Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/stsci/meetings/nhst/talks/ErikWilkinson.pdf Äàòà èçìåíåíèÿ: Tue Apr 15 23:44:12 2003 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 03:14:19 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: coma

Second-generation holographic grating t ec h n o l o g y
Dr. Erik Wilkinson University of Colorado

April 11, 2003

Innovative Designs for the Next Large Aperture UV/Optical Telescope

Dr. Erik Wilkinson (CU)


Outline
· · · · · · · Challenges of UV instrumentation Holographic gratings in general Aberration-control theory in a viewgraph First-generation holographic technology Second-generation holographic technology What SGHG technology might do for NHST Where should research be done

April 11, 2003

Innovative Designs for the Next Large Aperture UV/Optical Telescope

Dr. Erik Wilkinson (CU)


The technical challenges of UV instrumentation
· · One goal of NHST is ¥10 HST/COS sensitivity
~ 5.8 meter diameter HST at Lyman a

But component efficiencies at UV wavelengths are low
­ Optical Coatings: · ~90% for 2000å
·

The obvious strategy has been to use fewer optical elements
­ improves instrument sensitivity · For example, HUT, FUSE, COS ­ degrades image quality, especially for off-axis sources

·

Holographic grating technology has enabled new scientific missions, because it has allowed instrument designers to minimize the number of optical elements.
April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope Dr. Erik Wilkinson (CU)


Holographic Gratings in Review
· Photoresist is deposited onto grating substrate · Pattern is recorded in photoresist · Grating is then chemically etched to produce the diffractive structure · Advantages include:
­ Very low in-plane scatter <2x10-5/å. ­ Net efficiencies are improving, ~50% ­ The efficiency is more uniform ­ Lower risk fabrication ­ More flexible aberration-control
April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope Dr. Erik Wilkinson (CU)


A few words on aberration-control
· · Grating theory is generally based on applying Fermat's Principle to the light path function of a diffractive system. The exact expression of F is...

·

nml d Expand F into a power series in terms of w and l, the horizontal & vertical position on the grating. F = ( x - x ) + ( y - w) + ( z - l

[

2

2

)

2

][
2

1

+ ( x ' -x ) + ( y ' - w ) + ( z' - l

2

2

)

2

]

1

2

+



1 1 1 1 F = F000 + wF100 + w 2 F200 + l 2 F020 + w 3 F300 + wl 2 F120 + .... 2 2 3 2 where Fijk = M ijk ( r,a , r', b )

·

The advantage of this formulation is that each Fijk term is associated with a specific aberration
­ ­ ­ ­ ­ ­

(x,w,l)

F100 is the grating equation F200 is spectral focus F020 is spatial focus F300 is coma F120 slit curvature 4th order is spherical aberration

(x,y,z)

(x',y',z') Dr. Erik Wilkinson (CU)

April 11, 2003

Innovative Designs for the Next Large Aperture UV/Optical Telescope


Classic Analogs
· First holographic gratings were classic analogs... parallel grooves ­ Just replacements for mechanically ruled, parallel groove gratings...yawn. ­ Aberration control was only through controlling the geometry, i.e. Rowland circle, Wadsworth, or toroidal Rowland circle.

April 11, 2003

Innovative Designs for the Next Large Aperture UV/Optical Telescope

Dr. Erik Wilkinson (CU)


First Generation Holographic Gratings
· · Now assume rulings defined by the interference pattern of two coherent, stigmatic laser sources. The nth groove is then defined by....

nl0 = [ CP - DP ] - [ CO - DO
· ·

]
First Generation Holographic Grating

·


The groove pattern is then defined by the interference pattern generated by the laser sources and the substrate. This allows the designer to zero out aberrations inherent in the design by introducing equal and opposite aberrations with the groove pattern. ml Fijk = M ijk ( r,a , r', b ) + H ijk ( rc , g , rd ,d ) l0 FGHG designs have 4 unknowns (rc, rd, g, d) & thus 4 aberration terms can be zeroed out.

Noda, Namioka, & Seya, "Geometric theory of the grating," J. Opt. Soc. Am. 64, 1031-1036 (1974). April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope Dr. Erik Wilkinson (CU)


Second-generation holographic technology
· · The groove pattern is set by the interference pattern generated using at least 1 aberrated wave-front (i.e. non planar or spherical) SGHG designs provide more degrees of freedom for controlling aberrations.

April 11, 2003

Innovative Designs for the Next Large Aperture UV/Optical Telescope

Dr. Erik Wilkinson (CU)


The aberration coefficients
Aberration Coefficient
F
00

Geometric & Holographic Components
M 00 = r + r
'

Aberration
Basic Light Path



H 00 = rc - rd M10 = - sina - sin b

Note:
Grating Equation

F

10

H10 = - sin g + sin d
M 20 = cos2 a cos2 b 1 + - (cosa + cos b r r' R

F

20



)

Spectral Focus

cos 2 g cos 2 d 1 H 20 = - (cos g - cos d rc rd R

)
Astigmatism



F

M 02 =
02



111 + - (cos a + cos b r r' r
111 - - (cos g - cosd rc rd r

)

H 02 =

)
Type I Coma
)C
2



SGHGs can control higher order terms

F

30

é T (r,a ) ^ é T ( r' , b ) ^ M 30 = à ~ sina + à ~ sin b êr¯ ê r' ¯
é T ( r ,g ) ^ é T ( r ,d ) ^ 2( A10 H 30 = à c ~ sin g - à d ~ sin d R1 ê rc ¯ ê rd ¯ KC sin hC + 2( A10 R
2




)

2 D

K D sin hD

F

12

sin H12 = rC '

é S ( r,a ) ^ é S ( r' , b ) ^ M12 = à ~ sina + à ~ sin b êr¯ ê r' ¯
g õ 1 é rC ' ^ cos ì -à ~ í rC ' ê rC ¯ r
2

Slit Curvature (Type II Coma)
)C (B01 )C
2 VC sin hC - ( A10 r2

1. Aberrations scale with 1/(F/#), so fast systems exhibit larger aberrations. 2. NHST will use fast optics. 3. Aberration control of some sort will be needed to maintain image quality.

These terms controlled by FGHG



g sin g ° rD '

õ 1 é r ' ^ cos ì -à D ~ í rD ' ê rD ¯ r
C

d 2 + ( A10 ° r1

) D (B01 )
C

D

VC sin h

D

Auxiliary Equations


T ( r,a

)

=

cos a cos a r R

( A10 )

=-

cos g AC qC cosh

S( r,a
C

)

1 cosa =r r

( B01)

=

1 BC qC

April 11, 2003



Innovative Designs for the Next Large Aperture UV/Optical Telescope

Dr. Erik Wilkinson (CU)


SGHG technology has real potential
· The increased number of degrees of freedom available will provide enhanced aberrations control.
­ Better resolution, astigmatism control over a wider range of operational parameters
· e.g. field of view, band-pass

­ May allow simpler optics to be used in place of aspheric optics ­ SGHG technology has been demonstrated in a limited number of cases (Duban et al., Grange & Laget, Namioka & Kioke)

·

Much of FGHG technology is directly transferable to the development of SGHGs.
­ Techniques exist to create groove profiles (sinusoidal, psuedo-laminar, laminar, and triangular) ­ Efficiency has steadily been improving and will continue

·

The lithography industry is indirectly supporting holographic gratings
(photoresists, laser technology, and etching techniques)

·

SGHG performance capabilities are largely unexplored, especially for astronomical applications.
April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope Dr. Erik Wilkinson (CU)


A Potential Application
· We have designed a FGHG system for wide-field spectroscopic imaging, but the performance is limited by incomplete aberration-control. This design could be adapted to meet NHST goals. Off-axis Gregorian telescope
­ Minimum # of optics ­ Slit wheel for point source, long-slit, and multi-object spectroscopy ­ Possibly replace elliptical secondary with a spherical secondary (lower technical and programmatic risk) ­ Camera at n=0 could provide simultaneous imaging and spectroscopy of the same field.
· Compound lens or tertiary could correct image quality over a wide field.
April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope
Elliptical Grating

FGHG instrument l/Dl~500 l~950-1100å f.o.v =0.5°

·

Primary Focus/ Slit

Parabolic Primary

0 Order Focus

Diffracted Focus

Dr. Erik Wilkinson (CU)


Conclusions
· The capabilities of SGHGs are largely unexplored, because there has yet to be a general study of their uses.
­ There is every reason to believe they will work based on the few existing examples ­ There is real potential for enabling very capable instruments for NHST ­ SGHG technology could simplify optical substrates, reducing cost and risk

·

SGHG technology is low risk.
­ The technology to create a SGHG is in aligning the recording set-up and is comparatively low-tech (improving the diffractive facets is ongoing work)

·

What is industry doing?
­ Jobin-Yvon is increasing fabrication capacity...they see a long-term future. ­ Industry is providing significant R&D for photoresists and recording lasers to support the lithography industry for m-chip fabrication.

·

What steps should NASA take in the near future?
­ Study the uses of aberration-corrected gratings
· FGHG development has paid-off handsomely, now is the time to take the next step · It is unlikely that industry will do this...no need · Large format gratings will be needed
April 11, 2003 Innovative Designs for the Next Large Aperture UV/Optical Telescope Dr. Erik Wilkinson (CU)