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Cosmic Challenge: NGC 6803 and NGC 6804


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Cosmic Challenge:
NGC 6803 and
NGC 6804

August 2016

Phil Harrington

This month's suggested aperture range

Giant Binoculars (>=70mm)

3- to 5-inch (7-13cm) telescopes

 

Here's a two'fer for you, a pair of challenges found within 1° of each other in the constellation Aquila the Eagle. Both of these planetary nebulae present interesting tests for smaller apertures, each in its own way.

 

Above: Summer star map from Star Watch by Phil Harrington.

 

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

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

 

Despite their closeness to one another in our sky, NGC 6803 and NGC 6804 have no physical relationship and couldn't be farther apart in terms of appearance.

 

Let's begin with NGC 6803, found a little less than 4° west of Tarazed (Gamma Aquilae). The American astronomer Edward Pickering (1846-1919) was first to lay eyes on this tiny target on September 17, 1882, using the 15-inch refractor at Harvard College Observatory. Pickering is most famous for his work determining characteristics of stars by studying their spectra. Truth be told, much of Pickering's acclaim was due largely to the computational work performed by more than a dozen female astronomers who assisted him. Known in certain politically incorrect circles as "Pickering's Harem," his team of assistants included Annie Jump Cannon, Henrietta Swan Leavitt, Antonia Maury, and even his former maid, Williamina Fleming. Each went on to make many important contributions to the science in her own right.

 

You'll find NGC 6803 at the end of a meandering line of eight 8th-magnitude stars, just 10' south of the 9th-magnitude double star HD 183850. The problem is telling NGC 6803 apart from the rich surrounding star field, since its tiny disk measures a scant 6" across. Even viewing at 200x through my 4-inch refractor on a night of exceptionally steady seeing, it is still tough to tell which is the planetary and which are just faint surrounding stars without a little help.

 

Time to call in a narrowband nebula filter, such as a Lumicon UHC or Orion UltraBlock. An Oxygen-III (O-III) filter also works well, although it tends to dim the field far more than a UHC. For minuscule planetaries like NGC 6803, however, don't simply screw the filter into your eyepiece barrel. Instead, center your telescope on the field suspected of containing NGC 6803, hold your filter in between the eyepiece and your eye, and take a careful look. By alternately moving the filter in and out of the optical train, you will see the planetary "blink." Stars, which are broadband emission objects, will dim more noticeably than the planetary, which focuses its energy emissions only in a narrow portion of the visible spectrum. Do this back and forth rapidly, checking each stellar point as you go, and the planetary will have no choice but to reveal itself. After you capture NGC 6803, screw the nebula filter into your eyepiece's barrel to see if you can make out its disk. 

 

Leave the filter in place as we move on to part 2 of this challenge. From NGC 6803, slide 20' southeast to a 7th-magnitude star, and then another 30' due south to the binary star BU 976AB, a close-set pair of 6th-magnitude suns that are a nice resolution test for 3-inch telescopes. NGC 6804 is just 11' to their southwest.

 

While NGC 6803 is challenging for its tininess, NGC 6804 measures 35" in diameter. That's easily large enough to be distinguishable through my 4-inch refractor at 100x. My notes recall that I saw it "first with averted vision, then directly; a faint, homogenous disk of grayish light floating within a distinctive kite-shaped asterism." Larger apertures add several faint stars immediately around the planetary, producing it a ghostly, faux 3-dimensional effect that is quite striking. These same instruments may also show the nebula's 14th-magnitude central star.

 

It was probably this appearance, with the nebula framed by those unrelated stars, that led William Herschel to misclassify NGC 6804 as an open cluster when he discovered it in August 1791. Only after it was scrutinized more closely through the 100-inch reflector at Mount Wilson Observatory by Francis Pease in 1917 was its true nature uncovered.

 

Above: Sketch of NGC 6804 through my 4-inch 4/9.8 refractor at 100×.

 

These are but two of the 8 planetary nebulae in Aquila that are listed in the New General Catalog.  The others are NGCs 6741, 6751, 6772, 6781, 6790, and 6852.  Why not try your hand at all and see how many you can find through a small scope?  Or a large scope, for that matter!

 

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 test.  Contact me through my web site or post to this e-column's discussion forum.

 

Remember, 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 2016 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.

 




4 Comments

Between work, weather and family schedules I have had very little time at the eyepiece lately. I finally got out last night, 8-8-16, to take a stab at last months, M-13, challenge.

Armed with my Z-10 and your detailed descriptions of the area I went after it from my orange zone backyard. While I was only able to detect NGC 6207, and I mean "detect" (just the faintest hint of a core with AV) I really enjoyed searching the area. Thanks for providing a little "treasure hunt" motivation and adding a fun challenge to a evening of observing. 

Hoping for some clear skies and clear schedule to go after this months challenge.

    • PhilH likes this
Photo
osbourne one-nil
Aug 17 2016 08:54 AM

Well that's convinced me to buy the book - I love this sort of stuff where someone's done the hard work for me!

    • PhilH likes this

Well that's convinced me to buy the book - I love this sort of stuff where someone's done the hard work for me!

Hope it brings you better weather!  :)

Photo
osbourne one-nil
Aug 18 2016 08:17 AM

Working so far...well, this evening anyway. 

 

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