1. UVIS Calibration Update
Greg Holsclaw, Bill McClintock
June 23, 2008

2. 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

3. Recent UVIS Calibrations
The last Spica calibration occurred on
The most recent Spica observation UVIS_067IC_ALPVIR001_PRIME ( ) was lost due to DSN issues
Other star calibrations:
α Leo (Regulus),
Not yet examined in detail
Next Spica calibration

4. 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
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

5. α Vir / Spica Model Parameters
Spica is a double-lined spectroscopic binary system
Radii: 7.78, 3.99 (xRSun) [Morales et al, 2000]
Distance: light-years
derived from ” ± ” parallax measured by Hipparcos (7% uncertainty)
Photospheric temperature: 24500, Kelvin [Popper, 1980; Morales et al, 2000]
Gravity: log g = 3.69 [Morales et al, 2000]
Metallicity: [Fe/H] = 0 (assumed for both components)

6. 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

7. 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

8. 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

9. 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

10. UVIS Calibration paper outline
Working title: “The absolute spectral irradiance of Spica in the extreme and far-ultraviolet 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

11. 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= , ny= slit=2 (occ), y1=4, y2=57 ybin=27, int=1 sec

12. EUV Solar Occultations
This shows the average signal vs time for two solar occultations, illustrating the issue of varying baselines

13. EUV ring crossing background
Total signal vs scan number (or time in seconds)
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

14. 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.

15. Cassini Cosmic Dust Analyzer (CDA) Measurements
The Cassini-CDA 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]

16. EUV ring crossing background
Here the trajectory of the spacecraft is color-coded 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?
Saturn
Ring plane

17. 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?

18. 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 , 2000.
Popper, D. M., In: Annual review of astronomy and astrophysics. Volume 18, p ,1980.
Srama, R. et al, In situ dust measurements in the inner Saturnian system, Planetary and Space Science, Volume 54, Issue 9-10, p , 2006.