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 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

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 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

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 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]

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

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.