1. UVIS Calibration Update
Greg Holsclaw
Bill McClintock
June 27, 2011

2. Outline Recent stellar calibrations
Sensitivity decline and Spica variability
Alternative origin of the FUV flat-field, revisited

3. Recent stellar calibrations
EUV
FUV
These plots show the total signal on the detector as a function of star position along the slit

4. All stellar calibrations
EUV
FUV
These plots show the total signal on the detector as a function of star position along the slit

5. 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.

6. Data vs model
Total FUV signal with linear trend divided out. Also shown is the predicted variation in flux from the model.
Data vs 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 )

7. Background on Alpha Vir (Spica)
Spica is a non-eclipsing double-lined spectroscopic binary system
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:
Primary: B1V
Secondary: B4V
Spica is the brightest rotating ellipsoidal variable star
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
Orbital period is days
Amplitude of flux variation in V-filter ~3%
The primary of Spica is a Cepheid variable
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])

8. 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= (wavelength dependent “photometric distortion”)
M2/M1 = 1/1.59 (ratio of masses)
R = 7.6 Rsun = e6 km (polar radius of primary)
D = e7 km (mean separation between stars)
e = 0.14 (orbital eccentricity)
TA (true anomaly)
T0 = 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

9. Rethinking the FUV flat-field

10. 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?

11. Local mislocation of counts?
Hypothesis: photoevents that occur within the geometric area of an adjacent spatial pixel are erroneously counted
Say a photoevent located in this pixel is counted by the pixel above

12. Local mislocation of counts?
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.
80μm
100μm
140μm
60μm
120μm

13. 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]
Radiance [ph/s/cm^2/ster/nm]
Irradiance [ph/s/cm^2/nm]

14. 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?

15. 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

16. 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

17. Row width vs spectral column
Unclear why the apparent width increases toward shorter wavelengths

18. 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

19. Correlation of flat-field with row width
Fractional change in row width does not match the variation in the flat-field

20. 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

21. 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

22. 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

23. Ratio of UVIS to SOLSTICE
This plot shows the ratio of UVIS spectra (1.1nm bins) to SOLSTICE over nm
Some stars are located in starburned rows
WITH flat-field

24. 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

25. 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

26. To do Modify the current FUV calibration to better agree with SOLSTICE
Absolute calibration updates after every star calibration

27. Observation planning?