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UVIS Calibration Update
Greg Holsclaw
Bill McClintock
June 27, 2011
Outline
• Recent stellar calibrations
• Sensitivity decline and Spica variability
• Alternative origin of the FUV flat-field,
revisited
Recent stellar calibrations
EUV
FUV
These plots show the total signal on the detector as a
function of star position along the slit
All stellar calibrations
EUV
FUV
These plots show the total signal on the detector as a
function of star position along the slit
Decline in FUV in sensitivity over time
Total signal from Spica vs row position
of the image, for all calibration
observations.
Mean value of the signal when the star
was located between rows 18 and 22,
then normalized to the first.
Data vs model
Total FUV signal with linear trend divided out.
Also shown is the predicted variation in flux from
the model.
Variation in flux is given by [Shobbrook, 1969; Sterken et al, 1986]:
dE = A M2/M1 (R/D)3 (1+e cos(TA+Φ))3 (1-3cos2(TA+TA0+Φ) sin2i )
Data vs model
Background on Alpha Vir (Spica)
•
Spica is a non-eclipsing double-lined spectroscopic binary system
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–
–
Though not spatially resolvable, each component is detectable through measurements
of out-of-phase Doppler shifts in the constituent spectral lines
Non-eclipsing due to large apparent orbital inclination of ~70 degrees
Both stars are of a similar spectral class:
•
•
•
Spica is the brightest rotating ellipsoidal variable star
–
–
–
–
–
•
Primary: B1V
Secondary: B4V
The stars have a distorted ellipsoidal shape due to mutual gravitation effects
As the components revolve, the visible area (and thus the observed flux) changes with
orbital phase
Since this is a geometric effect, it should be roughly wavelength-independent
http://observatory.sfasu.edu
Orbital period is 4.01454 days
Amplitude of flux variation in V-filter ~3%
The primary of Spica is a Cepheid variable
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–
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Periodic variation in the pulsating primary star is much shorter than the system’s
orbital period and about a factor of 2 less in magnitude
Period is 4.17 hours
Amplitude of flux variation in V-filter ~1.5%
This short-term variation, identified in 1968, became undetectable in the early 1970’s
(but may return again due to precession of the primary’s rotation axis relative to the
orbital plane, which has a period of 200 years [Balona, 1986])
Ellipsoidal variation model
Variation in flux is given by [Shobbrook, 1969; Sterken et al, 1986]:
dE = A M2/M1 (R/D)3 (1+e cos(TA+Φ))3 (1-3cos2(TA+TA0+Φ) sin2i )
Where:
A=0.822 (wavelength dependent “photometric distortion”)
M2/M1 = 1/1.59 (ratio of masses)
R = 7.6 Rsun = 5.2858e6 km (polar radius of primary)
D = 1.92916e7 km (mean separation between stars)
e = 0.14 (orbital eccentricity)
TA (true anomaly)
T0 = 4.01454 days (orbital period)
TA0 = 150 degrees (apparent angle to line of apsides in
year 2005, has precession period of 128 years)
i = 65.9 degrees (orbital inclination)
Φ = empirical phase shift, a free parameter to match with data
One period of the expected variation in flux
from Spica
Rethinking the FUV flat-field
Extended source vs point source
• An extended
source
appears to
exhibit flat
field effects
• The total
signal from a
point source,
as a function
of position
on the
detector,
does not
• Why?
Local mislocation of counts?
Say a photoevent
located in this pixel is
counted by the pixel
above
• Hypothesis:
photoevents that
occur within the
geometric area
of an adjacent
spatial pixel are
erroneously
counted
Local mislocation of counts?
80μm
100μ
m
140μ
m
60μm
120μ
m
• The effect of these
mislocations is a
change in the
effective width and
position of spatial
pixels.
• The flat-field
variation is caused
not by changes in
QE, but by changes
in effective area.
Sensitivity
• Variations in sensitivity from row-to-row can be separated
into:
– Effective area
– Quantum efficiency (e.g. “burn” effects)
Sensitivity [counts/s per
radiance unit]
S
Radiance [ph/s/cm^2/ster/nm]
L

Irradiance [ph/s/cm^2/nm]

w slit  h pixel
f2
d
 At w pixel 
 T 
dx
C /t
S
C /t wslit  h pixel
E

S
f2
Consequences
• Images constitute an irregular grid of
nonuniform pixel size
• More accurate measure of unresolved targets:
– full-disk reflectance of icy satellites
– stars
• Similar behavior in the spectral dimension?
Evidence
• Row width is correlated with the row-to-row
variation from an extended object
• Row position is uneven in the FUV, more even
in EUV
• Row width is unaffected by the starburn
Spatial profile for a single pixel
• This shows the
detected counts
for a single pixel as
the star image is
slewed along the
detector
• We can measure
the time the star
crossed the center
of the pixel and
the width of the
profile
Row width vs spectral column
• Unclear why the apparent width increases toward
shorter wavelengths
Average row width
• Average of columns 600-1000
• Colors indicate different observations spanning 2005 to
2011
• Insensitive to changes in response at the starburned rows
Correlation of flat-field with row width
• Fractional change in row width does not
match the variation in the flat-field
Pixel center position
• We can find the
time at which
the image
crosses each
pixel
• Deviations from
a line indicates
either:
– Spacecraft
slew is not
smooth
– Pixel position
is not even
FUV rows appear unevenly distributed
• A strong correlation exists from column-to-column in the apparent
position of FUV pixels
• That is, entire rows systematically deviate from the expected
position
Future plans
• The response of each row is undersampled
because the image moves by ~90microns during
an integration period (currently 45 sec)
• This results in a poor determination of pixel
location and width
• Therefore, we would like to plan a stellar
calibration with either a shorter integration time
(10 or 20 sec) or a slower spacecraft slew rate
• The data volume would be 2-3x larger and the
time commitment 2-3x longer than the current
observation
Ratio of UVIS to SOLSTICE
• This plot shows
the ratio of UVIS
spectra (1.1nm
bins) to SOLSTICE
over 130-180 nm
• Some stars are
located in
starburned rows
WITH
flat-field
α Vir / Spica
Measurements vs Model
•
•
•
As calculated, the
Kurucz model exceeds
the measured
irradiance spectra by
~20%
This discrepancy is
likely due to
uncertainties in the
model parameters
(distance, radii, or
temperature)
Therefore, the Kurucz
spectrum will be
visually adjusted by a
factor of 0.8 to match
the measurements in
the FUV
α Vir / Spica
Measurements vs Model
• In the EUV, the
Kurucz model is in
rough agreement
with the EUVE
measurement
• However, the UVIS
and Rocket
measurements are
also in rough
agreement with
each other, but
significantly lower
than Kurucz and
EUVE
To do
• Modify the current FUV calibration to better
agree with SOLSTICE
• Absolute calibration updates after every star
calibration
Observation planning?