next previous
Up: Accurate positions and

3. Observations and reductions

To obtain identifications and accurate coordinates, two types of follow-up observations were obtained. For those objects were no finding chart was available at the time, CCD images were obtained with interference filters centered ON and OFF the Htex2html_wrap_inline1717 emission line, the images being centered on the published coordinates (see Sect. 3.1). This allowed to recover one (or more) emission-line objects in the field. For other PNe, the identification was taken from the literature. For all the identified PNe candidates, entire Schmidt plates were scanned, covering the whole LMC, to determine accurate coordinates and a rough broad-band magnitude.

3.1. CCD imaging

Images of about 50 PN candidates have been taken, from 1990 to 1991, with several telescopes in ESO La Silla (NTT, 2.2 m and the Dutch 90 cm) with respective fields of view of tex2html_wrap_inline1719, tex2html_wrap_inline1721 and tex2html_wrap_inline1723. A few objects turned out to be resolved in the Htex2html_wrap_inline1725 images (SMP011, SMP017, SMP024, SMP030, SMP060, SMP086, J26). All candidates in these fields could be recovered, with the exception of Sa108, Sa119, Sa125 and Sa127 which are therefore rejected as PNe candidates (see Sect. 2.2). Many broad band images were also recorded, just before spectroscopy, to allow a proper positioning of the objects on the slit. These images also provide high quality finding charts.

3.2. Scanning of Schmidt plates

We have scanned 5 SERC J (Blue) and 3 ESO (Red) Schmidt plates (to cover a maximum area of the LMC) with the MAMA (Machine à Mesurer Automatique) at Paris Observatory. MAMA is a linear scanner of 1024 diodes mounted on a stable marble table in a cooled room and provides excellent stability and accuracy (see Berger et al. 1991). The dynamics is 12 bits, therefore with very deep exposures all the stars of tex2html_wrap_inline1739 or brighter are already saturated. For stars brighter than tex2html_wrap_inline1741 = 12 the saturation is too strong, and then no precise centering can be done. This dynamics is however perfectly adequate for copy Survey Plates such as the ones we used, where the intrinsic dynamics is anyway limited. The J plates, with a field about 6tex2html_wrap_inline1743 by 6tex2html_wrap_inline1745, are mapped in a mosaic of 33 by 33, tex2html_wrap_inline1747 pixel images, while the mosaic is 28 by 28 in the case of the red plates. The total J image is tex2html_wrap_inline1749 pixels and occupies about 2.2 Gbytes of disk space. The pixel size is 10 tex2html_wrap_inline1751m or 0tex2html_wrap_inline175367. The main characteristics of these plates are shown in Table 1 (click here). The coordinates are the (tex2html_wrap_inline1755, tex2html_wrap_inline1757) of the plate center in J2000 equinox derived from our astrometry. The date of observation is also given. The last column presents respectively the initial and the final number of PPM (Röser & Bastian 1988) stars taken into account for the astrometric reduction (see below).

  table243
Table 1: Parameters of the used Schmidt plates

3.3. positions

The published coordinates for the various PNe candidates are given in several equinoxes (1950, 1975 and 2000). Most of the astronomical reductions have probably been done (although not explicitly said, except by Jacoby 1980) with earlier catalogues in the FK4 system, whose accuracy of positions and proper motions are no longer satisfactory.
Therefore, in order to make the comparison with our new positions, we converted all the coordinates to the J2000 equinox with the matrix suggested by Lieske (1979).
On each SERC or ESO plate, all the PPM references (in equinox J2000 and in the FK5 system) stars have been searched (the mean number is about 200), and their positions on the plates determined in (tex2html_wrap_inline1781). For all the (bright) PPM stars we used a spike centering method.
The Schmidt plate is then mapped with a bidimensional 3-degree polynomial in order to transform the (tex2html_wrap_inline1783) to (tex2html_wrap_inline1785, tex2html_wrap_inline1787) or to make the reverse transformation. The program is iterative, in the sense that it rejects the reference stars which deviate by more that 3 tex2html_wrap_inline1789 from the calculated position and recalculates the new coefficients of the fit. On the average, the program iterates only 3-4 times and rejects only about 10 stars randomly positioned on the plate (and not at the outer part as would be the case for a bad fitting on the borders).

  figure260
Figure 1: Differences between our new positions and previously published ones (new-old)

Figure 1 (click here) shows the differences between our new positions and the ones given in previous papers. In the case of Sanduleak et al. (1978) objects the original coordinates have been taken only if better one's were not available. The difference can be as large as tex2html_wrap_inline1793 and therefore the identification is made impossible in such dense fields from coordinates alone.
We used the same symbols for Figs. 1 (click here), 2 (click here) and 3 (click here): squares (Sanduleak et al. 1978, 1984; and Meatheringham et al. 1991a,b); crosses (Jacoby 1980); circles (Morgan & Good 1992; Morgan 1994). Figure 2 (click here) is an enlargement of the central part of Fig. 1 (click here) and shows only the points with better original coordinates (Jacoby 1980; Morgan & Good 1992; Morgan 1994; and Meatheringham et al. 1991a,b). The accuracy is still not satisfactory. Moreover we see a systematic difference in coordinates, both in tex2html_wrap_inline1795 and tex2html_wrap_inline1797, of respectively 1tex2html_wrap_inline1799 and 2tex2html_wrap_inline1801. The bulk of data is not centered at the origin indicated by the cross.

  figure281
Figure 2: Enlargement of the previous figure. Systematic difference in positions (new-old) of about tex2html_wrap_inline1803 is clearly visible. The cross indicates the center at position 0,0

The origin of this discrepancy is probably multiple. It could be due to the use of film copies for some earlier measurements, to systematic differences between astrometric catalogues (PPM versus SAO, Perth70, ...), to differences between reference systems (FK5 versus FK4, amounting to about tex2html_wrap_inline1805 in tex2html_wrap_inline1807 and tex2html_wrap_inline1809 in tex2html_wrap_inline1811 (Schwan 1988)), to unaccounted proper motions of standard stars, etc ... The measurements presented here are however based on the best available material (glass copies, digitized images, PPM stars).

The (tex2html_wrap_inline1813) positions of all objects have been measured with the 2 completely different methods. The first method is a Gaussian centering fit inside the MIDAS Image Processing Package of ESO. In very crowded fields, like the Bar, and/or with tightly blended stars, the fit does not converge properly and this measurement has not been retained.
The second is a home-made program (Alard 1995) which separates the blended stars, measures the central position and does the photometry of all the stars found in the field. This method failed only in a few peculiar cases were no position and photometry could be derived (7 objects in total from the BAR: SMP047, SMP048, J12, J22, J26, Mo15 and Mo23). These cases are indicated by a colon in Table 4 (Col. 3 after the flag number).

  figure288
Figure 3: Half difference between the two centering methods. No systematic effect can be found

Figure 3 (click here) shows that for all the objects, which could be measured by the two methods, the two determinations agree quite well. They gave similar results with a difference always less than 0tex2html_wrap_inline181530 and a tex2html_wrap_inline1817 of about 0tex2html_wrap_inline181909. Both methods have advantages and disadvantages. The first one, with manual identification, is indispensable when an object has poor coordinates but can be identified on a finding-chart. Its limits are reached in crowded fields not because of identification problems, but because the centering algorithm in MIDAS is not able to properly distinguish between sky background and adjacent objects. The second, automatic method is optimized for these difficult cases but, because no a-priori identification is made, the PN candidate cannot be selected out from the outputs when its initial coordinates are too poorly determined. Both methods are thus complementary and have been used in sequence.
We first identified the candidate, to center it in an extracted sub-frame of tex2html_wrap_inline1821 pixel (in order to produce the finding-charts). We then use first and second methods in this sub-frame only to determine the position and the magnitude of the PNe. In all the cases where the first method works properly, the two (tex2html_wrap_inline1823) position determinations agree perfectly. Therefore, in order to improve the accuracy and to reduce dispersion, the two determinations have been simply averaged. The final PNe coordinates are directly determined from the proper (tex2html_wrap_inline1825) to (tex2html_wrap_inline1827, tex2html_wrap_inline1829) transformation, and averaged if several positions were measured on different Schmidt plates. In the case of multiple determinations, if one is coming from a plate corner, check of the reliability is done automatically and the point is rejected if its distance to the mean value is greater than 0tex2html_wrap_inline183130 (see the final position Table 4 in Appendix). The definition of a corner is the area within 4 cm, in both directions, of the scanned border zone. Elsewhere, no distortion problem has been noted.

To look at the internal consistency, we use PNe in common on all plate pairs. The mean position error of the PPM stars at the LMC declination is 0tex2html_wrap_inline183311 and the estimated residual systematic deviation between PPM south and the FK5 system has a typical size of 0tex2html_wrap_inline183505.
The external consistency has been checked over all overlapping PPM stars on each pair of plates, and no systematic error could be found nor border fitting problems. The residual errors are of the order of 0tex2html_wrap_inline18372. Figure 4 (click here) shows the results of this test for the plate pair SERC 056J/SERC 085J.

Table 2 (click here) lists the number of PNe in common, as well as the corresponding sigma in coordinate differences. This test was done on the SERC plates only (overlap on ESO

  figure296
Figure 4: PPM position differences on Plates SERC 056J and SERC 085J

  table301
Table 2: PNe in common

plates is too small) but ensures the accuracy and consistency of the whole astrometry. Figure 5 (click here) represents the histogram of coordinates differences for all the overlapping PNe on the 8 plates. Very few objects fall outside 0tex2html_wrap_inline18575, and all the bad determinations came from a PNe pair with at least one lying in a plate corner. The average error is about 0tex2html_wrap_inline18593 and the corresponding mean standard deviation is about 0tex2html_wrap_inline186117. But these objects are taken from the borders of the plates, and therefore the expected errors in the inner part of the plates should be much smaller.

  figure314
Figure 5: Histogram of the position differences for all the PNe present on at least 2 Schmidt plates

3.4. Photometry

The automatic centering program (Alard 1995) also furnishes magnitudes, by profile reconstruction of point-like sources and integration inside the profile. This method has the advantage to give good results even when the central part of the star is saturated. We have therefore directly the magnitude (in B band) for all the PNe where crowding was not too important.
Calibration was obtained from B and V photometry of faint stars in the LMC (Linde et al. 1988) for very faint sources and from the Atlas of star clusters with color magnitude diagrams in the Magellanic Clouds (Alcaino 1975) from which we extracted 12 photometric sequences in LMC.
In total we used about 250 stars, covering a large magnitude domain from tex2html_wrap_inline1925 = 9.1 to tex2html_wrap_inline1927 = 22.7. B and V photometry has been used to produce tex2html_wrap_inline1933 magnitudes for the standard stars, following the recipe given Blair & Gilmore (1982)tex2html_wrap_inline1935.
Figure 6 (click here) shows the calibration curve with all the standard stars (except a few rejected outlying points due to confusion, saturation, etc...) and a fitted 6-degree polynomial, taken from two different plates (SERC 056J and SERC 085J). No systematic effect can be seen between these two plates. The linear domain extends from tex2html_wrap_inline1937 to 22 with a precision of about 0.20 magnitudes which is the expected internal error from the published measurements. The stars brighter than tex2html_wrap_inline1939 are saturated, due to the limited dynamical range of MAMA. This phenomenon is clearly seen in Fig. 6 (click here), where the slope at brighter magnitudes is smaller, and therefore the precision is diminished. But a magnitude determination is still possible down to 10-11th magnitude. From 114 stars which have been measured more than once, we derive an internal error smaller than 0.25 magnitude. Only for a few objects is the uncertainty larger, possibly due to peculiar local problems.

  figure331
Figure 6: Schmidt plates calibration


next previous
Up: Accurate positions and

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr