ironing out the wrinkles in space telescope imaging spectrograph t.r. ayres (casa)
DESCRIPTION
Ironing out the WRINKLES in Space Telescope Imaging Spectrograph T.R. Ayres (CASA) . Warning: this talk deals with lower rungs of J.J. Drake’s Ladder, where, sadly, sexiness is low . But, in fact, nothing on upper rungs of ladder would get done without good instrument calibrations . - PowerPoint PPT PresentationTRANSCRIPT
Ironing out the WRINKLES
in Space Telescope Imaging Spectrograph
T.R. Ayres (CASA)
Warning: this talk deals with lower
rungs of J.J. Drake’s Ladder, where, sadly,
sexiness is low.
But, in fact, nothing on upper rungs of ladder would get
done without good instrument
calibrations.
CALIBRATION
WRINKLES Project• OBJECTIVE: Test techniques for improving
echelle-mode wavelength scales of STIS, solar-system’s premier high-res UV spectrograph in
space (now in tandem with super-high-sensitivity, mod-res COS).
• METHOD: Measure deep exposures of Pt/Cr-Ne l*cal lamps (“WAVECALs”),
processed as if science images; compare with NIST/ECF laboratory wavelengths.
Raw Wavecal Images
E140H-1234 E230H-1763
low-ls
low-ls
WAVECAL Archive• Several hundred l*cals available,
mainly in primary tilts (darker colors).
Significant overlaps between settings.
Many secondary tilts have poor exposure
depth (yellow).
Laboratory Ref Wavelengths Excellent lists l < 1800
A (STIS lamp flight spare) from NIST/ST-
ECF collaboration (Sansonneti et al.
2004); but key STIS Cal Enhancement
extension to longer wavelengths (Kerber et
al. 2006) not yet completed. Earlier
NIST meas (black IDs) missing Cr I & II (blue prelim IDs); Wallace &
Hinkle (2009) KPNO FTS Cr I (red) l > 2360
A.
11 FUV-H + 26 NUV-H
template
“Self-Calibration*”
*Utilize STIS as its own laboratory spectrometer to enhance ref line list
l-meas
RESULTSReplacing CALSTIS Xcorr template
(below, red curve) has minor effect on H settings (upper left); virtually no
effect on M tilts (lower left). However, distortion correction makes factor of
~2 improvement in precision of echelle wavelength scales. (Note: full y-
axes are ~2/3 resol each).
Beyond post-facto Correction
• Properly orthogonal variables: CALSTIS dispersion polynomial: S= f(m,l), with terms up to fully bi-quadratic ([ml]2), although missing m2 and l2, but including extra term (ml)3 (coeffs not pop’d in CalLib, however). l has power in m direction; not properly orthog. k=ml (“grating parameter”) is orthog to m, but bi-quadr poly showed disappointingly little improvement over m,l combo.
Left panel compares current (‘orig’) CALSTIS disp coeffs, with newly derived values (‘new’) based on deep l*cals & new line
lists, and poly models (k,m) of increasing order: little improvement for n>2. Right panel adding terms to CALSTIS model. Single new term, m2, plus populating coefficients of
existing (ml)3, is as effective as full n=2.
Out-of-the-Box IdeasOn left side is map of E230H lamp lines in
“super-MAMA” detector coordinates versus
intensity (green=high brightness;
orange=intermed; open circles= low) ; right side shows residual red/blue
shifts after application of global model with terms in detector coordinates and dispersion space. Against
all odds, this actually works, with c2 not too
much larger than obtained for individual settings.
Suggests clear separation between detector
properties and dispersion characteristics.
At left: distortions from global modeling persistent in detector
coordinates. Distortions (max: +/-
2 km/s) very systematic between modes (M & H) and
cameras (FUV & NUV); apparently are fundamental property
of the MAMAs. Quadrupolar geometrical
distortions are unlike “pincushion” and “barrel” of typical optical systems.
Conclusions• STIS is a way excellent instrument, but capable of significantly
better wavelength performance than delivered by current dispersion models embedded in CALSTIS (although, to be sure,
the present precision well exceeds pre-flight specs).• Pipeline dispersion models could be significantly improved simply by re-deriving the coefficients for each setting using the upgraded ‘Deep Lamp’ material, as well as the new reference
‘self-laboratory’ line lists. Increasing exposure depth in current lightly exposed settings would be helpful in this regard.
(Cycle 18 GO cal program “Deep Lamp Too”).• Adding single term to existing CALSTIS polynomial, and
populating coefficients for an available, but unused term, would achieve significant additional improvement.
• Performing a geometrical correction prior to a robust dispersion model (k= k – k0[m]) could lead to a more stable global
solution.
Dispersion Relations Order by Order
Ripple Correction
Global Fit in Disp Coords, Only
Residual Wavelength Errors H-Spectrum & M-Spectrum, Cataloged
Lines