Download Sunlight Glinting from Water: not a hazard for WFC3 IR

Instrument Science Report WFC3 2011-12
Sunlight Glinting from Water:
not a hazard for WFC3 IR
P. McCullough
May 23, 2011
ABSTRACT
We analyze the potential hazard of an HST instrument inadvertently observing a glint of
sunlight reflected from water. During nominal operation, the detectors are protected
from such illumination. However, because of an anomaly on HST in which the
instruments were not commanded to their safe conditions, we wondered what could
happen in worst-case scenarios. This report examines one such scenario: what is the
likelihood and potential effect on WFC3’s IR detector if HST were to observe sunlight
glinting from water? As the irradiance on the detector from a (flat) sea-surface glint
potentially could be more than a thousand times greater than that of the reflection from
clouds, which we also derive and compare to NICMOS observations, and because the
likelihood of HST aiming at such a glint is not entirely negligible, we proposed a groundbased test on a flight-like detector. GSFC persons performed a test similar to the one
proposed herein, with no apparent damage to the tested IR detector, which was similar to
the WFC3 IR flight array.
Introduction
In this paper, we estimate the brightness of sunlight reflected first from water vapor
(clouds), then from liquid water (glints), and finally we show that neither is a hazard for
the WFC3 IR detector. As we calculate in this report, the glint’s irradiance on the
detector can be ~3000 times greater than the clouds’. Whereas the likelihood of HST
pointing at clouds is approximately unity on any given orbit, it is very unlikely to point
precisely at a smooth water surface at both a time and in the direction so as to catch the
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glint of sunlight. However, because HST is aimed in a deterministic manner, applying a
priori probabilities based upon a random pointing model may be inappropriate.1
The Brightness of Clouds
If we model clouds as Lambertian reflectors of geometric albedo A, then π times
their radiance (surface brightness) Bc equals A times the irradiance F = πB (T=5800 K) of
the surface of the Sun diluted by 1/R2 and by the cosine of the angle between the
sunlight’s propagation direction and the normal to the tangent plane of the Earth’s
surface:
πBc = A [πB (T=5800 K) (Rsun / 1 AU) 2 cos θ]
(1)
For the brightest clouds, A = 0.9, observed at the sub-solar point, cos θ = 1, at the
blackbody peak of ~5000 Angstroms, the radiance
Bc = 52 ergs/s/cm2/Angstrom/sr = 1.2E-9 ergs/s/cm2/Angstrom/arcsec2,
(2)
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which (for reasons unknown ) is 2.4 times the peak of the curve in Figure 1 of Cox et al.
(1987), even though both estimates used A=0.9. The irradiance at a focal plane
illuminated with a focal ratio f is
F = C π Bc / ( 2 f )2,
(3)
where C is the product of the wavelength-independent effect of the secondary mirror’s
obstruction (0.86) and the wavelength-dependent optical throughput to the instrument’s
focal plane.3 For WFC3 IR, f = 12, pixels are square with 18 micron pitch, and in this
work we approximate C ~ 0.5. Through F110W, a filter with bandpass 5000 nm wide
centered at 1.1 micron, bright clouds will produce an irradiance at the WFC3 IR detector,
F = 183 ergs/s/cm2 = 0.33E9 photons/s/pixel.
(4)
ν
ν
ν
ν
1
For example, program 11917 (WFC3 IR Earth Flats) intentionally proposed to aim HST
~60 degrees away from the Sun and toward the ecliptic plane in order to maximize
reflected (full) moonlight on the dark side of the Earth to produce flat fields in the wide
bandpass filters. If HST and WFC3 were to “freeze” during such a program and continue
exposing as HST traverse the sunlit side of Earth, the probability of exposing the IR
detector to sunlight glinting from water could be a concern.
2
For comparison, the LCROSS visible-light and infrared reflectance spectra (Figures 3
and 4) of the Earth’s western Pacific basin from 360,000 km shows a peak at 0.45 um of
520 W/m2/um/sr, which is identical to the value given in our Equation 2. On the other
hand, the maximum albedos modeled by Manalo-Smith et al. (1998, Figure 12) for low
solar zenith angles are 0.65 (clear snow) or 0.42 (cloudy). The latter albedo is ~2x less
than the 0.90 selected for our theoretical calculation for direct comparison with Cox et al.
Also, the extinction of the round-trip passage through the Earth’s upper atmosphere must
reduce the effective albedo below 0.9.
3
Relationships between radiance, irradiance, focal ratios are in many textbooks; the
section on “Image Radiance” on pages 45 ff of Arechi, Messadi, & Koshel 2007 is
available and searchable on line.
2
To enable an observational comparison, Bergeron examined all bright Earth flat fields
taken with NICMOS’s NIC2 camera under HST program 7775. He reports (2009, private
communication) that none of the narrowband observations were saturated in the zeroth
readout, but the brightest were close. Scaling appropriately from the brightest F212N
observation, i.e. accounting for the solar spectrum and the throughputs, bandpasses, and
(angular) pixel sizes, he estimates the F110W count rates would be 0.20E8 e-/s/pixel and
0.14E9 e-/s/pixel in NIC2 and NIC3, respectively. Scaling from NIC3 to WFC3 IR, using
ratios of throughput (3.0 = 0.4 / 0.13 from the respective Instrument Handbooks, Figure
A.193 of WFC3’s and Figure 4.3 of NICMOS’) and pixel angular area (0.4 =
0.121x0.135/(0.20x0.20)), we estimate the F110W count rate in WFC3 IR will be 0.17E9
e-/s/pixel, which is 52% of our theoretical prediction above. This level of disagreement
seems commensurate with the many approximations, or possibly that NIC2 never
happened to observe the brightest clouds at the sub-solar point, as assumed in the
theoretical prediction, or that our prediction is flawed by a factor of two.4
The brightness and duration of glints
The specular reflectivity of light from and air-water interface (Figure 1) can be
estimated with Fresnel’s law; a plot of the reflectivity of water as a function of angle of
incidence is available in McCullough (2006). Notably, the reflectivity averaged over two
orthogonal linear polarizations, as appropriate for a polarization-insensitive instrument, is
~5% for angle of incidence i = 60 degrees, and is 2% at normal incidence, i = 0 degrees.
At i ~ 60 degrees, i.e. near the Brewster angle, the reflectivity is ~10% in one linear
polarization and ~0% in the other.
The geometry of the Hubble Space Telescope (HST) in low Earth orbit is depicted in
Figure 2, in which the worst-case nominal observing geometry is depicted. The angle
separating the HST pointing vector and the direction of the Sun is never commanded to
be smaller than ~60 degrees, i.e. h > 60 degrees, implying the specular reflection’s angle
of incidence i = (180 degrees – h) / 2 < 60 degrees. This implies the polarizationaveraged reflectivity R from the water’s surface must be < 5%. Thus, writing in very
approximate terms, the point of view of HST is similar to a person on a cliff overlooking
the ocean: the Earth fills ~2pi steradians but because of HST’s sun-avoidance angle,
whenever it might be looking into the glint of the sunlight, the Sun’s altitude above
horizon is greater than 30 degrees. In the scenario that follows, we simply ignore the
facets of waves on the surface of the water and assume the water’s surface is a plane
mirror with 5% reflectivity. If we assume a 40% end-to-end throughput for HST, the 5%
4
Had we used Cox et al.’s Figure 1, or if we have used our equations and Manalo-Smith
et al’s A = 0.42 for cloudy conditions observed from orbit, we would have derived:
observed/expected = 1.1.
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reflectivity can be treated as a 2% reflectivity with an idealized telescope of 100%
throughput, which is what we will do in the following analysis.
An imager of focal ratio f observing an extended source of brightness B produces an
irradiance on its focal plane F = πB/(2f)2. For the Sun, a 5800 K blackbody, the brightness
is given by the Planck function, B (T=5800 K), and for WFC3 IR, f =12. If we were to
observe with WFC3 IR the Sun’s 2% reflection in F110W, a 0.5 micron bandwidth filter
centered at a wavelength of 1.1 micron, we estimate the irradiance
F = 0.047 Watts/cm2 = 1.5 ergs/s/pixel = 0.9E12 photons/s/pixel.
(5)
For comparison, the bolometric irradiance of direct sunlight on the Earth’s surface equals
~1000 W/m2, or 0.1 W/cm2. This suggests that direct sunlight, with no optics between the
Sun and the detector, will provide a convenient test, with ~2x margin, of the worst-case
sun-glint irradiance on WFC3 IR.
Due to HST orbiting the Earth every 96 minutes and aiming at a fixed5 celestial
coordinate, the “plane mirror” of the water’s surface appears to rotate 360 degrees every
96 minutes. The light reflected from any such rotating plane mirror scans the sky at a rate
twice that of the mirror’s rotation. The Sun’s diameter subtends 0.5 degrees, or 1/720th of
360 degrees, so due to the rotating mirror, a pixel in a focal plane of an HST instrument is
subjected to the reflected image of the Sun’s disk for ½ of 1/720th of 96 minutes, or 4
seconds. With an authentic ocean observed by HST, we expect the peak irradiance to be
lower, the duration to be longer, but the total energy deposited to be approximately equal
to the characteristics derived here for the glint from an idealized perfectly-flat ocean, i.e.
0.047 Watts/cm2 for 4 seconds.
ν
A Proposed Experimental Test
We proposed the following experimental test of the risk of WFC3 IR inadvertently
observing sunlight glinting from the water’s surface.
Take a dewar outside the lab into daylight, with a detector package similar to
WFC3, at operating temperature and under vacuum, with either no filter (2x
margin) or a filter similar to F110W, but no other re-imaging optics, and point the
dewar window directly at the Sun with a protective cover in place in front of the
dewar window. Verify the pointing by minimizing the silhouette of the dewar on
the ground, then remove the protective cover and expose the detector for ~4
seconds. Repeat this test a few times, for margin.
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We assume the HST’s pointing is fixed on the celestial sphere. It would be truly
pathological for the HST to be slewing from one astronomical target to another in such a
way as to maximize the duration of the glint into an HST instrument.
4
The information in this paragraph is from private communications with Randy
Kimble (GSFC). Analytic modeling by Richard Dame, not documented here, indicated
that a 1RG array should not break under the thermal stress of the experiment proposed
above. A similar test was performed in the detector lab at GSFC with an incandescent
lamp instead of the Sun, but with appropriate power incident upon the detector. The
detector survived with no apparent change in its characteristics. Indeed, data obtained
with the array before and after the test were basically indistinguishable, after persistence
decayed away.
Conclusions
We derive the brightness of sunlight reflected clouds and from oceans (glints). The
latter can be 3000 times brighter than the former, but even the bright glint of sunlight
from an ocean would not physically damage the WFC3 IR detector if it were to be
exposed to it inadvertently. The latter prediction was verified by analysis and by direct
experiment.
References
Arechi, A. V., Koshel, R. J., & Messadi, T. 2007, Field Guide to Illumination, SPIE
Press.
Manalo-Smith, N., Smith, G.L. Tiwari, S.N., and Staylor, W.F. 1998, Analytic forms
of the bidrectional reflectance functions for application to Earth radiation budget studies,
JGR, 103, No. D16, pages 19733-19751.
McCullough, P. 2006, Models of Polarized Light from Oceans and Atmospheres of
Earth-like Extrasolar Planets, ApJ, submitted, http://xxx.lanl.gov/abs/astro-ph/0610518.
Figure 1: Photograph of sunlight glinting of the sea near the island of Crete. The surface brightness
of the glint can be much greater than that of even the brightest clouds or snow. NASA photograph
STS61A-200-46.
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Figure 2: Geometry of a sea-surface glint of the Sun (S) viewed from HST (H) observing a fixed
celestial target at an angle h from the Sun. The angle of inclination i of the ray of glinting sunlight is
determined by the sum of the interior angles of the triangle: 180 deg = 2 i + h.
Figure 3: LCROSS reflectance spectrum of the cloud-covered western Pacific basin, taken Aug 1,
2009 from a range of 360,000 km, and posted on Aug 4, 2009 at
http://www.nasa.gov/mission_pages/LCROSS/multimedia/earth_look_images.html
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Figure 4: Same as Figure 3, but for the near IR.
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