Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/hst/training/events/InfraredInstrumentation/sept99tips.fm.pdf Äàòà èçìåíåíèÿ: Tue Apr 23 15:20:07 2002 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 09:35:30 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: zodiacal light

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1

SPACE TELESCOPE SCIENCE INSTITUTE NGST Straylight
Causes of straylight for a telescope

16 September 1999 Matt Lallo

· Instrumental sources reaching the detectors either directly, or via scattering. · Celestial sources outside FOV scattered or directly seen by the detectors. Approach to minimizing straylight for NGST · Completely shielding the observatory from the sun, earth, and moon. · Using baffles and stops to prevent direct illumination of detectors by off-axis sources. · Cooling the observatory (passively or actively) to minimize thermal self-emission. Considerations particular to NGST · No telescope tube and large, difficult-to-clean optics promote scattering. · Sunshield back presents large, comparatively warm surface to the open OTA.


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SPACE TELESCOPE SCIENCE INSTITUTE

16 September 1999 Matt Lallo

General Scattering from Surfaces
Bidirectional Reflectance Distribution Function (BRDF) · Pc ­ ­ ­ ­ = Pc Ps i PsBRDF(i,o) is the power scattered onto the collector is the power from a source is the solid angle subtended by the collector as seen from the source. and o are the incident and output (scattered) angles respectively.

· BRDF can be conceptualized as the fraction of power scattered per steradian as a function of i-o (the angle off-specular or scatter angle.)


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SPACE TELESCOPE SCIENCE INSTITUTE

16 September 1999 Matt Lallo

Scattering from Contaminated Optics
Mie scatter theory or experiment (Spyak and Wolfe1) gives BRDF function for mirrors with varying distributions of dust particle sizes and coverage. · Shape of BRDF varies with particle size distribution. We have assumed a particle size distribution following Military Standard 1246C2, an industry standard to which respectable clean rooms adhere. This is an important potential source of error since contamination of NGST's large mirror segments any time after leaving the clean room environment may produce unknown particle size distributions which will the affect the BRDF. On-orbit cleaning? · The BRDF simply scales linearly with the amount of dust (areal coverage fraction). Various fractions have been used as input, but 1% has been assumed for the results here unless otherwise noted. (HST's primary mirror is believed to be between 1 and 2%)


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SPACE TELESCOPE SCIENCE INSTITUTE

16 September 1999 Matt Lallo

Scattering from Contaminated Optics (cont'd)
figures for 1% dust coverage

BRDF function (sr-1)
10
0

5 106

Particlevs. Particle Size ution Np size distrib

6 4 10

1

1

=1µm
X

3 106

0.1

1

=20µm

2 106

0.01

1

1 106

0.0013 1 10 scatter angle 90

0

1

1

10 microns

10

100 100


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5

SPACE TELESCOPE SCIENCE INSTITUTE Thermal Emission

16 September 1999 Matt Lallo

Temperature distribution of sunshield back drives instrumental backgrounds · Heats the OTA mirrors. · Produces photons that can be scattered to focal plane. · 6 layer sunshield design is dynamic and a recent GSFC iteration3 has been used as a basis for this study.
14 meters node temps in Kelvin (end-of-life)
59 64 89 65 65 62 76

46 38 43 53 62 73 46 56 53 46 67 79 62 53 73 59

53

55 66 72 66 55

107 90

48 56 62 56 48

41 48 50 48 41

38

34

35

112 156 89 64 107 65

113 83 90 65 76 62

41

36

43
38

34

38

~2. et r 7m e s
X ~2. e e s 7m t r

14 meters

18 meters


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SPACE TELESCOPE SCIENCE INSTITUTE Results
Direct instrumental background

16 September 1999 Matt Lallo

· Calculations show sunshield heats primary mirror to ~35K at which temperature its emission is well below natural zodiacal background up to ~24 µm.
electrons /sec on detectors (2x2 arcmin)

· Secondary mirror ~18K and trivial contributor. · Proper baffles & stops ensure that no other sources are seen directly by the detectors. (e.g. shield shining through P.M.)

1.E+12
20% bandpass

1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04
0 5 10 15 20 25 30

Zodiacal light in field (max & min)

Mirror thermal (35K)

Microns


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SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered instrumental background

16 September 1999 Matt Lallo

· A MathCad algorithm was used to calculate geometry and power transfer via scattering from each warm sunshield source node to each of the 8 primary mirror petals, and to the secondary. · Dust-contaminated optics will scatter some amount of each source node's power to detectors.

Scatter of Scatterf off secondary m mirror secondary irror

Scatter off off Scatter primary m mirror primaryirror


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8

SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered instrumental background (cont'd)

16 September 1999 Matt Lallo

· Scatter paths were summed and totals were calculated over wide wavelength range.
1.E+10
electrons /sec at detectors (2x2 arcmin)

· Sunshield and zodiacal background "crossover" at 12µm · Sunshield emission found to be dominant background source.

20% bandpass

sunshade

1.E+09
Zodiacal light in field (@ eclip. pole)

1.E+08

1.E+07

1.E+06

1.E+05 0 5 10 15
Microns

20

25

30


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9

SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered instrumental background (cont'd)

16 September 1999 Matt Lallo

· Analyses were performed varying sunshield temperatures and contamination levels. Control of mirror dust & shield temperatures critical to background.
35 35

wherelshieldshade scatter=zodizodi where background =

where shield background = zodi

30 30 25 25 20 20 15 15 10 10

13 12 11 10 0 1 2 3 4 5 6

5 5 50 50 70 70 90 90
Max. shield

100 110

130 130

Maximum sunshade temperature temperature (K) visible

to mirrors

Areal percentage of dust on mirrors


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10

SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Background from off-axis celestial sources

16 September 1999 Matt Lallo

secondary mirror baffle

· Direct illumination of detectors are prevented by: ­ a baffle around the secondary mirror. ­ central baffle surrounding the return beam. ­ a stop at the first (intermediate) focus. · Properly designed, these can prevent direct rays from off-axis sources entering the instruments.

central baffle internal stop


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SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered celestial source background

16 September 1999 Matt Lallo

· MathCad scattering model similar to that used for the sunshield background was used to determine power scattered from celestial sources. · Astronomical sources were split into zodiacal light, and everything else (galactic/extragalactic). ­ Zodi-subtracted Mission Average (ZSMA) skymaps from COBE's DIRBE experiment provided reliable input over a range of wavelengths for the latter. ­ Zodiacal light model by Wright4 was adapted by us for the former. · For a given telescope pointing and time of year, the power transfer via scattering by the secondary and primary mirror petals to the detectors was calculated. The contribution from each skymap pixel visible to the mirror surfaces was summed for the total result.


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SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered celestial source background (cont'd) DIRBE maps5 Full sky map

16 September 1999 Matt Lallo

Zodiacal light removed


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SPACE TELESCOPE SCIENCE INSTITUTE Results (cont'd)
Scattered celestial source background (cont'd)
1.E+10
20% bandpass

16 September 1999 Matt Lallo

elec./sec at detectors (2X2 arcmin)

1.E+09
Zodiacal background in field (@ eclip. pole)

1.E+08
Scattered zodi (for mid-ecliptic pointings)

1.E+07

Detector noise

1.E+06

Scattered galactic light (for mid-galactic pointings)

1.E+05

1.E+04 0 5 10 15 20 25 30

Microns Microns


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SPACE TELESCOPE SCIENCE INSTITUTE Results (con't)
10000 1000 electrons / sec /pixel 100
sunshield
detector noise

16 September 1999 Matt Lallo

NGST Backgrounds
20% bandpass max & min natural in-field zodi
40K

between

10 1 0.1 0.01 0 5

30K

min & max zodi

&

min & max galactic light

& EOL

BOL

10

15 Microns

Primary

mirror

20

25

30


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SPACE TELESCOPE SCIENCE INSTITUTE Utility

16 September 1999 Matt Lallo

· Results in NGST Monograph #2, Straylight Analysis of the Yardstick Mission. · Is being implemented into the NGST ETC as part of the DRM simulator for better assessments of exposure and overall science mission durations. ­ Multiple node ("finite element") OTA and sunshield model with higher fidelity geometry provides better background estimates than earlier single node calculations. ­ Provides a flexible framework in which one can assess dynamic designs more readily than with large commercial straylight programs, though such products better model details (baffles, diffraction, minor components, etc.) ­ DIRBE maps give more accurate source inputs than previously used starcounts from Allen. ­ Realistic BRDFs as a function of wavelength, dust coverage, scatter angle, and surface microroughness enhance fidelity.


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SPACE TELESCOPE SCIENCE INSTITUTE References

16 September 1999 Matt Lallo

1. P.R. Spyak & W.L. Wolfe, "Scatter from Particulate-contaminated Mirrors," Optical Engineering, 31 (8), pp 1746-1784, 1991. 2. Military Standard 1246C, "Product Cleanliness Levels and Contamination Control Program," April 11, 1994 3. K. Parrish, personal communications, 1999 4. E.L. Wright, "Angular Power Spectra of the COBE DIRBE Maps," Astrophysical Journal v.496, p.1, 1998 5. M. Hauser & COBE DIRBE science team, STScI Press Release 98-01