I had 21.5 hours of data on the Squid Nebula and a decision to make: keep shooting it, or give up and point the scope somewhere else. Ou4 is one of the faintest things an amateur telescope can attempt. It is nicknamed the Squid Nebula for its long, ghostly, tentacle-like lobes. I couldn’t tell from the image whether I was close or nowhere near. So I did the math instead, and this post is that story.
The moment I thought I’d found it
After reprocessing everything from scratch (more on that below, for anyone who wants the exact steps), I ran a simple check: how much brighter is the sky right where the Squid is supposed to be, compared to empty sky nearby? The answer came back 3.1 times brighter than normal background variation. That looked like a real signal. For about an hour I believed I’d pulled off something genuinely hard.
Then I looked at what else was sitting inside that little patch of sky. A star. Not just any star, but V419 Cep, a bright variable star. Its glow spreads out well beyond its own point of light. My “detection” was starlight bleeding into the measurement, not the nebula at all.
Why Ou4 is brutally hard to catch
Most nebulae glow in hydrogen light (Hα), which is relatively common and relatively bright. Ou4 glows almost entirely in ionized oxygen (OIII), which is rarer and dimmer, and it sits inside a much larger, brighter structure called Sh2-129, the Flying Bat Nebula. Picture trying to photograph a faint gray smudge painted on a wall that’s already covered in bright red graffiti. The DWARF 3’s Duo-Band filter is built to catch both Hα and OIII at once. That’s great for most targets and a real handicap for this one.
Ruling out the star: two more tests
I didn’t want to take “it’s probably just the star” on faith, so I ran two more checks.
First, I tested the other two spots where the Squid’s arms are supposed to reach, away from any bright star. Both came back essentially flat: 1.5 and 0.3 times the background noise, well under the threshold for a real signal. If the nebula were actually there, at least one of those should have lit up. Neither did.
Second, and this is the part I find most interesting, I noticed the OIII image and a separate hydrogen-light image of the same field looked suspiciously alike. Same arc shape, same bright band along one edge. That shouldn’t happen if OIII and hydrogen are showing genuinely different gas. So I measured how much one channel was leaking into the other, using every patch of real structure in the frame as a data point. The answer: about half of whatever hydrogen light is in a spot also shows up in the OIII channel, whether or not any oxygen is actually there. I’d measured this leakage once before with a completely different method and gotten a very similar number. Two unrelated approaches landing in the same place usually means you’ve found something real.
Once I subtracted that leakage out of the OIII image and checked the Squid’s position again, both arms sat at essentially zero. Nothing left.
Making sure the test itself wasn’t broken
Before trusting a “nothing’s there” result, you have to prove your method can find something when something’s actually there. So I pointed the same measurement at Sh2-129 itself, the big bright nebula Ou4 lives inside, using the hydrogen-light data. It’s plainly there in real images. If my test missed it too, the problem would be the test, not the sky.
My first attempt at this came back flat too, which was a bad ten minutes. It turned out I’d guessed where to place the measurement boxes instead of checking the actual image, and I’d guessed wrong; the bright part of the nebula runs along a totally different edge of the frame than I expected. Once I put the boxes in the right place, the signal was unmistakable, five to ten times stronger than background noise across a big chunk of the field.
That was the reassurance I needed. The method works. It found the nebula that’s actually there and didn’t find the one that isn’t.
So, can a DWARF 3 catch the Squid Nebula?
Not from my backyard. Not with this filter. Whatever oxygen light Ou4 is putting out, it’s buried under less than about 1.4 percent of the sky’s own glow. That sky glow comes from streetlights and general light pollution (astronomers call this a Bortle 6 sky), not from anything I can subtract away with more exposure time. Doubling my hours wouldn’t have helped much. The noise floor barely moves with more subs, but the gradient and the color-channel bleed stay exactly the same size no matter how long I shoot.
The eight nights of data stay on the hard drive as a record of the attempt. If a proper narrowband OIII filter or a darker sky ever enters the picture, Ou4 is the first thing I’m pointing it at.
Clear skies.
Want to run this test yourself? Here’s exactly how
This part is for anyone who wants to check their own faint target the same way. The following is a step-by-step method for confirming whether a very faint deep-sky target is actually present in a stacked astrophotography image, using coordinate-based measurement instead of visual inspection.
Total Time: 3 hours
1. Rebuild the stack correctly
Group imaging sessions by sensor temperature rather than date, since that’s what actually determines which calibration files match. Calibrate each temperature group with a dark and flat frame shot at that same temperature, but don’t convert the files to color yet. Run a color-splitting extraction on the calibrated files first, so the two colors of light are separated before any pixel-blending happens, not after. Then register and stack each group normally.
2. Set up the box test, and decide your rule before you look at any results
Plate-solve your finished stack so you know the exact sky coordinates in the image. Place a small box, 200 pixels or so, right on your target’s known position. If the target has more than one region, box each one separately. Place four more boxes on plain background sky elsewhere in the frame, and four more further out as a backup set. For each box, take the sigma-clipped median, since that ignores stray bright pixels like stars so a single star in the box doesn’t throw off the whole reading.
3. Do the arithmetic
Average your background box values; that’s your background level. Find the difference between your highest and lowest background box; that’s your noise floor, since even “empty” sky varies a little from spot to spot. Subtract the background average from each target box’s value; that’s your signal. If signal is more than three times the noise floor, that’s a real detection. If it’s less than the noise floor, there’s nothing there. Anything in between means you need more data or a cleaner background measurement. Decide that rule before you look at the numbers, or it isn’t really a rule.
4. Watch for gradients
Check whether your background boxes agree with each other. If one side of your frame is simply brighter than the other, you likely have a smooth lighting gradient rather than random noise, which can be mistaken for a real signal if you don’t account for it. Fit and subtract a simple tilted-plane correction to flatten it out.
5. Prove your test works before trusting a null result, and check for filter bleed
confident is bright enough to see. If your method finds that, you can trust it when it says a fainter target isn’t there. Separately, if you’re shooting with a filter that captures two kinds of light at once, compare your two extracted channels for matching shapes. If they look alike where they shouldn’t, measure the relationship with a simple line-fit between the two, and subtract that fraction of one channel out of the other before trusting any faint structure in your result.
Frequently Asked Questions
Can a DWARF 3 image the Squid Nebula (Ou4)?
Not from a typical backyard sky. After almost 22 hours of shooting, careful measurement showed no trace of the nebula above the sky’s own background glow. It would take a specialized filter and a much darker sky to have a real shot at it.
Why can’t the DWARF 3’s built-in filter catch it?
The Duo-Band filter is designed to capture two kinds of light at once, which works well for most targets. Ou4 only glows in one of those two colors, and very faintly, so it gets lost in the sky glow the filter also lets through.
How do you know something isn’t there if you can’t see it in the photo?
By measuring instead of looking. Faint targets like this stay invisible on screen no matter how the image is stretched, so the only reliable check is comparing the target’s location to nearby empty sky using math, not eyes.
What does it mean if one color channel “leaks” into another?
Some filters that capture two colors of light at once don’t separate them perfectly. A little of one color shows up where it shouldn’t, which can trick you into thinking you’ve found something faint and rare when it’s really bleed-through from something bright and common nearby.
How much sky glow is too much for faint targets like this?
It depends on the target, but for Ou4 specifically, the nebula’s light would need to be stronger than roughly 1.4 percent of the background sky brightness to be measurable in this data, and it wasn’t. Suburban light pollution alone can be brighter than that.
Is more exposure time the answer for a target this faint?
Not always. Doubling the hours only reduces random noise a little, and it does nothing about steady problems like sky glow or filter leakage. Sometimes the right fix is different equipment or a darker location, not more time.
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