UVIS Calibration Update
Greg Holsclaw, Bill McClintock
Jan. 5, 2010
Outline
• Recent calibration observations
• Continued intrinsic variability of Spica
• Potential for a modified flat-field
corrector
Recent UVIS Calibrations
•
•
•
•
FUV2009_165
FUV2009_276
FUV2009_315
FUV2009_352
– Much data lost during downlink due to snow in Madrid
Spica variability
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:
•
•
•
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
Orbital period is 4.01454 days
Amplitude of flux variation in V-filter ~3%
http://observatory.sfasu.edu
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])
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
Normalized signal vs time
New data
• The left plot shows the total FUV signal vs time (normalized to the
mean), with a line fit
• The right plot shows the same data with this linear trend removed,
along with a theoretical model of the Spica ellipsoidal variation that
has been fit to the curve (optimizing only the magnitude and phase
offset parameters)
Data vs model
• The Spica
model
continues to
be consistent
with the
observed
variability
Primary UVIS calibration issues
• FUV flat-field
– Addressed by Andrew Steffl corrector
• Increase in sensitivity at FUV long-wavelengths (redresponse)
– Addressed by time-varying sensitivity
• Light-leak in EUV (mesa)
– Addressed through change in instrument setup
• Decrease in sensitivity at EUV and FUV central rows
(starburn)
– To be addressed through a modified flat-field corrector
• Decrease in sensitivity in FUV around Lyman-alpha
– There appear to be two components:
• Occultation slit - To be addressed through modified flat-field
corrector
• Low-res slit - To be addressed through update to time-varying
sensitivity
Approach to creating a modified
flat-field corrector
1. Create a summed spectrum for each Spica
calibration observation
–
Star is slewed along the slit at a fixed rate
2. Subtract a background estimate
3. Apply Andrew Steffl post-starburn flat-field
4. Normalize each detector column to the mean
value in a a select few rows
–
This creates an image that represents a relative
change to these rows
5. Evaluate how this evolves through time
Observation of Spica on 2005-295
Sum of all scans as the star is
slewed along the slit.
Spatial
average
Spectral
average
starburn
Application of Andrew Steffl’s flatfield.
Spatial
average
Significant reduction in row-to-row
and column-to-column variation.
Spectral
average
starburn
overcorrected
Divide an average spectrum
created from rows 18-23, 41-46
into each row.
Spatial
average
Spectral
average
Reference
rows
starburn
overcorrected
Lyman-alpha
Date: 2009-315
Spatial
average
Loss-of sensitivity around Lymanalpha looks like the FUV
occultation slit.
Spectral
average
starburn
undercorrected
Lyman-alpha
Row normalized
detector images
of Spica slewscans
time
Row normalized
detector images
of Spica slewscans
time
Some spectral features appear in
the normalized image
Ratio of row 31 to 20
• ~40% decrease
in sensitivity
125 – 135 nm
(relative to row
20)
How do we use these results as a
corrector?
• “First, do no harm”
– Make sure that a corrector does not cause
more problems than it is solving
• Linearly interpolate the corrector values as
a function of time?
• Need some validation process
Absolute spectra of Spica over time
Absolute spectra
Ratio of UVIS to SOLSTICE
SOLSTICE
• Overall sensitivity decline
• The time-varying sensitivity is overestimating the
adjustment for the most recent observations
Change in response due to burn-in
at Lyman-alpha due to low-res slit
• This shows the ratio
of several spectra of
Spica acquired over
the years, to one
obtained on 2005-295
• This represents an
average response
over several rows
• Since these are the
“reference” rows used
for the previous flatfield modifier, this is
an additional
correction required
HSP sensitivity decline from Miodrag