1. UVIS calibration update Greg Holsclaw Bill McClintock Jan 8, 2013 1

2. Outline Calibration observations – obtained, planned, future opportunities Calibration status EUV wavelength issues Data compression 2

3. Calibration observations Recently obtained standard cal – EUV2012_358_08_02_13_UVIS_177IC_ALPVIR001_PRIME – FUV2012_358_08_02_13_UVIS_177IC_ALPVIR001_PRIME Planned standard and STEFFL – UVIS_196IC_SPICARAST001_PRIME – 2013-230T14:00:00 to 231T08:30:00 – Target: Spica – Data volume: 320 Mbits 3

4. Comparison of recent stellar calibrations 4 These plots show the total signal on the detector as a function of star position along the slit EUVFUV

5. All stellar calibrations 5 These plots show the total signal on the detector as a function of star position along the slit EUVFUV

6. Decline in FUV in sensitivity over time 6 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.

7. Data vs model 7 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 (1-3cos 2 (TA+TA0+Φ) sin 2 i ) r = Linear Pearson correlation coefficient using IDL’s correlate() function, value ranges from -1 to 1

8. Future calibration opportunities Our desired approach – Two standard calibrations per year using Spica – One STEFFL calibration per year using Spica Look for opportunities within XD segments where either: – Saturn-Spica angle is < 30deg as seen from the s/c – Earth-Spica angle is ~90deg 8

9. XD periods where Saturn-Spica angle is below 30 deg period_name start end TWT date_min angle_min XD_183_184 2013-069T14:29 2013-078T13:45 XD 2013-070 // 12:00 15.6 XD_187_188 2013-113T11:32 2013-119T11:02 XD 2013-113 // 12:00 16.7 XD_196_197 2013-226T09:51 2013-254T08:03 XD 2013-230 // 12:00 13.5 XD_200 2013-362T01:47 2013-365T01:48 XD 2013-364 // 12:00 23.4 XD_200_201 2014-003T05:48 2014-031T23:51 XD 2014-031 // 12:00 17.0 XD_201_202 2014-037T17:07 2014-064T21:56 XD 2014-064 // 12:00 10.5 XD_203_204 2014-101T13:00 2014-133T10:46 XD 2014-131 // 12:00 4.7 XD_205_206 2014-170T14:29 2014-197T12:41 XD 2014-192 // 12:00 6.9 XD_206_207 2014-212T05:09 2014-229T10:37 XD 2014-223 // 12:00 7.7 XD_209 2014-279T01:01 2014-293T06:30 XD 2014-291 // 12:00 4.0 XD_209_210 2014-299T06:00 2014-340T21:30 XD 2014-339 // 12:00 0.7 XD_210_211 2014-351T03:15 2015-008T01:47 XD 2015-007 // 12:00 19.8 XD_223_224 2015-289T07:31 2015-301T00:30 XD 2015-300 // 12:00 29.4 XD_231_232 2016-038T00:48 2016-045T00:34 XD 2016-044 // 12:00 19.5 XD_232_233 2016-050T00:05 2016-068T22:52 XD 2016-067 // 12:00 3.9 XD_235 2016-109T18:43 2016-124T19:29 XD 2016-119 // 12:00 10.5 XD_235_236 2016-129T19:14 2016-155T17:15 XD 2016-148 // 12:00 17.9 XD_236_237 2016-161T10:30 2016-178T15:44 XD 2016-173 // 12:00 24.9 XD_237_238 2016-183T00:59 2016-202T07:28 XD 2016-197 // 12:00 24.9 9

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11. 2013 Solid line is the angle between Saturn and Spica 11

12. 2014 Solid line is the angle between Saturn and Spica 12

13. 2015 Solid line is the angle between Saturn and Spica 13

14. 2016 Solid line is the angle between Saturn and Spica 14

15. Notes s/c ephemeris kernel used: 110818AP_SCPSE_11175_17265.bsp The spacecraft ephemeris is sampled in 24 hour increments I obtained the XXM segmentation list (xxm_segments.xls) from this site: – https://cassini.jpl.nasa.gov/sp/xxmsp.html https://cassini.jpl.nasa.gov/sp/xxmsp.html – Labeled as "XXM segments/sequence spreadsheet: [XLS]" under the title "XXM References" on the right side – Retrieved on Oct 19, 2012 15

16. EUV wavelength scale issues The group at U of Arizona/LPL (Roger Yelle, Tommi Koskinen, Fernando Capalbo) have noted that the nominal EUV wavelength vector does not adequately fit solar occultation spectra. A linear shift would be expected given that the solar image is not constrained by the entrance slit, but there appears to be variation in dispersion as a function of spatial position. 16

17. Pointing stability of dataset for analysis Position of Sun in EUV solar occ port frame from SPICE Max drift in dispersion plane of 0.056mrad (20% of a spectral pixel) 0.25 mrad (1 spectral pixel) Data file: EUV2007_108_00_47_04_UVIS_043SU_SOL001_PRIME Objective: solar calibration Design: Point solar occultation port to sun. Slew 10 mrad in +Z direction to start. Slew 20 mrad in the -Z direction at 20 micro rd/sec 17

18. Data file: EUV2007_108_00_47_04_UVIS_043SU_SOL001_PRIME 584.3350, He I 629.7320, O V 703.8540, O III 949.7450, H I 977.0200, C III 1025.7240, H I 1031.9138, O VI 1084.5820, N II Emission line reference: “Predicted XUV Line Intensities CHIANTI database - Version 7.0” Measure of the spectral position of several solar lines at each detector row 18

19. Filled-aperture, filled-slit comparison 19 Sun occ-port Sun occ-port Venus telescope Venus telescope 58.4 nm102.6 nm

20. Performance check with raytrace Only a small fraction of the optics are illuminated by the solar beam Pickoff mirror is a cylindrical surface with R=500mm in the spatial dimension, with an effective area of ~1x1mm Telescope mirror grating detector Aperture stop 20

21. Raytrace results Slight spatial curvature. No evidence for spatially dependent dispersion. 21

22. One hypothesis for spatially dependent dispersion A spatially dependent groove spacing could explain the observed effect Because we do not see this effect for a filled aperture, this abnormality would need to be localized to the small part of the grating illuminated by the Sun (area of 3x3mm) Solar beam on grating Grating grooves 22

23. Data Compression The UA/LPL group have expressed concern regarding the impact of compression used for stellar/solar occultations This led to a realization that the compression algorithm is not adequately documented I wrote a short description of the algorithm and its effect on the data, with the intention to include this in the PDS user’s guide 23

24. Data Compression algorithm The default compression implemented is SQRT-9. The square root algorithm can be described by the following pseudocode: IF VALUE > 128 COMP. VALUE = ROUND( SQRT(VALUE * 2) ) + 128 ELSE COMP. VALUE = VALUE END IF 24

25. Quantization error Let Ni be the input number of counts Uncertainty in Ni: σ=sqrt(Ni) It can be shown that the uncertainty introduced by the compression is: σ/sqrt(2) 25

26. Summary No significant changes in FUV sensitivity STEFFL observation planned Many opportunities for future XD calibrations EUV solar occultation wavelength scale exhibits a spatially-dependent dispersion (~2 pixels), different than normal mode Compression algorithm is now documented To do: – 2D wavelength scale for EUV and FUV – Update PDS User’s guide with compression algorithm – Continue work on revised flat-field approach 26