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Cosmic Challenge: Abell 33

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Cosmic Challenge:
Abell 33


March 2023

Phil Harrington

This month's suggested aperture range:

10-inch (25-cm) to 14-inch (36-cm) telescopes












Abell 33


09h 39.2m

-02° 48.5'





Several of the challenges I have profiled over the years have involved hunting down tiny planetary nebulae. Many planetaries appear very small as seen from Earth, which can make them difficult to tell apart from surrounding stars. This also works in our favor, however, since their small size focuses all of the available light into small discs with high surface brightnesses. Their existence is also accented nicely by using narrowband and oxygen-III filters, which help suppress light pollution. That's why planetary nebulae are far better targets for urban observers than some other types of deep-sky objects.


Above: Evening star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington


Above: Finder chart for this month's Cosmic Challenge.


Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window



There are exceptions to every rule, however, and this month's challenge is one. Abell 33 was first spotted by George O. Abell in the 1950s as he and colleagues A.G. Wilson, Robert G. Harrington, and Rudolph Minkowski pored over the then-new Palomar Observatory Sky Survey photographic plates. Abell announced their collaborative discoveries as part of a paper he published in the April 1966 (vol. 144, p.259) issue of Astrophysical Journal entitled Properties of Some Old Planetary Nebulae. In the article's introduction, Abell noted that "these objects are large and faint and are probably at an advanced stage in their evolution as planetary nebulae; all 86 objectsare described as 'planetary nebulae,' with the cognizance of the fact that one or two of them may be improperly identified." Ultimately, four of his list's entries proved to be other types of objects.


Many of the Abell planetary nebulae stand today as some of the ultimate tests for deep-sky observers using the largest backyard scopes. Abell 33, nicknamed the Diamond Ring Planetary Nebula, is one of the easier members of this elite list. It can be detected through a 10-inch (25-cm) telescope from under suburban skies -- with a little help, that is.


Abell 33 is a perfectly spherical blue bubble expanding away from a 15th-magnitude central star. It lies an estimated 2,700 light years away. The central star is evolving into a white dwarf, just as our Sun will eventually. What makes this one unusual is that it appears to be a double star. Whether this is a true binary system or just another chance line-of-sght alignment has yet to be determined.


To spot Abell 33, first zero in on its location within its home constellation of Hydra, the Water Snake. That's actually quite easy to do. From the serpent's pentagonal head, slither southeastward along its winding body, past Theta (θ) Hydrae to Iota (ι) Hydrae. Abell 33 is 1.6° due south of Iota, right next to 7th-magnitude HD 83535 (aka SAO 137026).


That star, the diamond in the planetary's nickname, is beautiful in images, like the one below from Yuexiao Shen (CN member syxbach). But that same star is a big problem for visual observers. Abell 33's perfectly round gaseous shell, which spans almost 5 arcminutes in diameter, just overlaps that star. As a result, the stars glare easily overwhelms the nebula's faint glow, especially if a telescope's or eyepiece's optics are dirty or contaminated.

Above: Sketch of Abell 33 through the author's 18-inch (46-cm) reflector.


Below: An outstanding view of Abell 33 as captured by Yuexiao Shen (CN member syxbach). Click here for a full size image.

Visit his Flickr page to see more of his outstanding images. Used with permission.


Even with clean optics, seeing Abell 33 can be tricky. To improve the odds, many observers move the "diamond" off the edge of the eyepiece field. Problem is offsetting the glare will also bring the target to just inside the field edge where, depending on the eyepiece, distortion may blur it out of existence.


Instead, use an eyepiece fitted with an occulting bar across the center to block the detractor. While no company sells an occulting eyepiece to my knowledge, one is easy enough to make at home. To work correctly, the edge of the occulting bar must appear sharp in view, which means that it must lie at the eyepiece's focal plane. This usually coincides with the field stop near the field lens.


Begin by cutting a thin strip of opaque black photographic tape, no more than a third of the field lens's width and just long enough to span the inside diameter of the eyepiece barrel. (Recommendation: Use a low-power eyepiece with a relatively large field lens, and then insert it into a Barlow lens to get the high magnification needed for the task.) Holding the tape gently with tweezers, carefully lay it into place.


Then, when you aim toward the planetary, hide the star behind the occulting tape before looking for the nebula's faint glow. As a reference, a faint double star is superimposed on the nebula's northwestern edge, while several even fainter points litter the disk itself. With an O-III filter and occulting tape in place, I have been able to spot Abell 33 with direct vision through a 12-inch (30.5-cm) telescope. Remove the tape and filter, and the planetary quickly disappears, even with averted vision.


Spotting the central star, which only rates 15th magnitude, is a difficult chore through 10-inch (25-cm) to 14-inch (36-cm) scopes. Try this trick. Once you spot the nebula with a filter in place, remove the filter. The nebula will disappear, but the unfiltered view is your bet at seeing the central star.


Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's challenge.  Contact me through my website or post to this month's discussion forum.


Remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2023 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • Dave Mitsky, okiestarman56, John O'Hara and 2 others like this


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