Contains reports on three research projects.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Aeronautics and Space Administration (Grant NGL 22-009-421)National Science Foundation (Grant GP-20769)California Institute of Technology Contract 952568Sloan Fund for Basic Research (M. I. T. Grant 241

Barrett, Alan H.

Kebabian, P. L.

Myers, P. C.

Poon, Ronnie K. L.

Staelin, David H.

DSpace@MIT

II. RADIO ASTRONOMYAcademic and Research StaffProf. A. H. Barrett Prof. R. M. Price J. W. BarrettProf. B. F. Burke Prof. D. H. Staelin D. C. PapaDr. G. D. PapadopoulosGraduate StudentsR. H. Cohen P. L. Kebabian P. W. RosenkranzP. C. Crane C. A. Knight P. R. SchwartzM. S. Ewing K-S. Lam J. H. SpencerT. D. Halket K-Y. Lo J. W. WatersH. F. Hinteregger P. C. Myers A. R. WhitneyR. K. L. PoonA. FORMALDEHYDE ABSORPTION IN THREE DARKGALACTIC CLOUDS1. IntroductionA study of formaldehyde absorption at 4. 83 GHz in three dark galactic clouds wasmade in April 1971 with the 140 ft telescope of the National Radio Astronomy Obser-vatory in Green Bank, West Virginia. Each cloud was mapped extensively in orderto gain information about its formaldehyde distribution, its spatial structure, its groupand internal motions. This report discusses the observations and initial findings.2. Equipment and Observing ProcedureThe 140 ft telescope was equipped with a cooled 6-cm parametric amplifier and a413-channel autocorrelator. Spectral resolutions of 3. 25 and 6. 50 kHz were used. Thesystem temperature ranged from 78°K to 95'K, with 85°K a typical value. The radiom-eter employed frequency switching in the first local oscillator at a rate of 1 Hz. Scansacross Virgo A produced a peak continuum temperature of 17. 90 K and half-power beam-widths of 7. 1' and 6. 8' in Right Ascension and Declination, respectively. A mean aper-ture efficiency of 0. 47 was computed by adopting values of 78 flux units for Virgo Aand 20 flux units for DR 21.For each cloud, data were taken over a grid of positions, with a typical line detec-tion lower limit of 0. 1 K. A coarse grid of positions separated by several beamwidthswas made first; regions that yielded interesting spectra were then studied more care-fully by filling in the grid. The frequency switching scheme overlapped signal and com-parison bands by half a bandwidth, so that post real-time processing would yieldThis work was supported principally by the National Aeronautics and Space Admin-istration (Grants NGL 22-009-016 and NGL 22-009-421), and the National ScienceFoundation (Grant GP-20769); and in part by California Institute of Technology Con-tract 952568, and the Sloan Fund for Basic Research (M. I. T. Grant 241).QPR No. 102(II. RADIO ASTRONOMY)higher sensitivity in the radial velocity range where detection was expected. Twosample spectra are shown in Fig. II-1.0.5035.4+0.1 (1,0)0.00--0.50-10.00 000 o10.00oo 30.00US (a):00.40201.5+0.6 (0,0)z 0.200.00 --0.20-- 4.00.40.-40.00Fig. II-1. Spectra from 35. 4+0. 1 and 201. 5+0. 6 in thevelocity ranges where signal and comparisonbands overlap. The ordered pair is the off-set from the reference position in RightAscension, and in Declination, in units of6 minutes of arc. In (a), the apparent emis-sion feature at 6 km/s is actually an absorp-tion line at +44 km/s.0.00 40.00RADIAL VELOCITY/km/s(b)3. Results of Observations.The observational results are summarized in Table II-1. The velocity range listedis the total range spanned by the signal and comparison bands. In cloud 35. 4+0. 1, thetwo highest velocity features were detected because they fell in the comparison bandand appeared in the difference spectra as emission lines.19 00 S06 18Ih50m00--- a(1950.0)Later observations with aFig. II-2. Absorption line intensity distribution for 35. 4+0. 1, radialvelocity = 13 km/s. Circles represent half-power beamshapes located at the angular positions where data weretaken. The radio diagram is superimposed on a tracing ofa Palomar Sky Atlas photograph of the cloud.QPR No. 102I I-9^^^'Table II- 1. Summary of observations.Reference Position Velocity Frequency Positions Detection No. of LineSource (1950) Range Resolution Observed Velocities Detections Temperaturesa(h, m, s) 56(,, ") (km/s) (kHz) (km/s) (K)35. 4 + 0.1 18 53 07 02 09 49 -45 to 71 3. 25 78 9 4 -. 1 to -.313 38 -. 1 to -. 529 3 -. 1 to -. 244 13 -. 1 to -. 353 4 -. 1 to -. 3127.7 + 14.0 02 04 31 75 55 07 -55 to 61 3.25 56 -38 4 -. 1 to -.23 14 -. 1 to -. 38 3 -. 1 to -. 218 4 -. 1 to -. 219 3 -. 1 to -. 2201. 5 + 0.6 06 29 52 10 32 22 -112 to 120 6. 50 62 4 23 -. 1 to -. 3(II. RADIO ASTRONOMY)higher velocity signal band showed them to be absorption lines. A velocity of formalde-hyde detection is listed only if the feature was detected in three or more positions.The angular distribution of detected formaldehyde varies considerably from one cloudto the next. In 35. 4+0. 1, small clumps of formaldehyde appear off the southwest, north,and west edges of the optical cloud image at 9, 29, and 53 km/s, respectively. Form-aldehyde moving along the line of sight at 13 km/s coincides largely with the opticalimage; it is the strongest and most widespread feature observed. The distribution ofabsorption line intensity for this feature is presented in Fig. 11-2. At 44 km/s, a ringof detections appears, with a diameter approximately half the east-west diameter of theoptical image. The strongest detections appear off the edge of both the optical imageand the 13 km/s radio image.In cloud 127. 7+14. 0, formaldehyde detected at each velocity appears in clumps ineither the eastern or western parts of the cloud, or in both. The central region appearsdevoid of formaldehyde, except for a few features, each present in only one angular posi-tion. In 201. 5+0. 6, formaldehyde is present only at 4 km/s, and only in a sharply-defined region in the south central part of the optical image.4. Discussion of the Results.The location of 35. 4+0. 1 along a direction in the galaxy with much neutral hydrogenand dust suggests that the several detection velocities may correspond to severalclouds or clumps of gas, spatially separated along the line of sight. Only one radioimage, at 13 km/s, corresponds to the optical image; the others may be closer toearth, or farther away. The velocity distribution of formaldehyde in 127. 7+14. 0 isnot as easy to explain. The line of sight to this cloud is at a galactic longitude wherelittle neutral hydrogen is found. Furthermore, the high galactic latitude of b I I = 14. 0means that the line of sight is within the galactic disk for only approximately 900 par-secs, under the assumption of a disk thickness of 210 parsecs. This distance is lessthan 0. 1 of the in-the-plane line-of-sight length for the other two clouds observed. Theamount of intercepted material should therefore be much lower. If the galactic formal-dehyde distribution follows that of neutral hydrogen and dust, then fewer separate form-aldehyde clouds should be seen. In the case of 201. 5+0. 6, the single detection velocityis consistent with the low concentration of dust and neutral hydrogen along the line ofsight.A detailed analysis of these observations, as well as observations of the same cloudsat the 1. 667 GHz transition of the OH radical, will be published later.P. C. Myers, A. H. BarrettQPR No. 102(II. RADIO ASTRONOMY)B. STELLAR INTERFEROMETERDuring this quarter the duplicating filters described in our last report have beenconstructed, and operate as expected. Each unit cell occupies a 2. 5 in. X 4. 5 in. double-sided etched board. Although the dual 100-bit MOS shift registers that are used aredesigned for DTL or TTL voltage levels, the availability of RTL gates with a minimumopen-circuit output voltage of 2.6 V makes it possible for them to be used with RTL. Theshift registers are used with a substrate voltage of +5 V, and a clamping diode has beenprovided to prevent the substrate from being forward-biased if the 5 V supply shouldfail while the 3.6 V supply for the RTL is on. The lot of 35 shift registers obtained forthis project was tested initially to see if they could be used with the substrate at +3.6 V.Since the shift registers are P channel MOS, the limiting factor in this caseis that when the bipolar logic is in the low state, the voltage must be far enoughbelow the substrate that a channel is formed in the input transistor of the shiftregister. In half of the units tested, the threshold was low enough that operationat this substrate voltage would be possible. Therefore a design with the sub-strate at +3. 6 V would also require simple buffers to translate the input statevoltage to -- 1 V.Some progress has been made in studying the consequences of defining the dupli-cating property in terms of the limiting continuous function which is approachedby the descrete impulse response ahb in the limit of a - oo and the samplingtime, T, being reduced so that a T = constant. If it can be successfully imple-mented, this kind of definition should make it possible to decide the questionof the existence of other filter structures with a similar duplicating property,but so far there are several major difficulties to this approach.In the previous report, 3 the long shift registers should have length k - 1, and theaccompanying text should read k = 9.Filters using decimation of sampling rate have also been considered by Langenthaland Gowrinathan. 4 The time-domain properties such as duplication were not discussedby them, however.An autocollimator has been built for use in testing the other optical parts of thestellar interferometer. Since one of its purposes is to test the division of power intodifferent parts of the diffraction pattern formed by the beam splitter, a primary goalin the design was to minimize the scattering of light into the image plane by elementsbetween the light source and the element under test, especially the autocollimator'sbeam splitter.Figure II-3 is a schematic diagram of the autocollimator. For clarity, it has beendrawn cut in half, so that the light path is in or near the plane of the cut.QPR No. 102(II. RADIO ASTRONOMY)FOCAL POINT OF RETURNING BEAMhb STRAY LIGHT* /STOPTARGET FOR INITIAL SETTINGTELESCOPE OBJECTIVERIGHT-ANGLE N LENSPRISMMICROSCOPE OBJECTIVEBEAM SPLITTERABSORBERBAFFLELIGHT SOURCEFig. 11-3. The autocollimator.The beam splitter is a half-silvered prism, with another cemented to it to form acube. If the light from the source traveled through the cube perpendicular to the faces,then the reflection from the front face of the cube (the face farthest from the light source)would return to the observer and create a very bright background light in the focalplane of the light returning from the element that is being tested. A similar unwantedreflection could occur from the bottom face of the beam splitter, but it is eliminated bythe absorber. This background light is prevented by rotating the beam splitter throughangle a, 4 15 ' in this case. This causes the returning beam and the stray light beamto be deflected in opposite directions, and, as drawn, the stray light is absorbed by astop near the focal plane of the returning light.Rotating the beam splitter in this way would, by itself, introduce astigmatism intothe image that is finally formed, but this is removed by tilting the right-angle prism(the main purpose of which is to fold the light path and reduce the size of the autocolli-mator) through angle b, 3'. Stray light reflected from the faces of the right-angleQPR No. 102(II. RADIO ASTRONOMY)prism is similarly absorbed by the stop. This eliminates astigmatism because a and bare in perpendicular planes, and a € b because the path length in the right-angle prismis longer than in the beam splitter.The next element is a microscope objective, which is used to increase the effectivefocal length of the telescope objective. The microscope objective is 40X, n. a. = 0. 65,and the telescope objective is 10 in. focal length, f/5. Thus the effective focal lengthis 400 in.The oculars used are 10X and 20X, for an overall magnification of 400X and 800X.With such high magnification, it would be difficult initially to locate the image withoutsome auxiliary siting method. This is provided by the target located in the focal planeof the telescope objective. In normal use, the light passes through the hole in the cen-ter; for initial alignment, the ocular is removed, and a small lamp shines on the tar-get. In the telescope objective, greatly reduced by the microscope lens, the image ofpart of the target is seen, reflected in the mirror that is being tested. Once the mirrorhas been set to the center of the target, the ocular is replaced and the measurementcan proceed.Since the stray light from the beam splitter and right-angle prism has been elimin-ated, the only remaining source of significant stray light is reflections from theelements in the microscope objective. Although this light is perceptible, it is dimenough to be acceptable for present use.In the autocollimator only readily available and relatively inexpensive optical partsare used. The mechanical construction is simple and noncritical.P. L. KebabianReferences1. P. L. Kebabian, "Stellar Interferometer," Quarterly Progress Report No. 101,Research Laboratory of Electronics, M.I.T., April 15, 1971, pp. 1-11.2. Ibid., see Fig. I-5, p. 6.3. Ibid., see Fig. 1-10, p. 11.4. I. Langenthal and S. Gowrinathan, "Advanced Digital Processing Techniques,"CFSTI, AD-708-736.5. M. Habegger, "Astigmatism in Light Deflector Elements," J. Opt. Soc. Am. 60,326 (1970).C. INFRARED TRANSMISSION OF CO AND NO 0In our last report1 we proposed a method for evaluating the transmission of aQ branch arising from a particular vibration-rotation transition. In reality thetransition also gives rise to P and R branches. The complete spectrum containsQPR No. 102(II. RADIO ASTRONOMY)strong overlapping P, R, and Q branches arising from different vibration-rotationtransitions. For a homogeneous gaseous medium of path length u, the transmittanceaveraged over the channel Ac isdw fl exp -uk (})+k (,)+kQ(w)wherem mak (GO) = S m a1 iJ v 2J a+ W-W.)m = P, R, Qis the absorption coefficient of one branch. The subscripts iJ refer to a particularthline arising from the i vibration-rotation transition whose lower rotation number is J.The line is centered at frequency iJ, has integrated intensity SiJ, and assumes theLorentz profile of halfwidth a.We uncouple the different vibration-rotation bands and approximate Eq. 1 byPRQ'F T". T. T, (3)111where, omitting do in the equations to avoid excessive notation, we have7 = exp -uk m ) ,j1 AC e i iW m = P, R, Q.The same approximation is taken in the random Elsasser model. We do not, how-ever, determine T. by assuming an Elsasser band. Rather, the transmittance 7T isi 1 P R ievaluated as in our last report, and T. , T. are computed by the following method.P and R branches are alike in structure but differ from Q branches in having spec-tral lines more widely separated. Near the center of a line in the P and R branches, theabsorption is due mainly to the line itself and only slightly to the wings of neighboringlines. It is therefore expedient to treat the wing contribution as a correction term. Form = P or R, we havem l cT.:1 /Ac A exp {uSmiJ aT a + (C 2J 1iJjwIexp ua 2+ (-) 2miJ' 2Tr 2 a m2J' J a + -o i ,QPR No. 1027 =(II. RADIO ASTRONOMY)2where, by choosing J to be the value of J' that minimizes (-j N, we have dividedthe channel into intervals occupied by different lines. The separation thus far isLINEINTENSITY-- TOTAL PROFILEIS THE SUM OF----- i=1--- i =2FREQUENCYAu, CHANNEL WIDTHFOR i= 11 2 3 4SHADED PORTION IS THE WING CONTRIBUTIONUNSHADED PORTION IS THE LINE CONTRIBUTION,WHICH HAS LORENTZ LINE SHAPE; TRANSMITTANCE =-'1JFig. 11-4. Separating two overlapping bands.illustrated in Fig. II-4 for the case i = 1 and 2, and J running over 5 values wheni = 1.We further approximate Eq. 5 bym 1 -auiJT. - exp iJ a2 + (--ja0-iJ =J 'tJmmiJ 1IT _ 2S(w-c4Thmrepresents the wing contribution of lines adjacent to the one centered at wiJ We havemade use of the fact that the separation between lines is much greater than the halfwidth,so that iJ can be tabulated for each line in the channel. The integral in Eq. 6 repre-sents the transmittance of a single Lorentz line over a finite spectral interval. Its evalu-ation was given explicitly in our last report. 2This completes the algorithm for computing the transmittance of P and R branchesQPR No. 102where0.0 EXPERIMENT (REF. 5)SMI TH'S CALCULATION (REF. 3)3 - PRESENT CALCULATION0.2 u = 126 cm - atmT = 300 'K0.3 p =768 mm Hg0.40.50.60.70.80.91.0 -- - - - - - -6 0 650 690 730(a)0.0 EXPERIMENT REF. 5)PRESENT CALCULATION0.1 u=0344 cm-atmT=300 '0.2 p=575 mm Hg A)27 mm Ha B)0.00.0.20.3, B2300 2350 2400WAVE NUMBER (cm )(c)Fig. II-5. (a)(b)(c)(d)070.80'9218(TransmissionTransmissionTransmissionTransmissionu=6.3 cm-atmT 300 Kp= 15.6 mm HgEXPERIMENT (REF. 5)SSMITH'S CALCULATION (REF. 3)PRESENT CALCULATION510 650 690 730(b)-- EXPERIMENT (REF. 5)PRESENT CALCULATION5cmu=0.148 cm- atmT= 300 'Kp= 199 mm Hg (A)71 8 mm Hg (B)2200 2220 2240WAVE NUMBER (cm-')(d)CO2 near 15 [.CO2 near 15 Ki.CO2 near 4. 3 [1.N 2 0 near 3.9 2.QPR No. 1020.30.40.60.91.02250 2260(II. RADIO ASTRONOMY)over a homogeneous medium. With this procedure, we have evaluated the transmissionof CO 2 near 4. 3 a and 15 [, and N 2 0 near 3.9 [i. In Fig. II-5 some results are com-pared with experimental measurements and values calculated by using band models frompublished work. Agreement is satisfactory over most channels.1201101009080 -706030580 620 660 700 740 7WAVE NUMBER (cm' )Fig. 11-6. Atmospheric radiation: Experimental and calculated.Up to this point we have considered only homogeneous media. In the atmospherephysical conditions vary. An equivalent set of homogeneous conditions must be definedbefore the procedure outlined above can be applied to the atmosphere. We have adoptedthe Curtis-Godson approximation to obtain an equivalent pressure p and an equivalenttemperature T:p = f p du/f duT = f T du/f du.On May 8, 1966, a NASA balloon-borne interferometer measured several spectraof the upward infrared radiance at the 8 mb pressure level over Palestine, Texas. Themain absorbents in this frequency range are carbon dioxide and water vapor. In Fig. 11-6,the recorded values are compared with calculation performed under the assumptionthat the Earth radiates as a blackbody. The dotted lines were obtained by Smith3when he compared his polynomial fit with the quasi-random model.4 We retainedQPR No. 102BALLOON MEASUREMENTSSMITH'S CALCULATIONPRESENT CALCULATION5 c n'SI i I I - I I I(II. RADIO ASTRONOMY)his values for the H20 transmittance and multiply with it the CO2 transmittance calcu-lated by using this method. Our results appear to be in better agreement with experimentthan those using the polynomial fit for H20 and CO Z2R. K. L. Poon, D. H. StaelinReferences1. R. K. L. Poon and D. H. Staelin, "Transmission of Carbon Dioxide: Q Branch,"Quarterly Progress Report No. 101, Research Laboratory of Electronics, M. I. T.,April 15, 1971, pp. 11-17.2. Ibid., see Fig. I-11, p. 15.3. W. Smith, "A Polynomial Representation of Carbon Dioxide and Water Vapor Trans-mission, " National Environmental Satellite Center Technical Report, NESC 47 (1969).4. P. J. Wyatt, V. R. Stull, and G. N. Plass, "Quasi-random Model of Band Absorp-tion, " J. Opt. Soc. Am. 52, 1209 (1962).5. D. E. Burch, D. A. Gryvnak, and D. Williams, Appl. Opt. 1, 473 and 759 (1962).QPR No. 102

Radio Astronomy

http://dspace.mit.edu/bitstream/1721.1/56197/1/RLE_QPR_102_II.pdf

Radio Astronomy

Abstract

Similar works

Full text

Available Versions

DSpace@MIT