UVIS Calibra,on Update Greg Holsclaw Bill McClintock Jan 5, 2012 Outline •
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Miscellaneous updates A poin,ng curiosity Calibra,on used for PDS is old? Preliminary results from the recent slow-­‐slew calibra,on Miscellaneous updates •  IDL kernel finder rou,ne –  Bug fix related to properly iden,fying ,me ranges of c-­‐kernels •  Data cube visualiza,on –  Bug fix related to the sky projec,on, and how the FOV was drawn –  Now agrees with output from geometer geometer uvis_sim.pro Poin,ng curiosity •  Roger Yelle has no,ced what appear to be wavelength shiYs in EUV solar occulta,on spectra of Titan •  From the reconstructed c-­‐kernels, the Sun appears to move only slightly, but not enough to account for the spectral displacement •  The reported a\tude appears to be perfectly mo,onless in the iner,al frame A B
All spectra from t>650s Spectra at three ,mes B
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•  Wavelength of the spectral lines shiY during the occulta,on by ~0.2 Angstroms implying a poin,ng driY of ~0.064 mrad [from Roger Yelle] •  The poin,ng stability has been reported to be 0.011mrad [Pilinski and Lee, Poin,ng-­‐Stability Performance of the Cassini SpacecraY, 2009]. Reconstructed poin,ng •  ~20sec before poin,ng stabilizes •  DriY of 0.0005mrad in x (dispersion plane) •  DriY of 0.015mrad in y •  Pixel size: 0.25mrad (spectral) x 1mrad (spa,al) •  My understanding is that reconstructed poin,ng has been found to accurate to 0.14mrad based on comparisons with c-­‐
smithing AYer the first ~20 data points, the z-­‐axis of the spacecraY is perfectly fixed in iner,al space. Old UVIS calibra,on for PDS? •  UVIS calibra,on rou,nes used to produce PDS data for July 1, 2011 data and earlier (version 2) –  EUV: Get_EUV_03…, no flat-­‐field corrector applied –  FUV: Get_FUV_07…, no flat-­‐field modifiers (correc,on for regional declines in sensi,vity) Rou,ne stellar calibra,ons Low-­‐res slit •  This plot shows the posi,on of the star within the FUV low-­‐
resolu,on entrance slit •  Typical observa,on: –  Integra,on ,me: 45sec –  Slew rate: 0.02mrad/sec (0.9 rows/integra,on) –  ~78 full (non-­‐binned, non-­‐windowed) readouts •  Spectral pixel is 0.25mrad wide •  Spectral mo,on of image: •  Average: 6% of a spectral pixel •  Max: 30% of a spectral pixel Single readout Ver,cal sum •
Fit a Gaussian to the spa,al profile to derive: –  Sub-­‐pixel loca,on of image –  Image size (height) •
Compute the total signal at each image posi,on spa,al Detector image spectral Horizontal sum Sum all readouts •
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Simulated extended source (each row uniformly illuminated) Exhibits flat-­‐field nonuniformity (apparent row-­‐to-­‐row varia,on in sensi,vity) Single frames (anima,on) Single frames with total signal at image loca,on (anima,on) Cumula,ve sum (anima,on) Extended source vs point source •  An extended source appears to exhibit flat field effects •  The total signal from a point source, as a func,on of posi,on on the detector, does not The total signal in the two curves is equivalent, but the red line has more data points. Local misloca,on of counts? Say a photoevent located in this pixel is counted by the pixel above •  Hypothesis: photoevents that occur within the geometric area of an adjacent spa,al pixel are erroneously counted Local misloca,on of counts? 80μm 100μm 140μm 60μm 120μm •  The effect of these misloca,ons is a change in the effec,ve width and posi,on of spa,al pixels. •  The flat-­‐field varia,on is caused not by changes in QE, but by changes in effec,ve area. Single pixel response vs ,me (standard calibra,on) •  Slew rate: 0.02mrad/sec (0.9 rows/integra,on) •  Integra,on ,me: 45sec •  This plot shows the output from a single pixel as a func2on of 2me •  The curve represents the convolu,on of the spectral image with the pixel response •  The effec,ve width of the pixel response can be calculated from a gaussfit Correla,on of flat-­‐field with row width (standard calibra,on) EUV FUV •  Black line – row-­‐to-­‐row flat-­‐field •  Red line –row height Correla,on of flat-­‐field with row width (standard calibra,on) EUV FUV •  Black line – row-­‐to-­‐row flat-­‐field •  Red line –row height Effec,ve height of central rows unaffected by starburn event Single pixel response vs ,me (standard and slow-­‐slew) Fast •  Slew rate: 0.02mrad/sec (0.9 rows/integra,on) •  Integra,on ,me: 45sec Slow •  Slew rate: 0.004mrad/sec (0.18 rows/integra,on) •  Integra,on ,me: 45sec •  Slew rate: 0.02mrad/sec (0.9 rows/integra,on) •  Integra,on ,me: 45sec •  Slew rate: 0.004mrad/sec (0.18 rows/integra,on) •  Integra,on ,me: 45sec •  Higher contrast, improved correla,on with “flat-­‐field” •  Check imaging performance with raytrace? FUV Spa,al imaging varia,on •  Height of a single row varies systema,cally across the detector •  Due to lower imaging resolu,on, caused by higher energy of photo-­‐
electron at short wavelengths? •  Variable spectral/
spa,al resolu,on Implica,ons •  We can separate the effects of effec,ve area from quantum efficiency of the photocathode –  Spa,al variability (the flat-­‐field) caused by varia,ons in area –  Starburn caused by real decline in sensi,vity •  Characterizing varia,ons in spa,al row boresights (+/-­‐10%?) and height (+/-­‐20%) affects: –  Imaging resolu,on and effec,ve boresights (sampled areas) of aurora, satellites, atmospheres, rings… –  Absolute brightness measures of small objects •  Evil pixels caused by small effec,ve areas? –  Can we leverage a current dataset to characterize this (mul,ple passes along the slit) or design a new slow-­‐slew across the slit? –  May lead to new approach to dealing with evil pixels Andrew Steffl flat-­‐field corrector observa,on (anima,on) Future stellar calibra,on op,ons •
Con,nue “standard” stellar calibra,ons –
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Complete slit sampling (i.e. Andrew Steffl FF corrector) –
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single pass along slit Frequency: every ~90days Dura,on: ~3hrs Integra,on ,me: 45sec Data volume: ~12MB/channel ? Rou,ne sensi,vity calibra,ons, check of sudden changes in response Mul,ple slews along the slit (0.2mrad steps across slit) Frequency: Once/year? Dura,on: ~13 hour Integra,on ,me: 332sec Data volume: 18MB/channel ? Improved sensi,vity calibra,on ? Slow slew to characterize spa,al response –  Need ~3 more observa,ons to sample the en,re detector –  Resources for each observa,on the same as the “standard” calibra,on Slow-­‐slew single frames (anima,on) Slow-­‐slew cumula,ve sum (anima,on) To do •  Regenerate PDS data with current calibra,on approach •  Derive effec,ve row height (matrix) and row centers (vectors) •  Revise calibra,on rou,ne to separate row height (currently assumed to be constant) and sensi,vity More Recent stellar calibra,ons EUV FUV These plots show the total signal on the detector as a func,on of star posi,on along the slit All stellar calibra,ons EUV FUV These plots show the total signal on the detector as a func,on of star posi,on along the slit Decline in FUV in sensi,vity over ,me Total signal from Spica vs row posi,on of the image, for all calibra,on observa,ons. Mean value of the signal when the star was located between rows 18 and 22, then normalized to the first. Data vs model Total FUV signal with linear trend divided out. Also shown is the predicted varia,on in flux from the model. Varia,on 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 ) Data vs model r = Linear Pearson correla,on coefficient using IDL’s correlate() func,on, value ranges from -­‐1 to 1 Background on Alpha Vir (Spica) •
Spica is a non-­‐eclipsing double-­‐lined spectroscopic binary system –  Though not spa,ally resolvable, each component is detectable through measurements of out-­‐of-­‐phase Doppler shiYs in the cons,tuent spectral lines –  Non-­‐eclipsing due to large apparent orbital inclina,on of ~70 degrees –  Both stars are of a similar spectral class: •
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Primary: B1V Secondary: B4V Spica is the brightest rota,ng ellipsoidal variable star –  The stars have a distorted ellipsoidal shape due to mutual gravita,on 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 h|p://observatory.sfasu.edu –  Orbital period is 4.01454 days –  Amplitude of flux varia,on in V-­‐filter ~3% •
The primary of Spica is a Cepheid variable –  Periodic varia,on in the pulsa,ng 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 varia,on in V-­‐filter ~1.5% –  This short-­‐term varia,on, iden,fied in 1968, became undetectable in the early 1970’s (but may return again due to precession of the primary’s rota,on axis rela,ve to the orbital plane, which has a period of 200 years [Balona, 1986]) Ellipsoidal varia,on model Varia,on 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 distor,on”) M2/M1 = 1/1.59 (ra,o of masses) R = 7.6 Rsun = 5.2858e6 km (polar radius of primary) D = 1.92916e7 km (mean separa,on 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 inclina,on) Φ = empirical phase shiY, a free parameter to match with data One period of the expected varia,on in flux from Spica