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A solid state detector head for astronomical applications Lester, Timothy Paul 1977

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A SOLID STATE DETECTOR HEAD FOR ASTRONOMICAL APPLICATIONS by TIMOTHY PAUL LESTER B . S c , U n i v e r s i t y of V i c t o r i a , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Geophysics and Astronomy) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d , s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September, 1977 © Timothy P a u l L e s t e r , 1977 In present ing th is thes is in p a r t i a l fu l f i lment o f the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l ica t ion of th is thes is for f i n a n c i a l gain sha l l not be allowed without my wri t ten permission. Geophysics and Department of Astronomy The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date October 1977 i ABSTRACT A d e t e c t o r head f o r h o u s i n g s e l f scanned s i l i c o n p h o t o -d e t e c t o r a r r a y s has been c o n s t r u c t e d f o r use i n astronomy. I t employs a unique f r o z e n methanol cooling-"system and, a l -though b u i l t s p e c i f i c a l l y f o r the l i n e a r R e t i c o n d e v i c e s , i t i s c a p a b l e of h o u s i n g o t h e r l i n e a r or two d i m e n s i o n a l mono-l i t h i c a r r a y s . T h i s d e t e c t o r head i s used w i t h a c o n t r o l and d a t a a c q u i s i t i o n system t h a t was developed a t the U n i v e r -s i t y of B r i t i s h C o l u m b i a by the I n s t i t u t e of Astronomy and Space S c i e n c e . A p r e v i o u s d e t e c t o r head, used w i t h the above c o n t r o l system had i n c o r p o r a t e d a d r y i c e c o o l e d 2$6A/l7 R e t i c o n a r r a y i n a h o u s i n g c o n s t r u c t e d , s p e c i f i c a l l y f o r use on an F / l con-c e n t r i c m i r r o r s p e c t r o g r a p h camera. T h i s o l d e r d e v i c e had many problems a s s o c i a t e d w i t h i t w h i c h made i t d i f f i c u l t t o operate and u n r e l i a b l e . W i t h the 256A/17 R e t i c o n mounted i n the new camera head the s e problems have been e l i m i n a t e d . I n p a r t i c u l a r , f r o s t i n g no l o n g e r o c c u r s , dark c u r r e n t has been reduced t o a n e g l i g a b l e l e v e l , and the r e a d o u t n o i s e has been r e d u c e d by a f a c t o r of two. t As a r e s u l t of t h i s s u c c e s s , a 1024C/17 R e t i c o n has been s u b s t i t u t e d f o r the 256 element a r r a y and t h e r e are p l a n s f o r mounting a 100 x 1 0 0 two d i m e n s i o n a l a r r a y i n t h i s d e t e c t o r head. i i TABLE OF CONTENTS Page ABSTRACT i ACKNOWLEDGEMENTS i v Iv . .INTRODUCTION 1 I I . DESIGN FEATURES OF THE DETECTOR HEAD 8 1. The Hous i n g 9 2. C o o l i n g System 12 3. D e t e c t o r E l e c t r o n i c s 13 I I I . FINAL PERFORMANCE 16 IV. APPLICATIONS 18 BIBLIOGRAPHY 33 APPENDIX A APPENDIX B ' 36 APPENDIX C 37 i i i LISTCOF FIGURES F i g u r e s Page 1. S i m p l i f i e d s c h e m a t i c of a l i n e a r d i o d e a r r a y 2 2. Charge t r a n s p o r t i n a CCD 4 3. Readout of the CID 5 4. E x p l o d e d cut-away assembly d r a w i n g of the d e t e c t o r 19 head 5. D e t a i l e d drawings 20 6. D e t a i l e d drawings 21 7- D e t a i l e d d r awings 22 8. D e t a i l e d drawings 23 9. The completed d e t e c t o r head 24 10. P a r t i a l l y d i s m a n t l e d d e t e c t o r head 25 11. F r o n t view 26 12. The methanol c a n i s t e r and the r e a r of the 27 d e t e c t o r h o u s i n g 13• The d e t e c t o r head mounted on the F / l s p e c t r o g r a p h 28 14. The d e t e c t o r head mounted on the F / l s p e c t r o g r a p h 29 15. Dark c u r r e n t as a f u n c t i o n of temperature 30 16. Camera e l e c t r o n i c s 31 17. B l o c k diagram of R e t i c o n c o n t r o l and d a t a 32 a c q u i s i t i o n system i v ACKNOWLEDGEMENTS I would l i k e t o e x p r e s s my a p p r e c i a t i o n t o the many p e o p l e who c o n t r i b u t e d t o the c o n s t r u c t i o n of the R e t i c o n 'camera head. I want t o thank Dr. C h r i s P r i t c h e t and Dr. Bruce Campbell f o r t h e i r h e l p f u l comments and Ve r n B u c h h o l z f o r many h e l p f u l d i s c u s s i o n s . I am d e e p l y g r a t e f u l t o D i e t e r S c h r e i h e r f o r a l l o w i n g me t o i n t r u d e i n t o h i s machine shop, and f o r h i s u s e f u l s u g g e s t i o n s c o n c e r n i n g the c o n s t r u c t i o n of the d e t e c t o r h o u s i n g . Many thanks go t o my w i f e , C a t h i e , f o r the t y p i n g of t h i s t h e s i s and f o r h er p a t i e n c e and s u p p o r t . I would l i k e t o ex p r e s s my thanks t o my s u p e r v i s o r , Dr. Gordon Walker, f o r s u p p o r t i n g t h i s p r o j e c t and f o r g i v i n g encouragement and a d v i c e t h r o u g h o u t i t s p r o g r e s s . T h i s work was funded by a U n i v e r s i t y of B r i t i s h C o l u m b i a f e l l o w s h i p . - 1 -I . INTRODUCTION S e l f scanned s i l i c o n p h o t o d e t e c t o r a r r a y s have been i n r o u t i n e o p e r a t i o n as a s t r o n o m i c a l d e t e c t o r s f o r some time now, and many more groups are p r o p o s i n g or b u i l d i n g d e t e c t o r s em-p l o y i n g s o l i d s t a t e s e n s o r s ( L i v i n g s t o n 1976). The o p e r a t i o n of these d e t e c t o r a r r a y s i s s i m i l a r t o t h a t of the s i l i c o n d i o d e v i d i c o n except t h a t the e l e c t r o n r e a d i n g beam has been r e p l a c e d by on c h i p s c a n n i n g and a d d r e s s -i n g c i r c u i t r y . As a r e s u l t , t h e s e m o n o l i t h i c d e t e c t o r s are v e r y compact and t h e r e i s a b s o l u t e g e o m e t r i c s t a b i l i t y be-tween.\array elements. T h i s , t o g e t h e r w i t h s i l i c o n ' s h i g h quantum e f f i c i e n c y and l i n e a r i t y of r e s p o n s e , makes s e l f scanned s e n s o r a r r a y s v e r y a t t r a c t i v e f o r a s t r o n o m i c a l a p p l i c a t i o n s . The p h o t o d e t e c t o r a r r a y s i n common use today are of two b a s i c t y p e s , p h o t o d i o d e a r r a y s and charge t r a n s f e r d e v i c e s . 1) P h o t o d i o d e a r r a y s -- These s e n s o r s u t i l i z e t he d e p l e t i o n r e g i o n c a p a c i t a n c e i n p-n j u n c t i o n d i o d e s f o r the i n t e g r a t i o n of p h o t o g e n e r a t e d charge. The i n d i v i d u a l p h o t o d i o d e s are c o u p l e d s e q u e n t i a l l y t o one or a few v i d e o l i n e s v i a MOS s w i t c h e s and an on c h i p s h i f t r e g i s t e r . The u s u a l a r r a n g e -ment i s shown s c h e m a t i c a l l y i n F i g u r e 1. I n o p e r a t i o n , the d i o d e s a re f i r s t r e v e r s e b i a s e d t o a f i x e d p o t e n t i a l and the n i s o l a t e d . P h o t o g e n e r a t e d m i n o r i t y c a r r i e r s w i l l s u b s e q u e n t l y C L O C K S S T A R T ODD VIDEO COMMON EVEN VIDEO CLOCKS SHIFT REGISTER SHIFT REGISTER FIGURE 1. S i m p l i f i e d s chematic of a l i n e a r d i o d e a r r a y . •r. - 3 -d i s c h a r g e the i n d i v i d u a l diode c a p a c i t a n c e s . Readout o c c u r s when the d i o d e s are r e b i a s e d t h r o u g h the v i d e o l i n e , r e s u l t -i n g i n a t r a i n of r e c h a r g i n g c u r r e n t p u l s e s , each p r o p o r t i o n a l t o the amount of l i g h t t h a t had f a l l e n on the c o r r e s p o n d i n g d i o d e . A more d e t a i l e d account of p h o t o d i o d e a r r a y s i s g i v e n by White (1975). 2) Charge t r a n s f e r d e v i c e s -- These use a r r a y s of MOS c a p a c i -t o r s f o r the i n t e g r a t i o n of p h o t o g e n e r a t e d charge. The g a t e s of each c a p a c i t o r are b i a s e d so as t o d r i v e the b u l k s i l i c o n u n derneath i n t o d e p l e t i o n . The p o t e n t i a l w e l l s t h u s formed c o l l e c t and s t o r e the p h o t o g e n e r a t e d m i n o r i t y c a r r i e r s . Charge t r a n s f e r d e v i c e s (CTD's) i n c l u d e b o t h charge c o u p l e d a r r a y s (CCD's) and charge i n j e c t i o n a r r a y s ( C I D ' s ) . The o p e r a t i o n of each of t h e s e a r r a y s i s q u i t e d i f f e r e n t . I n CCD's, the charge p a c k e t s c o l l e c t e d under the a r r a y o f c a p a c i t o r g a t es are t r a n s p o r t e d a l o n g the s u r f a c e of the d e v i c e t o the i n p u t of an on c h i p a m p l i f i e r . F i g u r e 2 demon-s t r a t e s , w i t h the use of a p o t e n t i a l w e l l diagram, how the p r o p e r p u l s i n g of t h e gate p o t e n t i a l s can a c c o m p l i s h t h i s charge t r a n s f e r p r o c e s s . F o r more d e t a i l see S e q u i n (1975)-I n CID's, the i n d i v i d u a l s e n s i n g elements c o n s i s t of a p a i r of gate e l e c t r o d e s . I n a two d i m e n s i o n a l CID a r r a y the g a t e s are connected i n rows and columns. One gate from each element forms the rows, the o t h e r forms the columns. The row and column gate v o l t a g e s can "be used t o r e a d out a p a r t i c u l a r element, as shown i n F i g u r e 3- T h i s r e a d o u t i s n o n - d e s t r u c t -i v e s i n c e the charge p a c k e t i s s t i l l i n t a c t , t h e r e f o r e , many _ 4 -END OF E X P O S U R E 10 0 5 V I V 2 V 3 CHARGE T R A N S F E R 5 10 0 V I V 2 V 3 - 5 -10 Column Row (5) 10 Floating END OF E X P O S U R E COLUMN S E L E C T ( 5 - A V ) O Sense 111.*. .ij 1 . 1 . . . . . . J 1 . i . ROW S E L E C T AND NON D E S T R U C T I V E R E A D 0 — ' T / 7 7 ^ - \ • \ \ ( CHARGE INJECTION FIGURE 3. Readout o f th e CID. AV i s e q u a l t o the s i g n a l charge d i v i d e d by th e column c a p a c i t a n c e . - 6 -r e a d o u t s of the same charge p a c k e t can be made. The s e n s i n g element i s c l e a r e d f o r the n e x t i n t e g r a t i o n by removing the p o t e n t i a l on b o t h g a t e s s i m u l t a n e o u s l y . T h i s i n j e c t s the m i n o r i t y c a r r i e r charge i n t o the b u l k s i l i c o n where i t r e -combines. F o r more d e t a i l see Michon e t a l (1975)' T h e r m a l l y g e n e r a t e d m i n o r i t y c a r r i e r s w i l l s a t u r a t e CTD's and p h o t o d i o d e a r r a y s i n s e v e r a l seconds a t room temp-e r a t u r e . F o r t u n a t e l y , t h i s l e a k a g e c u r r e n t , commonly r e f e r -r e d t o as dark c u r r e n t , d e c r e a s e s e x p o n e n t i a l l y w i t h temper-a t u r e and i t i s p o s s i b l e t o make exposures of s e v e r a l hours w i t h a c o o l e d a r r a y . A .-.more d e t a i l e d account of t h i s d a r k c u r r e n t i s g i v e n i n Appendix A. I n astronomy, the l i n e a r diode a r r a y s , i n p a r t i c u l a r t h o s e of R e t i c o n C o r p o r a t i o n , have been used more th a n any o t h e r s e l f scanned a r r a y . These p a r t i c u l a r a r r a y s have proven t o be v e r y rugged, c a p a b l e of w i t h s t a n d i n g r e p e a t e d r a p i d c o o l i n g t o t e m p e r a t u r e s - b e l o w 173 K. The R e t i c o n d e v i c e s are easy t o o p e r a t e , r e q u i r i n g o n l y v e r y s i m p l e d r i v i n g c i r - r c u i t r y . They are a v a i l a b l e w i t h as many as 1872 elements and 430-750 m i c r o n wide a p e r a t u r e s , making them s u i t a b l e f o r a s t r o n o m i c a l s p e c t r o s c o p y . The d i o d e t o d i o d e s e p a r a t i o n i s ' f r om 15 t o 25 m i c r o n s and t h e r e i s x no dead space between s e n s o r elements as t h e r e i s i n CTD's. R e t i c o n a l s o makes a d u a l a r r a y w i t h two 370 m i c r o n wide a r r a y s s e p a r a t e d by 8 m i c r o n s , w h i c h would be u s e f u l i n cases where a sky sub-t r a c t i o n i s needed. - 7 -The p r i n c i p l e d i s a d v a n t a g e of R e t i c o n ' s d i o d e a r r a y s i s the l a r g e v i d e o l i n e t o s u b s t r a t e c a p a c i t a n c e , which makes low n o i s e s i g n a l e x t r a c t i o n d i f f i c u l t , ".and t h e e x i s t e n c e of a l a r g e v a l u e of r e s e t n o i s e . The o r i g i n of t h i s r e s e t n o i s e i s d e s c r i b e d i n Appendix B. F o r R e t i c o n a r r a y s the r e s e t n o i s e i s t y p i c a l l y 250 c a r r i e r s p e r d i o d e , a t 173 K. I t i s here where CTD's have a d i s t i n c t advantage, s i n c e r e s e t n o i s e can be e l i m i n a t e d by u s i n g a c o r r e l a t e d double s a m p l i n g r e a d -out t e c h n i q u e ( H a l l 1975)• The r e m a i n d e r of t h i s paper d e s c r i b e s a s o l i d s t a t e d e t e c t o r head w h i c h was d e s i g n e d and c o n s t r u c t e d by the au t h o r f o r a s t r o n o m i c a l a p p l i c a t i o n s . I t employs a unique s o l i d methanol c o o l i n g system, and a l t h o u g h b u i l t s p e c i f i c a l l y f o r the l i n e a r R e t i c o n d e t e c t o r s , i s c a p a b l e of h o u s i n g o t h e r l i n e a r or two d i m e n s i o n a l s e l f scanned a r r a y s . - 8 -I I . DESIGN FEATURES OF THE DETECTOR HEAD T h i s d e t e c t o r head was d e s i g n e d and b u i l t f o r use w i t h an e x i s t i n g c o n t r o l and d a t a a c q u i s i t i o n system which was de-v e l o p e d by the I n s t i t u t e of Astronomy and Space S c i e n c e a t the U n i v e r s i t y of B r i t i s h C o l u m b i a (Walker e t a l 1974). Two 1024 element R e t i c o n s and one 256 element R e t i c o n have been i n r o u t i n e use w i t h t h i s c o n t r o l system f o r the d e t e c t i o n of as-t r o n o m i c a l s p e c t r a (Walker e t a l 1976). The l i n e a r 256 R e t i c o n was c o o l e d w i t h d r y i c e i n a h o u s i n g b u i l t s p e c i f i c a l l y f o r use on the F / l Wynne s p e c t r o g r a p h camera of the Dominion A s t r o p h y s -i c a l O b s e r v a t o r y ' s 1.8 meter t e l e s c o p e . A l t h o u g h much u s e f u l astronomy had been done w i t h t h i s R e t i c o n camera head, t h e r e were many problems a s s o c i a t e d w i t h i t , and i t s o p e r a t i o n was not dependable. The main problems were: — E x c e s s i v e dark c u r r e n t due t o inadequate c o o l i n g by the dry i c e . — E x c e s s i v e r e a d o u t n o i s e . — Lack of a d j u s t m e n t s r e q u i r e d t o p r o p e r l y a l i g n the R e t i c o n . — F r o s t i n g . I t was hoped t h a t the new d e t e c t o r head would e l i m i n a t e t h e s e problems and p r o v i d e a dependable diode a r r a y system t h a t c o u l d e a s i l y be o p e r a t e d by o b s e r v e r s n o t f a m i l i a r w i t h t h e equipment.- A l t h o u g h t h e new d e t e c t o r head w a s " b u i l t f o r use on the Wynne camera w i t h the 256A/17 R e t i c o n , i t was de-s i g n e d so t h a t i t c o u l d e a s i l y be adapted f o r use on o t h e r i n -s t r u m e n t s and house o t h e r s o l i d s t a t e a r r a y s . - 9 -1. The Ho u s i n g A cut-away e x p l o d e d v i e w of the new d e t e c t o r h o u s i n g i s shown i n F i g u r e 4, and the d e t a i l e d drawings are shown i n F i g u r e s 5» 6, 7, and 8. The main f e a t u r e s of the l a y o u t are as f o l l o w s : — The R e t i c o n , i t s c l o c k d r i v e r s , and the p r e a m p l i f i e r a re . • . a l l on one e a s i l y removable p l u g - i n c i r c u i t b o ards. T h i s s i m p l i f i e s t he change over t o a d i f f e r e n t s e l f scanned a r r a y . — A l l the e l e c t r i c a l c o n n e c t i o n s are made t h r o u g h an a l u m i -num f r o n t p l a t e ( p a r t no. 3) on which the c i r c u i t b o a r d mounts. The c i r c u i t b o a r d , p l a t e , and c o n n e c t i o n c a b l e can be removed as a u n i t , as shown i n F i g u r e 10. T h i s s i m p l i f i e s the t e s t i n g and debugging of the e l e c t r o n i c s , and a l l o w s m o d i f i c a t i o n s t o the r e s t of the d e v i c e t o be made w i t h o u t h a v i n g t o d i s t u r b t h e R e t i c o n o r any of i t s e l e c t r i c a l c o n n e c t i o n s . — A t e f l o n s o c k e t h o l d s the R e t i c o n diode a r r a y w h i c h i s housed i n a cer a m i c d u a l i n - l i n e i n t e g r a t e d c i r c u i t pack-age. T h i s s o c k e t has a c e n t r a l s l o t t o a l l o w a copper c o l d probe t o come i n c o n t a c t w i t h the back of the R e t i c o n . When the d e t e c t o r h o u s i n g i s assembled, the copper probe b e a r s f i r m l y a g a i n s t the R e t i c o n , f l e x i n g the p r i n t e d c i r c u i t b o a r d s l i g h t l y . T h i s e n s u r e s good t h e r m a l con-t a c t and a r i g i d l y d e f i n e d f o c a l p l a n e . - 10 -The end of the copper probe i s removable so t h a t d i f f e r e n t t i p s can be used w i t h each a r r a y . T h i s i s a m o d i f i c a t i o n t h a t was made d u r i n g the c o n s t r u c t i o n and i s not shown i n the drawings. The q u a r t z window of the R e t i c o n ' s I.C. package l i e s 0.1 i n c h e s above t h e s u r f a c e of the f r o n t p l a t e . T h i s a l l o w s f i e l d f l a t t e n e r l e n s e l e m e n t s t t o be p l a c e d as c l o s e as .-: p o s s i b l e t o the a c t i v e s u r f a c e of the dio d e a r r a y . The f r o n t p l a t e has an 0 - r i n g groove and e i g h t t h r e a d e d mounting h o l e s so t h a t a v a r i e t y of windows, or f i e l d f l a t -t e n e r a s s e m b l i e s , may be f i t t e d t o the h o u s i n g . 0 - r i n g s e a l s ensure t h a t the i n s i d e of the h o u s i n g i s com-p l e t e l y s e a l e d o f f from the o u t s i d e . Two v a l v e s ( n o t shown i n the drawings but which can be seen i n F i g u r e 9) are p r o v i d e d t o a l l o w the i n s i d e of the hous-i n g t o be f l u s h e d w i t h v e r y d r y n i t r o g e n . T h i s i s neces-s a r y t o p r e v e n t f r o s t i n g of the dio d e a r r a y , or the i n s i d e s u r f a c e of the window or f i e l d f l a t t e n e r . The f i e l d f l a t t e n e r n e c e s s a r y f o r the F / l c o n c e n t r i c m i r -r o r s p e c t r o g r a p h c o n s i s t s of two a i r spaced l e n s e s (see F i g u r e 8 ) . I n o r d e r t o p r e v e n t the o u t e r l e n s from g e t t i n g c o l d and f o g g i n g , t h e s e two l e n s e s are mounted i n an evacu-a t e d s t a i n l e s s s t e e l c e l l . T h i s a l s o p r o v i d e s some t h e r m a l i s o l a t i o n f o r the R e t i c o n w h i c h l i e s v e r y c l o s e t o the i n n e r f i e l d f l a t t e n e r l e n s . The f i e l d f l a t t e n e r assembly - 11 -can be seen i n F i g u r e s 9. 10, and 11. The f i e l d f l a t t e n e r c e l l i s f i t t e d w i t h a h i g h vacuum v a l v e so t h a t i t may be r e - e v a c u a t e d p e r i o d i c a l l y . The assembled h o u s i n g s l i d e s i n t o a f l a n g e d aluminum s l e e v e ( p a r t no. 9). T h i s s l e e v e i s made t o mount on the F / l camera a f t e r the e x i s t i n g f i e l d f l a t t e n e r l e n s assembly has been removed. The s i x mounting h o l e s i n the f l a n g e are s l o t t e d , and a p o s i t i o n i n g screw i s p r o v i d e d so t h a t the d e t e c t o r may be a l i g n e d v e r t i c a l l y on the spectrum. T h i s i s n o t shown i n the drawings b u t can be seen i n F i g u r e 11. The d i o d e a r r a y may be r o t a t e d so as to", l i e p a r a l l e l t o the d i s p e r s i o n d i r e c t i o n , by a d j u s t i n g a s l o t i n the aluminum s l e e v e . A p i n on the h o u s i n g engages t h i s s l o t . T h i s arrangement can be seen i n F i g u r e s 9 and 13-An a d j u s t a b l e s t o p ( p a r t s 15 and 16) d e t e r m i n e s how f a r the h o u s i n g s l i d e s i n t o the aluminum s l e e v e and p r o v i d e s a means of f o c u s s i n g the d e t e c t o r . A q u i c k - r e l e a s e l a t c h on the aluminum s l e e v e l o c k s the de-t e c t o r h o u s i n g i n p l a c e once the a l i g n m e n t a d j u s t m e n t s have been made. The copper probe t h a t s u p p o r t s the R e t i c o n p a s s e s t h r o u g h the r e a r of the h o u s i n g t o the o u t s i d e where the c o o l i n g system c o u p l e s t o i t . The r e a r pl'ate on the h o u s i n g sup-p o r t s the copper probe. - 12 -— The re a r of the housing, seen i n Figure 12, was designed s p e c i f i c a l l y to accept the c o o l i n g system described below. However, there i s ample space around the copper probe to allow other c o o l i n g systems to be f i t t e d and t r i e d . 2. C o o l i n g System The c o o l i n g system was designed so as';to maintain the Re t i c o n array at a temperature of -90 C or lower. From Figure ure 15 i t can be seen that t h i s ensures n e g l i g i b l e dark current f o r exposure times of many hours. Such a low temperature r u l e d out the use of dry i c e as a coolant. However, an e q u a l l y sim-ple and inexpensive c o o l i n g method was devised which uses a sealed copper c a n i s t e r f u l l of f r o z e n methanol i n s t e a d of dry i c e . The methanol i s fr o z e n by c o o l i n g the copper c a n i s t e r i n an open mouthed dewar of l i q u i d n i t r o g e n . The c a n i s t e r i s then screwed i n t o the detector housing, p l a c i n g i t i n thermal contact w i t h the Reticon array. Methanol f r e e z e s at -98 C and the l a t e n t heat of f u s i o n i s 99 Joules/gram. With the estimated heat input of 5 watts, t h i s gives 20 seconds of c o o l i n g per gram of methanol, or 4.4 hours per l i t e r . F i g u r e s 4 and 12 show the d e t a i l s of the methanol can-i s t e r and how i t i s attached to the housing. The c a n i s t e r shown holds one l i t e r of methanol arid uses urethane foam f o r i n s u l a t i o n . The p a r t s numbered 18 are f i l l and vent plugs. The end of the c a n i s t e r that attaches to the copper probe i s - 13 -n o t i n s u l a t e d so t h a t the methanol may be f r o z e n by immersion i n t o l i q u i d n i t r o g e n . The urethane foam l i n i n g i n the r e a r s e c t i o n of the d e t e c t o r h o u s i n g p r o v i d e s i n s u l a t i o n f o r t h i s p a r t of the c a n i s t e r when i t i s screwed i n p l a c e . 3. D e t e c t o r E l e c t r o n i c s The d e t e c t o r e l e c t r o n i c s c o n s i s t of the R e t i c o n ' s c l o c k c i r i v e r s and the v i d e o p r e a m p l i f i e r . S i n c e the R e t i c o n 256A/I7 a r r a y used i n the o l d e r system was t o be made o p e r a t i o n a l i n the new d e t e c t o r head as q u i c k l y as p o s s i b l e , no attempt was made t o improve upon the e x i s t i n g e l e c t r o n i c d e s i g n . However, an attempt was made t o improve the l a y o u t of the d e t e c t o r e l e c -t r o n i c s i n o r d e r t o reduce the amount of f i x e d p a t t e r n s i g n a l and t o h o p e f u l l y a c h i e v e a lower r e a d o u t n o i s e . Tn the o l d e r system the r e a d o u t n o i s e was 36OO c a r r i e r s / d i o d e ( r m s ) . The o r i g i n and n a t u r e of the f i x e d p a t t e r n s i g n a l i s d e s c r i b e d i n Appendix C. F i g u r e 16 i s a sc h e m a t i c of the d e t e c t o r e l e c t r o n i c s , and F i g u r e 17 shows a b l o c k diagram of the complete c o n t r o l and d a t a a c q u i s i t i o n system. The 5 v o l t s f o r the R e t i c o n i s d e l i v e r e d from r e c h a r g e a b l e n i - c a d b a t t e r i e s . N o i s e on t h i s s u p p l y appears d i r e c t l y as n o i s e i n the r e a d o u t , and i t i s of utmost importance t h a t t h i s s u p p l y be low n o i s e and s t a b l e . B a t t e r i e s p r o v i d e such a low n o i s e s u p p l y . The o t h e r s u p p l y v o l t a g e s are d e l i v e r e d down a f i f t y f o o t c a b l e from remote power s u p p l i e s i n the c o n t r o l box. - 14 -The r i g h t and l e f t c l o c k p u l s e s are g e n e r a t e d " i n the v i d e o p r o c e s s i n g box f rom a b a s i c c l o c k s u p p l i e d down the l o n g c a b l e . The s t a r t p u l s e i s g e n e r a t e d i n the c o n t r o l box and s u p p l i e d t o the v i d e o p r o c e s s i n g box and t o the camera e l e c -t r o n i c s b o a r d . The v i d e o s i g n a l from the p r e a m p l i f i e r i s s e n t t o the v i d e o p r o c e s s i n g box w h e r e . i t i s i n t e g r a t e d , "Vr, sampled, and h e l d . I t i s t h e n d i g i t i z e d and s e n t down the f i f t y f o o t c a b l e t o the computer. The main improvements made t o the l a y o u t of the d e t e c t o r e l e c t r o n i c s are as f o l l o w : —- The R e t i c o n i s l o c a t e d i n a s p e c i a l l y made t e f l o n s o c k e t . — S h o r t t e f l o n i n s u l a t e d l e a d s are used f o r the two v i d e o l i n e s t h a t go t o the nearby a m p l i f i e r . These l e a d s l e a v e the R e t i c o n s o c k e t d i r e c t l y and n e v e r come i n c o n t a c t w i t h the p r i n t e d c i r c u i t b o a r d . T h i s d e t a i l can be seen i n F i g u r e 10. — The p r e a m p l i f i e r e l e c t r o n i c s are copper s h i e l d e d and p l a c e d away from the d r i v e r e l e c t r o n i c s . — The two s e t s o f d r i v i n g c i r c u i t r y a re l a i d out symmetrical-;-l y on each s i d e of the R e t i c o n . T h i s was done i n o r d e r t o reduce the amount of f i x e d p a t t e r n . I t r e q u i r e d the l e a d s on two of the IC's t o be b ent backwards so t h a t t h e y c o u l d be mounted u p s i d e down. — D e c o u p l i n g f i l t e r s were used on the s u p p l i e s t o a l l the I C ' s . - 15 -— S p e c i a l a t t e n t i o n was p a i d t o the g r o u n d i n g i n o r d e r t o reduce p i c k - u p . F o r example, the system ground was i s o l a t -ed from the h o u s i n g so t h a t a ground l o o p c o u l d n o t occur when the i n s t r u m e n t was a t t a c h e d t o the t e l e s c o p e . - 16 -I I I . ,FINAL PERFORMANCE F i g u r e s 13' and 14- show the d e t e c t o r head i n use on the t e l e s c o p e . The f i r s t o b s e r v i n g r u n w i t h the new d e t e c t o r head c l e a r -l y demonstrated i t t o be s u p e r i o r t o the o l d e r d e v i c e . Mount-i n g and a l i g n i n g the R e t i c o n "was? now a s i m p l e o p e r a t i o n a a n d a s i g n i f i c a n t l y b e t t e r f o c u s c o u l d be a c h i e v e d . F r o s t i n g of the diode a r r a y was no l o n g e r a p r o blem and the o u t e r s u r f a c e o f the f i e l d f l a t t e n e r d i d not f o g up. The new f r o z e n methan-o l c o o l i n g system was a b l e t o m a i n t a i n the R e t i c o n a t a temp-i e r a t u r e below -80 C f o r a p e r i o d of about t h r e e h o u r s , w h i c h i s warmer and s h o r t e r t h a n o r i g i n a l l y p l a n n e d . T h i s i s probab-l y due t o i n a d e q u a t e i n s u l a t i o n by the u r e t h a n e foam. E t h a n o l f r e e z e s a t -114 C and when s u b s t i t u t e d f o r the methanol i t was a b l e t o keep the R e t i c o n below -90 C, but f o r e v e n ^ s h o r t e r d u r a t i o n s . I t t a k e s a p p r o x i m a t e l y 45 minutes t o c o o l the c a n i s t e r f r om room temperature t o the p o i n t where a l l the methanol w i l l f r e e z e . However, i t t a k e s o n l y 15 minutes t o r e f r e e z e the can-i s t e r i f the t emperature has not been a l l o w e d t o r i s e above -70 C. A t p r e s e n t , t h i s - c a u s e s some l o s s n o f o b s e r v i n g time s i n c e o n l y one c a n i s t e r was c o n t r u c t e d . The new e l e c t r o n i c s r e s u l t e d i n a tremendous r e d u c t i o n of the f i x e d p a t t e r n s i g n a l , t o a l e v e l a p p r o x i m a t e l y 3f° of s a t u r a t i o n , and l o w e r e d the r e a d o u t n o i s e by a f a c t o r of two. - 17 -A ) G r a t i n g Coverage R e s o l u t i o n Speed The r e s u l t i n g performance of the dio d e a r r a y system, when used on the F / l s p e c t r o g r a p h of the 1.8 meter t e l e s c o p e , i s as f o l l o w s : 3 0 0 l.p.mm., "blazed a t 7500 A 5 , c e n t e r e d a t 7 5 0 0 A \ 5 , 0 0 0 R t o 1 0 , 0 0 0 R 16 S/diode s i g n a l / n o i s e = 2 0 0 / 1 w a v e l e n g t h = 7 , 0 0 0 R V = 14 . 0 f o r G or K s t a r i n t e g r a t i o n time 3 i 6 0 0 seconds 1 , 2 0 0 l.p.mm., b l a z e d a t 7 5 0 0 R, c e n t e r e d a t 6 5 0 0 R 6 , 0 0 0 R t o 7 , 0 0 0 R 3 . 7 ^ / d i o d e s i g n a l / n o i s e * 2 0 0 / 1 w a v e l e n g t h = 6 , 5 0 0 R V = 1 1 . 0 f o r G or K s t a r i n t e g r a t i o n time ^ 3 , 6 0 0 seconds B) G r a t i n g Coverage R e s o l u t i o n Speed - 18 -IV. APPLICATIONS P l a n s have been made f o r use of the d e t e c t o r head on o t h e r i n s t r u m e n t s b e s i d e s the F / l Wynne camera. A t the time of w r i t i n g , t he d e t e c t o r head, w i t r i a 1024C/17 R e t i c o n a r r a y i n s t a l l e d , has been used on the 32" coude s p e c t r o g r a p h camera o f the Dominion A s t r o p h y s i c a l O b s e r v a t o r y ' s 1.2 meter t e l e -scope. The 1024 R e t i c o n i s on i t s own c i r c u i t b o a r d , w i t h i t s own d r i v e r s and p r e a m p l i f i e r . P l a n s are a l s o i n p r o g r e s s f o r mounting the d e t e c t o r head on the 21" o f f - a x i s s p e c t r o g r a p h camera on the 1.8 meter t e l e s c o p e . I n a d d i t i o n i t i s thought t h a t the two d i m e n s i o n a l 50 x 50 element d i o d e a r r a y system a t the U n i v e r s i t y of B r i t i s h C olumbia might a l s o use t h i s d e t e c t o r head. When t h e y become a v a i l a b l e , a 1 0 0 x 4 0 0 R e t i c o n a r r a y c o u l d be sub-s t i t u t e d f o r the 5 0 x 5 0 . CAMERA ASSEMBLY DRAWING j 8 I \OMBY OATt - loyi/ty lots BY FIGURE 4. E x p l o d e d cut-away assembly d r a w i n g of t h e d e t e c t o r head. 2.#00 OIA. O O O o ff vo —i.noo D/A. — / .Soo 01 A.^ * . 002 o o O o 11 c r 3 5 0 0 OIA. 3.200 OM. It: • o o 0 M » -TOO OM.*— i IH M • 2 6 OA. i l . - » EQUAL L Y SPACED J t _ L P T / . FAEt MACMiMHVC, gTAIHLtLi STEEL. I « £ Q O . - A / » 0 # » V » o -1 U * . « 0 DJA. ' * . 0 © 2 I 2 &DO OM. •* ZOO OM. s s a . — — W W . o d d C - S I U M C - 2 0 X O. + -<o w«« -a* xo. 8 /VOJL£S E Q U A L L Y ALL KCIMAL D ' W £ V S / C W £ 1 . 3 0 3 RETICON DETECTOR FOR THE QA.O. SPECTROO-AAPH CAMERA HOUSING DETAILS Al SCAiJt - TWICE S ' Z g DW » V - T^A\nX~*. O FIGURE 5. D e t a i l e d d r a w i n g s . + - 4 0 U W C -It / & HOL£i, emuALXY 5FAC £ o 5\5i2 D / A . PT.+. MS. / A £ O 0 - / . O O O OA* ; M O D I I! L W C - / A x OS DU.P r - r i P T 7 . 2 0 G A G E S H E E T S T E E L 4 - K E Q D — . <>• I i- z . a i o -1 i O.SOO UA.—4 W « / D £ 2 i.o com*, m/mj FT 5. C O P P C * ; A £ Q O yAW.£6£ OTWSAW/se SPECIFIED ALL D W E » i l » S ± . 0 0 3 A L L MACHINE. S U R F A C E S RETICON DETECTOR FOP, THE bA.O. F/l SPECTROGRAPH CAMERA HOUSING DETAILS SCALE - TWICE H Z £ DATE - 5 / 1 / 7 7 A Z Du By DCS By 7 ^ /j^UT FIGURE 6 . ALL D C C f M A i . £ M £ « £ K W £ t.OOS ALL A U C H / M C i i / O F A C I S ALL AHCLS.Z. *. ^ ' RETICON DETECTOR FOR THE HA/). F/\ IRECTROORAPH CAMERA HOUSING DETAILS SCALE - fa/ICC | DH.tr TZm. DATE - t / l / 7 7 |at. » y 7 ^ -to ro FIGURE ?• FIGURE 9. The completed d e t e c t o r head. FIGURE 11. F r o n t v i e w . The R e t i c o n can "be seen b e h i n d t h e f i e l d f l a t t e n e r s . FIGURE 12. The methanol c a n i s t e r and t h e r e a r of the d e t e c t o r h o u s i n g . FIGURE 14. - 3 0 -o UJ to \ CO cr UJ tr cr < I 0 " r -10* 10 10 10' 10 10 -I 10' -2 /A / 4--/ / / / / ! « 1 -100 - 7 5 •50 - 2 5 0 + 2 5 FIGURE 15. Dark c u r r e n t as a f u n c t i o n of t e m p e r a t u r e . Open c i r c l e s a p p l y t o a R e t i c o n 256A w h i l e s o l i d c i r c l e s a p p l y t o a 256C. The dashed and d o t t e d l i n e s a r e t h e o r -e t i c a l c u r v e s d e r i v e d from .equation 2 i n Ap p e n d i x A. RIGHT CLOCK L E F T CLOCK + 5 V L E F T CLOCK S T A R T VIDEO OUT + 5 V RIGHT CLOCK FIGURE 16. Camera e l e c t r o n i c s . o > m > m r m o H 3J O z o CO CLOCK S T A R T V I D E O BAT. + 5V - H 5 V - I 5 V | 1 1 k | C O N T R O L | 1 DIGTITIZE INTE] G R A T E 1 I N T E G R A T O R | I I S / H A D C + 5V _ _ t _ +5V B A T T E R Y " ^ . -' AND P O W E R * I -I- -DIST. C L O C K S T A R T DATA DATA C L O C K + 5V + I5V - I 5 V VIDEO PROCESSING UNIT E X P O S . T I M E | C O U N T E R A N D 1 | MOV L I N E j | ^ S Y N C | STA JRT D A T A I I A C Q U I S I T I O N P I I + 5V r _ _ r _ _ ^ P O W E R I J S U P P L I E S I I I C O N T R O L UN IT S C O P E DATA C O M P U T E R FIGURE 17. Block diagram of Reticon cont ro l and data a c q u i s i t i o n system BIBLIOGRAPHY Campbell, B. 1976, ' . ' A p p l i c a t i o n of the DDO R e t i c o n System t o  G and K S t a r Abundance S t u d i e s and Nova C y g n i 1975' '• Ph.D. T h e s i s , U n i v e r s i t y of T o r o n t o . H a l l , J.A. 1975. i n . S o l i d S t a t e Imaging, eds. P.G. J e s p e r s , F. Van de W i e l e , and M.H. White (NATO Advanced Study I n s t i t u t e S e r i e s ; N o o r d h o f f : Leyden), p. 535-L i v i n g s t o n , W.C. 1976, i n P r o c . I.A.U. C o l l o q . no. 40, eds. M. Duchesne ...and G. L e l i e v r e (Paris-Meudon Obs., F r a n c e ) , p. 22-1. Michon, G.J. and Burke, H.K. 1975. i n S o l i d S t a t e Imaging, eds. P.G. J e s p e r s , F. Van de W i e l e , and M.H. White (NATO Advanced Study I n s t i t u t e S e r i e s ; N o o r d h o f f : Leyden p. 447. Sah, C.T., Noyce, R.N. and S h o c k l e y , W. 1957, P r o c . I.R.E., 45, 1228. S e q u i n , C H . 1975, i n S o l i d S t a t e Imaging, eds. P.G. J e s p e r s F. Van de W i e l e , and M.H. White (NATO" Advanced Study I n s t i t u t e S e r i e s ; N o o r d h o f f : Leyden), p. 233-Walker, G.A.H., B u c h h o l z , V.L., Camp, D., Isherwood, B . I . , G l a s p e y , J.W., C o u t t s , R., and Condal, A. 1974, Rev. S c i . I n s t r u m . , 45, 43. Walker, G.A.H., B u c h h o l z , V.L., Fahlman, G.G., G l a s p e y , J . , Lane-Wright, D., Mochnacki, S. and C o n d a l , A. 1976 i n P r o c . I.A.U. C o l l o q . no. 40,'--eds. M. Duchesne and G. L e l i e v r e (Paris-Meudon Obs.: F r a n c e ) , 24-1. White, M.H. 1975, i n S o l i d S t a t e Imaging, eds. P.G. J e s p e r s , F. Van de W i e l e , and M.H. White (NATO Advanced S t u d y I n s t i t u t e S e r i e s ; N o o r d h o f f : Leyden), p. 165. - 3^ -APPENDIX A F o r s e m i c o n d u c t o r s w i t h wide energy gaps, such as s i l i c o n , t he dark l e a k a g e c u r r e n t i s due almost e n t i r e l y t o the h o l e e l e c t r o n p a i r s b e i n g c r e a t e d t h e r m a l l y i n the space charge r e g i o n . From Sah e_t a l (1957), the magnitude of t h i s dark lea k a g e c u r r e n t i s g i v e n by: 2 V * 0 where, = lea k a g e c u r r e n t d e n s i t y = w i d t h of the d e p l e t i o n r e g i o n n^ = i n t r i n s i c c o n c e n t r a t i o n of f r e e c a r r i e r s t = m i n o r i t y c a r r i e r l i f e t i m e i n the d e p l e t i o n r e g i o n S q = s u r f a c e l e a k a g e parameter The dark c u r r e n t parameters t and S q can be tempera-t u r e dependant, however, e m p i r i c a l l y i t has been found t h a t t h e y are e f f e c t i v e l y c o n s t a n t . Advances i n s i l i c o n d e v i c e t e c h n o l o g y have made p o s s i b l e v e r y low v a l u e s of s u r f a c e l e a k a g e c u r r e n t and the b u l k l e a k a g e term predominates down t o t e m p e r a t u r e s below -50 "JC f o r p r e s e n t l y a v a i l a b l e d e t e c t o r a r r a y s . N e g l e c t i n g s u r f a c e l e a k a g e the temperature dependance of l e a k a g e c u r r e n t i s a p p r o x i m a t e l y g i v e n by: J d ( T ) = C n i ( T ) = C T 3 / 2 exp(-7015/T) 2 I n F i g u r e 15 t h i s t e mperature dependance i s p l o t t e d a l o n g w i t h some a c t u a l measurements on two R e t i c o n l i n e a r a r r a y s ( f r om Campbell 1976). I t i s e v i d e n t t h a t f o r these d e v i c e s , s u r f a c e - 35 -l e a k a g e i s s t i l l n e g l i g i b l e a t t e m p e r a t u r e s as low as -80 C. The t h e r m a l dark c u r r e n t i s not l i n e a r b u t i s a f u n c t i o n of the r e s i d u a l charge on the p h o t o s e n s i t i v e elements. T h i s n o n l i n e a r b e h a v i o r i s due t o the parameter i n e q u a t i o n 1 above. The w i d t h of the d e p l e t i o n l a y e r t h a t forms the s t o r -age c a p a c i t a n c e i n the p h o t o s e n s i t i v e element i s a f u n c t i o n of the s t o r e d charge, and i t i s t h i s same c a p a c i t a n c e t h a t i s d i s c h a r g e d by the p h o t o g e n e r a t e d h o l e e l e c t r o n p a i r s and by the t h e r m a l d a r k c u r r e n t . Such n o n l i n e a r b e h a v i o r makes a c c u r a t e dark s u b t r a c t i o n s v i r t u a l l y i m p o s s i b l e and i t i s t h e r e f o r e d e s i r a b l e t o c o o l s e l f scanned a r r a y s t o such a low t emperature t h a t dark c u r r e n t i s t o t a l l y n e g l i g i b l e . - 36 -APPENDIX B When a s w i t c h i s used t o s e t the v o l t a g e a c r o s s a c a p a c i t o r , Johnson n o i s e i n t h e s w i t c h r e s i s t a n c e r e s u l t s i n an u n c e r t a i n t y i n the c a p a c i t o r v o l t a g e . A s i m p l e e q u i v a l e n t c i r c u i t f o r t h i s s i t u a t i o n i s shown below; = 4kTR Af R i s the s w i t c h r e s i s t a n c e and i s i t s a s s o c i a t e d n o i s e v o l t a g e g e n e r a t o r . P r o v i d e d t h a t the s w i t c h i s c l o s e d f o r a time t » R C, t h e n the bandwidth A f of t h i s s i m p l e s e r i e s RC c i r c u i t i s g i v e n by: dw • .1 k f - 2 2 2 = n 4R C n T h e r e f o r e , V 2 nc kT C Qnc = kTC When the s w i t c h i s opened a g a i n the i n s t a n t a n e o u s v a l u e of t h i s n o i s e charge i s h e l d on the c a p a c i t o r . An e x p r e s s i o n f o r the r e s e t n o i s e i s t h e r e f o r e : ( N E C ) r = ^ -y/kTC e l e c t r o n s . A t y p i c a l v a l u e f o r C i n s e l f scanned a r r a y s i s 0.5 p f and R i s - g e n e r a l l y l e s s t h a n 200 ohms, t h e r e f o r e , RC 10 -9 seconds and the assumption t h a t the r e s e t time t >> RC i s a good one. - 37 -APPENDIX C I n R e t i c o n d i o d e a r r a y s u n a v o i d a b l e p a r a s i t i c c a p a c i -t a n c e s a r e p r e s e n t between the s h i f t r e g i s t e r s and the v i d e o l i n e s . These c a p a c i t a n c e s c o u p l e the t r a n s i e n t s of the c l o c k d r i v e r s i g n a l s on'to the v i d e o l i n e . Two complementary c l o c k waveforms are used f o r each s h i f t r e g i s t e r i n o r d e r t o reduce the n e t charge i n d u c e d . However, th e c a p a c i t i v e c o u p l i n g g e n e r a l l y d i f f e r s s l i g h t l y f o r each of these c l o c k s i g n a l s and a n e t f i x e d p a t t e r n on the v i d e o output i s observed. P r o v i d e d the c l o c k waveforms are v e r y s t a b l e and the temper-a t u r e i s s t e a d y , t h i s n e t i n d u c e d charge i s a f i x e d c o n s t a n t f o r each diode and may be c a l i b r a t e d out or removed w i t h a second dark -'readout. Any changes i n c l o c k t i m i n g or a r r a y temperature w i l l r e s u l t i n a r e s i d u a l f i x e d p a t t e r n and f o r t h i s r e a s o n i t i s i m p o r t a n t t o make the o r i g i n a l f i x e d p a t t e r n as s m a l l as p o s s i b l e . I n a d d i t i o n , a l a r g e amount of f i x e d p a t t e r n w i l l r e d u ce the dynamic range of the d e v i c e . 

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