beta Pictoris-like disks around stars, galactic material around quasars, or
faint stellar companions. These require a minimal, smoothly distributed amount of
scattered light. Despite the lack of a coronographic mode or an occulting disk, scatter
was expected to be low based on results from WF/PC-1, and PSF structure could be
subtracted using models or observations of a reference star. The antiblooming feature
of the WFPC2 CCDs would conne the bleeding from saturated pixels into a narrow
range of columns, allowing details to be seen close to a bright object.
associated with the WFPC2 IDT
JOHN E. KRIST
F555W
F675W
F814W
1"
Fig. 1. Saturated WFPC2 PC images of stars in dierent lters, showing the change in the halo
caused by the zonal scatter.
The only important expected sources of scattered light were the zonal errors and
microroughness of the HST optics. Zonal errors are circular patterns on the primary and secondary mirrors which were left by the polishing tools. Diraction of light
by these zones creates a low intensity halo around an object. The light is not uniform, consisting of rings around and streaks radiating from the star. Because these
are diraction features, they change signicantly with wavelength. At short wavelengths (about 200 nm), the halo is about 1" in radius, growing to about 4" at long
(about 800 nm) wavelengths. However, since the amount of light contained in the
halo remains about the same independent of color, the ux per pixel is greater at
shorter wavelengths, thus the halo is more prominent in bluer lters. Figure 1 shows
how this halo changes with wavelength. Table 1 indicates the amount of light in this
and other parts in the PSF.
TABLE I
Distribution of ux in F555W PC
PSFs
Core (<0.3")
86%
Zonal scatter (0.3 - 2.5") 4%
CCD scatter (2.5 - 15") 3%
Spiders
7%
The scatter caused by the zonal errors is fairly well modelled by the Tiny Tim PSF
generator (Krist, 1995), especially with the addition of recently revised zonal error
maps (Krist & Burrows, 1995). Model PSFs can be used to subtract the halo, if no
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WFPC2 GHOSTS, SCATTER, AND PSF FIELD DEPENDENCE
T Tauri (WFPC2 PC)
F555W
F675W
F814W
- Tiny Tim
- Observed
Raw
5 arcsec
Fig. 2. Images of T Tauri and nebulosity taken in WFPC2 PC through dierent lters. An observed
star at a position 4" away was used, as were Tiny Tim models.
suitable observed PSFs are available (though observed PSFs are still preferred). Figure 2 illustrates a case where observed and model PSFs are used in PSF subtractions.
The streaks in the halo vary with position and focus.
Analyses of highly saturated WFPC2 images of stars (Krist & Burrows, 1994)
indicate that there is an unexpected source of scattering, causing light levels to be
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JOHN E. KRIST
Flux / sq. arcsec (total=1.0)
10-2
10-3
10-4
10-5
PC Data
10-6
10-7
WF4 Data
10-8
0
20
40
Radius from Star (arcsec)
60
Fig. 3. Azimuthal average plot of a star centered in the PC. The background is higher in the
camera containing the star due to CCD scatter. The total ux is normalized to one.
about a magnitude higher than expected at large distances (>3") from a star (it
should be pointed out that this scatter is only visible in very highly saturated images).
In these exposures, a star was placed in the center of one of the CCDs and the
average azimuthal intensity measured out into the adjoining chips. As shown in gure
3, the large angle scattered light, measured in ux per square arcsecond (which is
independent of detector pixel size), is much higher in the camera in which the star
falls. The ux measured in the other cameras is caused by microroughness in the HST
optics and is at the expected level.
The fact that this scatter is limited to the camera containing the star indicates
that something in WFPC2 is the cause. Most likely, light is being scattered o the
surface of the CCD, and is then scattered by other surfaces in the camera (the back
of the WFPC2 primary, perhaps). The CCDs used in WFPC2 are front-illuminated,
which means that the electrode structure is exposed, giving the surface a matte-like
texture. The WF/PC-1 CCDs, however, were back-illuminated, and the surface was
smooth and shiny. Recent analyses by Bill Fastie, Dan Schroeder, and colleagues at
Johns Hopkins University with a ight spare camera and detector agree with this
assessment (Schroeder, private communication).
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WFPC2 GHOSTS, SCATTER, AND PSF FIELD DEPENDENCE
2.1 PSF Subtraction
If this additional light formed a smooth, uniform halo, then it could be subtracted out
without too much diculty. Unfortunately, it instead forms numerous streaks which
vary with position on the detector. These are very dicult to remove and impossible
to model. Only by using an observed PSF of comparable saturation and close to
the same position (optimally within 0.5") can the PSF be adequately subtracted.
This, of course, requires additional exposures of a star with no known surrounding
material, adding at least an orbit to a program (if no suitable exposures exist in the
archive). If the expected circumstellar material was properly oriented (an edge on
disk, for instance), then the telescope can be rolled to provide images of the same
object which could then be subtracted from one another.
The positioning of the reference PSF is crucial, and if it is more than an arcsecond
away from the position of the object, signicant residuals will be present in the subtractions. Unfortunately, the average absolute pointing error of HST is 0.8" (Burrows
1995), assuming correct input coordinates and proper motions. Even larger errors,
up to 1.5", sometimes happen. Most of this is apparently due to errors in guide star
positions. A roll as part of an observation sequence can also cause an additional shift
of up to 1.5". The only guaranteed way of placing the reference star at the exact
place is to use an interactive acquisition, but that requires a signcant overhead (on
the order of an orbit).
While the excess scattered light is conned to the camera in which the star is
located, the lower, more uniform scattered light in the other cameras provides a less
cumbersome background for the imaging of circumstellar material. The star can be
placed in the corner of a chip (near the OTA axis), so that the material may be
imaged in the other chips. A reference PSF at the same location is required, but
the smoother background results in reduced residuals from subtraction. The main
problem with this method is that the elds of adjacent cameras overlap because the
spherical aberration is corrected after the pyramid. A star placed too close to the
edge of one chip will also be imaged in another. Based on experience, a pixel position
of (110,110) on the PC is appropriate. Anyone considering this or any other method
described here are urged to contact the Institute for the most up to date information
on combating scatter.
As show in gure 7, there is an electronics problem which may be mistaken as
a scatter eect. When the o-chip electronics encounter a saturated pixel during
readout, the system is overwhelmed, and a capacitance eect causes a decaying signal
to be added to the remaining pixels being read. The resulting images have enhanced
background levels to the right of saturated pixels, wrapping around to the other side
as the signal decays. The signal is additive up to a certain saturation level, so that
more saturated pixels in a row will result in higher background values in that and
remaining rows. This eect can be well characterized with an exponential function,
but the parameters such as decay time appear to vary on the order of an orbit. There
is no way to avoid this eect, except to avoid saturated pixels. It does appear to be
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JOHN E. KRIST
Fig. 4. WFPC2 PC observation of SN 1987a in F656N. The Lucy-Richardson deconvolution used
a Tiny Tim PSF. The deconvolution removes the illusion that the interior of the inner ring is lled
with nebulosity.
somewhat lower at a gain of 14 than it is at 7.
2.2 Removal of Scattered Light Using Deconvolution
While many consider deconvolution to be a dirty word when applied to WFPC2
images, it can sometimes help distinguish PSF eects from nebulosity. Figure 4 shows
a raw and deconvolved image of the rings around SN1987a. The inner ring in the raw
image appears to be lled with nebulosity. However, deconvolution shows that it is
simply an eect of the convolution of the ring with the PSF. Remember, 15% of the
light in a PSF is outside of the core.
3. Ghosts
There are a number of ghosts in the WFPC2 cameras, some of which could (and have)
seriously aect science objectives. These are also discussed in Biretta et al (1995).
The rst is shown in gure 5. It is likely caused by an internal reection in the
eld attener. In the Planetary Camera, the ghost looks essentially like a star, so care
must be taken not to mistake it as such. In the WFs, it looks like a small doughnut.
It is about 6 to 8 magnitudes fainter than the star.
At the center of the chip, the ghost lays on top of the star. As the star moves away
from the center, the ghost moves away from the star along a line from the camera
center through the star. The further the star is from the camera center, the further
the ghost is from the star. In the PC, the distance of the ghost from the star is cDstar,
where Dstar is the distance of the star to the camera center and c is approximately
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WFPC2 GHOSTS, SCATTER, AND PSF FIELD DEPENDENCE
PC1
WF2
Fig. 5. On the left is a WFPC2 PC (F555W) coadded image of the same star at two positions on
the chip, with arrows pointing to eld attener ghosts. The crossmark indicates the camera center.
On the right is a WF2 image in F547M showing the doughnut shaped ghost. The PC image is 11"
on a side and the WF image is 8" in y.
0.25 to 0.30.
When two lters are used, light bouncing between them results in a highly defocussed ghost, as shown in gure 6. The size of this ghost depends on the distance
between the two lters (which varies depending on the lter wheel). Such a ghost
would seriously interfere with observations of bright objects, and there is no way
around it, except for using a single lter instead.
Another ghost, which is seen only in narrow band interference lters is shown in
gure 7. This one is caused by reections within the lter. The ray oset caused by
the bounce results in coma induced by the corrective optics in WFPC2, producing the
arrow shaped patterns. Multiple reections inside the lter result in multiple ghosts.
Finally, a ghost appears in the PC when a bright enough star is located on the
section of the pyramid facet not imaged by the camera. It forms an arc in the corner
near the OTA axis.
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JOHN E. KRIST
Fig. 6. An entire WFPC2 PC eld showing a bright star imaged with a lter and a polarizer. The
reections between the two create the ghosts. Also visible are streaks caused by CCD scatter.
4. PSF Field Dependence
The PSFs in WFPC2 vary with eld position due to obscuration shifting, eld- dependent aberrations, and scattering. This complicates photometry, PSF subtraction, and
deconvolution.
In WFPC2, like in WF/PC-1, the obscuration patterns in the cameras (secondary
mirror and spiders) appear to shift with respect to those in the telescope depending on
position. These shifts caused signicant changes in the aberrated WF/PC-1 PSFs, but
have a much smaller eect in WFPC2. The spiders interact with the light diracted
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WFPC2 GHOSTS, SCATTER, AND PSF FIELD DEPENDENCE
Fig. 7. Entire WFPC2 PC eld of a bright star imaged in F502N. The two ghosts to the lower right
of the star are caused by multiple internal reections within the lter. Also visible are the streaks
from the CCD scatter and saturation eects. Taken at a gain of 7.
by the previously mentioned zonal errors in the HST mirrors, causing the streaks
in the scattering halo to vary in position and intensity. The interacting diraction
between the WFPC2 and OTA spiders causes ripple patterns in the spiders, which
vary as the two spiders shift relative to each other. These result in residuals in PSF
subtractions, like those shown in gure 2. Currently, the spider ripple patterns are
not well matched by Tiny Tim model PSFs.
Recent analysis by Jon Holtzman (Lowell Observatory, private communication) has
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JOHN E. KRIST
WFPC2 WF2
(400,400)
(750,750)
+aber
(750,750)
a
b
c
c-b
Fig. 8. Tiny Tim WF models with and without measured eld dependent aberrations : a) model
at (400,400) with appropriate aberrations; b) model at (750,750) with same aberrations as (a); c)
model at (750,750) with proper aberrations applied; d) dierence between (c) and (b), with the
brightest and darkest pixels each representing an error of four percent of the total ux.
shown that the coma and astigmatism aberrations vary signicantly within a camera
across the eld. These variations are simply part of the optical design, and generally
agree with ray trace analyses conducted by the author. At the extreme corners of the
WFs, away from the OTA axis, there is about 1/5 waves of astigmatism ( referenced
at 633 nm), decreasing to nearly zero near the eld center. Astigmatism at this level
causes the PSF core to become elliptical and slightly less sharp. Coma also varies, but
to a much lesser extent. In the PC, astigmatism and coma changes are considerably
less than in the WFs.
Focus also varies across a chip, in part due to the optics but also because the CCDs
are slightly tilted. The largest focus variations are equivalent to about 8 microns of
secondary mirror defocus.
Another artifact of the optical design is distortion, which is well described in the
WFPC2 Instrument Handbook (Burrows 1995) and in Holtzman et al (1995).
The signicance of these variations are illustrated in gure 8. Obviously, prole
tting photometry would be aected. Holtzman is currently investigating the use of
photometry procedures which would t for the focus of the frame, then t stars using
models which include all of the above variations. He reports that initial results look
very promising.
Currently, the only eld dependent eects the Tiny Tim PSF generator includes
are obscuration shifts and focus variations. The aberrations for the camera centers
are used for all locations in the elds. The software will likely be updated when
parameters describing the variations have been nalized.
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Acknowledgements
This work was funded by the Wide-Field and Planetary Camera 2 Investigation Definition Team (NASA contract NAS7-1260).
References
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Space Telescope', P.A.S.P. 107, 156.
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the Pacic : San Francisco, 349.
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Telescope images', Applied Optics 34, 4951.
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