Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 4356c Report Date: 09-May-96:19:33 Version: ********** Check-in Date: ********** 1.Proposal Title: UV SPECTROSCOPY AND IMAGING IN THE CRAB NEBULA - CYC3HIGH ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID INTERSTELLAR MEDIUM GO Sub Category SN & SNR ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Prof. Kris Davidson UNIVERSITY OF MINNESOTA USA 612-624-5711 ------------------------------------------------------------------------------------ 7. Abstract The Crab Nebula is the most observable SNR that is composed mainly of processed ejecta rather than swept-up interstellar matter. This makes it one of the most critical of all astronomical objects for SN and nucleosynthesis theory. However, despite its fame, the Crab is NOT understood and does not yet give much support to stellar evolution and nucleosynthesis theory. A shortage of nitrogen is particularly troubling, among other unresolved puzzles. Meanwhile, chemical abundance variations and correlations with bipolar structure have recently become interesting with reference to other objects (e.g. SN1987A). ...These problems absolutely require UV observations of the Crab which can be done only with HST. UV spectra with FOS are most obviously needed, mainly to obtain C, N, Mg, and Si abundances. UV images with FOC are also highly desirable, in case there are surprises such as (e.g.) carbon-rich UV-bright condensations. No such data will have been obtained yet by the end of HST Cycle 2 -- not even a brief reconnaissance. Here we propose a suitable program of exploratory FOC and especially FOS observations. These observations are appropriate for Cycle 3 (i.e., before COSTAR) because their spatial resolution is limited by S/N, not by telescope optics. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 5.58 par: 0 10. Num targs pri: 8 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: FOS ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 4356c PI: Prof. Kris Davidson Proposal Title: UV SPECTROSCOPY AND IMAGING IN THE CRAB NEBULA - CYC3HIGH ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Pi Kris Davidson UNIVERSITY OF MINNESOTA USA Alan K Uomoto JOHNS HOPKINS UNIVERSITY USA Gordon M Macalpine UNIVERSITY OF MICHIGAN USA William P Blair JOHNS HOPKINS UNIVERSITY USA Richard Bc Henry UNIVERSITY OF OKLAHOMA USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. We propose FOS spectroscopy of 4 selected target condensations in the Crab Nebula, using the largest ordinary FOS aperture (1" sq) and the FOS/BL detector. We concentrate on the spectral range 1230--2000 A where the most important emission lines are, mainly using grating G160L but also using G130H and G190H for the brightest target. Spectral range 2000--3300 A is given less emphasis, using G270H. Longer wavelengths are also sampled with short exposure times using grating G400H, in order to normalize fluxes measured in ground-based data (not yet obtained). Exposure times range from 3 minutes (G400H only) to almost an hour each. ... In increasing order of difficulty, our goals are (1) to measure the CIV 1548, HeII 1640, and CIII 1908 emission lines to estimate the carbon abundance; (2) to detect NIII 1750 in order to make nitrogen estimates far more secure; and (3) to detect other lines of NIV, SiIII, SiIV, etc., all in the same wavelength range. In the lower-priority 2000-3300 A range we hope to discover whether MgII 2798 and/or other lines are present. ... The target condensations are diffuse, with typical diameters of roughly 2". Our target selection criteria are (1) each target condensation is among the 40 brightest local peaks in the Crab Nebula in [OIII] light; (2) each target is a relatively well-defined conensation, or a sector of a narrow filament; and (3) we sample the morphologically different parts of the SNR as widely as possible with this small a number of targets. ... Acquisitions will be by offset from reference stars. We have measured positions of the targets and offset stars using high- quality ground-based CCD images; pointing accuracies of 0.1" or even 0.2" are acceptable and we have measured the required offsets to slightly better than 0.1" (rms dev). We have taken the small expansionary motion of the SNR into account in the planned offsets. ... Originally we hoped to use the FOS/RD detector in addition to FOS/BL, to provide better efficiency at most wavelengths. However, FOS/BL is definitely needed for the shortest-wavelength lines and there is not enough available time to make side-switches practical. ...... Our Phase I proposal also included UV imaging with the FOC. However, we and the TAC agree that spectroscopy has a higher priority, and with the limited amount of time that has been allotted we have deleted the FOC images. (One reason is that the FOS is better-suited than the FOC for observations of a low-surface-brightness extended object like the Crab.) ...... Altogether, this project should be viewed as a reconnaissance rather than a full detailed study. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. HST is needed because these are UV observations of faint, small objects. ... A major effort was made to observe the Crab Nebula with IUE, with marginal results reported by Davidson et al. (1982) Ap.J. 253, 696. The IUE was capable of measuring just the three brightest UV lines (CIV 1548, HeII 1640, CIII 1908) integrated over the brightest large amorphous region in the Crab. The story for ASTRO (HUT and UIT) is similar, with even less spatial resolution for the spectroscopy. With HST we can observe particular condensations (this is important for theoretical modeling), the sensitivity is adequate to observe additional locations in the SNR (morphological variations are known to exist there), and we hope to detect important fainter lines, especially NIII 1750. ... We will supplement the HST/FOS observations with specific ground-based spectroscopy of exactly the same targets at longer wavelengths. We hope to obtain these ground-based data in late 1993. ... Example of expected count rates and exposure time: Based on available IUE and ground-based data, combined with ionization models, we expect the CIII 1908 emission line to produce a flux of the order of 1.e-15 erg cm-2 s-1 through the 1" square FOS aperture at one of our targets. With the efficiencies listed in the FOS handbook this will give somewhat less than 200 counts per hour with G160L. NIII 1750 is "expected" to be several times fainter. Therefore exposure times approaching an hour are needed for this part of the spectrum (even though one motivation for the project is the hope that some regions will be brighter than expected in some UV emission lines). ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. We have no special scheduling requests. There is no need for any particular orientation of the FOS aperture. There is no need for the targets to be observed all on the same day or even the same month. ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. No special calibration requirements are needed. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. These FOS results will be emission-line spectral data of an operationally conventional type (although the numbers of counts may be unsatisfying). However, we must connect these data to special ground-based spectrophotometry of the same somewhat-diffuse targets. To do this we include short FOS exposures with grating G400H, overlapping the ground-based data so that we can estimate the relative normalizations of the FOS vs. ground-based data (using mainly the bright OII 3727 and NeIII 3868 lines). --- Interpretation of the spectra, particularly abundance analyses, will employ ionization models calculated with MacAlpine's codes and others. All of us have experience with models and analyses of this type. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. During January 1993 we have measured the precise target offsets and other acquisition data, using several different CCD images and plates obtained in 1986- 1991. --- Precisely-positioned violet-to-red spectrophotometry of our FOS targets will be obtained with the McGraw-Hill 2.4-m telescope and (we hope) with one or more larger telescopes also. Page 4 ------------------------------------------------------------------------------------ 9. Description of previous HST work. --GO 2338, "Spectroscopy of...Eta Carinae", Davidson is PI; not related to this project. --GO 2416, "Imagery and spectroscopy of super metal poor galaxies", Davidson was a CoI. Not related. --GO 3815, "UV observations of Hubble-Sandage variables...", Davidson is a CoI. Not related. --GTO 1062, "A Search for Extra- Solar Planets", Uomoto is a CoI. Not related. --Uomoto and Davidson are also assisting some GTO programs, but not as CoIs. --Data from these programs: FOS data were obtained in GO 2338, but are not ready for publication yet because of the complexity of the data. GO 2416 observations began in August 1992 and are still in progress; no publications yet. GTO 1062 is a "cycle 9" project, no data yet. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). Most importantly, the McGraw-Hill 2.4-m telescope will be used for ground-based spectrophotometry of our FOS targets. The usual computer infrastructure is provided by each of our institutions. Students at several of our institutions will probably be involved in the data reduction and analysis and in the ground- based observations. The APS at the University of Minnesota was already used several years ago to assess reliability of the GSC acquisition and guide stars in the field of the Crab Nebula. ------------------------------------------------------------------------------------ 11. Address Information Name: KRIS DAVIDSON Category: PI Institution: University of Minnesota Address: UNIVERSITY OF MINNESOTA ASTRONOMY DEPT. 116 CHURCH ST. S.E. City: MINNEAPOLIS State: MN Zip Code: 55455 Country: USA Telephone: 612-624-5711 Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 4356c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 10 WM09- A,140 RA = 5H 34M 27.20S +/- J2000 1 V = 14.5 +/- 0.2 053427+22004 0.04S, 2 B-V = 1.1 +/- 0.3 7-STAR- DEC = +22D 0' 47.7" +/- OFFSET CRAB- 0.3", NEB-STAR- PLATE-ID=000M 071X-008Y- OFFSET Comments: WM REF STARS--WYCKOFF AND MURRAY 1977, MNRAS 180, 717. SP TYPES ARE UNKNOWN, COLORS ONLY EDUC GUESSES. LOCAL X,Y COORDS (ARCSEC, EQ 2000) ARE GIVEN IN NAME_2. -- RE. PLATE-ID: OUR COORDS WERE NOT MEASURED WITH GSSS, BUT ARE CONSISTENT WITH GSC PLATE 000M, AND NOT WITH PLATE 0075 WHICH DISAGREES BY 2". ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC1952- G,506,925,927 R = 29.48", J2000 1 SURF(V) = 20.0 +/- 1.0 053429+22003 PA = 109.8D, 2 SURF-LINE(1548) = 16 +/- 7 E-16 7 CRAB-NEB- FROM 10 2 W-LINE(1548) = 1.0 +/- 0.5 KNOT-043X- 3 SURF-LINE(1908) = 16 +/- 7 E-16 018Y CRAB- 3 W-LINE(1908) = 1.0 +/- 0.5 NEB-BF1 4 SURF-LINE(2798) = 4 +/- 4 E-16 4 W-LINE(2798) = 1.0 +/- 0.5 5 SURF-LINE(3727) = 160 +/- 70 E-16 5 W-LINE(3727) = 1.0 +/- 0.5 Comments: CONDENSATION IN BRIGHTEST FILAMENT OF 6 SIZE = 2 +/- 1 CRAB NEB. FLUXES ARE GUESSES BASED ON CCD IMS AND IUE, DEFINED SO THAT FOS APERTURE THROUGHPUT IS INCLUDED. W-LINE = 1.0 IS NOMINAL VALUE, REAL LWIDTHS ARE UNKNOWN, PRODUCT (SURF-LINE * W-LINE) IS OK. ------------------------------------------------------------------------------------------------------------------------------------ 30 WM17- A,140 RA = 5H 34M 33.54S +/- J2000 1 V = 15.25 +/- 0.2 053433+22022 0.04S, 2 B-V = 0.8 +/- 0.3 9-STAR- DEC = +22D 2' 29.7" +/- OFFSET CRAB- 0.3", NEB- PLATE-ID=000M STAR+017X+09 4Y-OFFSET Comments: SEE COMMENTS FOR TARGET 10 ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC1952- G,506,925,927 R = 27.49", J2000 1 SURF(V) = 20.0 +/- 1.0 053433+22020 PA = 189.55D, 2 SURF-LINE(1548) = 8 +/- 4 E-16 2 CRAB-NEB- FROM 30 2 W-LINE(1548) = 1.0 +/- 0.5 KNOT+013X+06 3 SURF-LINE(1908) = 8 +/- 4 E-16 7Y CRAB-NEB- 3 W-LINE(1908) = 1.0 +/- 0.5 N2 4 SURF-LINE(2798) = 2 +/- 2 E-16 4 W-LINE(2798) = 1.0 +/- 0.5 5 SURF-LINE(3727) = 80 +/- 30 E-16 5 W-LINE(3727) = 1.0 +/- 0.5 Comments: EAST SIDE NORTHERN LOOP, CRAB NEB. SEE TARGET 11 COMMENTS. TARGET LIST a) Fixed Targets ID = 4356c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------ 40 WM21- A,140 RA = 5H 34M 35.71S +/- J2000 1 V = 15.5 +/- 0.2 053436+22003 0.04S, 2 B-V = 1.1 +/- 0.3 7-STAR- DEC = +22D 0' 37.3" +/- OFFSET CRAB- 0.3", NEB- PLATE-ID=000M STAR+048X- 019Y-OFFSET Comments: SEE COMMENTS FOR TARGET 10 ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC1952- G,506,925,927 R = 26.27", J2000 1 SURF(V) = 20.0 +/- 1.0 053434+22005 PA = 302.6D, 2 SURF-LINE(1548) = 7 +/- 4 E-16 1 CRAB-NEB- FROM 40 2 W-LINE(1548) = 1.0 +/- 0.5 KNOT+025X- 3 SURF-LINE(1908) = 7 +/- 4 E-16 005Y CRAB- 3 W-LINE(1908) = 1.0 +/- 0.5 NEB-E2 4 SURF-LINE(2798) = 2 +/- 2 E-16 4 W-LINE(2798) = 1.0 +/- 0.5 5 SURF-LINE(3727) = 70 +/- 30 E-16 5 W-LINE(3727) = 1.0 +/- 0.5 Comments: IN FILAMENT E OF PULSAR, CRAB NEB. SEE TARGET 11 COMMENTS. ------------------------------------------------------------------------------------------------------------------------------------ 50 WM18- A,140 RA = 5H 34M 33.07S +/- J2000 BKG 1 V = 16.4 +/- 0.2 053433+21595 0.04S, 2 B-V = 1.3 +/- 0.2 1-STAR- DEC = +21D 59' 51.7" +/- 3 SURF-BKG(V) = 20.1 +/- 0.2 OFFSET CRAB- 0.3", NEB- PLATE-ID=000M STAR+011X- 064Y-OFFSET Comments: SEE COMMENTS FOR TARGET 10 THIS IS OUR FAINTEST ACQ STAR BUT HAS BRIGHTEST BACKGROUND ------------------------------------------------------------------------------------------------------------------------------------ 51 NGC1952- G,506,925,927 R = 19.67", J2000 1 SURF(V) = 20.0 +/- 1.0 053434+21595 PA = 85.9D, 2 SURF-LINE(1548) = 9 +/- 4 E-16 3 CRAB-NEB- FROM 50 2 W-LINE(1548) = 1.0 +/- 0.5 KNOT+030X- 3 SURF-LINE(1908) = 9 +/- 4 E-16 063Y CRAB- 3 W-LINE(1908) = 1.0 +/- 0.5 NEB-S1 4 SURF-LINE(2798) = 2 +/- 2 E-16 4 W-LINE(2798) = 1.0 +/- 0.5 5 SURF-LINE(3727) = 90 +/- 40 E-16 5 W-LINE(3727) = 1.0 +/- 0.5 Comments: IN BRT FIL S OF PULSAR, CRAB NEB. SEE TARGET 11 COMMENTS. ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 4356c [ 7] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10.1 DEFINE # FOS/BL ACCUM 1.0- G130H STEP- 1 55M 11 2 1 CYCLE 3 / 10.1- FOSEQ-1 PAIR PATT=SINGLE 10.4 GROUP 10.1-10.4 NO GAP Comments: QUOTED S/N RATIO REFERS TO TOTAL EMISSION LINE, MORE THAN ONE DIODE. THIS SEQ FOR BRIGHTEST TARG ONLY. ------------------------------------------------------------------------------------------------------------------------------------ 10.2 DEFINE # FOS/BL ACCUM 1.0- G190H STEP- 1 55M 18 3 1 FOSEQ-1 PAIR PATT=SINGLE Comments: SEE COMMENTS FOR LINE 10.1. ------------------------------------------------------------------------------------------------------------------------------------ 10.3 DEFINE # FOS/BL ACCUM 1.0- G270H STEP- 1 20M 10 4 1 FOSEQ-1 PAIR PATT=SINGLE Comments: SEE COMMENTS FOR LINE 10.1. ------------------------------------------------------------------------------------------------------------------------------------ 10.4 DEFINE # FOS/BL ACCUM 1.0- G400H STEP- 1 5M 38 5 1 FOSEQ-1 PAIR PATT=SINGLE Comments: G400H USED FOR NORMALIZATION OF GROUND-BASED DATA. SEE COMMENTS FOR LINE 10.1. ------------------------------------------------------------------------------------------------------------------------------------ 11.1 DEFINE # FOS/BL ACCUM 1.0- G160L STEP- 1 60M 10 2 2 CYCLE 3 / 11.1- FOSEQ-2 PAIR PATT=SINGLE 11.2 GROUP 11.1-11.2 NO GAP Comments: QUOTED S/N RATIO REFERS TO TOTAL EMISSION LINE, MORE THAN ONE DIODE. TARGETS FOR THIS SEQ ALL HAVE SIMILAR EXPECTED BRIGHTNESS. ------------------------------------------------------------------------------------------------------------------------------------ 11.2 DEFINE # FOS/BL ACCUM 1.0- G400H STEP- 1 5M 30 5 2 FOSEQ-2 PAIR PATT=SINGLE Comments: G400H USED FOR NORMALIZATION OF GROUND-BASED DATA. SEE COMMENTS FOR LINE 11.1. ------------------------------------------------------------------------------------------------------------------------------------ 110 WM09- FOS/BL ACQ/ 4.3 MIRROR 1 25S 1 1 CYCLE 3 / 110-111 053427+2200 BINARY ONBOARD ACQ FOR 47-STAR- 111 SEQ 110-111 NO GAP Comments: TARGNUM 10 ------------------------------------------------------------------------------------------------------------------------------------ 111 USE NGC1952- FOSEQ-1 053429+2200 37 Comments: TARGNUM 11 SEE SEQ DEF FOR S/N ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 4356c [ 8] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 130 WM17- FOS/BL ACQ/ 4.3 MIRROR 1 35S 1 2 CYCLE 3 / 130-131 053433+2202 BINARY ONBOARD ACQ FOR 29-STAR- 131 SEQ 130-131 NO GAP Comments: TARGNUM 30 ------------------------------------------------------------------------------------------------------------------------------------ 131 USE NGC1952- FOSEQ-2 053433+2202 02 Comments: TARGNUM 31 SEE SEQ DEF FOR S/N; TARGNUMS 31,41,51 HAVE SIMILAR FLUXES. ------------------------------------------------------------------------------------------------------------------------------------ 140 WM21- FOS/BL ACQ/ 4.3 MIRROR 1 65S 1 2 CYCLE 3 / 140-141 053436+2200 BINARY ONBOARD ACQ FOR 37-STAR- 141 SEQ 140-141 NO GAP Comments: TARGNUM 40 ------------------------------------------------------------------------------------------------------------------------------------ 141 USE NGC1952- FOSEQ-2 053434+2200 51 Comments: TARGNUM 41 SEE LINE 131 COMMENT ------------------------------------------------------------------------------------------------------------------------------------ 150 WM18- FOS/BL ACQ/ 4.3 MIRROR 1 3M 1 2 CYCLE 3 / 150-151 053433+2159 BINARY ONBOARD ACQ FOR 51-STAR- 151 SEQ 150-151 NO GAP Comments: TARGNUM 50 ------------------------------------------------------------------------------------------------------------------------------------ 151 USE NGC1952- FOSEQ-2 053434+2159 53 Comments: TARGNUM 51 SEE LINE 121 COMMENT ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 4356c [ 9] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ACQ/BINARY ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters STEP-PATT=SINGLE ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 5.58 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category SN & SNR ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CYCLE 3 / 10.1-10.4; GROUP 10.1-10.4 NO GAP CYCLE 3 / 11.1-11.2; GROUP 11.1-11.2 NO GAP CYCLE 3 / 110-111; ONBOARD ACQ FOR 111; SEQ 110-111 NO GAP CYCLE 3 / 130-131; ONBOARD ACQ FOR 131; SEQ 130-131 NO GAP CYCLE 3 / 140-141; ONBOARD ACQ FOR 141; SEQ 140-141 NO GAP CYCLE 3 / 150-151; ONBOARD ACQ FOR 151; SEQ 150-151 NO GAP ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130H G190H G270H G400H G160L MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names WM09-053427+220047-STAR-OFFSET CRAB-NEB-STAR-071X-008Y-OFFSET NGC1952-053429+220037 CRAB-NEB-KNOT-043X-018Y CRAB-NEB-BF1 WM17-053433+220229-STAR-OFFSET CRAB-NEB-STAR+017X+094Y-OFFSET NGC1952-053433+220202 CRAB-NEB-KNOT+013X+067Y CRAB-NEB-N2 WM21-053436+220037-STAR-OFFSET CRAB-NEB-STAR+048X-019Y-OFFSET NGC1952-053434+220051 CRAB-NEB-KNOT+025X-005Y CRAB-NEB-E2 WM18-053433+215951-STAR-OFFSET CRAB-NEB-STAR+011X-064Y-OFFSET NGC1952-053434+215953 CRAB-NEB-KNOT+030X-063Y CRAB-NEB-S1 ------------------------------------------------------------------------------------------------------------------------------------