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NGC 3918
ESO170-PN013, PK294+04.01
RA 11:50:16
Dec -57° 04.5'
Planetary
nebula |
This planetary nebula was discovered by Sir
John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope.
He recorded it as "A perfect planetary disc 6 arcsec diameter; quite sharp,
with not the least haziness. It is of a most decided independent blue colour
when in the field by itself, and with no lamp light and no bright star. About
10' north of it is an orange coloured star 8th mag. When this is brought into
view the blue colour of the planetary becomes intense. Shown to my attendant,
John Stone, who, on being asked what colour, said at once 'blue.' " His
next observation was recorded as "planetary nebula - in light to a star 7th
mag; diam. in RA = 1.3 seconds; viewed with all the powers; very beautiful;
decidedly blue." On the next occasion he described it as a "planetary
nebula, colour a beautiful rich blue, between prussian blue and verditter green.
The light is fully = that of a star 8th mag diameter; 2 seconds in RA; 12 arcsec
by careful estimation. When kept steadily at rest its outline is sharp and
clean, and perhaps a very little elliptic. A feeble lamp light gives it a deep
indigo contrasted colour. Ditto if a red star N.p., about 10' distant, be
brought into the field with it. My attendant saw it and declared proprio motu
that the light has quite a green cast in it. About 90 stars are in the field,
none above 11..12 mag, and only one of that magnitude (meaning when the neb. is
central). It has none but stars 16th mag near it." Then, on 26 February
1835, after completing the sweep for the night, he viewed it with Maclear: "Blue
colour very conspicuous. Has one small companion certain (pos by diagram about
290 ) dist = full diameter, another nearly certain (at about half the distance
by diagram, and pos about 135 ) The field is full of stars. Total light of
planetary = star of 6th magnitude or 6' m. A very little oval in position about
160 or 165." Some while later, he recorded it as "planetary, diameter
8..10 arcsec, perfectly round and well defined, and of a fine blue colour."
His final observation was recorded as "Planetary nebula. Perfectly round,
very planetary; colour fine blue; a very little ill defined at the edges; has no
'satellite stars'; very like Uranus, only about half as large again and blue.
Diameter in RA = 1.5 seconds."
Hartung writes "it is so bright that even
a 2-inch will show the small disk ... this is round, well-defined, vivid pale
blue about 10 arcsec across and lies in a fine starry field."
Houston, observing with a 6-inch RTF, writes: "NGC
3918 was recognizable as an 8.5 mag planetary nebula, once its tiny 13 arcsecond
disk was ferreted out."
Gerd Bahr-Vollrath (Noosa Heads, Queensland,
Australia) observing with an 8-inch f/12 SCT, writes in the The Webb Society
Nebulae and Clusters Section Report No. 10, July 1992: "Quite bright and
large irregular disc with fainter envelope; appears 'spikey'."
ASV Journal Vol 24 No 3 June 1971: "visible
in 1-inch 8x finder."
6-inch f/8.6 Newtonian: A
colourless, perfectly round and smooth disk-shaped planetary. In the field with
an 8th mag star north-northwest. The nebula, just fainter than 8th mag, is easy
to identify; simply rack the eyepiece out of focus, and then start focusing it.
As focus is approached, the disk-shaped images of both objects shrink in size.
But at an obvious moment, the star continues to shrink, but the planetary
remains disk-shaped. The disk is clear at 72x, and possibly even at 40x.
(exurban skies) [AS]
10-inch f/5 Newtonian: This
is a very interesting object; a 10-inch f/5 at 120x shows it as a small, neat
planetary nebula, very bright with a smooth disk and sharply terminated edges.
It reminded me of a luminous coin. There is a reddish 8th mag star closeby to
its north, and when viewing both in the same field, it is not impossible to
imagine the nebula as having a bluish cast. Definitely a good object to study at
higher powers. (suburban skies) [AS] |
