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VIRTIS Operations at Lutetia

This timeline provides details on the first and second light curves, as well as the operative parameters for the internal scan phases during the VIRTIS operations at asteroid Lutetia.

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VIRTIS Operations at Lutetia

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  1. VIRTIS Operations at Lutetia VIRTIS Team

  2. Timeline

  3. General • First Light curve: a S/C offset in the Y direction as been introduced to avoid the damaged region of the filter • Second Light curve: during this LC we must consider the S/C flip happening between 11:34 and 12:14.

  4. The Second Light Curve will continue until 14:42 (start of) Asteroid Fly By Mode

  5. First Light CurveVIRTIS-MVIS Signal evaluation Signal at start of First LC (Lutetia sub-pixel (0.36)) Signal at end of First LC (Lutetia sub-pixel (0.58))

  6. T=200K T=200K First Light CurveVIRTIS-MIR Signal evaluation Signal at start of First LC (Lutetia sub-pixel (0.36)) Signal at end of First LC (Lutetia sub-pixel (0.58))

  7. Local Time Vs Surface Temperature

  8. T=170K T=220K Light CurveVIRTIS-M IR Signal evaluationas a function of T

  9. First Light Curve: Operative Parameters • 15 lines for each acquisition (+2dark) • 5 s repetition >> 85s >> 340 punti • 10 s repetition >> 170s >> 170 punti • 20 s repetition >> 340s >> 85 punti Selected • Lutetia rotates with angular speed of 0.0125 deg/s • With 20s repetition we get one LC point every 4.25deg • Compression wavelet 2; factor 8 • Binning Mode • Modo reduced slit: 3x1 • Modo all pixel : 1x1 Selected • Integration Times 4-5s VIS e 2s IR • Verify Temperature increase of CCD due to the selected integration time

  10. Parameters for ITL First Light Curve • 20s repetition • 15lines (start angle and stop angle need offset) • Dark frequency one every 15 acquisitions • Wavelet 2 compression • Mode full window (1x1) • 8 hours operations start 09 July 19:00 stop 10 July 03:00 • DATA VOLUME  660Mbit = 82.5MBytes • Integration times 4s for VIS and 2s for IR • Every hour stop acquisition to limit size of cube: we shall get 8 cubes in total

  11. Second Light CurveVIRTIS-MVIS Signal evaluation Signal at start of Second LC (Lutetia sub-pixel (0.75)) Signal at end of Second LC (Lutetia 4pixels)

  12. T=220K T=220K Second Light CurveVIRTIS-MIR Signal evaluation Signal at start of Second LC (Lutetia sub-pixel (0.75)) Signal at end of Second LC (Lutetia 4pixel )

  13. Second Light Curve: Operative Parameters • 15 lines for each acquisition (+2dark) • 5 s repetition >> 85s >> 340 punti • 10 s repetition >> 170s >> 170 punti • 20 s repetition >> 340s >> 85 punti Selected • Compression wavelet 2; factor 8 • Binning Mode • Modo reduced slit: 3x1 • Modo all pixel : 1x1Selected • Integration times 1.5s VIS e 1s IR

  14. Parameters for ITL Second Light Curve • 20s repetition • 15lines (Alice Narrow Slit Boresight) • Dark frequency one every 15 acquisitions • Wavelet 2 compression • Mode full window (1x1) • 8h 33min operations: start 10 July 06:09 stop 10 July 14:42 • DATA VOLUME  700Mbit = 87.5MBytes • Integration times 1.5s for VIS and 1s for IR • Every hour stop acquisition to limit size of cube: we shall get 8 cubes in total

  15. Internal Scan – First part • The reason for the scan rely on the absence of longitudal motion between 15:00 and 15:40, this suggest to use an internal scan to cover the full area of the asteroid. • On the other hand as we approach the asteroid the pixel resolution increases; this imply that the maximum number of acquisition per scan shall be limited by the change in resolution. This should be less than about 15%. • The S/C will point with the AL narrow slit centre. VIRTIS-H can operate in this case. • In the first part from 15:15 to 15:30 we shall perform full surface scans.

  16. Internal Scan – Second Part • After 15:30 we shall not be able to image the full asteroid without introducing distortion in the resulting image. • But the Longitude variation shall still be minimal until about 300s before CA. • We have then introduced a second scan phase, limited to a portion of the asteroid surface, limited to a scan of 10lines • This will have the additional advantage that we could properly sample the zero phase passage. • This phase will last from 15:32 to about 15:43 (about 450s before CA)

  17. Lutetia S/C footprint over surface

  18. The sub S/C longitudedoesn’t vary considerably until 300s before CA;

  19. To keep the resolution variation across one image scan image less than 15% we split the scans in two parts • From 2250 to 1250 seconds before CA: full surface scans • From 1250 to 450 seconds before CA: limited surface scans

  20. Parameters for ITL Internal Scan Phase First Part • First 7 scans (continuous scan; produces a single cube) • 5s repetition • Integration times 1.5s for VIS and 1s for IR • 20lines • Dark frequency one every 20 acquisitions • Wavelet 2 compression • Total duration 750s • Mode full window (1x1) • DATA VOLUME each scan 70Mbit = 8.75MBytes • Scan 8 and 9 • 5s repetition • Integration times 1.5s for VIS and 1s for IR • 30 lines • Dark frequency one every 20 acquisitions • Wavelet 2 compression • Total duration 315s • Mode full window (1x1) • DATA VOLUME each scan 16Mbit = 2MBytes

  21. Parameters for ITL Internal Scan Phase Second Part • Scans 10 to 18 (continuous scan; produces a single cube) • 5s repetition • Integration times 1.5s for VIS and 1s for IR • 10lines • Dark frequency one every 20 acquisitions • Wavelet 2 compression • Total duration 475s • Mode full window (1x1) • DATA VOLUME each scan 45Mbit = 5.5MBytes

  22. Pushbroom Scan • During this part of the sequence, VIRTIS-M will operate in pushbroom mode, relying only on the S/C motion over the surface. • Both VIRTIS-M and H will operate. • Between 450s and 150s there will be oversampling the instrument ground resolution being larger than the relative shift of the sub S/C point on ground. • During the 300s around CA on the contrary the S/C motion shall be faster than the instrument resolution giving undersampling conditions (See next slides) • This phase shall last from 15:45 until 16:00

  23. S/C at t=0 S/C at t=5s CA OperationsGround Resolution and Surface track The S/C is at distance D from the asteroid, and its track moves of angles  and  in one repetition time over the asteriod surface; We assume Virtis Slit perpendicular to S/C ground track; Ground Resolution = D * IFOV S/C track surface projection

  24. Pixel Resolution OVERSAMPLING Ground track shift over 5s • At ±2min around CA there will be coverage gaps between subsequent acquisition, due to the combined S/C motion and repetition time

  25. Parameters for ITL Pushbroom Phase • Continuous scan from 380s before CA until 520s after CA • 5s repetition • Integration times 1.5s for VIS and 1s for IR • No scan mirror motion • Dark frequency one every 20 acquisitions • LossLess compression • Total duration 900s • Mode full window (1x1) • DATA VOLUME each scan 330Mbit = 41MBytes

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