Object of the Week February 7th 2016: Abell 33, The "Engagement Ring" Planetary Nebula

Object of the Week February 7th 2016: Abell 33, The "Engagement Ring" Planetary Nebula

Abell 33
PK 238 +34.1

Hydra

RA 09h 39m 09.2s
Dec -02 48 35

Planetary Nebula

Magnitude 13.9

Diameter 4.5 arc-minutes

The night sky is full of jewels. Open clusters such as the Pleiades, the Double Cluster in Perseus, or NGC 2362 in Canis Major, have rightfully been named “jewels of the sky”, and there is a “Jewel Box” cluster (NGC 4755) in the deep southern skies. There is even an “engagement ring asterism with Polaris as the “diamaond”. Yet, to my eyes, none of all these compares to the beautiful “engagement ring” image conveyed by Abell 33, a big planetary nebula in the constellation of Hydra. Especially in photographs, it is one of the most striking images in the night skies. At the same time, it is attractive enough for visual observers as well.

This large planetary (almost 4.5 arc-minutes across) is located 2700 light years away near the celestial equator in the constellation Hydra, thus it is accessible from both northern and southern localities (except the far north and south). Abell 33, also known as PK 238+34.1 has a magnitude 13.4, which makes it a very low surface brightness object, with a 15.5 magnitude central star. Yet, as we shall soon see, it is not as difficult an object as it sounds. The main problem with observing this planetary is the 7th magnitude star HD 83535, which is located right at the edge of the planetary (in our line of sight of course!). At the same time, this very star assumes the role of the sparkling “diamond” in the “engagement ring” and makes this object such an attractive quarry for both the observer and the photographer.

My own interest in this planetary began after reading an article on “The best Abell Planetaries” by Eric Honeycutt in the May 2002 issue of Sky and Telescope. The photo on page 100 of the article certainly does not make justice to the appearance of Abell 33 in more recent photos that I have seen, but at the same time the rather gloomy appearance of the object somehow captivated me. I remember attempting it with the 10” equatorially mounted reflector I owned at the time but I failed to see it.

Attachment 1987
Abell 33 from the Deep Sky Survey

My next attempt on this planetary occurred in the spring of 2009, this time using a 12” dobsonian from a mountain dark location. I had prepared beforehand a locator chart made from the Deep Sky Survey images, showing that the planetary was located right next to the middle star in a line of three 7th magnitude stars. To my surprise, using an OIII filter on a 32mm Plossl eyepiece I was able to detect an ill-defined circle of faint light touching the aforementioned 7th magnitude star. I could hold this faint circle of light using both averted and direct vision. Yet, with direct vision it remained an extremely faint ring of light, while with averted vision I could detect that the inside part of the circle was (just barely) brighter than the outer space around the nebula (somewhat like a huge diffused version of the Ring nebula). Needless to say, the nebula was invisible without the filter.

The not-so-exciting appearance of Abell 33 in my old 12” pushed it out of my observing projects for a few years, until I happened to stumble across some of the fine recent photos of the planetary, such as this photo by the ESO astronomers (http://scitechdaily.com/esos-large-t...bula-abell-33/). By the time I discovered these photos I had upgraded to my current scope, and my 18” Obsession definitely helped me to finally enjoy the subtle beauty of this planetary.

Attachment 1988

Under dark skies, using an OIII filter on my 31mm Nagler, I was stunned to see a perfectly round sphere of subtle light touching the bright star. But the best view was obtained using my 13mm Ethos. The edges of the nebula became much better defined, and there seemed to be slight variations in the light visible within the sphere, the eastern half slightly brighter than the western. Also, the eastern edge of the nebula appeared distinctly brighter. This appearance did indeed remind me of the “engagement ring” appearance I had seen in recent photos of the object. By placing the 7th magnitude star just outside the field I could more certainly detect the subtle variations in the brightness inside the sphere (albeit with the loss of the “engagement ring” effect). I could detect a very faint star near the center of the nebula but I am not sure whether it was the central star.

Attachment 1989

Removing the OIII filter almost eliminated the subtle appearance of the nebula. It was barely detectable as a faint patch using a 31mm Nagler. By increasing the magnification with the 13mm Ethos I could no longer detect the subtle patch of light, but I could see (using averted vision) the eastern edge of the nebula. Also, by removing the filter I could detect a few more very faint stars in the area of the sphere that were not seen using the OIII filter, one of which was presumably the central star.

Searching through the internet I found several reports stating that PK 238 +34.1 was seen through apertures as small as 8 inches. So, I decided last week to attempt this planetary through my 6” achromatic refractor. Using it at low power with and without filters I could see no hint of the nebula. Increasing the magnification to 67X using the Ethos 13, and by adding my trusty OIII filter, I managed to detect a barely visible patch of diffuse faint light at the correct position. It was very ghostly, and I could not even define its shape, it was just a patch of light. With averted vision it became slightly more defined, sort of round, but still it was a borderline observation.

Attachment 1990

Using my 120mm achromatic refractor nothing was seen, with or without filter. So under my good (but not perfect) skies, this planetary requires at least six inches of aperture just to be detected, and I would say at least 10 inches to really squeeze some detail. But, the “engagement ring” appearance needs several inches. It was not seen as such with 12 inches, but it was evident with 18, so my guess would be that at least 15 inches are required. On the other hand, under extremely dark and clear skies, Abell 33 might be visible in even four inches, and maybe the “engagement ring” might show in lesser apertures.

So, next time you are out under a nice dark winter sky, turn your scope to Abell 33, and you might be pleasantly surprised. And, as always,

GIVE IT A GO AND LET US KNOW!

This is a nice planetary, good choice! Here's my one observation from 2010 with the 28 inch:

"Easier to see than Abell 31 (which has) a more definite perimeter. (Abell 33) is brightest along the edge overlapped by the brightest filed star. The center of the PN is evenly illuminated. Best at 136x, SQM 21.28, no filters."

Attachment 1991 Attachment 1992

Evidently I saw the central star but didn't place it the center of the PN in my sketch. Perhaps that's because I didn't see the outer perimeter, and threw off the proportion a little. It certainly has me interested to have another go at it now.

For sure my favorite Abell PN. Nice choose. The double beside the CS is a very interesting detail. I have to revisit it with the 27".

8", 31x-80x, [OIII], NELM 6m5+
with 31x (AP 6,5mm) faint glow directly NE of a 7mag star; looks like a ghost of the star, but the offset position and the missing of other stars makes it clear that the glow is the PN; still visible with averted vision with higher power (80x = AP 2,5mm); exactly round disc with 4' diameter and well defined edges; at the NW border a faint 12mag stars shines through the filter; another 12mag star is 2' N; structureless

16", 129x, [OIII], NELM 6m0+
also noticeable without filter; with [OIII] and 129x a large, round glow; can hold it with direct vision; no structure within the glow; sharply defined
Attachment 1993

I haven't looked for Abell 33 with a smallish refractor, but here a couple of observations with 5" and 4" refractors that I've copied below.

Sue French: I was surprised I could see the nebula immediately without checking its exact position (with a 5.1" refractor). *A narrowband filter offers some improvement in contrast, while an OIII filter gives a better boost. *At 63x, Abell 33 is about 4.5' across, and a faint star adorns its NNW rim. With the help of an OIII filter, Abell 33 appears round an subtly annular through the 10" scope at 118x and the 15" at 133x. *The latter also reveals a companion to the faint star on the NNW rim. *Under dark skies, folks have nabbed Abell 33 in telescopes as small as 3", but I've never heard of the central star being seen in anything less than an 18" scope.


John Tatarchuk: Abell 33 (first observation through 4" refractor) - this object in Hydra (mag 12.6v, 275”x260” across) a slight step up in difficulty from Abell 31, but still definitely in the top 10 easiest Abells. It’s main claim to difficulty is the magnitude 7 star that is superimposed on the SW edge. An easy catch in my 18” when I first saw it several years ago, but would the refractor be able to handle it? On 12/21, I gave it a try. Once I placed the refractor on the 7th magnitude star, I popped in the 19mm panoptic (37X) and OIII filter to see what I could see. The nebula was easy, and I was able to hold it in adverted vision. It looked as if some of the light from the magnitude 7 star had spilled out and formed a circular puddle to the northeast.

I'll also add my last observation with my 18-inch

Using 73x and an OIII filter, Abell 33 was immediately visible with direct vision as a very large, well-defined 4' disc attached to a mag 7.2 star at the SW edge. *The rim is slightly brighter, noticeably along a 120° arc on western side to the north of the bright star, giving a mild annular or bubble appearance. Removing the filter and upping the magnification to 280x, the planetary was still faintly visible and a 12" pair of stars was embedded at the NNW edge of the rim. At this magnification, the mag 15.5 central star was faintly visible (collinear with the double star).

I had a look at this planetary nebula tonight with my 20" from SQM 20.8 skies. The nebula didn't appear round to me but oval instead. The eastern rim was brightest up to where it connects with the star. It vaguely reminded me of Barbapapa...