Table 2 (click here) displays line intensity measurements taken from Acker
et al. 1993 (hereafter Acker93) and Cahn et
al. 1992 (CKS92) as well as other published parameters collected from
the literature for seven PN in study. Peimbert & Peimbert
1983 defined as type I PN those having 
or N/O 
, and which generally display bipolar structures.
Table 2 (click here) summarizes the measured parameters (PN dimensions, ionized
mass in solar mass, and absolute fluxes).
The 
and [OIII]5007 Å images display a nearly circular and identical
shape (Figs. 1 (click here)). However, this circular symmetry seems to be
distorted
by the interaction with the interstellar medium. The diameter
measured on the image (
) agrees with the value of 174''
reported by Cahn & Kaler (1971) (CK71). [NII]6583 emission was
not detected. At the distance of 1200 pc (CKS92), the observed diameter
corresponds to a linear value of 1.0 pc.
This object was not detected with the filter [OII]3727 (1800 s), and
curiously HeII (600 s) as well, but we strongly suspect the effets of the
bad weather conditions during these observations. The 
images are not
usable for investigating internal absorption. Nevertheless, all the other
images (Figs. 2 (click here)) display outstanding peripheral features, the
nebular dimensions being definitely larger than 35'' quoted. The
observed ( and [NII]) images show a rather complex structure. Even the
bright core actually spreads over 60''. Bubbles and wisps extend over 6
arcminutes. Due to the faint jet-like emissions on both sides of the
bright waist, we can hardly assign to Sh 1-89 any of the available
models (bubble, butterfly, jet?) to explain such a peculiar morphology.
The overall dimensions are 
along the major axis. Sh 1-89
would be one of rare planetary nebulae which have linear diameter larger
than 1 pc, since its main bubble-like structure spreads over 230''
(
).
Figure 1:
a) Top: H
. The planetary nebula shows a nearly circular shape.
At the distance
of 1200 pc, A 39 has a linear diameter of 
. The overall
field of view covers 
arcseconds. North is towards the top
and East at left for all figures.
b) Bottom: The [OIII] emission is much stronger than that in
Figure 2:
a) Top left: The image was obtained with a RCA-CCD. Due to the limited
dimensions of the detector (
i.e. 
)
the extended western filament is not shown here. A substantial contribution
from the nitrogen emission line should be taken into account into the
48 Å wide bandpass.
b) Bottom:
Obtained with a Tektronic 
CCD: the bright centre spreads
over 
. The total field of view covers 
square
arcseconds. Note the "jet-like'' structure on both sides of the main
bubbles marked by filamentary network. The major axis extends up to 
, whereas the main bubble-like nebula spreads over 230'' (2.16 pc).
c) Top right:
Only the bright centre is seen here, surrounded by a rather diffuse emission
Figure 3: a) Top: H.
The overall dimensions spread over 250'' or 1.2 pc. The bright elongated
core is
surrounded by a faint envelope.
b) Bottom: The outer [NII]6583 emission is more
conspicuous than that seen in
Figure 3: c) Top: [OIII]5007.
The outer halo seen in the two previous emission lines is not detected in
[OIII]. d) Bottom: [SII]. A faint emission is detected southwards
Figure 4: a) Top left.
The limited dimensions of the RCA-CCD detector did not allow to display the eastern
region of NGC 650-1.
b) Top right: ([NII]). Same remarks as for . c) Bottom:
The [OIII] image displays the same morphology as in
Figure 5: a) Top: H.
NGC 6302 was observed at the MJUO 1-m telescope. North is towards the right
and east is at the top.
b) Middle: [NII]6583. Same orientation as for .
The knotty westward emission is more extended in [NII].
c) Bottom: Only the bright core is seen in [OIII]
Figure 6: The low and high ionisation potential ions apparently occupy the same
volume. a) Top left: H. b) Middle left: [NII]
emission shows a more conspicuous filamentary
structure. c) Top right: The [OII] emission region is similar
to the [OIII] ones. d) Middle right: 
ions are seen in the same region as for
[OII] and [SII]. The inner ring in the left side is the ghost image of the
bright centre. e) Bottom: The [SII] emission is fairly weak in comparison with the
other ionic species
Figure 7: a) Top: H
The overall dimensions of the Ring nebula are 
square
arcseconds. The image shows an outer circular "ring'' not seen in
[NII]. b) Bottom:
The outer [NII] emission is more diffuse than in
The , [NII]6583 Å, [OIII]5007 Å (Figs. 3 (click here)), , and 39 Å-wide
[SII]6737 Å images show a rather elongated and bright central nebula
(
), whereas an outer faint emission spreads over a
larger area 
than previously quoted (100'', CK71).
The extensions reported in this study are even much larger than that
reported by Jewitt et al. 1986 who considered Sh 2-71 as an
excellent example of PN with halo even though these authors observed only
the bright elongated core. The nitrogen emission is more prominent along the
radial structures outwards.
The , [NII]6583 Å images display a bright elongated nebula (
), with faint emission spreading over a more extended area
at the distance of 997 pc
- (CKS92, 
), much larger than quoted so far.
The [OIII]5007 image is smaller, limited to the bright (
) elliptical ring, without the outer emission detected in the and
[NII] images.
This planetary nebula (Figs. 4 (click here)) was imaged with the
RCA-CCD. The images in , [NII]6583 Å and [OIII]5007 Å display an
apparent morphology which shows strong similarities with Sh 1-89. The
overall extension is certainly larger than the dimensions revealed by the
CCD in use (
).
This young bipolar PN is characterized by a high interstellar absorption
since the 
) ratio implies E(B-V) = 0.96.
The excitation is very high (class 10+); the helium abundance He/H is
twice (0.182) the average value for known PN. Furthermore, nitrogen
(
) has 10 times the average abundance
(CKS92).
Our monochromatic images (Figs. 5 (click here)) in , , [NII]6583 Å and [OIII]5007 Å obviously reveal a butterfly
shape with an "explosive'' structure around a bright compact central core
separated into two areas by an absorbing lane.
The and [OIII]5007 images display similar structures,
whereas the [NII]6583 image shows a more knotty morphology. The present
observations have detected even larger dimensions (> 250'') than that
reported by Ashley (1990) (Proc. ASA, 8, 360). The narrowband [NII]6583 filter
helps to outline conspicuous filaments more extended (NW) than in .
This is one of the most outstanding PN among the so-called multiple-shell
planetary nebulae, displaying a large faint envelope (
in diameter).
It is obviously of interest to investigate the morphological nature of the
surrounding giant halo. We have observed (Figs. 6 (click here)) this PN through the
, [NII]6583 Å, [OIII]5007 Å, [OII]3727 Å, and 39 Å-wide [SII]
filters. All images show conspicuous ejecta far from the main nebula. The
global filamentary structure appears up to 270'' from the bright core, in
agreement with Middlemass et al. (1989) who considered them as
low surface brightness areas and probable density enhancements,
illustrating the point that ionizing photons are escaping from the system.
The overall diameter in the observed images is 340'' including the faint
outermost structures. This dimension corresponds to 
at
the distance given by CKS92 (982 pc). This overall extension raises the
question of the age of the nebula. The present estimated value of 3000
years probably involves solely the bright central nebula of 
wide. The actual dimensions of NGC 6543, along with the expansion velocity
in the outer region, 
, would infer a much larger age
(
). Presumably, we see previous ejecta by the AGB
progenitor.
The Ring Nebula is certainly one of the most extensively investigated
planetary nebula since its discovery (Duncan 1937). Moreno &
Lopez 1987 showed that the Ring Nebula has dimensions definitely
larger than the optical 76'' of the bright elliptical core (Moreno
& Lopez 1987; Bryce et al. 1994). Our monochromatic
observations (Figs. 7 (click here)) through narrow (, [NII]6583, [OIII]5007)
and moderate (, 
, [SII]
,
) bandwidth filters confirm these outermost
features surrounding the filamentary network.