naic visiting committee meeting february 15-16, 2005 telescope performance pointing the telescope....

NAIC Visiting Committee MeetingFebruary 15-16, 2005

Telescope Performance

• Pointing the telescope. • Gain curves for all receivers• Recent high frequency results.• Beam widths for all receivers.• Things to work on.

• More info at: http://www.naic.edu/~phil– -> pointing– -> system performance


Pointing the Telescope

• To point the telescope you use:– Pointing model– Azimuth, za drive systems.– Turret drive system.– 6 Distomats (laser rangers)– Tiedown control system.– Temperature sensor on the platform.– Tertiary drive system (installed but not in use).

• Beam widths of receivers range from:• 900 arc seconds 327 MHz• 28 arc seconds at 10 GHz

• 1/8 inch motion at the horn is 5 arc seconds on the sky.

• Make sure that any errors are repeatable.


The pointing model• 13 term model in az and za with a za lookup table. • Latest model created may04 after:

– Alfa added (2000+ lbs)– New compressors on dome (450*4 lbs)– Kevlar cables added to stiffen feed tower.– Dome side rollers tightened on 1 side, loosened on the other.

Model history (errors in arc seconds).

Date Az err asec Za err asec Total asec

1998 3.82 2.37 4.49

1999 5.4 3.1 6.2

2000 5.68 3.45 6.65

2003 5.38 3.48 6.41

2004 4.20 2.41 4.84


The distomats and tiedowns

• The distomats and tiedowns correct for temperature variation.• 6 distomats (around the rim road) are used to measure the position

of the platform. • Tiedown cables connect each platform corner to a computer

controlled jack in the bowl.

• Operation:– The distomats measure the average platform height every two

minutes.– The computers then drive the tiedowns so the average platform

height remains at a fixed value.• We keep the average height of the platform fixed. We do not keep

the platform level.• Plot shows platform height 05feb05.


PL Platform hght vs hour


Distomat failure …

• The distomats fail if it is raining, or if the platform has tilted enough so that the targets are no longer in the distomat beam.

• On failure the computer switches to a temperature sensor on the platform. The differential temperature change from the last good reading is then used to control the tiedowns.

• A 5 deg F change will move the platform 1 inch.


Tiedown failure

• Large platform tilts and higher temperatures cause a tiedown cable to go slack.

• On 27may04 tension in td 4 was lost during a calibration run:– 10Am, 84degF, za=16.5 az=155.– The td4 jack still had about 8 inches of throw.

• The plot shows the pointing error became > 85 arc seconds.

• The problem is magnified because the computer continues to move the 3 jacks (but only two of them are now moving the platform). This causes the platform to tilt even more.

• Day/evening observing is affected by this. Dome weight is the main culprit.


Pnt err when td4 looses tension


Pre 2004 gain Curves


Curvature of the hi freq gain curves

• The curvature in the high frequency gain curves comes from collimation and focus errors that are a function of az and za.

• A 1 lambda focus error is a 3db loss in gain. At 10 Ghz 3cm is 3db

Theodolite surveyJul03 after shimming


Recent hi freq measurements

• Latest reflector adjustments:– Tertiary: 12oct to 30oct04– Secondary: 7jun to 29jun04– Primary: nov04 to jan05

• Data taken 01dec04 thru feb05• The plots contain:

– Gain (K/Jy)– Tsys (K)– SEFD (Jy)– Average beamwidth (Asecs)

• A single freq/rcvr is plotted versus za.• Color/symbols are different sources.


PL Gtsb cb5000


PL Gtsb cbh7200


PL Gtsb xb9000


• System performance vs freq.• Za range limited to 5-14 deg• 4th order poly fit to data• Table contains median values• Scatter from optics.

RcvGainK/Jy

TsysK

SefdJy

Cb4500 8.6 29.0 3.4

Cb4860 8.7 29.9 3.4

Cb5000 8.5 30.6 3.6

Cb5400 8.1 34.5 4.3

Cbh6600

6.0 26.8 4.5

Cbh6900

5.7 25.6 4.6

Cbh7200

5.1 24.1 4.6

Cbh7400

5.0 24.4 4.8

Xb8500 5.0 27.9 5.8

Xb8800 5.0 28.4 5.9

Xb9000 5.0 29.2 6.1

Xb9200 5.0 30.3 6.3


Beam widths vs. wavelength

− HalfPowerBeamWidth = K x lambda/Diameter− HPBW is not sensitive to Cal errors or Flux errors.− HPBW is sensitive to: extended source size, focus and surface errors.− The plots show HPBW measured during 2004 (11600 points).− The illumination lets you trade Tsys for Gain.

Observations:– The dipole feeds have a larger illumination than the horns.

– SBN, CBH horns are under illuminating the dish/tertiary. This gives a smaller Tsys at the expense of the gain.

− The tertiary skirt would let us increase the illumination of the tertiary without a large increase in Tsys. It would also lower Tsys for those receivers that are over illuminating the tertiary.


Things to work on

1. The dome weight is causing the tiedown/distomats to fail. We need to:– Make the load cell tension measurements more robust– When low tension in 1 tie down, disable tracking of average height.– Get the tertiary drive system online. Use it to change the focal

length

2. Install the tertiary screen so that receivers that over illuminate the tertiary do not have such a large Tsys (of course this adds weight!!)

3. Measure the pitch/roll/focus errors of the current configuration and then make a new model to try and correct the pitch, roll, focus errors.

4. Continue monitoring the system performance of all receivers for maintenance purposes.

naic visiting committee meeting february 15-16, 2005 telescope performance pointing the telescope....

Documents

platform corner

platform level

average platform height

za range

average height

za drive systems

mhz28 arc seconds

tiedownsthe distomats