THE PARKES CATALOGUE OF RADIO SOURCES DECLINATION ZONE -20 TO -60 By J. G. BOLTON,* F. F. GARDNER,* and M. B. MACKEY*

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1 THE PARKES CATALOGUE OF RADIO SOURCES DECLINATION ZONE -20 TO -60 By J. G. BOLTON,* F. F. GARDNER,* and M. B. MACKEY* [Manuscript received April 9, 1964] Summary A catalgue f 297 radi surces between declinatins -20 and -60, has been cmpiled frm bservatins with the Australian 210-ft telescpe. The surces were selected frm a survey at 75 cm wavelength as having flux densities in excess f 4 X W m-2(cjs)-1. The survey did nt cver a small area near the galactic plane. Additinal measurements were made at wavelengths f 21 and 11 cm. Results n surce identificatin, spectra, and plarizatin are discussed. 1. INTRODUCTION This paper cntains the results f the first part f a survey fr radi surces between declinatins +20 and -90 being made with the 210-ft reflectr f the Australian Natinal Radi Astrnmy Observatry at Parkes, New Suth Wales. The survey is in fur znes, f which bservatins are cmplete fr declinatins -20 t -60 and _60 t -90, almst cmplete fr +20 t 0, and in prgress fr 0 t -20. Sme areas near the galactic plane are nt cvered in this survey but are the subject f a separate investigatin. The catalgue is being cmpiled t prvide a basic list f the mre intense surces fr subsequent detailed measurements f parameters such as precise psitin, brightness distributin, spectrum, and plarizatin. Althugh such a catalgue fr the suthern hemisphere already exists (Mills, Slee, and Hill 1958, 1960, 1961)-subsequently referred t as MSHit was felt advisable t repeat this wrk fr the fllwing reasns. (I) The wavelengths at which the 210-ft telescpe can be used t the greatest advantage are cnsiderably shrter than that f the MSH survey. (2) Mre accurate psitins than thse in the MSH catalgue are desirable fr bservatins with a narrw pencil-beam instrument. (3) There are sme discrepancies between the MSH catalgue and ther bservatins (Kellermann and Harris 1960; Bennett and Smith 1961). The bservatins cnsisted f an initial finding survey at a wavelength f 75 em, measurements f flux densities and psitins at 21 cm, measurements f flux densities and psitins at II cm, and determinatin f the plarizatin f the 63 mst intense surces at 21 cm. The flux density scales have been adpted t agree with thse in the recent cmpilatin f surce spectra by Cnway, Kellermann, and Lng (1963)-subsequently referred t as CKL. The psitin calibratin f the 210-ft telescpe is discussed in sme detail. Psitins f abut ne-third f the surces have been examined n Palmar Sky Survey prints, r plates made with the 74-in. Munt Strml reflectr. Lg N-lg S cunts are presented at the three * Divisin f Radiphysics, CSIRO, University Grunds, Chippendale, N.S.W. Aust. J. Phys., 1964, 17,

2 THE PARKES CATALOGUE OF RADIO SOURCES 341 survey wavelengths, and als at 350 cm. Spectra f surces and the relatin between plarizatin and spectral characteristics are discussed. Finally, differences between the present results and the MSH survey are investigated. II. THE BASIC SURVEY AT 75 CM The basic survey was made at the nminal wavelength f 75 cm (actually 408 Mc/s) where the beamwidth f the telescpe is 48' f arc. The receiver, cnstructed by F. Tnking (Mackey 1964), has a duble-sideband crystal mixer with an input temperature f abut K and a bandwidth f 8 Mc/s. The receiver is switched between the aerial feed and a reference lad at liquid nitrgen temperature. 22h1Sm 22h 20m' 22h 22m 22h 24m 22h26m 22h 28m _600 Fig. l(a).-facsimilies f five adjacent declinatin scans at 75 em frm the finding survey. The five surces identified n the recrds are included in the catalgue. With a 2-secnd time-cnstant, peak-t-peak nise fluctuatins are abut 0 5 K. At the maximum drive rate f the telescpe in equatrial crdinates f 2t /min, the time t traverse the beam width is 20 s, r 10 times the chsen time-cnstant. Even at the maximum drive rate, used thrughut the 75-cm survey, the sensitivity f the instrument was limited by cnfusin effects due t faint surces and variatins in backgrund radiatin rather than by system nise. The bservatinal prcedure was t make a series f scans in declinatin at intervals f 2 min in right ascensin. These scans were therefre 28' apart at -20

3 342 J. G. BOLTON, F. F. GARDNER, AND M. B. MACKEY declinatin and 15' apart at -60 declinatin. Frm the recrds f these scans bjects were selected which were ;> 2 5 K in aerial temperature (,..."" 4 X lo-26 W m-2(c/s)-1) and which were nt nticeably brader than the aerial beam in either crdinate. The survey was principally aimed at extragalactic bjects, nly a few f which have diameters in excess f 20' f arc, necessary t prduce such bradening. I T E It) E It) 5 5 '0 0 '0 0 " N " N I I SOURCE A=21CM INCREASING DECREASING 0-21"05' a DEC.REASING a INCREASING a O1n15m02s Fig. l(b).-examples f a 21-cm recrd cmprising frward and reverse scans in right ascensin and declinatin. Psitin markers are at intervals f 1 min in R.A. and 10' in declinatin. The value f the declinatin f a surce culd be determined fairly precisely frm individual scans and an estimate f the right ascensin was made frm cmparisn f amplitudes n adjacent scans. Figure l(a) cntains facsimiles f the 75-cm survey scans fr right ascensins 22h 18m t 22h 28m. Five surces indicated in this figure are included in the catalgue. They cmprise three surces previusly catalgued t 2r E It) In "0 N I SOURCE A =11 CM a INCREASING 0-21"03(5 a DECREASING 1) INCREASING 0 DECREASING a 01n15m06s ' Fig. l(c).-examples f an ll-cm recrd cmprising frward and reverse scans in right ascensin and declinatin. Psitin markers are at intervals f 20 s in R.A. and 5' in declinatin. by Mills, Slee, and Hill (ne f which, 22-51, is reslved int a duble at 21 cm) and tw nt previusly catalgued. The 75-cm survey was discntinued near the galactic plane wherever the aerial temperature due t backgrund is mre than,..."" 200K abve the regins f minimum temperature in the survey area. The area cvered in the finding survey is apprximately 2 7 steradians; the regin nt cvered is shwn in Figure 2.

4 THE PARKES CATALOGUE OF RADIO SOURCES 343. THE 21-M OBSERVATIONS All the surces selected frm the 75-cm survey were re-bserved at a nminal wavelength f 21 cm (actually 1410 Mc/s). The feed system permitted simultaneus bservatin at bth 75 and 21 cm. The 21-cm receiver (Gardner and Milne 1963), a degenerate parametric with a 10 Mc/s bandwidth and system temperature f K, has peak-t-peak nise fluctuatins f nly 0 15 K with the 2-secnd utput time-cnstant used in the bservatins. In this receiver a backward-lking sky hrn is used as the reference element. 20" " 12h "h 10" / sh?./' _/I...i/ yl/ / _/--,r / Fig. 2.-The regin f the sky between declinatins _20 0 and _60 0 which is nt cvered in the present survey. Hatched area is the excluded regin. The dashed line indicates the new galactic equatr. / The bservatinal prcedure was t set the telescpe n the psitin fund at 75 cm and abut 0'5 away frm the apprximate right ascensin. It was then scanned past the surce at a rate f '" 2m X sec 1) per minute. Frm drives in bth directins an imprved value f right ascensin was fund. With this right ascensin, declinatin scans were made acrss the surce at O 5 /min and a mre accurate value f declinatin was btained. If the new value f declinatin differed by mre than 5' frm the 75-cm value, repeat right ascensin scans were made at the revised declinatin. Examples f 21-cm recrds are shwn in Figure l(b). Finally, a right ascensin scan, several degrees in extent, was made thrugh the surce;

5 344 J. G. BOLTON, F. F. GARDNER, ANDM. B. MACKEY this bservatin cmbined with the.. survey scans in declinatin was used t estimate the 75-cm flux density. The 21-cm bservatins shwed that sme 5-10% f the surces selected frm the 75-cm survey were prbably backgrund variatins and these were rejected frm the catalgue. Twelve f the surces were clearly reslved int tw bjects f the same rder f intensity. Their significance as physical r nn-physical dubles is discussed later. IV. THE 11-CM OBSERVATIONS All the surces in the 21-cm list were bserved again at 11 cm (2650 Mc/s) using the same prcedure as at 20 cm. Hwever, the declinatin and right ascensin scan rates were carefully set t values f 0 25 and 1m xsec I) per minute s that beam bradening, and thus angular size effects, culd be easily detected. The 11-cm receiver is f the degenerate parametric type (Cper, Cusins, and Gruner 1964) and switches between the aerial feed and a sky hrn reference. The verall system temperature is,..., K and the i.f. bandwidth abut 40 Mc/s. With a 2-secnd utput time-cnstant as used in these bservatins peak-t-peak nise fluctuatins are,..., K. Mst f the surces measured had aerial temperatures in excess f 0 4 K. A typical 11-cm recrd is shwn in Figure l(c). The 11-cm bservatins prvided an independent check n numerical errrs in the reductin f the 21-cm psitins. One cmpnent f a surce fund t be duble at 21 cm was further reslved int tw surces. Beam bradening, indicating surce sizes f the rder f 3' f arc r greater, was detected in a further 20 cases. V. POLARIZATION OBSERVATIONS AT 21 CM Sixty-three f the mst intense surces were examined fr linear plarizatin at 21 em using techniques previusly described by Gardner and Whiteak (1962). The measurements were made with a special single-wavelength feed equipped with a rtating caxial jint. N plarizatin was detected fr 15 surces; plarizatin in excess f 2% was fund fr 28 surces, f which 11 were plarized in excess f 5%. Only the percentage plarizatin is reprted in this catalgue; a full investigatin f plarizatin characteristics such as psitin angle, dependence n wavelength, and Faraday rtatin is being made by F. F. Gardner and R. D. Davies and will be published separately. The 21O-ft telescpe is n an altazimuth munting and thus the psitin angle f the feed changes with hur angle and declinatin. The psitin and intensity measurements were made withut regard t chang in psitin angle. Fr surces nt specifically investigated fr plarizatin, pssible linear plarizatin represents a surce f errr in the flux density measurements; hwever, the average errr frm this cause is prbably nly f the rder f 1 r 2%. VI. DETERMINATION OF FLux DENSITIES Cnway, Kellermann, and Lng (1963) have recently ratinalized the flux density scales in use at bservatries in the nrthern hemisphere. Rather than use independent calibratin in this wrk we have attempted t adpt the same

6 THE PARS CATALOGUE OF RADIO SOURCES 345 scales thrugh bservatin f a number f surces in the "CKL list. Receiver calibratin fr each bservatin was made by injecting a knwn nise signal frm a discharge lamp thrugh a directinal cupler int the line between the aerial feed and the r.f. switch. The values f these signals, against which the aerial ' -100 :. '", -50 b c >- z w a >< -20 :>.J "-.J " U -10. / / V' / 275.l / / e278 / e270 k73 HY,? / /.It:ER (aj 'L...\!! 50 f '" 20 b c... >- Ui t a >< :>.J "-.J " U / / / 353e HYO.E.273 / (b) TA AT 21 CM (OK) 2 (e) 5 10 TA AT 11 CM (OK) Fig. 3.-Cmparisn between flux densities f surces frm measurements by Cnway, Kellermann, and Lng (CKL) and bserved aerial temperatures fr the 210-ft telescpe. (a) 75 cm,(b) 21 cm, and (e) 11 cm. temperature due t a surce was measured, were calibrated by B. F. C. Cper against mre fundamental standards. Figure 3 shws the relatin between CKL flux densities and apparent aerial temperatures f the 21O-ft telescpe. At 75 cm

7 346 J. G. BOLTON, F. F. GARDNER, AND M. B. MACKEY (Fig. 3(a)) the CKL values frm clumn 13 f their table were used. These had been determined frm the best fitting curve t all pints in the range frm 20 t 870 cm. Similarly, at 21 and 11 cm (Figs. 3(b) and 3(c)) we have read ff values frm the curves f best fit in the range frm lo t 40 cm. The multiplying factrs t nvert frm the 2lO-ft aerial temperatures t flux densities n the CKL scales are 1'47, 1'55, and 1 80 at 75, 21, and 11 cm respectively. These imply crrespnding aerial efficiencies f 58, 55, and 47% fr the feeds in questin. This variatin in efficiency with wavelength is cnsistent with an r.m.s. deviatin f 4-5 mm in surface accuracy deduced frm direct survey measurements f the basic structure and measurements f individual skin panels (Bwen and Minnett 1962). The factr f 1 55 at 21 cm refers t the single-wavelength feed used fr plarizatin and intensity cmparisn measurements. Fr the dual cm feed used in mst f the survey wrk the cnversin factr is 6% higher and the aerial efficiency rrespndingly lwer. Having decided n relative scales f flux density, we have t cnsider the reliability f the individual measurements with respect t these scales. There are tw types f errr, thse that are prprtinal t the flux density, arising frm scaling factrs, and thse that are fixed in flux density. The latter cmprise errrs frm nise fluctuatins and cnfusin effects. The average errr frm nise fluctuatins, when the mean f fur scans thrugh the surce is used, is nt likely t exceed half the peak-t-peak fluctuatins. At 11 and 21 cm this crrespnds t 0 1 flux units. At 75 cm, where nly tw scans were used, it is abut 0 5 f.u. Cnfusin errrs are negligible at the shrt wavelengths but can be quite severe at 75 cm. The average difference between flux densities judged frm scans in declinatin and right ascensin is abut 1 f.u., which is f the same rder as the backgrund variatins shwn in the recrds f Figure l(a). The average errr frm this cause wuld be abut 0 5 f.u., especially if ne takes accunt f pssible plarizatin f the backgrund. The prprtinal errrs cmprise errrs due t variatins in calibratin signal-r aerial gain, unknwn plarizatin, and angular extent. Repeated bservatins n strng surces during the survey wrk revealed differences in their aerial temperatures in terms f the calibratin signal f up t ±5%. The exact cause f this is nt knwn. As pinted ut in the previus sectin, unknwn plarizatin f the weaker surces culd result in errrs as high as 15% at 11 r 21 cm in an extreme case, but the average wuld be nly 1 r 2%. At 75 cm errrs frm this cause are negligible. Surces:;;;' 3' f arc in angular diameter shw nticeable beam bradening at 11 cm. These are indicated in the "remarks" clumn f the catalgue. Except where specially stated, the values f flux are the peak values n the recrds and thus represent lwer limits. Surces belw 2' f arc have negligible errrs in their estimate f flux density due t reslutin. Hwever, weak surces between 2' and 3' f arc, whse bradening culd nt be detected, culd be underestimated by 7-16% at 11 cm and 2-5% at 21 cm. Frm available statistics n the diameters f surces, we wuld expect 15-20% f surces t be in this range.

8 THE PARKES CATALOGUE OF RADIO SOURCES 347 When bth the fixed and prprtinal effects are cmbined the fllwing estimates f average and extreme errrs result:,\ (cm) Average Errr Extreme Errr 11 ±0 07 f.u. ±7% ±0 17 f.u. ±21% 21 ±0 05 f.u. ±6% ±0 15 f.u. ±12% 75 ±0 6 f.u. ±5% ±1 6 f.u. ±5% At the level f flux density exceeded by 70% f the catalgued surces (the apprximate turnver pint in the lg N-Ig S curve f Sectin X), the crrespnding values wuld be:,\ (cm) Flux Density Average Errr Extreme Errr ± ± ±0 9 ±0 38 ±0 39 ±1 9 VII. POSITION CALIBRATION The perid f this survey has als been the perid in which the pinting errrs in the 210-ft telescpe have been evaluated and t a large extent vercme. The evaluatin f these errrs has depended in part n the bservatin f radi surces f knwn r assumed knwn psitin. Near the nrthern limit f the telescpe, surces whse psitins have been determined precisely by the Caltech and Cambridge bservatries have been used as calibratrs. In the suthern hemisphere there are n such calibratrs. One f the reasns fr this survey was t btain suthern identificatins (see Sectin IX), sme f which have been used in the calibratin prgram. The verall pinting calibratin prgram has been directed by Dr. J. A. Rberts, wh has been respnsible fr many f the bservatins and nearly all the evaluatin. Subsequent adjustments t the telescpe have been made by Mr. A. J. Shimmins. As the basic axes f the telescpe are altazimuth and thse f the master cntrl system equatrial (Bwen and Minnett 1962), pinting errrs ccur in bth crdinate systems. Errrs in the master equatj'ial arise frm misalignment f the plar axis and deflectins in its structure. The plar axis was riginally set during cnstructin t within 15" f the crrect azimuth and zenith angle. Subsequent checks have been made by bserving the apparent psitins f FK3 stars bth near the ple and in the general field. These bservatins shwed that the master equatrial, which is independently munted n a part-steel, part-cncrete clumn 80 ft high, is stable ver lng perids f time. The initial azimuth and zenith angle errrs have been reduced, in a series f adjustments, t nly a few secnds f arc. It is believed that within ±4 hr hur angle, the master equatrial pinting errrs are nw cmparable t the last digit in the readut indicatrs (ls in R.A., 0 1' in declinatin).

9 I TABLE 1 SOURCE CATALOGUE Psitin (1950) Annual Precessin Flux Density Spectrum Catalgue Number R.A. Il da +M : ' : Remarks (Ang. Size, Identificatin, etc.),..,,1 'NS PO 2% >1/NS,..,,20 H EW >1/NS 741 Pl'8% 15 H EW < 15 H EW >1/NS 40 H EW 74IIP8'8% NGC Omag.Sc P<I'5% 23 H EW > 40 H EW ext in Il at 11 cm, may be duble Galactic Crdinates bil MSH Cat. N <.; Cd.21?;J?;J I 1;;1 Jd t:1 Cd 1;;1 > '85[ <20 H NS PO 6% II 741II

10 !) <20 NS N P11% >45 EW mag.EO N ext at 11 em' ' >!'NS? <20 NS? N < 15 N EW NS II P3% N ext t N r 2nd surce at cm >40 EW 270 N >40 EW N ', <20 NS N P4 5% <10 EW 2nd surce r ext N t S at 21 and 11 em >40 EW 275 N ,' NGCl316 PI0 and 12%

11 TABLE 1 (Cntinued) Psitin (1950) Annual Precessin Flux Density Spectrum Catalgue Number I ll-+ R.A. 8 Lla I I ll II II ll II Hi II l Remarks (Ang. Size, Identificatin, etc.) >45"EW 40"EW >l'ns+fs II P3'3% >40"EW 74II P<1'5% Pl% 35"EW > 1. 5' + fs < 20"NS P3 7% II I 16mag. EO Pl 3% cnf.w at 75 cm P4 9% Galactic Crdinates I bii MSH Cat. N <:i:) Ol <.; td t-< >-3,21 >rj 7l Ij 21 f Ij td lei > ext in RA at 21 cm II see P5%

12 II fs < 20"NS 2nd surce 15'N at 21 em 20"EW ext at 11? II>I'NS Pietr-A 19mag. galaxy ext at 11 em P3% <20"NS 20"EW P3 5% >30"EW 2nd surce 15'N at 21 em l P<l% P<2% Pl 7% P14% Pl 3% 20"EW PO 8% PO 5% II fs <20"NS>5' in Il at 11 em P6%

13 Catalgue Number Psitin (1950) Annual Precessin \ R.A. Il Aa +AIl ; n I tl OS I-'ll TABLE 1 (Ontinued) -"-- Flux Dens lty I O " " Spectrum I-I & I-I Remarks (Ang_ Size, Identificatin, etc_) >45"EW 35"NS ext in RA at 11 cm P<I% ext in RA at 11 cm P4-.()% 875 and 8350 incl. 2nd surce P<I% 875 and 8350 incl. 2nd surce ext at 21 and 11 cm Galactic Crdinates bii " MSH Cat. N_ e..:> I:;l1 '-< Cd Z Z t:l Cd I I surce 2 min. earlier at 21 cm NGC2467 ext at 21 and 11 cm neb P<I%

14 0807-3S OS OS 07 4S OS OS OS OS S S 'S S II II II 7l II II OS ext t later RA at II and em P<I% I'S O'S >1'NS OS ext r multiple surce OS-24 OS OS OS OS OS S' '35 30 S S O'S >1'NS? OS >30 H EW 875 and OS-53. inel. 2nd surce O'S O'S OS O'S O'S EW' H P3 2 % : ,20' at 21 em Pl 9%? RA may be 20. earlier >30 H EW 2nd surce 'N at 21 em S 'S l S >1'NS : , <10 H EW 30 H NS ext J at II em 10II ;L II S -18 II 4., O'S 0 7 2nd surce 23'S at 21 em P<I 6% O'S P2 9% O'S II03-24 II03-20 II II II II 'S ext at II em II II ext at II em in RA II-23 r duble

15 TABLE 1 (Cntinued) Psitin (1950) Annual Precessin Flux Density Spectrum Catalgue Number I I 11-+ R.A. 3 6.u i l l I '0l Remarks (Ang. Size, Identificatin, etc.) P2 4% ext at 11 em {probably 1 ne cmp ex ext at 11 em b' Ject P<l% Galactic Crdinates I bii MSH Cat. N lo Ol H>- <..; <:;:2 I:d Z "'J <:;:2 t:i Z t;j j;1 t:i I:d t;j > and 8350 incl Pl% < 15"EW NGC mag. Sc.P<l% S

16 I ext at 11 em NGC mag.Se P<I% ,...,15"EW P<l% NGC mag.SO Pay 6% includes P4 /c {IC mag.E P11; Als includes NGC Omag.Se P<4% P20% ext at 11 em P4% "EW ext at 21 and em NGC mag.EO <12"EW P2 5%

17 Catalgue Number Psitin (1950) Annual Precessin R.A. 0 a +M , TABLE 1 (Ontinued) Flux Density Spectrum 75->- 21->- 11-> I I Remarks (Ang. Size, Identificatin, etc.) Galactic Crdinates bii ext in 0 at 11 cm >30"EW ext in 1) at 11 cm Pl 3% P3 2% <15"EW P2 6%? nd surce sf at 21 cm MSH Cat. N C;:l 01 '-< P C:l t< >-3 Z "'l ;<l «:l Z t;j,pj 1::1 C:l Pi t;j > >30"EW

18 II ext at 11 em? <10"NS 18"EW Pl 2% Peculiar spectrum P < 1 % >1'NS 30"EWext in RA at 11 em? P<l% II>1'NS >40"EW P<1'5% II1,...,30"NS,...,20"EW >35"EW >1'NS 74II ext at 21 em >l'ns 741 ext in Sat llcm <15"EW 74II <lo"ew P8% >1'NS I 16mag.E <15"EW mag.galaxy IC mag.SO II "EW >80"NS I I 17mag.E I

19 TABLE 1 (Ontinued) e;" Ot (Y:) Catalgue Number Psitin (1950) Annual Precessin R.A. S!!..a +!!..S Flux Density Spectrum Remarks (Ang. Size, Identificatin, etc.) >50"EW >1'+f8 <20 H NS I 16mag.E P<l% 74II1 >40"EW P4% U 30"EW PI % 74II Galactic Crdinates bii MSH Cat. N '" Cd. "'l t:>j }d tj f'= Cd t:>j ><l

20 I , ",15HEW O p "EW P2 1% ,.15 H EW >1'NS HEW,",,":,30 H NS PO 7% >40 H EW l II HEW 7411 P4'9%

21 360 J. G. BOLTON, F. F. GARDNER, AND M. B. MACKEY Errrs in the altazimuth system (az-altitude and ax-azimuth) arise frm the fllwing causes: (1) Misalignment between the electrical phase centre and the mechanical centre f the feed (affects bth az and ax). (2) Misalignment between the mechanical centre f the feed and the intersectin f the axis f the reflectr and the fcal plane (affects bth az and ax). (3) Misalignment between the axis f the errr detectr and the axis f the reflectr (affects bth az and ax). (4) Deflectin f the reflectr and the feed supprt structure with changing zenith angle (affects az nly). (5) Refractin, which, with an altazimuth munt, we can chse t cnsider a telescpe errr (affects az nly). The varius cmpnents are nt easy t determine separately and fr crrectin purpses it is nly necessary t knw the effect f the sum. Hwever, it is in practice desirable t reduce each t the minimum pssible. (I) can be determined by measuring the apparent psitin f a surce at different feed angles and (2) by direct survey techniques. At the zenith (3) can be determined frm the variatin in utputs f the errr detectr as the main telescpe is rtated with bth it and the master unit pinting near the zenith. The resultant errrs in right ascensin and declinatin may be written aa (in secnds f time) = 4 sec 8 (ax cs q+az sin q), a8 (in minutes f arc) = -ax sin q+az cs q, where az = j(z) and ax are in minutes f arc and q is the parallactic angle (psitive befre transit). ax and az fr z R:i 0 can be determined frm bservatins near transit f a surce which passes clse t the zenith but whse psitin is nt necessarily accurately knwn. Near transit q changes frm t while the zenith angle hardly varies.!:::..z as a functin f z can be determined frm bservatins f the same surce r by bservatin f the apparent declinatins f calibratr surces near transit. These bservatins, f curse, must be crrected fr the knwn master equatrial errrs. In practice errrs (1) and (2) have been reduced t negligible prprtins. The sum f the zenith angle cmpnents f (3), (4), and (5) nw changes frm 0 t 1 7' between zenith angles 0 0 and 60 0 and cmpensatin is injected int the errr detectin system t allw fr it. Cmpensatin will als be intrduced t allw fr the azimuth cmpnent, which varies frm 0 t 0 4' with changing zenith angle. There are tw ther small surces f errr intermediate between the tw systems. One is due t a small lag in the serv system. The amplitude f this errr is directly indicated n the cntrl desk and, except in the case f high wind gusts, des nt exceed a few secnds f arc. The secnd is due t variatin in the cllimatin between the axis f the ptical telescpe in the master unit used in its calibratin and the nrmal t the reflecting plane mirrr. Its amplitude is f the rder f 5/1 f

22 THE PARKES CATALOGUE OF RADIO SOURCES 361 arc between +4 and -4hr hur angle; hwever, it exhibits sme hysteresis and at present n systematic crrectins are applied t cmpensate fr it. Very few f the survey psitins have been measured with the instrument in its present state and crrectins f as much as 1 5' have been applied t the varius sets f bservatins frm crrectin tables prvided by Rberts. Telescpe adjustments based n deductins frm the crrectin tables have generally resulted in a reductin f errrs f the rder expected. Nevertheless, errrs f as much as 0 3' fr surces with declinatins nrth f "":'40 0 and perhaps twice as much near _60 culd be due t an inadequate knwledge f the crrectins. 0 In view f the magnitude f these errrs we did nt apply secnd-rder precessin crrectins, giving a further small surce f errr. A cmparisn between crrected psitins btained at 21 and 11 cm at different times shwed median differences f 0 45' in bth crdinates. These differences wuld include differential errrs in the crrectin tables, differences in secnd-rder precessin, and effects due t bth signal and cnfusin nise. The latter effects d nt appear t be serius, as the median differences between the 21 and 11 cm psitins are almst independent f surce intensity, being abut 0 40' fr the 100 mst intense and 0 45' fr the ttal 300. Taking int accunt all the surces f errr discussed, we believe that the errrs in the catalgue psitins d nt exceed 0 6' fr 90% f the surces ver mst f the zne. Between declinatins and the uncertainty may increase t,... d O'. V. NOTES ON THE CATALOGUE Table 1 lists the details f the 297 surces in the declinatin zne _20 0 t The table is largely self explanatry. Details f the individual clumns are as fllws: Clumn 1.-Catalgue number cnsisting f the hurs and minutes f right ascensin and the sign and degrees f declinatin. This has ne mre digit than in the MSH r IAU system but has the advantages that it specifies psitin quite clsely and permits interplatin f additinal surces with n basic change in the numbering system. Clumns 2 and 3.-The psitin fr epch Fr a discussin f errrs see Sectin VII. Clumns 4 and 5.-The annual precessin in right ascensin and declinatin. The sign f the declinatin crrectin is indicated at the tp f clumn 5. Clumns 6, 7, 8, and 9.-Flux densities in units f W m-2(c/s)-1 at wavelengths f 350, 75, 21, and 11 cm. The value at 350 cm is taken frm MSH; where a surce is a blend at the lnger wavelengths and reslved at the shrt wavelengths this fact is indicated by placing the 75- and 350-cm flux densities midway between adjacent rws, r by a nte in clumn 13. In the case f "extended" surces where tw values f flux density are quted in MSH the smaller r "peak" value is used if the surce is nt reslved at the shrt wavelengths. Peak rather than integrated

23 362 J. G. BOLTON, F. F. GARDNER, AND M. B. MACKEY values fr extended surces are als given at 21 and 11 cm except where nted in clumn 13. Fr a discussin f the flux density scales and errrs see Sectin VI. Clumns 10, 11, and 12.-The spectral indices* fr the ranges 11-21, 21-75, and cm. If the flux densities are in dubt wing t pssible extensin r blending, the spectral indices fr the apprpriate ranges are mitted. Clumn 13.-Miscellaneus data. These data invlve the classificatin f the ptical field f the surce (Rman numerals-see Sectin IX), data n angular sizes, plarizatin, and identificatins. Angular sizes dented EW are taken frm MSH and refer t interfermetric bservatins frm twp east-west base lines. Angular sizes dented NS are preliminary estimates frm interfermetric bservatins at fur nrth-suth base lines. These were kindly supplied by Dr. P. A. G. Scheuer in advance f publicatin. Plarizatin is indicated by a P fllwed by a percentage and refers t 21 cm bservatins nly. Clumns 14 and 15.-New galactic crdinates. Clumn 16.-MSH catalgue numbers. Abbreviatins used in clumn 13.- I, II,, IV-field class n 48-in. Sky Survey Plate, 74 fllwed by ne f the abve-field class n 74-in. Strml Plate, EW-east-west angular size, NS-nrth-suth angular size, N-nrth, S-suth, P-plarizatin, S75-flux density at 75 cm etc. cnf. w-cnfused with, ext-extended, f-fllwing, fs-fine structure, p-preceding. IX. IDENTIFICATIONS The psitins f all f the 297 surces in the catalgue have been cmpared with the psitin f galaxies and emissin nebulae in the NGC. One bject agrees in psitin with an einissin nebula, NGC 2467, and the agreement between the radi and ptical dimensins and the radi spectrum makes the identificatin certain. Nine f the surces agree in psitin with galaxies brighter than phtgraphic magnitude These include the tw well-knwn identificatins NGC 5128 and NGC 1316; the remainder have been previusly suggested as identificatins by Mills, Slee, and Hill. The psitins f 68 surces have been examined n prints f the Palmar Sky Surveyr n plates taken with the 74-in. Munt Strml reflectr. We are indebted t Prfessr Bart J. Bk and the staff f Munt Strml Observatry, in particular Dr. Bengt Westerlund, fr taking these plates and als t Miss Lindsey Smith fr measuring psitins n them. The surce fields have been classified in the system first used by Harris and Rberts (1960). * Previusly the spectral index has been derived frm S = r, where x is the spectral index. We have adpted the ppsite sign thrughut this paper t avid the 'cntinuus use f a minus sign.