2
Cosmic Challenge: IC 418
Discuss this article in our forums
|
|
Cosmic
Challenge:
|
January 2023 |
Phil Harrington |
This month's suggested aperture range:6- to 9.25-inch (15- to 24-cm) telescopes |
|
|
|
Target |
Type |
RA |
DEC |
Constellation |
Magnitude |
Size |
|
IC 418 |
Planetary |
05h 27.5m |
-12° 41.8' |
Lepus |
9.6 |
12" |
If you have ever glanced at a compendium of images taken with the Hubble Space Telescope, then chances are you have seen this next challenge. You may not know it by its catalog number, IC 418, but instead may recognize it by its nickname, the Spirograph Nebula. That nickname came about because the Hubble images show an amazingly complex cloud of entangled filaments that create a strange, oval cloud that looks like it could have been drawn using a child's Spirograph toy. Remember those? You would trace intertwining arcs by rolling a color pen in a circle along the inside or outside of another circle.
|
|
|
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
|
Despite its disk shining at magnitude 9.6, IC 418 remains an underappreciated target among amateur astronomers. Why, I don't know. It might be that there is just so much to look at in the winter sky that nobody pays much attention to a planetary nebula that even the Herschels missed during their sky surveys. That could lead some to think that any planetary listed in the Index Catalog is probably so difficult to see that they don't even try. Too bad because they are missing a nice catch.
To find IC 418, drop 4° southward from Rigel [Beta (β) Orionis] to a keystone of four 4th- and 5th-magnitude stars -- Iota (ι), Kappa (κ), Lambda (λ), and Nu (ν) Leporis. By tracing a line from Iota southeastward through Nu, and continuing that line an equal distance further beyond, you'll come right to the field of IC 418. Look for a close-set pair of 12th-magnitude stars lies just 10' to its west-northwest.
Once you have the nebula in view, switch to as high a magnification that seeing conditions will allow for the best view. My notes jotted down several years ago at the eyepiece of my 8-inch reflector at 203x evoke memories of a "small, bright disk, perhaps a greenish-gray, surrounding an obvious central star. Although clearly not a ring, averted vision suggests a darker central area adjacent to the central star, just to its north and south." The sketch below was also made at the time.
|
|
|
Above: IC 418 through the author's 8-inch (20.4-cm) f/7 reflector.
Below: Hubble Space Telescope image that led to the "Spirograph Nebula" nickname. Purists still prefer to call it the "Raspberry Nebula." |
|
|
What color is IC 418? In its most famous Hubble rendition, IC 418 shows off a burnt orange edge fading into a purplish disk. A blue inner disk surrounds the white hot progenitor star buried within. That color, however, is false, induced to accent subtle contrasts in the Spirograph-like structure.
How about you? What color do you see when you look at IC 418? Observers seem to disagree. Some, like me, see a gray disk with just a hint of a greenish tinge. Others recall a pinkish or reddish tint. The issue appears to boil down to aperture. The larger the instrument, the more distinct IC 418's ruddy hue. Magnification also plays a role. While higher magnifications are needed to see the planetary's disk, they tend to dilute any coloring. To see the reddish or pinkish effect that has led to IC 418's original nickname, the Raspberry Planetary, stick to magnifications below about 175x.
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'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. |
|
- Mike Allen, okiestarman56, John O'Hara and 3 others like this
29 Comments
I gave it a shot last night from a site called the Antennas Site, often used by the Saguaro Astronomy Club of Phoenix. This site is about 100 miles from Phoenix and is reasonably dark. SQM readings ranged from 21.67 to 21.76 throughout the night. In my 100 mm f/9 ED refractor at 45x, it was seen as a bright "star", but I easilly picked it out by blinking with an OIII filter. At 200x, a definite grayish disk with a brighter middle. I noted no bluish or greenish color even at low power, which is so typical of planetaries. Howver, I do have slight catarcts.
I love my little ED refractor, but I confess wish for access to one of my larger scopes in storage back east.
John
I have seen the orange ring once clearly and several times poorly. On 11/30/2016 at a dark sky site in central TX I spied the ring for the first time with a 25" F5 Obsession. This is what I wrote in my log.
A very interesting object. Takes magnification well. With the 5.5 mm eyepiece at 664x there is a prominent central star. Darker patches around central star. O-III enhances the contrast of the edges only. Hint of sharp lines at outermost edge of the nebula. Distinct blue green without the filter. 9-10" across. Bright and circular, well defined. O-III seems to reduce the level of observable detail except for sharpening the edges. With the 183x (20 mm) eyepiece I saw a prominent greenish central region surrounded by a reddish-orange ring. I almost did not want to believe I was seeing this. Almost as if someone drew this with a very fine pen. Two structures: the fine reddish-orange circular feature and a greenish central region with a star. Very nice.
3 years later at the same observing site I recorded this:
Very bright and very small: 10" across. Possibly a slight n-s elongation. Prominent central star easily seen. Green center visible in all 4 eyepieces: 183x (20 mm), 261x (14 mm), 406x (9 mm), and 664x (5.5 mm). However, the edge of the nebula is a fine orange line best seen at 261x and 183x (20 mm). It gets washed out at higher power. Only visible during periods of very good seeing. Poor seeing results in a muddy appearing edge. O-III filter does not improve visibility much especially at high magnification. Edge definition is improved at lower magnification. Interior is pretty uniformly bright: no noticeable detail. I thought the best views were at 261x and 183x (20 mm).
Dave Wickholm
I should also say I did not have any prior knowledge of the orange color. I was quite surprised. Campbell's Hydrogen Star had a similar appearance. On Nov 17, 2017 I wrote this:
Also known as PK064+5.1, HD184738, or Campbell's Hydrogen star. Extremely small, bright, and circular. 6" across. Has a central star but no annularity. Nebula fades rather quickly to a soft edge that is moderately poorly defined at high magnification. Best view was at 664x (5.5 mm) with an H beta or UltraBlock filters. The O-III filter has no effect. Without a filter the central star is visible within a fairly uniform 3" disk that then fades quickly. At 183x (20 mm eyepiece) and 261x (14 mm) the nebula is nearly stellar and shows a distinct orange color. Sometimes I thought I saw a very fine orange ring around the star None of the other stars showed this - it is unique to this object.
On July 17, 2020 I wrote this:
Campbell's Hydrogen star. Located with star chart. Bright and nearly stellar, but oddly hard to focus. At 183x there is a red-orange ring around a "fat" star or extended region. Ring not visible at higher magnification. No response to O-III filter. At 664x (5.5 mm) the center appears to be at least 2 stars or knots separated by ~1.5 seconds and maybe 3 surrounded by a faint, poorly defined haze. Haze could be as much as 10" across.
Both observations were with the 25" F5 at our club's dark sky site. These are the only nebulous objects I have viewed that showed reddish-orange colors. Very odd ducks.
Dave
A few weeks ago we (me and some visitors) looked at IC418 in my 25". Seeing and transparency were quite good. We could all clearly see the inner blue section along with the outer halo which was clearly reddish.
At the same time we also observed the PN in my 6" finder. At 100x the nebula was easily visible but the color was very muted compared to the 25".