national radio astronomy observatory charlottesville,...

National Radio Astronomy Observatory

To:

From:

Charlottesville, Virginia

Addressee November 8, 1972

P. Napier ENG/NEE /ZING MEMO 11/4—

Subject: Cassegrain Feed System for NRAO 140-ft. Antenna

A design for a multifeed, nutating subreflector system for the 140' telescope

is attached. The system is essentially identical to the proposed VLA feed

system. It is hoped to put the system on the telescope in August 1973.

I would be glad to hear any comments . you may have.

Addressee:

S. WeinrebH. HvatumJ. FindlayW. HorneC. MooreG. BehrensC. BrockwayK. KellermanF. CrewsH. BrawnJ. PayneM. BalisterB. Clark

CASSEGRAIN SYSTEM FOR 140' TELESCOPE

P. J. NapierNovember 8, 1972

A. Multifeed Cassegrain System

The "multicone" feed system used by JPL on their 210-ft. antenha l appears

to be the best way to utilize a multiple cassegrain feed system. It is pro-

posed to use the same system for the NRAO 140 ft. telescope. The advantages

of this system are that it has no phase error or boresight shift and should

have good spillover performance.

In a "multicone" feed system the feeds are located in a circle around the

parabola axis. The subreflector is tilted about the parabola focus to point

towards the feed in use. To change feeds the subr'eflector is simply rotated

about the parabola axis.

B. Cassegrain Geometry

Figure 1 shows the proposed cassegrain geometry for the 140 ft.

Paraboloid F/D = 0.4286Parabola Half Angle = 60,510

Average Hyperbola Diameter = 10.53 ft. (126.4")Focus to Feed Di:stance = 45 ft. (540")

Average Hyperbolic Half. Angle = 7.14°Magnification = 9.348

Eccentricity = 1.240

Focus to Hyperbolic Vertex .4.35 ft. (52.19")Angle Between Parabola Axis and Hyperboloid Axis = 4.1°

Cassegrain System for 140' Telescope -2-

C. Subreflector

Figure 2 shows the coordinate system for the subreflector. Table 1 details

the hyperbolic profile of the subreflector. The hyperboloid axis of symmetry

is colinear with the horn phase center. Spillover will be minimized by

-truncating the symmetrical hyperboloid asymmetrically. This truncation is

determined by the boundary formed by the cone, which has its apex at the

paraboloid focus and symmetrically circumscribes the edges of the paraboloid.

Table 2 details the edge of the subreflector. The hyperboloid will be

rotated about the paraboloid axis to focus on the feed being used. The sub-

reflector will weigh approximately 150 lbs.

Subreflector tolerances:

(a) Subreflector Surface Deviation - The ENS departure from a best fit

hyperboloid must be less than 0.01".

(b) Subreflector - AxialTocus - This should be remotely adjustable over

a range of 4- 4" with a resetability and stability of 0.1" to keep

gain error < 5% at A = 1.35 cm.

(c) Subreflector Tilt - The subreflector axis must be parallel to the

line between the paraboloid focus and the feed phase center to within -

+ 0.1° and must be stable . (or known through a readout sensor) to--

within + 40 arc-seconds (+ .03" at edge).

(d) Subreflector'Lateral Focus - Ther vertex of the subreflector must

be on the line between.the parabola focus and feed phase center to

within .2" and must be stable to within + .01" to keep pointing

errors less than + 3 arc. sec.


(e) Rotation About Parabola Axis To focus on the feed to be used,

the subreflector should be able to be remotely rotated about the

paraboloid axis to a predetermined position with an accuracy of

q- 1 * and must be stable to within + 3 arc. min.

D . Nutation

The purpose_of nutating the antenna beam is to provide cancellation of

atmospheric noise. Other functions such as locating a source or mapping of

an extended object can 13 performed by scanning the antenna.

A square-wavo movement of the beam is desirable in order to provide

optimum signal-to-noise. The frequency of the square-wave should be

> 2 cps and as high as 5 cps, if feasible, in order to cancel atmospheric

effects and mechanical instabilities in the receiver. This is evident from

our own experience and from the wörk of Slobin2

. A maximum of 1/8 of the

cycle should be spent in moving from one position to the other; thus the

transition time must be < 31 ms and preferably < 12 ins.

The amplitude of the beam deviation must be large enough to make the

beams exclusive and thus must be at least a few beamwidths at the longest

wavelength of operation. The beam deviation, de, for an a rotation of the

hyperboloid about its vertex is approximately 2a/M, where M is the

Cassegrain magnification. (If the hyperboloid is pivoted at a point located

at a distance, it, behind its vertex, the beam deviation is approximately

2 a/M k /F, where F is the paraboloidal focal length 720"; thus ka/F

is negligible for 2, < 14".)

Preferable 1Transition Wavelength li perboloid

Time for La = 3 EdgeT Beamwidths Movement

Beam HyperboloidDeviation Rotation

Le a

4"

1.3"

3.6'

18'

6'

1

6 cm

2 cm

1.2 cm 0.8"


On the basis of the above considerations the-following table of

transition time, T, vs. angular movement, a,.is recommended:

Nutation should be in the plane containing the axis of the paraboloid and

the axis of the subreflector.

Feed Package

Initially:. four (4) feeds will be positioned on the telescope: L Band,

C Band, Ku Band and K Band. There is room on the feed circle for more feeds

if required at a later date (e.g. S Band, X Band). The four. (4) feeds

are being made by Rantec and are described in detail . in "Specifications for

Cassegrain Feeds" - NRAO October 31, 1972. Approximate dimensions for the

feeds are as follows:

FeedApertureDiameter

(- 10%)Length(.1- 10%)......

Weight(-1- WA).....

L Band •

C Band

Ku Band

K Band

62"

17"

6"

4"

85"

82"

82"

82"

1

1

•

1000 lb.

200

100

100

Cassegrain System for 140' 'Telescope

Figure 3 shows the feed arrangement and equipment room.

The feeds must be directed to the center of the subreflector area (not

the subreflector vertex). This requires that they be tilted at an angle

of 4.7 * to the paraboloid axis. The centers of all feed apertures lie on

a circle with radius 38.4" about the paraboloid vertex. The center of this

feed circle is 15.5' (186") above the paraboloid vertex on the paraboloid

axis.

The equipment room below the feeds should be circular with a diameter

of 10' The room should be 7.5 ? high with its floor 34" above the parabola

vertex. In this position the ends of the feeds should project approximately

20" into the room.

Mountings for up to five 2 ft. X 2 ft. X 2 ft. floor mounted racks with

a total weight of 2000 lbs. should be provided.

Feed Tolerances

(a) Feed Axial Focus - The center of the L Band feed 'aperture should

not be behind the feed circle described above, but may extend up

to 6" in front of it. The center of the C, Ku and K Band horns

should all lie in the same plane within + 1" of the feedS circle.

(b) Feed Lateral Focus - The center of the K Band feed aperture should

be within + 1" of the line through the paraboloid focus and the

subreflector vertex and should be stable to 4° .1". The tolerances

for the other frequencies are proportionately greater.

Cassegrain System for 140' Telescope

(c) Feed Tilt - All feed axis should lie on a line at 4.7° to the

paraboloid axis and their tilt in two orthogonal planes should

be locally adjustable to + 3°. The feed tilt should be stable

to + 40 minutes of arc.

To allow the telescope to operate at both C and Ku Band simultaneously

a dichroic optics system shown in Figure 4 will be positioned above the

C and Ku Band feeds. A supporting system is needed to hold the ellipsoidal

reflector over the Ku Band horn and the dichroic reflector over the C Band

horn. The support system must provide the necessary positional accuracy

and also be capable of swinging the reflectors out of the path of the

feed radiation when dual frequency operation is not needed (see photographs

of the JPI, dichroic-optics system and reference 3). The ellipsoid and

dichroic reflectors will be approximately 12" in diameter and will weigh

approximately 5 lbs. They will be provided by the feed manufacturer.

Dichroic Optics Feed Suport Tolerances

(a) Axial and Lateral Focus - The ellipsoidal and didhroic reflectors

must be positioned on a specified point on the axis of the Ku and

C Band horns with an accuracy of .1" and should be stable to + .01".

(b) Reflector Tilt - The reflectors should be positioned at the correct

specified angle with respect to the feed axis - this angle should

be locally adjustable over + 3° and should be stable to + 20 minutes

of arc.


References:

1. The following parts of IPISpas...e_Lograms Summary give details of the

design and construction of the JPL 210-ft. tricone system.

37-45, Vol. III, pp. 48-51

37-56, Vol. II , pp. 121-124

37-57, Vol. II„ pp. 160-165

37-61, Vol. II , pp. 108-113

2. Slobin, S. a., "A Study of Asymmetrical Cassegrainian - Antenna Feed

Applications to Millimeter-Wavelength-Radio-Astronomical Observations,"

Univ. of So. California, Elec. Science Lab., USC EE Report 321,

December 1968.

3. The following JPL publications give details of the design and construc-

tion of the JPL dichroic optics system.

JPL DSN Progress Report, TR-32-1526, Vol. VI, pp. 139-1 1.It II

2 TR-32-1526, Vol. VIII, pp. 53 . 60.If If t If TR-32-1526, Vol. IX, pp. 141-146.If If TR-32-1526, Vol. X, pp. 135-142.If If U It TR-32-I526, Vol. X, pp. 186-190.

In la

verieY,

goy." el-Nase.ca-4-re.11

32•6-$

0

70

we i , 140 fi CraKSS ci( a "" Ceoiv‘e

0

t et CCA C < CI r"*" titr,

U. — 0 le&

— Pr

v jt c:› c>,,, VC -re

" 51N , Cre- e ho l•

GA r c

(R)cp, z)

•

CeAl l

15:5" ff.

_ *4 it

rrter

POO rv't

ft

PbolQr. .4‘ v rkc

K bar,c,1

horos stAotAJA

14.4

to 1

5.4

-GF

Iz H

orn

OK

I

A

14.

5 to

5.0

-GH

z H

orn

DIC

HR

OIC

PL

AT

E(T

ran

spare

nt

at

C-b

and;

opa

que

at K

u-i

pan

cl)

POL

AR

IZE

R

400. " *

aro.

Ell

‘pso

.ic,tv

,tPH

ASE

CO

RREC

TO

RO

F B

EA

M W

AV

EG

UID

E

A L. C--) I\ Lii%i L(1,1u.k..# • :J.O 1....)0 0.Quq.) .!. •L,,; j.JII.)_:o .Qu C.k;334 44,,io ji()o./ _)•.), j•i,),,,

oeu,i 0.153) i.uu 0•4V.jv ,J•J,i j.e.i.J.,"-j . k./ 0.3A63 1J•UU j.4t4:.1J Li•u,J

14.kiti 0.t151 13•uu O.(Liu l'i-ev.d1. -..6.-QQ C11t.;633 13.0u i.t. t Li•: i.L)euI. J.OU 1.3d16 19.uj i.:).)00 el,"•,)," i. /,; t -t-2.1. 1. 34 221 ) %J.J■t4 - 4J•0,,i 2.L:)1.&-t.td0 2.1i5J3 25..uu 4.: ._o i4t-4/4,uk.1 3.6S67 43.;.)J -., ... 0 4..'jskJ,) . i...WoD-iv.uu 3.3161i...)•,,,, 4.611)0 ..: /i.JU 4-.o=',J4 J..)•1/4),) ).1/0'.)iAa.k.i,i 5.4054 27.oj .)•ilvl

'..;, J.kiu s'a . i2li i - --tu•OU U.I'tjLt ,ri•AJ‘i i.ji:›344.vv 7.92ji 4:-.1 .j() 1.111.:A 4.-T•uu

'E.'.).. ULJ 6.4J/3 4o.uu Jejtiu,4,)... C.6-42') 49.ou 1. Q.U4-Ji :) J•v.,., iti. m:.):Ji.Uki LO.c;56/1 52.u,) 1. 1., d..)•.,/ ii./J2-.)1) .t.,ije -12.136o i.i.ju i.,./. . 571v :/v•,10 i...14,..; 2

13.4621 56.vt.; 1.3.Vq4-di :)%iiikiki i-t•-ti'4.1ti,J.Uu .4. 4.6'117 61.0u i:). 1)1 . .•1/4JJ 1:i.oci 10.). o 16.3646 t)li.U1) io•,..i. 6o.v ...) i•.695 67.uu 1o.4 68.4J.) 1. 61-Y:›00

1-0kVe, . 1) 0\ek pr-cyClie

cb

-0,J.JtJ,J-ok).u:JJ

-uv.%))

-1"--teJvki

-44-t.vJJULij

-11-Q.OUV

V;Jj-44.0JO-4tv.Jti‘j

•-_14t.OkdO

-Zo.QJJ•

-4.4.4JQO

-0.yOU

-2.000

zc.760

tz.C/1-2

6j.496

O3.225cc.12266.C13

oa 7. L.

▪

'1O37

37.235c

•

CZ-,7

(.: 6.171c6.616c. .45164-.. 2o I<3 • 1G9

.3 3i5lt572

tD.20Cci. Ci2

1.

6.4.241t4.C4o

3.45863. 262

z (R)

iu.264It.). 1 31. 6.j1.;

1!).:))ID.467

1.4t.i‘t.VUb

14-.1C;3

14.o4V

1. 4.4,)£t• 3';/vi.iLt.Z3/1. 4.1C114.1,4-o1. 4.107

14.4.6v2

£3.941

1. 3.0ot;i3. o:3

q5p 1 Rt3.01

24.,./v‘j v2.4;7ou.0%),/ 62.466

62.29547. 2.1u5ti.cA

14,..vvo

1. :).%)00 C1.36640.0vj 61,1.66

LJ%JU 61.C124:4.v j O 6G.Zi4-0

6C.67060.50400.342

.1b36C.C2O

Jo.uou 5916785C.731

44, u.ouu 5c.b095S.4515C.31.85G.1905';.C66

.1)0.w1u 5d.S46:34.k)vO 58.E34

58.-12656.623

:)4.).ouj 5.526uo.uou 5E4.434

58.3470 ,t.tiou 5 266

56.191oo.vuO 56.122

141.04)j 5 .00114. .ULJU 57.S49

51.'3C557.63

QUdy,)

51.46.1,c).v ‘du 57.-165ao..JvJ 57.750

2,. Foly- coordiActies oc5,Aloijlector.,

national radio astronomy observatory charlottesville,...

Documents