SMA Mosaic Image Simulations

Transcription

1 l To: Files From: Douglas Wood Date: Monday, July 23, 1990 Subject: SMA Technical Memo #23 SMA Mosaic mage Simulations Abstract This memo presents some preliminary results of SMA image simulations using the SDE software package. t examines the question of a Y vs. 0 configuration for D a.rray and concludes that no significant advantage can be seen for an 0 given only 6 antennae. Random pointing errors are introduced into simulations of compact and extended sources. For the SMA case random pointing errors of 1 and perhaps as much as 3 seem acceptable for SiMA observations. These conclusions are preliminary, however, and should be investigated with more extensive study. ntroduction have used the Software Development Environment described in Tech. Memo #22 to conduct some simulations of SMA imaging characteristics. There are a wide range of parameters that one might consider when simulating an SMA observation and image restoration. A partial list includes: UV Coverage Antenna positions HA limits Elevation Limits Observatory Latitude Declination of source Frequency Mosaic Spacing of samples

2 2 ntegration time per pointing Observing strategy Primary beam pattern Guard Band Pointing Errors (random and global) Central element flux (if any) Target Source structure Brightness Largest angular diameter Required dynamic range for science mage Reconstruction Reconstruction algorithm Weighting Selfcalibration have decided to focus this study on two important problems: Y vs. 0 layout for smallest configurations and the effects of random pointing errors. There are many other aspects of the SMA imaging problem that will have to be addressed. This memo and memo #22 will pave the way for future studies Y vs. 0 for D array t is expected that mosaicing is likely to be performed in the smallest configurations. t is also likely that the most extended configurations wil require a Y or similar station layout for economic and environmental reasons (obviously there is much less track/road/waveguide etc. with a Y). NRAO s MMA is currently designed around the concept of an 0 layout. Tim Comwell has shown in the MMA Technical memo series that an 0 configuration is a better optimized than Y configuration for MMA observations. His simulations, however, have used 40 antennae rather than the 6 antennae that will be available in the SMA. With 6 antennae the U,V plane is not well sampled so it is of interest to know whether a Y vs. 0 configuration has any significant effect.

3 3 The basic Y configuration (MK12.DAT, see Tech Memo #22) uses the inner 2 stations on each arm only. ts maximum base line is 27m. A crystaline6 array was also constructed (MKCRY6.DAT) using based on Cornwell s optimization. choose the diameter of the circle to be the same as the maximum baseline of the Y. The components of the model image (testc.mod) are listed in the attached table. A simulated mosaic was created on a 3 x 3 grid with spacing 12. A full synthesis was run with a shadowing limit of 20. The results of these simulations are presented in Figures 1 and 2. Although differences in the two images can be seen, the basic components of the model source are approximately the same. The peak flux in the two simulations is nearly the same (527 Jy/pixel for the Y array vs..476 Jy/pixel for the 0 array). The rms of the residuals produced by MOSAC were 536 Jy/pixel for the Y and.642 Jy/pixel for the 0 array. [Note: n general these simulations produced very large residuals with structures that are clearly source related. More work needs to be done on running MOSAC to improve the quality of the restoration.] have run another simulation (RUN #S) which compares the response of the two sources to an extended gaussian source (30 = 1 primary beam at 345GHz). The results are seen in Figures 3 and 4. n this case it appears that the 0 array has done a better job of maintaining the circular symmetry of the source. A slice through each image is shown in figures 5 and 6. There is no significant difference in the FWHM of the source above the background. [Note: These images do not go to zero at the edge. This is probably due to the fact that didn t include a large enough guard band in the simulated observations. was unable, however, to get the current version of SDE to produce images larger than 128 square. am sure that this problem can be over come.] conclude that there appears to be no major advantage in an 0 vs. a Y array for the SMA D array, but that these preliminary results should be checked with more extensive investigation. Random Pointing Errors Random pointing errors during a mosaic observation have been shown by various MMA memos to reduce the dynamic range of MMA observations. SMA run #5 introduced random pointing errors of 3 while run #6 included errors of 1. They should be compared with run #4 that had no pointing errors. Figures 7 and 8 show the results. There appears to be little difference in the images. The residuals for the 1 pointing error case are Jy/pixel compared with

4 Jy/pixel for the no pointing error case, an insignificant difference. For 3 pointing errors the rms of the residuals is higher, Jy/pixel. Pointing errors for an extended source were introduced in run #lo shown in Figure 9. Figure 9 should be compared with Figure 4, but note that the peak level is different in the two images and the background does not go to zero so the contours are not exactly at the same levels in both plots. Still, it is clear that the extended source is not remarkably distorted even with these large random errors. From these preliminary simulations alone, conclude that random pointing errors of 1 are acceptable for SMA observations and that errors as large as 3 may be tolerable. This contradicts the extensive work done on MMA simulations and so caution that it needs to be investigated more completely. suggest that pointing errors are more significant in the MMA case because that telescope will produce images with much greater dynamic range than the SMA. Pointing errors may only be important in high dynamic range cases. have used crude methods to compare simulation results with and with out point errors. t would be better to compare the synthesized image with the true image, especially as a function of spatial frequency. mages of real sources and the effects of receiver and atmospheric noise should be included. t has also been suggested that global rather than random pointing errors are more important.

5 % more testc.mod 'PON' 'GAUS' 'PON' l 0 'PON' l.l 0 'PON' l.1 0 'PON' l 0 'PON' 'PON' l 0 'PON' l 0 'PON' 'PON' l 0 'PON' l.l 0 'PON' l 0 'PON' l.1 0 'PON' l.l 0 'PON' 'GAUS' 'GAUS' 'GAUS'

6 PLot f i l e v e r s i o n 1 c r e a t e d 25JUL :58:00 TEST MHZ MKCRY6#4VM.CLN.l r 15 D : L N A 00 T C :, N /n \ \ 30 t 00 0; P3.!Zl R GHT ASCENS ON Peak flux E01 JY/PXEL Levs = E03 * ( , 2.00, 1.00, 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00) 58.0

7 PLot f i l e v e r s i o n 5 c r e a t e d 25JUL :57:04 r! TEST MHZ MK12#4VM.CLN.l 15 D E L N A 00 T :, N is t ZB.!B RGHT ASCENSON Peak flux = E01 JY/PXEL Levs E03 * ( , 2.00, 1.00, 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00)

8 PLot f i l e v e r s i o n 1 created 25JUL :00:58 TEST MHZ MKCRY6#8VM.CLN.l C t / m RGHT ASCENSON Peak flux = E03 JY/PXEL Levs = E05 * ( , , , 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00)

9 PLot f i l e v e r s i o n 2 c r e a t e d 25JUL1990 2O:OO:Ol TEST MHZ MK12#8VM.CLN.l ml RGHT ASCENSON Peak flux = E03 JY/PXEL Levs = E05 * ( , 5.000, 10.00, 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00)

10 2.6 Plot file version 3 created 25JUL :02:05 TEST MHZ MKCRY6#8VM.CLN.l y 2.2 i ; 2.0 P X ; ARC SEC Center at RA ROT DEC

11 P l o t f i l e v e r s i o n 3 c r e a t e d 25JUL :03:20 TEST MHZ MK12i~8VM.lcLN.l M 2.6 i i 2.4 J Y p 2.2 X E L ARC SEC Center at RA DEC ROT

12 PLot f i l e v e r s i o n 1 c r e a t e d 25JUL :59:58 TEST MHZ MK12#6VM.CLN.l 1 15 D E C L N A T 0 N 00 3 C t m.!b RGHT ASCENSON Peak flux E01 JY/PXEL Levs E03 * ( 3.00, 2.00, 1.00, 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00) 58.p

13 PLot f i l e v e r s i o n 1 c r e a t e d 25JUL :58:58 TEST MHZ MK12#5VM.CLN.l m.b RGHT ASCENSON Peak flux E01 JY/PXEL Levs E03 * ( , , , 1.000, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 20.00, 30.00, 40.00, 50.00, 60.00, 70.00, 80.00, 90.00)