HAT CREEK INTERFEROMETER POINTING FITTING Melvyn Wright 03-Jan-90 ABSTRACT Pointing data for 3 configurations Nov-Dec 1989 is analysed. The effect of antenna structure and solar heating on the pointing is also discussed. The parameters derived from optical pointing are reproducible after an antenna move. There is a large dispersion in the parameters derived from radio pointing, even with large data sets. The pointing parameters should not be changed based on small data sets. The radio pointing can be based on the parameters derived from optical pointing without significantly degrading the rms. Using the refraction computed from temperature and relative humidity does not significantly change the rms. INTRODUCTION The antenna azimuth and elevation are corrected by the following expressions: DAZ = APC1*COS(EL) + APC2 + APC3*SIN(EL) + COS(EL)*( APC4*SIN(AZ) + APC5*COS(AZ) + APC6*SIN(2.*AZ) + APC7*COS(2.*AZ) ) DEL = EPC1 +EPC2*SIN(EL) + EPC3*COS(EL) + EPC4*SIN(AZ) + EPC5*COS(AZ) + EPC6*SIN(2.*AZ) + EPC7*TAN(1.5708-EL) where DAZ = delta azimuth * cos(elevation) and DEL = delta elevation. The pointing parameters, A(1:7) and B(1:7), represent the following: APC1 - Azimuth encoder + antenna mount offset from the meridian APC2 - Collimation error of optical or radio axis from the mechanical axis (orthogonal to the elevation axis). Note that the optical and radio pointing will differ in this term. APC3 - Misalignment of elevation axis orthogonal to azimuth axis. APC4 to 7 - Fit to azimuth encoder errors. EPC1 - Elevation encoder offset. EPC2,3 - Elevation encoder errors. The radio pointing will contain an additional contribution in B(3) due to subreflector deflection. EPC4,5 - tilt of azimuth axis. EPC6 - Fit to residual tilt in azimuth bearing. EPC7 - Refraction term. Different for radio and optical pointing. Although the functions in the above equations are not orthogonal, additional corrections may be added to the pointing constants already in use provided that sufficient pointing data are taken to separate the terms. A correlation matrix printed for each fit, indicates the degree of independence of the parameters fitted. Fitting more parameters will improve the rms for any data set but a large number of data points must be obtained to obtain significant results. After moving the antennas only the azimuth offset, APC1 should change. The remaining pointing constants have been determined from independent measurements of mechanical and encoder misalignment and confirmed with more than 12 hours of data. If the antennas have been levelled, then they SHOULD NOT BE CHANGED BASED ON SMALL DATA SETS. Between optical and radio pointing we expect differences in collimation (APC2 and EPC1), Subreflector deflection (EPC3), and refraction (EPC7). These terms should be consistent from configuration to configuration. We might also hope that the encoder terms (APC4,5,6,7 and EPC2,3,6) might not change with time. If this is all true then the best way to determine the pointing parameters is to measure the parameters from optical pointing, which should be constant apart from small adjustments to EPC4 and 5 due to antenna tilt of order 5", and the azimuth offset APC1. We should then determine the radio collimation (APC2 and EPC1) and subreflector deflection (EPC3) from the radio pointing. DATA The data files below contain all the pointing data for the b1, b2 and b3 configurations. The data is in Hat::du:[data.point]. Number of points Ant1 Ant2 Ant3 Data file optical.B3 255 279 0 optical.B2 57 150 21 optical.29dec (B1) 60 125 0 onestar.29dec 0 39 0 radio.B3 127 151 143 radio.B2 175 189 183 radio.B1 (cop.01jan,29,30,31) 95 101 100 The TV camera on 3 needs focussing, and the cover spring needs replacing. The CCD chip was burned out because the spring is stretched and will not return the cover. After replacing the CCD chip the camera is now out of focus. The power to the cover motor is now disconnected with the cover closed. To open the cover, connect the cable; to close, disconnect and pull gently on spring. Camera 1 is quite noisy (hum) and only finds the brightest stars. RESULTS The tables summarizes the pointing for optical and radio data. The pointing parameters in arcmin; residual rms in arcsec. Antenna number: 1. ------------------- Optical data - Azimuth Pointing parameters (arcmin) rms notes B3 7p fit -30.73 -0.50 0.14 -1.08 -3.32 0.11 0.03 10 B2 5p fit -47.01 -0.68 0.77 -1.08 -3.32 0.11 0.03 6 B1 3p fit -26.21 -0.82 0.87 -1.08 -3.32 0.11 0.03 7 Radio data - Azimuth rms B3 7p fit -29.59 0.47 -0.01 -1.59 -5.17 0.29 0.83 13 1 B3 6p fit -30.97 0.54 -0.07 -1.61 -3.14 0.32 0.03 13 1 B3 opt + 2p -30.73 0.38 0.14 -1.08 -3.32 0.11 0.03 15 ** B2 opt + 2p -47.22 0.96 0.14 -1.08 -3.32 0.11 0.03 12 night only -47.22 0.96 0.14 -1.08 -3.32 0.11 0.03 12 B2 7p fit -47.11 0.47 0.77 -1.16 -3.27 0.21 0.01 11 Current APC -26.55 0.96 0.14 -1.08 -3.32 0.11 0.03 15 Optical data - Elevation rms B3 7p 12.33 0.33 0.04 -0.06 0.13 0.04 0.82 11 B2 5p 12.18 0.20 0.03 -0.21 0.06 0.04 0.82 10 B1 1p (from B3) 12.04 0.33 0.04 -0.06 0.13 0.04 0.82 11 B1 1p (from B2) 11.77 0.20 0.03 -0.21 0.06 0.04 0.82 9 Radio data - Elevation rms B3 7p fit 11.07 0.35 0.24 -0.25 -0.30 0.04 0.91 29 B3 6p fit 11.15 0.26 0.31 -0.27 -0.32 0.05 0.87 30 B3 opt + 3p 10.97 0.19 0.18 -0.06 0.13 0.04 0.87 31 B2 opt + 3p 10.65 0.72 0.20 -0.06 0.13 0.04 0.99 18 B2 opt + 3p 10.86 0.47 0.42 -0.21 0.06 0.04 0.87 14 2 B2 7p fit 10.81 0.66 0.25 -0.32 -0.11 0.05 0.97 13 night only 10.81 0.66 0.25 -0.32 -0.11 0.05 0.97 11 B2 6p fit 10.96 0.45 0.43 -0.33 -0.10 0.05 0.87 13 2 B1 fix epc7 10.84 0.67 0.28 -0.31 -0.05 -0.02 0.87 16 B1 5p 10.17 0.29 0.26 -0.24 -0.10 0.04 0.87 15 ** Current EPC 10.92 0.67 0.28 -0.31 -0.05 0.04 0.87 19 2 Antenna number: 2. ------------------- Optical data - Azimuth rms B3 7p fit -21.72 -4.60 0.84 -3.49 -1.73 0.17 0.36 11 B2 7p fit -33.94 -4.72 0.86 -3.47 -1.75 0.12 0.35 13 3 B1 7p fit -38.63 -4.67 0.80 -3.55 -1.75 0.25 0.38 6 3 Radio data - Azimuth rms B3 7p fit -21.33 2.84 0.84 -3.74 -2.10 0.67 0.51 16 4 B3 opt + 2p -21.65 2.90 0.84 -3.49 -1.73 0.17 0.36 20 B2 opt + 2p -34.00 2.90 0.84 -3.49 -1.73 0.17 0.36 20 ** B2 7p fit -34.37 3.30 0.32 -3.92 -1.42 0.52 0.23 18 4 B2 night only -34.00 2.90 0.84 -3.49 -1.73 0.17 0.36 18 Current APC : -39.17 2.90 0.84 -3.49 -1.73 0.17 0.36 21 Optical data - Elevation rms B3 7p fit 72.94 0.42 0.35 -0.15 0.38 0.12 0.86 9 3 B2 7p fit 72.91 0.35 0.32 -0.13 0.31 0.12 0.85 7 3 B1 7p fit 72.85 0.28 0.37 -0.27 0.25 0.16 0.83 7 Radio data - Elevation rms B3 opt + 3p 71.12 0.53 0.07 -0.15 0.38 0.12 0.98 17 B3 opt + 3p 71.29 0.31 0.30 -0.15 0.38 0.12 0.87 18 B3 7p fit 71.32 0.47 0.26 -0.31 0.12 0.18 0.92 16 4 B2 7p fit 71.49 0.02 0.31 -0.28 0.26 0.27 0.87 13 **4 night only 71.49 0.02 0.31 -0.28 0.26 0.27 0.87 11 B2 7p fit 71.45 0.03 0.51 -0.34 0.30 0.34 0.77 14 B2 6p fit 71.27 0.27 0.34 -0.34 0.31 0.35 0.87 15 Current EPC : 71.23 0.02 0.31 -0.28 0.26 0.12 0.87 19 Antenna number: 3. -------------------- Antenna 3 was not moved between B2 and B3; combined radio data has 326 points. Night (8pm-7am) has 117 points. Large scatter at sunrise (see below). Radio data - Azimuth rms b3 7p fit 18.23 2.21 0.98 -0.23 -0.45 0.47 0.14 10 b2 7p fit 18.56 2.10 1.13 0.13 -0.96 0.23 0.44 12 b2+b3 7p fit 18.48 2.10 1.12 0.01 -0.81 0.31 0.35 15 ** night only 7p 18.19 1.97 1.21 0.20 -0.32 0.15 0.22 9 exclude 8-10am 18.19 1.97 1.21 0.20 -0.32 0.15 0.22 13 Current APC : 18.89 2.10 1.12 0.01 -0.81 0.31 0.35 13 Radio data - Elevation rms B3 7p fit 5.21 -0.54 1.43 -0.24 -0.20 -0.55 0.76 15 B2 7p fit 5.06 -0.45 1.61 -0.31 -0.08 -0.46 0.69 17 B2+B3 7p fit 5.08 -0.47 1.53 -0.38 -0.08 -0.40 0.73 18 B2+B3 6p fit 4.89 -0.20 1.27 -0.39 -0.07 -0.38 0.87 18 6 night only 7p 5.44 -0.80 1.27 -0.39 -0.17 -0.48 0.79 17 B1 6p 4.95 -0.29 1.30 -0.65 -0.22 -0.22 0.87 21 B1 (B2+B3 fit) 4.89 -0.20 1.27 -0.39 -0.07 -0.38 0.87 22 ** Current EPC 4.86 -0.20 1.27 -0.39 -0.07 -0.38 0.87 25 NOTES ** This fit is the basis for the current pointing parameters. 1) This data set gives a high correlation (-.918) between APC5 and 7. The 6p fit is more realistic. 2) Use optical pointing for B2 configuration with refraction fixed to 0.87 3) For antenna 2 we have three good data sets for optical and for radio pointing. The two optical data sets for antenna 2 are very consistent. 4) The 7-parameter fits for the radio data are not so consistent, but the rms pointing is not much degraded by using the optical pointing with radio offsets. The 7-parameter fits may still be suffering from errors in the data. We await more optical data before making similar comparisons for antennas 1 and 3. 5) Using nighttime data only, there is a significant improvement in rms for antenna 3 azimuth. Most of the daytime degredation is 8am-10am. Antenna 3 elevation, and antennas 1 and 2 are not so bad. 6) Fixed refraction term for Temp=0 C and RH=50% humidity. See Jack's memo. 7) The subreflector deflection (difference between optical and radio EPC3) is not large on antennas 1 and 2, as expected for these antennas; it may be quite large on antenna 3, which has longer subreflector support legs. 8) EPC2 is probably describing an encoder error, rather than a subreflector deflection, and should be the same for optical and radio data. 9) No compelling evidence for 1 or 2 degree period for onestar.29dec data on antenna 2 azimuth or elevation. No data for other antennas. CONCLUSIONS The parameters derived from optical pointing seem to be reproducible after an antenna move. The cameras on antenna 3 and 1 need fixing so this result can be confirmed for these antennas. There is a large dispersion in the parameters derived from radio pointing, even with large data sets. The pointing parameters SHOULD NOT BE CHANGED BASED ON SMALL DATA SETS. Observers should only change the azimuth offset, APC1 after an antenna move. The radio pointing can be based on the parameters derived from optical pointing without significantly degrading the rms. Using the refraction computed from temperature and relative humidity does not significantly change the rms. Larger variations in refraction are expected in summer, and it seems desirable to compute the refraction (with suitable restraints against unreasonable values). The time dependence of antenna 1 azimuth is presumed to be a temperature dependence, and may now be fixed. The noise from the camera on antenna 1 needs to be fixed so that this can be checked. The antenna tilt is a function of azimuth for antennas 1 and 2. There is also some elevation dependence; a much cleaner residual tilt of the azimuth bearing is obtained for antenna 1 with elevations less than 90 degrees. The azimuth dependence results from the inadequate support of the azimuth bearing, and can be characterized by a transition at azimuth 30 degrees west corresponding to when the yoke is over a corner of the upper triangle of the beams supporting the azimuth bearing. The elevation dependence probably results from some antenna unbalance. The azimuth dependence of antenna elevation is being fitted by the pointing parameters EPC4,5,6. Using more complex functions did not result in a significant improvement in the rms in the radio data. The poor pointing on antenna 3 after sunrise can probably be improved by closing the sides of the beam supporting the feed legs, and blowing air through the resulting box structure as is currently done for the feed legs.