The SOLSTICE Observing Technique
Martin Snow, William McClintock, Gary Rottman, & Thomas Woods
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder CO 80303
Snow@lasp.colorado.edu
The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE)
observes an ensemble of bright, early-type stars to track the loss of
sensitivity over time. The stars were chosen to be stable over
timescales of centuries, and by using multiple stars, we remove the
effect of any intrinsic stellar variability.
SOLSTICE observes both the sun and stars with the same optical path.
A factor of 108 in dynamic range is achieved by changing apertures
(~104), bandpass (~101), and exposure time (~103).
Solar-Stellar Irradiance Comparison
1015
Stellar Spectral Resolution:
Thermospheric hydrogen
scatters sunlight at 121.6 nm.
The wide field of view of the
instrument in stellar mode
allows off-axis light to get to
the detector as if it were onaxis light at a shifted
wavelength. The diffuse
airglow shows up as a very
broad line that is highly
variable with time and viewing
geometry.
Solar Irradiance
1014
Here is the solar spectrum along with a
typical stellar spectrum. The ratio of the
solar to stellar irradiances will be a
constant since it depends only on
apertures and integration times.
1013
1012
Eta UMa Irradiance(×109)
Measurements:
Wavelength Coverage:
Solar Spectral Resolution:
Challenges of the Lyman alpha Airglow Region (115-130 nm)
The SOLAR-STELLAR
CHALLENGE
1011
115-320 nm
0.1 nm (FUV)
0.2 nm (MUV)
1.1 nm (FUV)
2.2 nm (MUV)
Spectral Scan of a Dark Region
1010
109
The spectrum of a hot star in
this wavelength range can
have significant structure.
Small wavelength offsets due
to pointing or the grating
drive must be corrected using
the measured spectrum. In
the airglow region, we use
IUE spectra convolved with
the SOLSTICE instrument
profile to correct for spectral
shape.
108
150
200
Wavelength (nm)
250
300
The stellar observations track changes in the
instrument sensitivity at 40 discrete
wavelengths across the entire UV band.
Wavelengths in the FUV marked with
diamonds require special techniques to
overcome the Lyman alpha airglow
background.
Stars in the Far Ultraviolet (FUV)
IUE Spectrum of eta UMa
Stars in the Mid Ultraviolet (MUV)
The top panel shows the
uncorrected stellar
irradiances as a function
of time. The units are 104
photons/cm2/s/nm.
MUV stellar irradiances.
The dashed line shows the
beginning of a near realtime measurement of the
background (see below).
The bottom panel shows
those same irradiances
after normalization for
stellar brightness, along
with an exponential fit to
the long-term trend.
The small inset plot
shows the distribution of
corrected stellar
irradiances. The width of
this distribution is about
1.5%, which is consistent
with photon counting
statistics.
Challenge in the MUV:
Dark Rate
The dark rate of both MUV detectors is much
higher on orbit than it was pre-launch. The
rate is highly variable and shows a steady
upward trend with time.
Challenge in the FUV:
Spectral Shape
The observed wavelength for each stellar
observation can vary due to pointing offsets, and in
the FUV the stellar irradiance can be a strong
function of wavelength, even at the low-resolution
of stellar mode. Correcting to a common
wavelength is one of the dominant factors in
reducing the spread of stellar measurements.
Spectrum of Spica from SOLSTICE stellar scans. The dots are
the measurements from six individual scans and the line is the
merged spectrum at the stellar mode resolution of 1 nm.
The solution is to couple each stellar
measurement with a brief measurement of the
dark rate. This change went into effect in
August, 2004 and is indicated by the dashed
line in the plot above.
Observations of dark regions of the sky. The green dots are
the individual integrations, the asterisks are the medians of
the observations, and the blue line is a smooth fit to the data.
The special observational technique used to remove
the airglow contamination includes three separate
measurements. The first observes a dark region of
the sky, so that the signal is entirely airglow
background. The instrument then slews through a
small angle to observe the star plus background, and
finally the first dark region is re-observed to
complete the background measurement. The entire
observation is reduced with a least-squares fit to a
polynomial background plus an offset for the star.
The polynomial coefficients and offset are fit
simultaneously. The resulting stellar observation has
similar statistical noise properties as the normal
stellar observations.
The Sun
Solar Irradiance
corrected for
degradation by the
stellar measurements.
Here is a 1-nm binned
time series of solar
irradiance at the
wavelength
corresponding to the
stellar degradation curve
in the FUV panel to the
far left. The dashed
curve is the uncorrected
irradiance.
Challenge for the solar data:
Field of view
The fundamental assumption of the
SOLSTICE technique is that changes in
the measured stellar irradiance reflect
changes in the instrumental sensitivity in
solar mode. The ratio of solar to stellar
observations depends only on the ratio
of the apertures and exit slits.
In the plot at left, MUV channel data are
shown in red, FUV in blue. The crosses
are UARS SOLSTICE observations, the
diamonds are from SORCE SOLSTICE.
This is a preliminary validation of the
pre-launch aperture measurements.