NATIONAL RADIO ASTRONOMY OBSERVATORY. I have read through Chapters I, I I, I I I, I V, V, V I, V I I,

Size: px

Start display at page:

Download "NATIONAL RADIO ASTRONOMY OBSERVATORY. I have read through Chapters I, I I, I I I, I V, V, V I, V I I,"

Marsha Greer
5 years ago
Views:

1 VLBA CORRELATOR MEMO VC 010 VLBA CC Memo No. /a NATIONAL RADIO ASTRONOMY OBSERVATORY Charlottesville, Virginia November 1 7, 1983 MEMO TO: H. Hvatum FROM: K. Kellermann SUBJECT: VLBA Volume I I I I have read through s I, I I, I I I, I V, V, V I, V I I, V I I I and list my comments below, I note the rather uneven treatment due to the m u l t i p l e a u t h o r s h i p, i. e. f i g u r e numbering, acknowledgements, cost t a b l e s, e t c. I suggest you issue guide l i n e s for t h e s e t h i n g s. Perhaps budget estimates should be put in a separate chapter. Do we need an introduction explaining what Vol. Ill is and that costs were done independently of Vol. I, and are in $1983? More important, some parts of the report are already obsol e t e, and do not reflect current thinking. I comment on specific e x a m p l e s b e l o w, but it is not clear to me how to treat t h i s without continually rewriting things. Perhaps the report should be backdated to August, but then it will realistically represent our plans. I. Configuration Section I I. The most important thing here w i l l be to at least mention Pietown under 1 0. Perhaps more emphasis on Los Alamos vs. Bernal. It might be a p p r o p r i a t e tentative order of construction on. I I. in I or I I to give the that we now more or l e s s agree Sites Note that all costs and planning have always assumed the AOC is at a current NRAO f a c i l i t y (Section I I and Red B o o k ). I d o n ' t t h i n k t h i s is generally appreciated by those giving us advice. Section I I I. B. Section I I I. E. Give humidity range i. e. 40 ± 10% Should the leased telephone lines be included here? Why i s t h e p a r k i n g a r e a so b i g? Do we w a n t to r e d u c e the s i z e of the antenna c o n t r o l b u i l d i n g? Do we need the s e p a r a t e computer room? Main Operating B u i l d i n g - I hope we can get

2 2 something other than the Green Bank-Charlottesville "Box" design. Cost e s t i m a t e : the caveat about contingency, management, etc. should either be removed or included everywhere, or d i s c u s s e d in an introduction. TIT. Antenna Elements I would l i k e to see more emphasis on the "Advanced D e s i g n. " Bill H o m e is too busy with the RFP to waste time w r i t i n g and we have agreed that I will try to do t h i s. TV. Electronics S e c t i o n I I. A. It i n i t i a l l y implement the options. Only that the later. This is not clear i s my understanding that we w i l l not 5 / 2 2, 1 0 / 4 3, and 2 2 / 4 3 GHz d i c h r o i c feeds w i l l be positioned to allow this from the text. S e c t i o n I I. B. I am concerned that we may not meet our s p e c i f i c a t i o n s for 22 and 43 GHz performance. Nasers are i n common use for 1. 3 cm VLBI with K system temperatures being routinely achieved in good w e a t h e r. We can not a c c e p t a 1989 goal of 130 K. I fear that in two y e a r s, the HEMT and SIS systems will not offer the required performance, the maser will be f e l t unreliable and we may be forced to a disappointing performance. I t seems to me not u n l i k e l y that the 2 K refigerator will also have reliability problems and so the S I S s o l u t i o n may not be r e a l i s t i c. Should we not work on the maser r e l i a b i l i t y problem now, (if there is such a problem) and try to make this very attractive and demonstrated low noise device, with which we have considerable experience meet our reliability requirements. The VLB A 15 GHz r e c e i v e r s p e c i f i c a t i o n is 40 K, not 55 K. 55 K is given i n Volume I as the current performance. 40 K was p r e d i c t e d by Predicted noise temperatures always seem to increase as the target date approaches. Section D. It may be appropriate cussion in view of the Pietown site. to modify the link dis- S e c t i o n E. Is the VLA water vapor radiometer program a high priority one? While i t is c e r t a i n l y appropriate to defer production of the 10 radiometers, we s t i l l need a lot of work to develop a stable radiometer, and to understand how to relate the radiometer measurements to path length fluctuations.

3 3 IV, Construction P l a n. Napier has s u g g e s t e d t h a t the VLA 8 GHz systems be b u i l t f i r s t so that where p o s s i b l e they can be i n s t a l l e d during the normal o v e r h a u l. I f they can be b u i l t at a f a s t e r rate then some can be put on VLBA antennas. V. Data Acquisition T h i s chapter goes into more d e t a i l than o t h e r s. I still have trouble with t h i s c h a p t e r, as I would not be prepared to defend some of the discussion, particularly relating to VCiyinstr c o s t s, o p e r a t i o n s, and f e a s i b i l i t y. I suggest c h a n g i n g a l l reference to "red-cover" report to Volume I. All figures can be reduced to 1 / 2 page or l e s s and should go in place in the text. S e c t i o n C. Subchannel Format. I do not understand what is meant by "nondata replacement" format. Section F, line 2. VLB I array should read VLB Array. S e c t i o n G. The second sentence s t r a i n s the c r e d i b i l i t y of the whole p r o j e c t. " I t i s hoped that both systems will be ready for some operational tests starting 1 September.." Changing the date w i l l e a s i l y f i x up the report, (too e a s i l y ) but that does not change the fact that we are way behind in the c r i t i c a l area. At t h i s point we are probably not considering reel-to-reel VCRs r e a l i s t i c a l l y. Volume I I I is supposed to be a p r o j e c t plan not a design study, and I suggest deleting this discussion. We may need to fall back to the system with a "large number" of manually changed V C R s, (particularly if we keep putting off starting development of an automatic c h a n g e r ) ; but the impact to operations is s i g n i f i c a n t! Change to end of 1984 Table G-l. (last line of Section G ). How about including 4 head VCR system. Design Costs. I don't understand the 100k for MKIIIA development and 35k for M&S if NASA supports t h i s for the CDP. On the other hand I d o n ' t see any VCR development. This chapter should reflect cost to the VLBA Project, I think, without distinction as to what is done at NRAO or Haystack. Section c a l l e d Comments (MKIIIA v s. VCR). VCR numbers taken from " r e d book" a r e 1982$ whereas new numbers are About half of the difference is due to escalation.

4 4 Section J. T h i r d l i n e s t a t e s " t y p i c a l l y " the u n i t s a r e s e p a r a t e d by m. Why i n c l u d e the word " t y p i c a l l y? " Everything i s i d e n t i c a l. Gbps) I t e m B s u g g e s t s t h a t we could have super for k. This is misleading! HDR mode The V i d e o Converter Parts L i s t i s unnecessary, cluded, items 3 1, 3 2, and 33 are missing. (1.5 but if in- I f I u n d e r s t a n d c o r r e c t l y V I a l l o w s for only 1 6 input channels per station whereas V g i v e s 3 2. Either we should delete 1 / 2 of the video converters or add input channels to correlator. V I. Monitor and Control Section B. Fringe V e r i f i c a t i o n bigger than 2 Mbits? S e c t i o n D. Hardware: important - Underline. Cftapter V l l r Buffer: Comment about Should redundancy this is be very Cprrtlatps S e c t i o n C._ It i s s t a t e d that the c h o i c e between c u s t o m n-mos V L S I c h i p and other a l t e r n a t i v e s does not make a l a r g e impact on cost or c o r r e l a t o r d e s i g n. Why then i n c u r the r i s k of t h e V L S I d e v e l o p m e n t? What is cost and time s c h e d u l e if VLSI development i s unsuccessful and we have to r e t r e a t to PAL or g a t e a r r a y d e s i g n. Or w o r s e, and perhaps more p r o b a b l e, w i l l V L S I q u e s t i o n remain u n r e s o l v e d a f t e r say 1 y e a r, w i t h a s o l u t i o n l o o k i n g good after a f u r t h e r 6 months work. There is a tendency for that sort of thing to go on and on. Does the Correlator system provide for the autocorrelations for each s t a t i o n r e q u i r e d f o r s p e c t r o s c o p y. Is t h i s i n c l u d e d in cost? If not, what is cost increase? Will it be feasible to play back m u l t i p l e narrow band recordings simultaneously. Can we e i t h e r r e c i r c u l a t e data to make b e t t e r use of the " f a s t " c o r r e l a t o r s, or b u f f e r e a c h " s l o w " data s t r e a m, and m u l t i p l e x several " p a c k e t s " through the fast correlator. Note the Volume I d e s i g n e a s i l y a l l o w e d for t h i s mode w i t h the r e c i r c u l a t i n g system. It is not clear how important t h i s m u l t i p l e playback w i l l be to make up for o t h e r w i s e l o s t c o r r e l a t o r time and/or implementing more than 512 spectral channels.

5 The cost of the correlator less computer (420k) and playback units (660k) is $ 2. 6 6 M. This may be compared with $898k estimated about 18 months ago ( p g. V I I of V o l. 1 ).

5 5 The cost of the correlator less computer (420k) and playback units (660k) is $ M. This may be compared with $898k estimated about 18 months ago ( p g. V I I of V o l. 1 ). I (and perhaps t h e NSF i f they read t h i s ) would l i k e to understand the reason for this large descrepency. The two designs are comparable in performance s p e c s, although the Volume I design has several advantages (allows multiple VCR playback, gives a u t o c o r r e l a t i o n s, no risky VLSI development). V I I I. Data Processing What is the impact on computing requirements i f we allow for 16 or 32 s e p a r a t e f r e q u e n c y c h a n n e l s. Note in the middle of page 3, 4 to 6 frequency channels are indicated. Section I I I. A. 2. L a s t s e n t e n c e has the word " d e s p i t e " twice. T h i s s e c t i o n might be i n t e r p r e t e d to c l a i m t h a t t h e VLBA needs the same amount of computer power as the VLA. This has not been demonstrated and we are not asking for i t. Section I I I. C. l. In a d d i t i o n to the common use of phase c l o s u r e by the VLA and V L B A, the VLBA w i l l a l s o make use of phase r e f e r e n c i n g, as done normally for the VLA. Also the inner VLBA baselines only exceed the longer VLA b a s e l i n e s by a f a c t o r of 2, so t h e r e i s a real continuity in baseline length and thus post-processing techniques. Section I I I. C. 3. Paragraph 3. I would p r e f e r to delete the f i r s t 2 sentences. Geodetic as w e l l as astronomy data w i l l be a n a l y z e d in and out of h o u s e. O u t s i d e astonomers may also want to develop their own s o f t w a r e and we may have an in-house geodicist. L e t ' s t r e a t astronomers and geodicists in the same way. Section IV. AIPS. I t i s s u g g e s t e d t h a t the c o r r e l a t o r put out a FITS Tape. Goodl Why doesn't the VLA DEC 10? Section V. I would have thought t h a t the VLA experience is directly relevant to the q u e s t i o n of s e p a r a t i n g development from o p e r a t i o n s. Of c o u r s e, there i s no agreement on the VLA experience 1 cc: M. KIK/cw Roberts

43 and 86 GHz VLBI Polarimetry of 3C Adrienne Hunacek, MIT Mentor Jody Attridge MIT Haystack Observatory August 12 th, 2004

43 and 86 GHz VLBI Polarimetry of 3C454.3 Adrienne Hunacek, MIT Mentor Jody Attridge MIT Haystack Observatory August 12 th, 2004 Introduction Quasars subclass subclass of Active Galactic Nuclei (AGN) Extremely