12M Manual Chapter 4


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Tracking, Pointing, and Focus



 

4.1 Tracking Capabilities

 The 12 Meter Telescope can track both stationary (sidereal) sources or fast moving sources such as the Sun, Moon, planets, comets, or satellites (called “ephemeris objects"). The positions of stationary sources may be entered into source catalogs by the observer or directly into the on-line system by the operator. Stationary source positions may be given as

  • Apparent equatorial positions (already precessed),
  • Equinox B1950 or J2000 equatorial positions, or
  • Galactic coordinate positions (lII, bII).

An example of the 12m catalog format is the following:

18:27:17.3       01:13:23.0       B1950 SERMM1        8.0       LSR     RAD

18:27:28.0       01:10:45.0       B1950 SERMM2        8.0       LSR     RAD

18:27:27.3       01:11:55.0       B1950 SERMM3        8.0       LSR     RAD

18:27:24.7       01:11:10.0       B1950 SERMM4        8.0       LSR     RAD

18:27:18.9       01:12:02.0       B1950 SERMM5        8.0       LSR     RAD

18:27:25.4       01:12:02.0       B1950 SERMM6        8.0       LSR     RAD

18:27:22.0       01:12:30.0       B1950 SERCTR         8.0       LSR     RAD

 

where

column 1 = right ascension (HH:MM:SS.S) or galactic longitude (DDD.DDDDD),

column 2 = declination (DD:MM:SS.S) or galactic latitude (DD.DDDDD),

column 3 = coordinate epoch (B1950, J2000, APPARENT, or GALACTIC),

column 4 = source name,

column 5 = source velocity,

column 6 = source velocity reference frame (LSR, HEL, GEO, or TOP), where

LSR = local standard of rest velocity reference frame,

HEL = heliocentric velocity reference frame,

GEO = geocentric velocity reference frame, and

TOP = topocentric velocity reference frame,

column 7 = source velocity type (RAD, OPT, or REL), where

RAD = radial velocity type, 

OPT = optical velocity type, and

REL = relativistic velocity type

 

The sky frequency equations corresponding to the above velocity types are given in Equations 5.4.

 

4.1.1 Ephemeris Objects

To track ephemeris objects (comets, asteroids, etc.), the control system requires three positions, centered upon the present time. The tracking algorithm then makes a cubic spline interpolation to determine the source position at the specific time of observation.

The observer must prepare a catalog in the correct format in order to observe ephemeris objects other than the planets and the Moon. The basic format needed by the control system is show below.

OBJ Hyakutake

VELFRAME LSR

VELTYPE RAD

#

# Ephemeris for Hyakutake, 0.000 H 9 APR 1996 to 23.700 10 APR 1996

#

D  45.7745  D  41.3975  J  50182.000720  0.00  55.84

D  45.7705  D  41.3822  J  50182.017387  0.00  55.86

D  45.7662  D  41.3670  J  50182.034053  0.00  55.89

D  45.7620  D  41.3517  J  50182.050720  0.00  55.91

D  45.7580  D  41.3363  J  50182.067387  0.00  55.93

D  45.7540  D  41.3212  J  50182.084053  0.00  55.94

D  45.7500  D  41.3058  J  50182.100720  0.00  55.95

D  45.7463  D  41.2907  J  50182.117387  0.00  55.96

D  45.7422  D  41.2755  J  50182.134053  0.00  55.97

D  45.7385  D  41.2603  J  50182.150720  0.00  55.97

D  45.7347  D  41.2452  J  50182.167387  0.00  55.97

D  45.7313  D  41.2300  J  50182.184053  0.00  55.97

where the first three lines are descriptors for the object name, velocity frame, and velocity type. The next three lines are comments. Each column thereafter is defined as follows:

D 45.7745 = the topocentric right ascension in degrees.

D 41.3975 = the topocentric declination in degrees.

NOTE: You can also use a geocentric right ascension and declination in the last two locations, but if you do you must enter the correct horizontal parallax (see below).  

J 50182.000720 = the Modified Julian Date (mjd) of this record in the TT (Terrestrial Time, which used to be called Terrestrial Dynamical Time, or TDT) system. Note that  TT = UTC + 63.184 sec after June 30, 1997.

0.0 = the horizontal parallax (the angle from you to the object to the center of the earth in arc seconds). The horizontal parallax is zero in this case since we are using topocentric coordinates. If one uses geocentric coordinates, the horizontal parallax must be non-zero and is given by:

HP = arcsin

 geocentric distance of object

55.84 = the topocentric velocity. Non-planets default to topocentric velocities. Planets default to geocentric positions and velocities.

After you create the ephemeris put it in a file with the extension eph in the observers home directory - /home/obs/ini/comet.eph for example.

If you do not have an ephemeris generation program, we have a copy of the “TBEG4" two body ephemeris generator program written by Don Yeomans which you can use. This program will produce a 12m-format ephemeris file given a set of orbital elements. Contact Tom Folkers if you would like to use this program.

 

4.2 Tracking Limits

4.2.1 Elevation Limits

Mechanical limits constrain the movement of the telescope on both the azimuth and elevation axes. Under normal source-tracking operation, the control system software prevents the telescope from moving into these limits. The telescope is also equipped with fail-safe hardware limit switches that will turn the drive motors off and apply the brakes before the telescope can be damaged. When the telescope reaches a lower elevation of 15.0º, it begins to depress a safety spring on the elevation stop. Tracking can continue until the elevation drive motor begins drawing excessive current; this usually occurs at an elevation near 14.5º. A final limit switch is tripped at 13.8º elevation.

The control computer will allow a source to be tracked up to an elevation of 90º. The pointing equations diverge at the zenith, however, and elevations greater than 88º should be strictly avoided during observations. For routine observations, we recommend that you avoid elevations >80º if possible, since both tracking and pointing degrade in that range. For maintenance purposes only, the telescope can be driven to an elevation of 92º before tripping a final limit switch.

 

4.2.2 Azimuth Limits

To prevent the over-wrapping of cables, the telescope has hardware and software azimuth limits. The control system will not allow the telescope to rotate through 66.8º azimuth to acquire or track a source. As they rise, sources with declinations between 27º and 38.75º will pass through 66.8º azimuth above 15º elevation. When a rising source reaches an azimuth of 66.8º, the telescope will rotate 360º to re-acquire the source. The region of the sky where azimuth transitions occur is displayed in Figure 4.1. If an integration is in progress when the transition azimuth is reached, the integration is halted while the telescope rotates around to re-acquire the source. The integration is resumed when the source is reached again. If it is undesirable for this to happen, you should cease the integration before reaching the transition azimuth.

 

4.3 Tracking Error Tolerance

The 12 Meter Telescope is vulnerable to tracking errors produced by wind gusts. The tracking software has a provision for rejecting from the integration any data sample that was taken with the telescope off source. The basic timing cycle of the 12m control system is 100 milliseconds (0.1 sec). After each 100 ms interval, the system checks for error conditions such as tracking errors or a loss of frequency phase lock. If an error condition is detected, the data sample collected during the last 100 ms interval is not averaged with the rest of the data.

You can choose how much tracking error you are willing to accept. This number is called the “tracking tolerance" and is displayed on the on-line status monitor in the lower left-hand corner under the heading “TOL". The 12m typically experiences 3-4” tracking errors under calm conditions. The tolerance is usually not set to less than 5” for this reason. Typical choices for the tolerance are 5” for 1.3 mm and shorter wavelength observations, and 10” for 2 and 3 mm observations. Figure 4.2 shows the amount of signal that is lost by a given pointing error, expressed as fractions of a FWHP beamwidth.

 

4.4 Sequence of Position Computation Operations

The sequence of computer operations that is executed when seeking and then tracking a source is as follows:

 

  1. Input RA-DEC (B1950, J2000, or current epoch) or galactic (lII,bII).

  2. If B1950 position, precess to current date. If galactic coordinates, first precess to epoch B1950 The precessed position of a source is computed only when the source is first accessed.

  3. If source is an ephemeris object, interpolate to current UT.

  4. Do spherical coordinate conversion from RA-DEC to AZ-EL.

  5. Add azimuth and elevation encoder corrections as computed from Equations A.1, A.2, A.9, and A.10 (Appendix A). 

  6. Add azimuth and elevation pointing corrections.

  7. Command telescope to the correct AZ-EL position.

  8. Once every 10 seconds, go back to spherical coordinate conversion. Between loops, extrapolate AZ and EL drive rates every 100 ms to compute positions.

 

Figure 4.1: Local hour angle and declination versus azimuth and elevation for the 12m. The hatched region indicates the range of source declination where the telescope will undergo a 360º azimuth transition while tracking a rising source.

 

      

Figure 4.2: Loss of signal due to pointing error.

 

4.5 Subreflector Beam Throw

The 12 Meter Telescope is equipped with a nutating (chopping) subreflector that is used for beam-switched observations in both the spectral line and continuum modes. Other observing modes, such as position switching, are made with the subreflector locked in place. Obviously, the throw position of the subreflector, whether it is chopping or locked in place, affects the pointing of the telescope. The current 12m subreflector switches only in azimuth, although mount misalignments may produce a small component in the elevation direction (usually these are less than 1-2”). The pointing equations for the 12m are set up for 0 offset in subreflector position, i.e. with the subreflector axis aligned with the electrical axis of the primary reflector. When the subreflector is offset so that the telescope must be moved in the positive azimuth direction to bring the source into the beam, the subreflector is said to be in the “+BEAM”. When the telescope must be moved in the negative azimuth direction to acquire the source, at least with respect to the +BEAM, the subreflector is in the “-BEAM”. Figure 4.3 illustrates this convention. Both the spectral line and continuum backends are configured so that the +BEAM signal is positive and the -BEAM signal negative.

 

Figure 4.3: Subreflector +Beam/Beam conventions.

 

The control system software takes account of the subreflector throw setting when positioning the telescope. The computer does not read the subreflector setting automatically, however; it depends upon the operator to manually set the throw value. The subreflector is physically set by turning two dials on the subreflector control chassis. Default values of subreflector throw are stored in the computer: for example, ±2′ for 3 mm and ±1′ for 1 mm. You can check the beam throw by making a continuum cross-scan on a strong source.

Continuum mapping rows are explained in detail in Chapter 6.

 

4.6 Pointing

    You are responsible for determining the residual azimuth and elevation pointing offsets, relative to the nominal telescope pointing. These offsets are usually of the order 10″- 20″ and often vary across the sky. In particular, the offsets often show an elevation dependence. The pointing of the 12m may drift over a period of several months by 10″ or so, and you are cautioned not to assume that old pointing data are still valid. If a project requires accurate pointing, observers should budget 5 - 10% of their total observing time to pointing checks. High frequency projects, for which the FWHP of the main beam is 30″ or less, obviously require more attention to the pointing.

You may perform pointing checks in either continuum or spectral line mode. Pointing sources must have an angular brightness distribution that is compact compared to the antenna beam. In addition, they must have well-determined positions and must be strong enough to point on in a reasonable amount of time, e.g. 10 minutes or less. The best pointing sources are the major planets Venus, Mars, Jupiter, Saturn, and Uranus, observed in continuum mode. These sources have strong flux densities throughout the millimeter band, although Uranus is faint at 3mm and longer wavelengths. To obtain complete sky coverage, particularly north of the zenith, the planets must be supplemented with other pointing sources. A tabulation of these sources and approximate flux densities at 3 and 1 mm is given in Table 4.1. This table includes the sources traditionally used at the 12m together with sources found useful at the IRAM 30m telescope. The sources with no listed flux density are on the order of 1 Jy in strength. It should be noted that the flux densities of many of the extragalactic sources in the table can be highly time-variable. For an up-to-date listing of measured 3mm fluxes, see http://dopey.haystack.edu/cmva/quasar.list.

The spectral line emission from a number of sources can be used for pointing purposes. Emission from rotational transitions of CO, HCN, and SiO are often strong enough and angularly compact enough to provide good pointing results. A list of spectral line pointing sources and their strong molecular emission lines are listed in Table 4.2.

 

4.6.1 Continuum and Spectral Line Five-Point Measurements

The five-point mapping option is the standard observing technique for determining the antenna pointing of the 12 Meter Telescope. Continuum five-point measurements can also be used to measure the flux density of a strong source with the highest precision, or the flux density of a source whose position is not exactly known. The five-point map consists of five successive ON/OFF sequences (for continuum and beam-switched spectral line measurements), or five successive ON and OFF measurements (for position-switched spectral line measurements), or five successive ON measurements with one associated OFF measurement (for total power spectral line measurements). The five sequences or ON measurements are positioned in azimuth and elevation relative to the nominal source position, as shown in Figure 4.4. The numbers identifying the positions give the time order in which the sequences are taken. The offset for each position (“HP” in Figure 4.4), is a selected offset (in arcsec) that is usually chosen to be close to half the HPBW of the telescope at the observing frequency. HP should be somewhat larger if the source is extended; for a planet that is resolved by the beam, chose HP to be approximately the semi-diameter of the source.
 

Table 4.1: Galactic and Extragalactic Continuum Pointing Sources

Source

(B1950)

(B1950)

Sv(3mm)(Jy)

Sv(2mm)(Jy)

0003+380

00:03:22.300

38:03:33.00

1.4,0.7

 

0048-097

00:48:10.000

-09:45:24.00

1.8,0.5

 

0106+013

01:06:04.500

01:19:01.00

0.9,0.3-0.6

 

0112-017

01:12:43.900

-01:42:55.00

 

 

0119+041

01:19:21.400

04:06:44.00

1.2,0.4

 

RSCL

01:24:40.020

-32:48:06.80

Good

 

0133+476

01:33:55.100

47:36:13.00

2.8,0.4-1.5

 

0135-247

01:35:17.100

-24:46:09.00

1.3,

 

0202+149

02:02:07.400

14:59:51.00

3.7,1.5

 

MIRA

02:16:49.110

-03:12:22.40

Very

 

W3(OH)

02:23:16.500

61:38:57.00

3.8,8.3

 

0224+671

02:24:41.200

67:07:40.00

0.8,0.8-1.9

 

0235+164

02:35:52.600

16:24:04.00

1.3,0.8-3.5

 

0234+285

02:37:52.406

+28:48:08.99

 

 

0300+471

03:00:10.100

47:04:34.00

1.5,0.6

 

3C84

03:16:29.600

41:19:52.00

5.5,1.8-8.0

 

0316+413

03:19:48.160

+41:30:42.10

 

 

0322+47

03:22:59.100

47:21:22.00

Good

 

GL490

03:23:39.220

58:36:35.60

,2.9

 

NGC1333

03:26:05.030

31:03:13.00

3

 

0336-019

03:36:59.000

-01:56:17.00

2.1,0.9-1.3

 

NRAO150

03:55:45.300

50:49:20.00

3.0,1.0-2.0

 

0355+508

03:59:29.747

+50:57:50.16

 

 

3C111

04:15:00.610

37:54:19.50

3.7,1.3-9.8

 

0420-014

04:23:15.801

-01:20:33.07

4.1,0.8-4.2

 

0426-380

04:26:54.700

-38:02:52.00

2.9,

 

L1551IRS5

04:28:40.200

18:01:42.00

,1.7

 

3C120

04:30:31.600

05:15:00.00

2.0,0.5-1.6

 

0430+052

04:33:11.096

+05:21:15.62

 

 

0439+360

04:39:33.800

36:01:15.00

1.1,2.6

 

0439+36

04:39:34.040

36:01:16.00

Very

 

0454-234

04:54:57.300

-23:29:28.00

2.1,0.4-0.5

 

0456+56

04:56:40.600

56:06:28.00

OK

 

0458-020

04:58:41.300

-02:03:34.00

1.9,0.6-0.8

 

0521-365

05:21:12.900

-36:30:17.00

4.6,2.7-4.2

 

0528+134

05:28:06.800

13:29:42.00

5.6,1.9-3.5

 

0529+075

05:29:56.490

07:30:38.10

,1.3

 

TAUA

05:31:30.000

21:59:00.00

 

 

ORIONA

05:32:47.000

-05:24:21.00

 

 

0552+398

05:52:01.400

39:48:22.00

2.3,0.7-0.9

 

0605-085

06:05:36.000

-08:34:20.00

1.6,0.6-1.1

 

0607-157

06:07:26.000

-15:42:03.00

4.8,2.7-5.9

 

0642+449

06:42:53.000

44:54:31.00

1.3,0.5

 

0716+714

07:16:13.000

71:26:15.00

2.5,0.3-1.5

 

VYCMA

07:20:54.700

-25:40:12.00

 

 

0716+714

07:21:53.448

+71:20:36.36

 

 

0727-115

07:27:58.100

-11:34:53.00

2.4,0.3-1.1

 

0735+178

07:35:14.100

17:49:09.00

3.0,0.7-3.0

 

0736+017

07:39:18.034

+01:37:04.62

2.8,0.6-3.5

 

0804+499

08:04:58.400

49:59:23.10

,1.3

 

0805-077

08:05:49.600

-07:42:22.00

1.2,0.7

 

0814+425

08:14:51.700

42:32:08.00

1.3,0.3

 

0820+560

08:20:53.200

56:02:27.00

1.0,0.3

 

0823+033

08:23:13.560

03:19:15.40

,1.7

 

0827+243

08:27:54.400

24:21:08.00

 

 

OJ287

08:51:57.300

20:17:58.00

2.0,1.0-4.0

 

0851+202

08:54:48.875

+20:06:30.64

 

 

0923+392

09:23:55.300

39:15:24.00

6.2,0.5-3.0

 

RLEO

09:44:52.200

11:39:40.00

 

 

CWLEO

09:45:14.840

13:30:40.50

,1.5

 

IRC+10216

09:45:14.890

13:30:40.80

Very

 

1013+30

10:13:10.940

30:49:16.70

Very

 

1034-293

10:34:55.800

-29:18:27.00

3.0,1.5

 

1039+811

10:39:27.800

81:10:24.00

1.0,

 

1044+719

10:44:49.700

71:59:27.00

1.4,0.3

 

VHYA

10:49:11.300

-20:59:04.90

 

 

1055+018

10:55:55.300

01:50:03.00

3.2,0.8-1.7

 

1055+018

10:58:29.605

+01:33:58.82

 

 

1116+128

11:16:20.800

12:51:07.00

1.0,0.5

 

1156+295

11:56:57.800

29:31:26.00

2.5,0.4-1.3

 

3C273

12:26:33.200

02:19:43.00

20.1,6.0-24.0

 

3C273B

12:26:33.200

02:19:43.00

20.1,6.0-24.0

 

VIRA

12:28:17.600

12:40:02.00

7.9,1.9

 

1226+023

12:29:06.700

+02:03:08.60

 

 

1244-255

12:44:06.700

-25:31:27.00

2.0,1.1

 

3C279

12:53:35.800

-05:31:08.00

20.0,4.0-21.0

 

1253-055

12:56:11.167

-05:47:21.52

 

 

1308+326

13:08:07.600

32:36:40.00

2.0,0.5-1.5

 

1313-333

13:13:20.000

-33:23:10.00

1.4,0.6-1.4

 

CENTA

13:22:31.800

-42:45:30.00

9.4,6.1-8.0

 

1334-127

13:34:59.800

-12:42:10.00

5.6,2.1-5.5

 

WHYA

13:46:12.100

-28:07:09.00

 

 

1354+195

13:54:42.100

19:33:44.00

1.2,0.3

 

1413+135

14:13:33.900

13:34:17.00

2.6,0.2-2.2

 

1418+546

14:18:06.200

54:36:58.00

1.0,0.3-0.8

 

1502+106

15:02:00.200

10:41:18.00

1.0,0.3

 

1504-166

15:04:16.400

-16:40:59.00

1.1,0.5

 

1510-089

15:10:08.900

-08:54:48.00

1.3,0.6-1.7

 

1514-241

15:14:45.300

-24:11:23.00

1.2,0.5-1.7

 

1546+027

15:46:58.300

02:46:06.00

4.1,0.5

 

1548+056

15:48:06.900

05:36:11.00

2.0,0.3

 

1611+343

16:11:47.900

34:20:20.00

3.2,1.0-1.4

 

1622-253

16:22:44.100

-25:20:52.00

2.4,

 

1629-242

16:29:21.050

-24:22:15.50

,5.3

 

1633+382

16:35:15.493

+38:08:04.50

2.5,0.7-1.0

 

1637+574

16:37:17.400

57:26:16.00

1.1,0.5

 

1638+398

16:38:48.200

39:52:30.00

1.0,0.6

 

3C345

16:41:17.600

39:54:11.00

5.7,0.8-6.0

 

1641+399

16:42:58.810

+39:48:36.99

 

 

1655+077

16:55:44.000

07:46:00.00

1.0,0.5

 

1657-261

16:57:47.700

-26:06:29.00

1.8,0.8-1.9

 

1704-24

17:04:54.600

-24:40:39.00

OK

 

NRAO530

17:30:13.500

-13:02:46.00

10.2,3.4-7.0

 

1732+389

17:32:40.500

38:59:47.00

1.0,0.6

 

1730-130

17:33:02.706

-13:04:49.55

 

 

1739+522

17:39:29.010

52:13:10.40

,1.2

 

1741-038

17:43:58.856

-03:50:04.62

2.2,1.1

 

1742-29

17:45:40.038

-29:00:28.07

 

 

1749+096

17:51:32.819

+09:39:00.73

6.4,0.9-4.2

 

1757-240

17:57:26.800

-24:03:57.00

,14.0

 

1803+784

18:03:39.200

78:27:54.00

2.1,1.3-1.6

 

1807+698

18:07:18.500

69:48:57.00

1.2,0.4-0.8

 

VXSGR

18:08:44.500

-33:52:00.50

 

 

1823+568

18:23:14.900

56:49:18.00

1.0,1.4-1.8

 

1828+487

18:28:13.500

48:42:41.00

2.5,0.5

 

1830-211

18:33:39.930

-21:03:40.77

 

 

G34.3

18:50:46.100

1:11:12.60

?,25,?

 

G45.1

19:11:00.420

10:45:42.90

,3.7

 

1921-293

19:21:42.200

-29:20:26.00

11.6,3.8-8.0

 

1928+738

19:28:49.300

73:51:45.00

1.2,0.5

 

CHICYG

19:48:38.530

32:47:09.90

Very

 

1954+513

19:54:22.500

51:23:46.00

1.1,0.6

 

CYGA

19:57:44.500

40:35:46.00

 

 

1958-179

19:58:04.600

-17:57:17.00

2.9,1.4-1.5

 

K3-50A

19:59:50.100

33:24:19.00

6.5,7.5

 

2005+403

20:05:59.600

40:21:02.00

1.5,0.3

 

2007+777

20:07:20.400

77:43:58.00

1.5,0.6-1.0

 

2007-06

20:07:47.400

-06:25:11.00

OK

 

ON1

20:08:09.910

31:22:42.30

,2.9

 

2013+370

20:15:28.730

+37:10:59.52

 

 

MWC349

20:30:56.850

40:29:20.20

,1.7

 

2037+511

20:37:07.500

51:08:36.00

3.0,0.7

 

DR21

20:37:14.370

42:08:56.50

15.6,

 

VCYG

20:39:41.420

47:57:43.20

Very

 

EGGNEB

21:00:19.900

36:29:45.00

Very

 

NGC7027

21:05:09.400

42:02:03.00

5.2,3.9

 

2105+42

21:05:09.500

42:02:04.00

Very

 

2128+50

21:28:15.100

50:50:47.00

Very

 

2128-123

21:28:52.700

-12:20:21.00

1.0,0.3

 

2134+004

21:34:05.200

00:28:25.00

2.1,0.5-0.7

 

2135+78

21:35:52.650

78:23:58.70

Very

 

2145+067

21:48:05.459

+06:57:38.60

6.2,1.9-3.0

 

BLLAC

22:00:39.400

42:02:09.00

3.3,2.0-8.0

 

2201+315

22:01:01.400

31:31:06.00

3.0,0.4-0.9

 

2200+420

22:02:43.291

+42:16:39.98

 

 

2210-257

22:10:14.100

-25:44:22.00

2.1,0.2

 

2216-038

22:16:16.400

-03:50:41.00

1.2,0.3

 

2223-052

22:23:11.100

-05:12:18.00

4.0,0.8-4.1

 

2230+114

22:32:36.409

+11:43:50.90

3.7,0.8-2.0

 

2234+282

22:34:01.700

28:13:23.00

1.2,0.3

 

2243-123

22:43:39.800

-12:22:40.00

2.3,0.8-4.5

 

3C454.3

22:51:29.500

15:52:54.00

7.1,1.9-10.8

 

2251+158

22:53:57.748

+16:08:53.56

 

 

CEPA

22:54:19.000

61:45:47.20

,4.9

 

2255-282

22:55:22.500

-28:14:26.00

2.4,0.5-3.0

 

NGC7538

23:11:36.700

61:11:50.00

2.1,5.5

 

2316+16

23:16:42.400

16:55:10.00

Very

 

2318+049

23:18:12.100

04:57:23.00

1.1,0.3-0.8

 

CASA

23:21:11.000

58:32:48.00

 

 

2345-167

23:45:27.700

-16:47:53.00

2.2,0.5

 

RCAS

23:55:52.070

51:06:37.30

Good

 

 

To make a five-point map proceed as follows:

 

Continuum Five-Point:

  1. Select an appropriate source to point on, such as a planet or other strong continuum source.

  2. Using the pointing charts, make an initial guess for the azimuth and elevation pointing offsets.

  3. Ask the operator to setup for a continuum five-point measurement on the chosen source. The operator will set the integration time per sample and the number of OFF-ON-ON-OFF sequence pairs to measure on the source. For the stronger sources, 5 seconds and 1 pair is sufficient. For the weaker sources, 10 seconds and 2 or 3 pairs may be needed. If the source is somewhat extended, ask the operator to set the grid spacing to be the semi-diameter of the source. All other parameters will default to reasonable values. Once everything is set, ask the operator to start the measurement.

 

Spectral Line Five-Point:

  1. Select an appropriate source. The source listing in Table 4.2 is by no means all inclusive, but if another source is used, take care that it is really suitable for use as a pointing source. In particular, it should have an angular size that is smaller than, or at least comparable to the antenna beam size and possess an accurately known position.
  2. Ask the operator to setup for a spectral line five-point measurement on the chosen source. Set the integration time per sample, the number of ON/OFF pairs to measure on the source, the number of scans per calibrate, and for a total power five-point the ON and OFF integration time. Depending on the source size, you may also have to set the grid spacing appropriately. All other parameters will default to reasonable values.

  3. Choose an azimuth offset angle for the reference, to be used for position switching. 

  4. Using the pointing charts, make an initial guess for the azimuth and elevation pointing offsets.

  5. Ask the operator to start the measurement.

To process these measurements, there are a number of condar and line procedures available.

Table 4.2: Spectral Line Pointing Sources

 


 

Figure 4.4: Five point mapping scan sequence.

 
4.6.1.1 Continuum Five-Point Analysis

Continuum five-point measurements are processed automatically by the on-line dataserver. The results from these on-line fits are sent automatically to the operator for entry into the control system. To process a continuum five-point measurement manually, use the condar resident procedure five, which can be abbreviated as f. You can look at the results for each channel separately by using the command

      Condar > scan number f

Channel 1 is selected by specifying subscan number .01, while channel 2 is selected with subscan .02. The display (Figure 4.5) provides the observer with the best-fit positional offsets in azimuth and elevation, along with the estimated peak flux density of the source, both for a constrained fit of Gaussian half-width HP and an unconstrained solution. The optimum pointing offsets (labeled CORRECTED OFFSETS) are also printed, assuming the given position of the source to be correct.

 

4.6.1.2 Spectral Line Five-Point Analysis

The display of the map and the fit for pointing offsets is somewhat more involved than for a continuum five point. There is a standard line procedure which processes all types of spectral line five-point measurements. To process a measurement, within line type:

Line> center scan number fivel1 (for channel 1)

Line> center scan number fivel2 (for channel 2)

You will be asked some questions by the routine:

 

AVERAGE THE TWO RECEIVERS? (1=YES, 0=NO) - This question is asked only if the data were taken in series mode. If the question is answered “yes”, the routine averages the first and second filter bank to improve sensitivity. You should answer “yes” only if you are reducing the first filter bank; you will usually not need to reduce the two filter banks separately if averaging the two. If the data were taken in parallel mode, the two polarization channels in each filter bank are automatically averaged.


 

Figure 4.5: Continuum Five-Point Analysis Example


 

Figure 4.6: Display of each scan produced by the line procedure fivel.

 

USE OLD BASELINE & INTEG. REGIONS? (1=YES, 0=NO) - The observer must define baseline fitting and spectral line integration regions for this routine. If these regions have already been accurately set, as, for example, in a five-point of the same source performed immediately prior to the present one, you can answer “yes” and speed up the reduction process considerably. If you answer “no”, the spectrum at the center of the map is displayed on the screen, and the question ENTER # OF BL SEGS is printed. This part of the f routine is the same as the bset procedure. The observer must enter the number of baseline segments to be used to fit a least-squares baseline. [NOTE: The order of the fit is determined by whatever nfit is set to (the default is 1 - see the line data reduction manual for a description of the baseline fitting facilities).] When the number of segments to be fit is entered, the graphics crosshairs appear and you must mark off the two ends of each segment with the crosshairs (move the crosshairs to the desired position and strike any mouse button). When this operation is finished, the baseline is removed from the center spectrum and the spectrum is replotted. The prompt ENTER INTEG. REGIONS is printed and the crosshairs reappear. You should then mark off the two velocity/frequency extremities of the spectral line (or whatever region you wish to integrate over).

DISPLAY EACH SCAN? (1=YES, 0=NO) - If you answer “no”, the map and the fit for new offsets are printed out. If you answer “yes” each scan of the map, with baselines removed, is displayed on a single plot, one spectrum on top of the other (see Figure 4.6).

TYPE 1 TO CONTINUE OR 0 TO STOP - If you approve of the fits, answer 1 and the map and fit for new offsets will be displayed. An example of the five-point map with explanatory annotations is given in Figure 4.7.

 

4.6.2 Pointing Model Equations

In addition to coordinate conversions, the telescope tracking program incorporates pointing corrections. The pointing corrections take into account resolver zero offsets, differences between electrical and mechanical axes, axis tilt, etc. The coefficients of the pointing model equations are determined during dedicated optical and radio pointing source measurements several times during the observing season. The pointing models used for the 12m are described in detail in Appendix A.

 

4.6.3 Pointing Data Analysis Program

Each time a continuum five-point measurement is made, the on-line data server performs a fit to the results and writes them to a permanent file for later analysis. A program called

gpoint is available on any of the mountain workstations for display and analysis of the recent pointing history of the telescope.

To start the program, type gpoint from any login prompt. gpoint is menu-driven, so you need only select the appropriate entries in each menu to configure the data selection and display parameters to suit your needs. An example of a gpoint plot is given in Figure 4.8.

 

4.7 Focus

4.7.1 Axial Focus

To bring the incident radiation to a focus at the receiver feed horn, the subreflector of the 12m may be moved in and out along the electrical axis of the telescope. The focus exhibits an elevation dependence which is automatically corrected by the control system computer, according to the equation

              F(E) = F0 + 2:8 sin(E)                                           (4.1)

where F(E) is the focus setting in millimeters, E is the elevation, and F0, the subreflector focus setting at 0º elevation, is to be determined by the observer. F0 is given in millimeters, and larger values of F0 represent greater distances between the dish surface and the subreflector.

An automatic procedure called a focalize (this word really is in the dictionary!) will determine the best value for F0. An analysis routine of the same name exists in condar and line for displaying and fitting the results. Instructions for performing a focalize are given in §4.7.2.
 

Figure 4.7: Spectral line five-point analysis example.


 

Figure 4.8: GPOINT display example. Each pointing source is indicated by a unique symbol, while the arrow on each symbol indicates whether the source was rising or setting.

 
 

4.7.2 Determining the Axial Focus

The axial focus of the telescope should be checked occasionally during an observing run.

It is often most convenient to make a focus check after a continuum five-point pointing measurement since both measurements use the same kinds of sources (strong continuum point sources). It is particularly important to check the focus after nightfall and daybreak.

Focusing is performed via the focalize procedure. This steps the radial distance of the subreflector from the telescope vertex through seven positions centered on the nominal best setting (F0). Larger values of F0 represent greater distances between the dish surface and the subreflector. The incremental movement between positions (WL) is set to half of the observing wavelength by default, but can be set by the observer.

Perform a focalize as follows: 

  1. Choose as bright a continuum source as possible. The planets Jupiter, Venus and Saturn are ideal. If the source is extended relative to the telescope main beam, the accuracy of the focalize will be diminished, however.

  2. Make a five-point map of the source to optimize the antenna pointing (see §4.6.1).

  3. Reasonable default values for the axial subreflector position (F0) have been assigned to each receiver, but, if you find it necessary you can give the operator a value for F0.

  4. The default integration time per sample is 5 seconds, but if you are performing a focalize measurement on a weak source, you may need to increase this to 10 -15 seconds. 

  5. If you are satisfied with the setup, ask the operator to start the measurement.

 

Focalize measurements are processed automatically by the on-line dataserver, the results of which are sent automatically to the operator for entry into the control system. To reduce the focalize in condar use the commands,

Condar > scan number focalize

where each channel is selected by specifying either subscan .01 for channel 1 or .02 for channel 2. The output from the focalize procedure is shown in Figure 4.9. The fit should be smooth and “quasi-Gaussian". (If not, repeat the focalize. Increase the value of WL if you think this is necessary.) The display will give a best-fit value for F0. Where appropriate, you should average the values for the two channels.

 

4.7.3 Lateral Focus

The position of the subreflector for optimum gain shifts in the elevation direction, with changing elevation angle. The change in antenna gain produced by this effect is not significant at 3mm wavelengths but could be as much as 20% or more at 1.3mm and shorter wavelengths. The 12m is equipped with lateral “north-south” and “east-west” translation stages at the prime focus to eliminate this loss in antenna gain. The computer is able to automatically control the positioning of these stages, simultaneously applying an appropriate correction to the telescope pointing in elevation and azimuth, respectively.

This correction to elevation pointing is 31.5″/mm (which is the prime focus “plate scale” derived in Appendix G) of movement in the north-south focus or east-west focus stages.

In the future, the coefficients necessary to control the movement of the north-south and east-west translation stages will be installed in the computer and applied automatically. At this time, the north-south and east-west translation stages can be set to a given position, but are usually left fixed in this position. Automatic procedures for fitting for maximum north-south or east-west gain, called nsfocal and ewfocal are available. These routines operate analogously to the focalize procedure. Normally, only staff astronomers use these procedures; we do not recommend that observers spend time trying to optimize this further.

 

Figure 4.9: Focalize display example.
 


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Last updated: 11/08/11.