Abstract
Precise (1" or better) positions have been determined or verified for all
the planetary nebulae appearing the NGC and IC catalogues. The table includes
new positions measured on large-scale plates as well as positions from
visual micrometry, astrometric catalogues, and two digitised sky surveys.
These data provide many substantial corrections to existing catalogues.
Introduction
It was common as recently as a decade ago for professional astronomers to
arrive at the telescope bearing a wad of note cards. On the cards were
positions for astronomical objects of interest to 1 arcminute accuracy along
with a finding chart, often photocopied from the BD atlas or more recently
from the Palomar Observatory Sky Survey prints (or their southern equivalents).
These were necessary because the telescopes themselves did not point
accurately around the sky, especially with setting circles, no matter how
huge, but often even when the axes were encoded and had coordinate readouts.
So one ended up looking through an eyepiece or perhaps a TV guider to actually
set the telescope on a target of interest.
Now that most larger professional telescopes have no facility for viewing
directly through an eyepiece, it became necessary for telescope pointing and
tracking to become far more accurate. As a consequence it has become
imperative that the astronomer now arrive with precise coordinates to one
arcsecond accuracy or better, often with proper-motion taken into account.
Yet for many deep-sky objects, even bright ones, no precise positions are
determined nor published in major catalogues or anywhere in the literature.
So part of preparing for an observing run is the task of getting good
positions for each object to be sought, be it a program target, standard star,
and even blank sky regions.
One such set of objects needing improved positions is the class of
planetary nebulae. Despite publication in 1990 of the "Strasbourg-ESO
Catalogue of Planetary Nebulae", many of the objects there are given with
either imprecise or simply incorrect coordinates. As part of a larger project
to improve positions for deep-sky objects, I have collected "best" positions
for all the planetaries appearing in the NGC and IC catalogues. These are
among the most extensively studied objects in all of astronomy, many having
hundreds of literature citations in bibliographic databases.
Sources
As a start, I knew that most of the bright, well-defined objects had
been measured micrometrically by visual observers around the turn of the
Century, and at first I thought their work would still be useful. Indeed,
it turns out that these old determinations are quite good when compared to
modern measurements. In many cases their errors are less than an arcsecond
when compared to modern high-precision positions. But the best way to use
these measurements was to make a full re-reduction of the original micrometer
offsets from astrometric reference stars, using modern positions for those
stars, and making appropriate corrections for precession and proper motion.
In general, this proved too time-consuming---as much as I would have liked to
highlight the sometimes poor quality of recent publications with 100-year-old
visual micrometry.
Arguably the best positions for the very brightest objects are those that
appear in contemporary astrometric reference catalogues, such as the PPM
(Roser & Bastian, 1991) or ACRS (Corbin et al. 1991). The mean errors of both
these catalogues are about 0".3 at the present. Such positions were used
without hesitation.
The majority of the smaller, well-defined objects appear also in the
Space Telescope "Guide Star Catalogue" (GSC), which I have relied upon for
many of the positions listed here. One must be careful in using this
catalogue to insure that the GSC object actually corresponds with the centre
of the nebula or central star.
A more recent digitised sky survey comes from the U. S. Naval Observatory
Flagstaff Station, where the entire sky is being scanned from Schmidt survey
plates in two colors and at two epochs. In the northern hemisphere, the scan
series includes a set of short-exposure plates taken especially for the
project at Palomar Mountain. These will allow accurate measurement of the
bright (mag. 8-10) reference stars (which are grossly overexposed on the deep
survey plates and hence unmeasureable) as well as a substantial number of
fainter stars that are well-exposed on the deep survey plates, and thus permit
a positional tie-in between the two. Early results from this latter survey
are issued on a CD-ROM, called the "UJ1.0" disc. It includes positions for
millions of objects covering about 70 percent of the sky north of +20 Dec.
Because the astrometric work for UJ1.0 was done more carefully than for the
GSC, I have used these positions alone or in combination with other sources.
Finally, for about a dozen bright objects, I have made new measurements
on plates taken at Lowell Observatory by Carl Otto Lampland. These were taken
with a 1-meter f/5.5 Newtonian telescope erected at the observatory in 1909.
Most of the plates I measured were taken between 1914 and 1925. The image
scale of 37"/mm on 4x5-inch plates allowed the central stars of the nebulae to
be seen clearly on short exposures. The fairly wide field also included
enough PPM reference stars to make a tie-in to RA and Dec. I measured the
plates on Lowell's PDS scanning microdensitometer. Typically about ten stars
were measured along with the central star; the accuracy is probably about
+/-0".5 overall, although the internal errors from plate-to-plate were very
small, only 0".1 - 0".3. In any case, these provide precise positions of what
are unambiguously the central stars of the nebulae.
For all the objects, particularly those with GSC or UJ positions, I
examined the Digitised Sky Survey using the SkyView facility at Goddard Space
Flight Centre in Maryland, USA. The DSS represents scans at a resolution of
1".7 per pixel of the original POSS-I red-light plates (northern sky) and the
UK Schmidt blue-light survey plates (in the south). This excellent
tool is available on-line via the World Wide Web at the URL:
http://skview.gsfc.nasa.gov/cgi-bin/v2.2/skyview_advanced
Unlike other on-line implementations of the DSS, SkyView allows catalogue
objects to be marked and a coordinate grid to be overlain on the image. This
permitted me to check the positions against an actual picture of the sky. For
instance, often a GSC object near the location of a planetary nebula applies
not to the centre of the glow or the central star, but instead to a bright
knot at one edge of the nebula, or perhaps a superposed field star. Even this
check wasn't always the last word, since many bright objects are overexposed
on the DSS, and the central star is simply not visible. Many of these problem
cases were solved by using the Lampland plates mentioned above.
Results
The table shows the objects in Right Ascension order, along with the
positions and sources for each. A few objects beyond the NGC/IC list are
added. The positions are given to a precision of 0s.01/0".1 when the
accuracy is likely to be below an arcsecond. In cases where I feel the
results are less accurate, they're given to just 1" precision. The sources
are listed in abbreviated form; the main ones are:
- PPM = positions from the PPM star catalogue
- GSC = Guide Star Catalogue, version 1.1
- UJ = UJ1.0 survey
- BSkiff = new positions from Lampland plates or measurement of POSS-I prints,
as specified
- microm = visual micrometry from old sources in the literature
- SkyView = determined by eye using SkyView
When positions have been averaged, the source list is enclosed in angled
brackets [...]. I also include GSC identifications as available. The GSC is
widely used in commercial software for amateur observers. Having the nebulae
identifications here allows the database to be amended to show their location
explicitly.
Notes on Individual Objects
| Designation | Notes |
|---|
| NGC 40 | The historical micrometric and GSC positions match the UJ position
within errors. |
| NGC 650/51 | New position for the central star measured by Harold Corwin on
the POSS-I prints. |
| NGC 1360 | Micrometric positions and recent determinations are in error. |
| NGC 1535 | The position was measured very accurately via micrometry in 1909 by
Winnecke (Strassburg Obs. Annalen, vol. 3, 189), whose result was confirmed by
later micrometry by Porter and by Silbernagel, whose results average to:
4 14 15.87 -12 44 21.4, i.e. only a few tenths of an arcsecond from my new
position from the Lampland plates. |
| NGC 2440 | The average of micrometric and GSC positions have rms errors of
+/- 0".2 and 0".6 in RA & Dec, respectively. |
| NGC 2818 | Kohoutek (1986 A&A 162, 232) has measured the position of the
planetary's mag. 17 central star on plates taken with the ESO 3.6-m telescope
relative to SAO stars. This should provide sufficient image scale to make
it definitive. The GSC position clearly refers to the centroid of the nebula,
but also obviously matches that of the central star within an arcsecond.
The best published position for the open cluster (also NGC 2818) is in
the ESO/Uppsala Survey of the ESO (B) Atlas, first listed by Holmberg et al.
(1978 A&AS 34, 285)---but later compiled by Lauberts into a single volume:
oc N2818: 9 16 11 -36 37.7, which I confirmed using SkyView. |
| NGC 2899 | The GSC object is the central star. |
| NGC 5189 | All published positions apply to stars or other non-central places.
The GSC object/position is for the hub of the bright bar that looks roughly
like a late-type barred spiral galaxy, which is slightly north of the
geometric centre. The nebula is quite asymmetric however, so this may be
about as good as one can do. |
| NGC 5844 | GSC position is not centered on central star. |
| NGC 6302 | The GSC entry is for the brightest patch east of geometric centre.
The SkyView position given in the table is for the place between the two
hyperbolic/parabolic arcs of this peculiar nebula, whose central star is still
unidentified. |
| NGC 6337 | GSC 7883-0798 is the star on the annulus. |
| NGC 6369 | Mean of my new measurement with one published by Kohoutek and a
third measured by my colleague Bobby Bus on a Palomar Schmidt plate. The
objects GSC 6826-0239 and -0343 are not for the centre. |
| NGC 6445 | GSC 6257-0457 and -0272, which average to: 17 49 14.8 -20 00 40.
The GSC entries apply to bright patches on the N & S sides. |
| NGC 6543 | the object IC 4677 is bright knot in the outer halo to the west,
not a background galaxy (MCG +11-22-17). |
| NGC 6563 | A position published by Kohoutek is close: 18 12 02.5 -33 52 05. |
| NGC 6567 | The GSC entry includes close companion stars; the SkyView position
is more exact. |
| NGC 6644 | = ESO 522-G33(!). |
| NGC 6720 | Amazingly, none of the published positions are even close to the
central star except an old micrometer position by Lorentz. The GSC entry is
fairly good. |
| NGC 6772 | The GSC position with the larger uncertainty is more exact and has
been averaged here with new positions from the POSS-I prints. |
| NGC 7008 | The GSC entry is the central star. |
| NGC 7009 | The mean of micrometry also very close: 21 04 10.82 -11 21 48.0,
+/- 0".3, 0".8. |
| NGC 7094 | The central star was included in a proper-motion study of the
nearby globular cluster M15, which gives the best position. |
| IC 3568 | = UGC 7703(!). |
| IC 4599 | also GSC 7860-1550; one of the two -0074 positions omitted in the
average. |
| IC 4642 | GSC object seems to correspond to the central star. |
| IC 4670 | = AS Sgr. |
| IC 5148-50 | The GSC is slightly off, perhaps corresponding to centre of the
nebula rather than central star. |
Conclusions and Future Work
Although all the objects now have positions determined to at least one
or two arcseconds accuracy, many could benefit from being measured on large-
scale photos or images taken so as to show the central stars without
overexposing the surrounding nebulae. I plan to extend this work to include
additional bright planetaries.
Readers eager to get the UJ survey data are urged to wait: they're
already up to version 1.3 for in-house use, and the final astrometric
reductions are not yet done. So far the detection list for the entire sky
includes 500 million objects! (Only 20 million objects are on UJ1.0.) Since
the project has been paid for by American taxpayers, the data will eventually
be available inexpensively and in the public domain.
This work was started during a summer 1995 stay at the Strasbourg
Observatory's Astronomical Data Centre; I appreciate the hospitality and
encouragement of the staff there. Harold Corwin pointed out several omissions
in the original list, and spurred me to make some final touches as well.
Precise Positions for NGC/IC Planetary Nebulae
Version: 02 October 1996
Name RA (J2000) Dec s GSC Remarks
NGC 40 0 13 00.98 +72 31 19.3 U 4302-1297
NGC 246 0 47 03.32 -11 52 19.3 G 5272-1890
NGC 650/51 1 42 19.7 +51 34 35 - pos from HGCorwin, POSS-I
IC 1747 1 57 35.82 +63 19 19.9 U 4036-0705
IC 289 3 10 19.2 +61 19 01 - pos from PK67
NGC 1360 3 33 14.63 -25 52 18.6 G 6450-1071
IC 351 3 47 32.95 +35 02 48.3 U 2364-0855
IC 2003 3 56 22.01 +33 52 30.4 U 2365-0203
NGC 1501 4 06 59.41 +60 55 14.5 U
NGC 1514 4 09 17.00 +30 46 33.3 + 2358-0056 <PPM+UJ1.0>
NGC 1535 4 14 15.78 -12 44 21.6 L 5318-0563 BSkiff (Lampland, n=7)
IC 418 5 27 28.19 -12 41 50.4 P 5340-0684
NGC 2022 5 42 06.18 +9 05 10.3 G 0714-0101
IC 2149 5 56 23.85 +46 06 17.2 + 3361-1662 <GSC+UJ1.0>
IC 2165 6 21 42.67 -12 59 14.0 G 5371-1137 also = GSC 5371-2086
NGC 2242 6 34 07.42 +44 46 37.5 - pos from SIMBAD
NGC 2346 7 09 22.54 -0 48 24.1 P
NGC 2371/72 7 25 34.71 +29 29 26.3 + 1922-1320 <GSC+UJ1.0>
NGC 2392 7 29 10.77 +20 54 42.9 P 1372-1287 PPM
NGC 2438 7 41 50.59 -14 44 07.1 L 5422-0587 BSkiff (Lampland, n=3)
NGC 2440 7 41 55.37 -18 12 31.4 + 5984-0213 <M+GSC> (0".2/0".6), A0.9: 55.43/30.9
NGC 2452 7 47 26.22 -27 20 07.7 G 6548-0205
NGC 2610 8 33 23.38 -16 08 57.2 G 6011-1176
IC 2448 9 07 06.53 -69 56 31.1 G 9199-1420
NGC 2792 9 12 26.52 -42 25 41.1 G 7686-1242
NGC 2818 9 16 01.55 -36 37 37.5 - 7164-3813 pos: Kohoutek+ 1986 A&A 162,232
NGC 2867 9 21 25.35 -58 18 41.5 G 8596-0189
NGC 2899 9 27 03.01 -56 06 21.7 G 8588-0317
IC 2501 9 38 47.25 -60 05 31.1 G 8941-0543
NGC 3132 10 07 01.77 -40 26 11.7 P 7711-0963
IC 2553 10 09 20.93 -62 36 48.5 G 8947-1602
NGC 3195 10 09 21.07 -80 51 30.8 G 9409-1163
NGC 3211 10 17 50.41 -62 40 13.9 G 8960-1688
NGC 3242 10 24 46.11 -18 38 32.6 P 6065-0671
IC 2621 11 00 20.21 -65 14 57.6 G 8966-1209
NGC 3587 11 14 47.71 +55 01 07.7 L BSkiff (Lampland, n=5)
NGC 3699 11 27 57.2 -59 57 29 S
NGC 3918 11 50 17.84 -57 10 56.5 G 8639-2315
NGC 4071 12 04 15.3 -67 18 35 - Acker/Milne
NGC 4361 12 24 30.79 -18 47 05.1 L BSkiff (Lampland, n=5)
IC 3568 12 33 06.87 +82 33 49.0 P 4633-1523
IC 4191 13 08 47.52 -67 38 37.2 G 9241-0477
NGC 5189 13 33 32.91 -65 58 26.6 G 9003-0669
NGC 5307 13 51 03.27 -51 12 20.8 G 8274-2298
NGC 5315 13 53 57.09 -66 30 50.3 G 9016-1038
IC 972 14 04 25.95 -17 13 41.4 G 6141-0189
IC 4406 14 22 26.0 -44 09 04 S
NGC 5844 15 10 40.7 -64 20 24 S 9029-2732
NGC 5873 15 12 50.73 -38 07 32.9 G 7822-0489
NGC 5882 15 16 49.95 -45 38 58.0 G 8294-0398
NGC 5979 15 47 41.12 -61 13 04.3 G 9023-1592
NGC 6026 16 01 20.95 -34 32 39.2 G 7337-0355
NGC 6058 16 04 26.55 +40 40 59.3 + 3064-0555 <GSC+UJ1.0>
IC 4593 16 11 44.53 +12 04 17.2 P 0953-0381
NGC 6072 16 12 58.21 -36 13 47.6 G 7355-0076
IC 4599 16 19 23.16 -42 15 36.3 G 7680-0074
NGC 6153 16 31 30.60 -40 15 13.2 G 7858-0685
NGC 6210 16 44 29.48 +23 47 59.8 + 2045-0022 <PPM+UJ1.0>
IC 4634 17 01 33.58 -21 49 33.6 G 6231-1753
IC 4637 17 05 10.42 -40 53 07.3 + 7873-0897 <GSC + radio source>
IC 4642 17 11 45.26 -55 24 00.4 G 8731-0931
NGC 6302 17 13 44.1 -37 06 14 S 7373-0818
NGC 6309 17 14 04.33 -12 54 38.5 G 5653-0197
NGC 6326 17 20 46.41 -51 45 15.8 G 8353-1062
NGC 6337 17 22 15.65 -38 29 01.4 L BSkiff (Lampland, n=2)
NGC 6369 17 29 20.43 -23 45 35.1 + <Koh+SJB+BSkiff> (Lampland, n=1)
IC 4663 17 45 28.66 -44 54 17.6 G 7897-0182
NGC 6439 17 48 20.32 -16 27 33.8 G 6249-0626
NGC 6445 17 49 15.1 -20 00 34 L 6257-0457 BSkiff (Lampland, n=1); also = GSC 6257-0272
IC 4670 17 55 07.01 -21 44 40.7 G 6262-0349
NGC 6543 17 58 33.37 +66 37 59.1 + 4212-0508 <PPM+UJ1.0>
IC 4673 18 03 18.42 -27 06 23.5 G 6850-0377
NGC 6537 18 05 13.04 -19 50 34.2 G 6259-2412
NGC 6565 18 11 52.51 -28 10 42.9 G 6855-1998
NGC 6563 18 12 02.6 -33 52 06 S
NGC 6572 18 12 06.37 +6 51 13.0 P 0443-1482
NGC 6567 18 13 45.1 -19 04 34 S 6272-1627
NGC 6578 18 16 16.47 -20 27 02.4 G 6273-1407
IC 4699 18 18 31.98 -45 59 01.7 G 8358-1206
NGC 6620 18 22 54.19 -26 49 18.1 G 6852-0337
NGC 6629 18 25 42.42 -23 12 10.0 G 6857-0814
NGC 6644 18 32 34.71 -25 07 43.7 G 6861-2786
IC 4732 18 33 54.62 -22 38 40.6 G 6858-0905
IC 4776 18 45 50.73 -33 20 34.3 G 7412-1226
NGC 6720 18 53 35.09 +33 01 44.5 L 2642-1202 BSkiff (Lampland, n=4)
IC 1295 18 54 37.05 -8 49 37.0 S
NGC 6742 18 59 19.8 +48 27 57 S
NGC 6741 19 02 37.04 -0 26 57.7 G 5128-0058
NGC 6751 19 05 55.53 -5 59 31.9 G 5140-3497
NGC 6765 19 11 06.5 +30 32 45 S
NGC 6772 19 14 36.36 -2 42 24.4 + 5133-0901 <GSC+B> (POSS-I, n=2)
IC 4846 19 16 28.26 -9 02 36.8 G 5708-0961
IC 1297 19 17 23.41 -39 36 46.7 G 7922-0271
NGC 6778 19 18 24.93 -1 35 46.6 + 5130-2197 <GSC+B> (POSS-I, n=2)
NGC 6781 19 18 28.3 +6 32 23 - pos from Kaler83
NGC 6790 19 22 56.99 +1 30 47.0 G 0465-0361 also = GSC 0465-2099
NGC 6803 19 31 16.49 +10 03 21.7 G 1059-1140
NGC 6804 19 31 35.43 +9 13 30.6 M pos from microm
NGC 6807 19 34 33.5 +5 41 02 - pos from AK90; A0.9: 34.49/02.4
NGC 6818 19 43 57.76 -14 09 11.4 G 5736-2071
NGC 6826 19 44 48.17 +50 31 30.4 + 3565-0205 <PPM+UJ1.0>
NGC 6833 19 49 46.58 +48 57 40.2 U
NGC 6842 19 55 02.3 +29 17 21 - pos from Kaler83
NGC 6853 19 59 36.33 +22 43 16.0 + 2141-0287 <GSC+UJ1.0+BSkiff> (Lampland, n=4)
NGC 6852 20 00 39.21 +1 43 41.2 G 0494-1859
NGC 6884 20 10 23.65 +46 27 39.6 + 3559-1827 <GSC+UJ1.0>
NGC 6879 20 10 26.6 +16 55 23 S
NGC 6881 20 10 52.45 +37 24 41.8 U
NGC 6886 20 12 42.82 +19 59 22.4 + 1626-0429 <GSC+UJ1.0>
NGC 6891 20 15 08.85 +12 42 15.2 G 1081-0805
NGC 6894 20 16 23.97 +30 33 54.9 L BSkiff (Lampland, n=5)
IC 4997 20 20 08.80 +16 43 53.6 G 1631-1785
NGC 6905 20 22 23.02 +20 06 16.4 L 1639-1907 BSkiff (Lampland, n=5)
NGC 7008 21 00 32.81 +54 32 35.5 G 3956-0738
NGC 7009 21 04 10.78 -11 21 48.8 L BSkiff (Lampland, n=5)
NGC 7026 21 06 18.55 +47 51 08.0 U 3592-2931
NGC 7027 21 07 01.69 +42 14 10.1 + 3176-0798 <GSC+AGK2+UJ1.0>
NGC 7048 21 14 15.2 +46 17 19 - pos from PK67
NGC 7076 21 26 24.1 +62 53 28 S
IC 5117 21 32 30.97 +44 35 48.1 U
NGC 7094 21 36 52.97 +12 47 19.5 - 1128-0057 pos from LGDC
NGC 7139 21 46 08.4 +63 47 30 S
IC 5148/50 21 59 35.1 -39 23 08 S 7986-0552
IC 5217 22 23 55.70 +50 58 00.5 + 3619-3823 <GSC+UJ1.0>
NGC 7293 22 29 38.35 -20 50 13.2 L BSkiff (Lampland, n=3); A0.9: 38.38/13.3
NGC 7354 22 40 19.94 +61 17 08.0 + 4265-0889 <GSC+UJ1.0>
IC 1454 22 42 24.5 +80 26 33 S
NGC 7662 23 25 53.93 +42 32 06.1 + 3238-1111 <PPM+UJ1.0>
Position Source Codes (s):
B = BSkiff, from sky survey plates/films/prints
G = GSC
L = BSkiff, from Lampland plates
M = visual micrometry, specified in remarks
P = PPM
S = SkyView
U = UJ1.0
+ = means of sources specified in remarks
- = other from literature specified in remarks
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