UVIS Calibration Update
Greg Holsclaw, Bill McClintock
June 23, 2008
Outline
• Recent calibration observations
• Kurucz model compared with UVIS and
other measurements of Spica
• UVIS Calibration / Spica flux paper
• EUV baseline drift during solar
occultations
Recent UVIS Calibrations
• The last Spica calibration occurred on 2007-362
• The most recent Spica observation
UVIS_067IC_ALPVIR001_PRIME (2008-129)
was lost due to DSN issues
• Other star calibrations:
– α Leo (Regulus), 2008-076
– Not yet examined in detail
• Next Spica calibration 2008-266
Kurucz Stellar Models
• The Kurucz model stellar irradiance at any wavelength is
computed in a coarse grid of three variables:
temperature, gravity, and metallicity
• A convenient routine for accessing these models is
available from:
– IUEDAC IDL Software Libraries
– http://archive.stsci.edu/iue/iuedac.html
• The routine KURGET1 allows a user to specify a
measured temperature, gravity, metallicity and angular
size to arrive at a model spectral irradiance from any
star:
–
–
–
–
–
Wavelength range: 9.1 nm to 106 μm
Total wavelength steps: 1221
Resolution: ~1nm in the EUV, FUV, MUV
Default units: [ergs/cm2/s/Angstrom]
Allows linear interpolation between model grid points
α Vir / Spica
Model Parameters
• Spica is a double-lined spectroscopic binary
system
• Radii: 7.78, 3.99 (xRSun) [Morales et al, 2000]
• Distance: 264.5 light-years
– derived from 0.01244” ± 0.00086” parallax measured
by Hipparcos (7% uncertainty)
• Photospheric temperature: 24500, 17200 Kelvin
[Popper, 1980; Morales et al, 2000]
• Gravity: log g = 3.69 [Morales et al, 2000]
• Metallicity: [Fe/H] = 0 (assumed for both
components)
α Vir / Spica
Model Output
• This shows the
interpolated
Kurucz model
spectral irradiance
for each binary
component of the
Spica system
using chosen
stellar parameters
• The primary
component is ~15x
brighter than the
secondary for λ >
140 nm
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
α Vir / Spica
Comparisons with SOLSTICE
•
•
•
SOLSTICE is a well-calibrated UV spectrometer onboard the SORCE spacecraft, and was
developed here at LASP under the direction of Bill McClintock, and provides accurate measures of
FUV stellar spectral irradiances
Over most of the FUV spectral range, the UVIS measurement is within ~10% from the spectrum of
SOLSTICE
The spectral shape of the Kurucz model using the assumed parameters is a good match over the
FUV range with the measured irradiance values
UVIS Calibration paper outline
•
Working title: “The absolute spectral irradiance of Spica in the extreme and farultraviolet from Cassini-UVIS”
1. Spica background
•
–
•
Binary star system, 4 day orbital period, Cepheid primary, etc
2. Mission and instrument overview
–
•
Mission timeline, UVIS specs
3. Calibration (Lab)
–
•
telescope scattered light, wavelength scale, spectral resolution, radiometric sensitivity,
detector linearity
4. Data reduction and methodology
–
•
Sensitivity vs time adjustment, evil pixel deletion, I/F calculation
5. Observations
–
•
Dates during cruise and orbit, differences in geometry of scans
6. Observations from other investigators
–
–
•
Wavelength range, resolution, platform
BMF rocket, Voyager, IUE, EURD, SOLSTICE, Kurucz model
7. Results
–
–
Measurement of ellipsoidal orbital variability in the FUV, correlation with visible model
Present absolute fluxes, fractional differences
EUV Solar Occultations
• This shows a
measure of total
signal in the EUV
as a function of
scan number for
all USUNOCC
observations
• Signal is total in
spatial
dimension,
average in
spectral
Instrument setup:
nx=1024.00, ny=2.00000
slit=2 (occ), y1=4, y2=57
ybin=27, int=1 sec
EUV Solar Occultations
• This shows the
average signal
vs time for two
solar
occultations,
illustrating the
issue of varying
baselines
EUV ring crossing background
A
A
B
Total signal vs scan number
(or time in seconds)
•
•
B
Average of 100 spectra at two
positions
Here the most recent ring crossing solar occultation is examined
The background contribution during eclipse is seen to be mostly flat across
the detector, with some structure at the edges
EUV ring crossing background
Raw total signal vs time
•
•
total signal vs time after subtraction of solar
spectrum in non-occulted regions
It is thought that the detector counts occur due to charging of the spacecraft from
dust impacts.
The background is clearly correlated with spacecraft ring crossings, and exhibits a
bimodal character.
Cassini Cosmic Dust Analyzer
(CDA) Measurements
• The CassiniCDA has
previously
saturated during
ring crossings
[Helfert, 2005]
• However, this
plot shows that
there is
evidence the
CDA has
measured a
similar
distribution of
dust [Srama et
al, 2006]
[Srama et al, 2006]
EUV ring crossing background
•
Saturn
•
Ring plane
•
Here the trajectory of
the spacecraft is colorcoded by an estimate of
the EUV background
Was the Cassini
Cosmic Dust Analyzer
(CDA) acquiring data
simultaneously?
Other ring crossing
events where UVIS
acquired data?
Further Work
• Continue writing UVIS Calibration / Spica paper
• Comparison of spectral irradiances of stars other than
Spica between UVIS, SOLSTICE, and Kurucz model
• Characterization of Lyman alpha sensitivity changes
• Resolve point-spread function differences between
LASP and new results from Don
• Integrate EUV flat field into calibration routine
• Additional Spica photometry with:
– ISS
– UVIS housekeeping?
References
• Helfert, S., Ring Plane Crossings with the Cassini CDA
Instrument: Saturation Analysis and Deadtime
Correction, Workshop on Dust in Planetary Systems
2005.
• Morales, C. et al, Far-Ultraviolet Absolute Flux of α
Virginis, The Astrophysical Journal, Volume 530, Issue 1,
pp. 403-407, 2000.
• Popper, D. M., In: Annual review of astronomy and
astrophysics. Volume 18, p. 115-164,1980.
• Srama, R. et al, In situ dust measurements in the inner
Saturnian system, Planetary and Space Science,
Volume 54, Issue 9-10, p. 967-987, 2006.