NATIONAL RADIO ASTRONOMY OBSERVATORY SOCORRO, NEW MEXICO. VLA Test Memorandum No. 117 POINTING RUN - FEBRUARY 15-16, E. Fomalont.

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1 NATIONAL RADIO ASTRONOMY OBSERVATORY SOCORRO, NEW MEXICO VLA Test Memorndum No. 117 POINTING RUN - FEBRUARY 15-16, 1977 E. Fomlont Februry 1977 Two twelve-hour periods of identicl pointing runs were mde on Februry to check the consistency of the telescope pointing. Three sources, 3C84, 3C120 nd 3C273 were observed in pointing mode t C nd K-Bnds. 1. Dy to Dy Pointing Consistency Pointing observtions were mde for 3 sources t C nd K-Bnd for telescope #2. The sources nd frequencies were interleved. A comprison of the two dys re shown in Figure 1. The difference in pointing between the two dys ppers to be function of time with excursions of -18" over severl hours. This behvior is independent of the source (i.e., position in the sky) nd frequency. The sctter bout smooth curve is ^±4". Pointing observtions of two dys for telescope #6 t C-Bnd only re shown in Figure 2. In this plot we hve shown the individul source observtions. The sctter is lrger here thn for telescope #2; however, there is systemtic difference of -12" in zimuth between the two dys. No significnt source dependence is suggested. 2. Frequency Dependence of the Pointing The pointing offset between C nd K-Bnds were mesured for telescope #2 nd #1. It is expected the offsets should be constnt. Results re given in Figure 3. For telescope #2 the pointing offsets re resonbly constnt over the two dys with sctter of -8", For telescope #1, however, the C to K elevtion offsets systemticlly chnges by ~40" over ten hours. The chnge does not pper to be significntly source dependent but is temporl in nture. The difference is minly cused by C-Bnd chnge in the elevtion pointing. Unfortuntely, telescope #1 operted on only one dy. The zimuth behvior is quite good on telescope #1.

2 3, Bem Offsets Between Right nd Left Circulr Polrizing The bem offsets were clibrted for severl ntenns nd frequencies nd re given below. (R-L) POINTING POSITIONS (ARC-MIN) C-BAND K-BAND Elev. Azim. Elev. Azim. Tel #1 #2. #6 #1 #2 #6 #1 #2 #1 #2 Offset ±0.02 ±0.02 ±0.04 ±0.02 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 ±0.01 As expected, the offsets re nonvrible nd scle roughly with wvelength. 4. Tenttive Conclusions With this limited dt it is cler tht the ntenn pointing behves in vriety of wys. ) In telescope #2 nd #6 there re temporl vritions of zimuth nd elevtion of ~18" on time scle of severl hours to dy. Lrger vritions over longer durtion re possible. These vritions seem to be independent of frequency for telescope #2. b) The elevtion offset in pointing between two frequencies vried by ~40" over 10 hours for telescope #1. This offset chnge ws temporl in nture nd seems to be unrelted to detiled subreflector motion or position in sky. However, such n effect plys hvoc with the determintion of collimtion differences between frequencies. The collimtion differences between C nd K for telescope #2 nd zimuth #1 ws well behved, i.e., constnt.with sctter of ±10". c) The Right-Left point difference is very stble nd 2

3 virtully identicl for ech telescope. It does not scle precisely with observing wvelength. These results suggest tht most of the pointing problem is temporl in nture nd my be relted to heting effects on dish nd subreflector, encoder drifts Detiled subreflector motion errors do not pper to be importnt. 5. Hlf-Power Bem-widths The bem-width t K-Bnd is nomlous for ntenn #1. The following tble shows the derived hlf-power width. HALF-POWER BEAM-WIDTHS (ARC-MIN) Telescope Freq. Elev. Width Azim. Width 1 C K C K C K-Bnd Elevtion Dependence Bseline 2-4 shows well-defined gin - elevtion i dependence. Observtions of two dys were combined with, 1 three sources ech scled inversely by their flux density rtio. This plot is shown in Figure 4. The sctter bout men line is ~5%. The dependence shows mrked decrese for elevtions less thn 25%, pek sensitivity t elevtion nd moderte drop towrds zenith. t low elevtion is bout equl to q»10% tmospheric ttenution (1+q sec z)/(l+q). The drop off Bseline 2-1 shows much poorer gin - elevtion dependence. This is nticipted by the strnge K-Bnd bem shpe for telescope 1 discussed in Section 5, is n zimuth dependence of the "gin There elevtion" behvior. Correction for system temperture chnges hve not been in ve s t i gt ed. 3

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