260709 - Ou4 "Giant Squid" nebula
ASTRO:
type=Emission nebula
mag=(unk)
const=Cepheus
dist=2300 ly
size=50 ly
IMAGE:
location=Emerald Bay community, Lake Palestine, TX, USA
BrtlCls=4
moon=31% WnCr
exposure=CMOS OSC; 51x300s (4.3h)
palette=HOO
EQUIPMENT:
optics=ES102CF APO / Starizona "L" 0.65x rdcr, FL(eff.)=464mm, f/4.55
camera=ZWO ASI2600MC-Pro
filter=Optolong L-eXtreme Duo NB
mount=Celestron AVX
guiding=Orion 60x240mm scope, ZWO ASi224MC camera
SOFTWARE:
acquisition=NINA, PHD2, Stellarium
processing=PixInsight (RCAstro, SetiAstro), Photoshop (APF-R, Lumenzia), LrC
click image to enlarge
Ou4, colloquially known as the “Giant Squid” nebula, was only recently discovered in 2011 by French amateur astronomer Nicolas Outters. It is an incredibly faint, giant bipolar outflow structure located roughly 2,300 light-years away in the constellation Cepheus. At that distance, this ghostly, teal-colored marvel lives within our home galaxy, the Milky Way; and, it spans nearly 50 light-years from end-to-end. It sits directly inside a massive, contrasting red hydrogen cloud known as Sh2-129, the “Flying Bat” nebula.
The cosmos has managed to hide this giant structure right inside our galactic backyard until the 21st century. Professional telescopes missed it for decades because Ou4 emits light at an exceptionally low surface brightness. It only reveals its distinct, elongated silhouette when imaged through specialized, narrow band filters for hours of exposure time.
The Science Behind the Glow:
• The striking blue hue of the Squid isn't just a beautiful artistic artifact - rather, it is a distinct chemical signature: ...doubly Ionized Oxygen (O²⁺, also known as OIII). The gas glows because oxygen atoms have had 2 electrons violently stripped away, leaving those atomic particles in an unnatural state of highly energized ionization. As the atoms seek to return to their more natural, "relaxed" state, they eventually encounter and recombine with 2 free electrons to fully “relax”. As they do, they release energy in the form of light photons. These photons are released at 2 very specific wavelengths belonging to elemental oxygen – a primary emission at 500.7nm and a more faint secondary emission at 495.9nm. We perceive these wavelengths with our eyes as a faint cyan, or blue-green color.
• The stellar engine driving this high-energy ionization process is HR 8119, a ferocious, powerful triple-star system located at the geometric center of the Squid.
• The cosmic jet stream created by this star system is somewhat of a rare occurrence. Instead of creating a planetary nebula (a dying star's final breath), current research shows the Squid is a spectacular, high-velocity material outflow created by these relatively young, massive and powerful central stars.
This DSO presents a compelling study of cosmic contrast and scale. We are viewing a massive, translucent oxygen structure expanding inside a vast cloud of elemental hydrogen, both being energized by the 3-star system at its core. It is a stunning example of how interstellar forces and specific elements can shape highly organized structures out of deep-space gases. It is also a pristine example of how the use of today’s narrow-band filters create visual contrast based on periodic elements.
IMAGER'S COMMENTS - Initial Thoughts:
For some time now, I’ve been looking at images of this DSO captured by other, more experienced amateurs; and, their write-ups and exposure vitals always tell a similar story: …that this object is EXTREMELY faint, and that dozens of hours of exposure time are required to capture it. Until now I’ve dismissed trying to capture it for myself; because, my f/7.0 refractor didn’t seem up to the task (...it just isn't "fast" enough).
Using a recently acquired technique to input images into the PC planetarium software app "Stellarium", I came to realize that although faint, this is a VERY BIG object. At my prime FL=714mm, there’s no way to capture a respectable framing of it. So, without the willingness to fork-out big-$bucks on a new, shorter scope, I determined a reducer is required; and a very substantial reducer at that.
About a year ago, I acquired a 0.65x reducer sold by Starizona. I don’t use it often, because when paired with my ES102 refractor, it suffers from significant coma around the edges of the FOV. But putting that aside, it also reduces the f/ratio of my scope down to f/4.55. Despite the coma, it seemed to be just the tool needed. From my location, the “Squid” is just now approaching its annual achronycal rise date. As such, I found it as I was looking for potential targets in Stellarium that were at or near their achronycal rise. My decision to go after the Squid boiled-down to very simple reasoning, “Other than time, what have I got to lose?”
I am astounded at the result. With only 4.3h integration, and just a little bit of processing "trickery", the resulting image is truly beautiful. Yes, I had to correct a number of mis-shaped stars around the outer edges (you may still see a few I missed); but overall, it’s a descent image.
IMAGER'S COMMENTS - Follow-up: Now having taken time to more fully consider WHY this image turned-out the way it did, it has "dawned" on me there is a reason... I've read in several online forums that the Mono guys routinely have trouble with extra faint objects like this one; because, the NB filters they often use are 3nm in bandwidth. The 3nm are the "latest Tech" in filters; so, everyone has bought them. Unfortunately, their total light transmittance is also very low. While 3nm filters are well known for improving image detail, this limitation in total transmittance is challenging when applied to faint objects. That, together with the need for 3x the imaging time due to separate filters for each wavelength (Ha, SII and OIII), makes for those extremely long exposure times.
As an aside, I've always felt the common reporting of accumulated imaging time for Mono exposures, where the time is reported as the cumulative time for all the color filters, is a mis-representation of the "true" effective integrated imaging exposure; because, the R, G and B channels of an RGB image blend together in "parallel" fashion (for lack of a better way to describe it). It is absolutely factual to say a 1 hour image shot with a OSC camera is equivalent in every way to 3 hours of exposure time shot in Mono with Red, Grn and Blu filters, which are then merged to form an RGB image of equivalent content to the OSC. So the equivalence ratio between them is 1:3. And then when you add 1 hour of Luminance into the mono mix, the equivalence goes up to 1:4. The most appropriate way to report effective Mono integrated imaging time is to report the actual expended integration time for only the Luminance channel. Most Mono imagers will collect more time on Luminance than they do the colors; because, it is the component that provides the detail in the image. I fully understand the time commitment it takes to create Mono images; but that still does not warrant or justify the typically inflated reporting of imaging time actually captured in the image.
(BTW - the same argument also holds true for mosaics.) With that said, I shot this object with my OSC camera using an Optolong L-eXtreme Dual NB filter which has 7nm band-pass characteristics in Ha & OIII. That means appx 2x the light gets thru when compared to Mono's 3nm versions, and in 1/3 the time requirement. The "trickery" mentioned above involved the use of Pix's PixelMath process to add the Blu and Grn photo channels together with each at full strength. So, the "Squid" came thru at 2x normal exposure. What makes that possible is, because the wavelength of OIII falls squarely in the middle between the Blu and Grn photographic color channels, adding them together allows them both to contribute to the exposure in an additive way (...as opposed to the standard "parallel" blending of photo color channels). So with the L-eXtreme, what I got was (effectively) 4.3h of Ha, and 8.6h of OIII, and with both being at 2x the transmittance rate of "slower" 3nm filters. If this image had been captured by someone using a Mono camera, they would have reported their exposure as, 4.3h in Ha, 4.3h in SIII (...which BTW are both Red), and 4.3h in OIII = almost 13h total. ...Not including any effort with Luminance, that's about 3 nights worth of (Mono) imaging effort condensed into 1 night with OSC. What-da-ya-know! ...chalk one up for OSC!
IMAGER'S COMMENTS - Follow-up: Now having taken time to more fully consider WHY this image turned-out the way it did, it has "dawned" on me there is a reason... I've read in several online forums that the Mono guys routinely have trouble with extra faint objects like this one; because, the NB filters they often use are 3nm in bandwidth. The 3nm are the "latest Tech" in filters; so, everyone has bought them. Unfortunately, their total light transmittance is also very low. While 3nm filters are well known for improving image detail, this limitation in total transmittance is challenging when applied to faint objects. That, together with the need for 3x the imaging time due to separate filters for each wavelength (Ha, SII and OIII), makes for those extremely long exposure times.
As an aside, I've always felt the common reporting of accumulated imaging time for Mono exposures, where the time is reported as the cumulative time for all the color filters, is a mis-representation of the "true" effective integrated imaging exposure; because, the R, G and B channels of an RGB image blend together in "parallel" fashion (for lack of a better way to describe it). It is absolutely factual to say a 1 hour image shot with a OSC camera is equivalent in every way to 3 hours of exposure time shot in Mono with Red, Grn and Blu filters, which are then merged to form an RGB image of equivalent content to the OSC. So the equivalence ratio between them is 1:3. And then when you add 1 hour of Luminance into the mono mix, the equivalence goes up to 1:4. The most appropriate way to report effective Mono integrated imaging time is to report the actual expended integration time for only the Luminance channel. Most Mono imagers will collect more time on Luminance than they do the colors; because, it is the component that provides the detail in the image. I fully understand the time commitment it takes to create Mono images; but that still does not warrant or justify the typically inflated reporting of imaging time actually captured in the image.
(BTW - the same argument also holds true for mosaics.) With that said, I shot this object with my OSC camera using an Optolong L-eXtreme Dual NB filter which has 7nm band-pass characteristics in Ha & OIII. That means appx 2x the light gets thru when compared to Mono's 3nm versions, and in 1/3 the time requirement. The "trickery" mentioned above involved the use of Pix's PixelMath process to add the Blu and Grn photo channels together with each at full strength. So, the "Squid" came thru at 2x normal exposure. What makes that possible is, because the wavelength of OIII falls squarely in the middle between the Blu and Grn photographic color channels, adding them together allows them both to contribute to the exposure in an additive way (...as opposed to the standard "parallel" blending of photo color channels). So with the L-eXtreme, what I got was (effectively) 4.3h of Ha, and 8.6h of OIII, and with both being at 2x the transmittance rate of "slower" 3nm filters. If this image had been captured by someone using a Mono camera, they would have reported their exposure as, 4.3h in Ha, 4.3h in SIII (...which BTW are both Red), and 4.3h in OIII = almost 13h total. ...Not including any effort with Luminance, that's about 3 nights worth of (Mono) imaging effort condensed into 1 night with OSC. What-da-ya-know! ...chalk one up for OSC!