An OSC Epiphany?

2025/02/27. I was planning to start a tour here of the winter sky modelled on the "Grand Circle of National Parks." I figured there were equivalents of the Grand Canyon, Zion, Bryce, Arches, and Mesa Verde, five oft-cited members of trhe Grand Circle. The membership changes with the decades and includes some "lesser" venues and nweer ones (Petrified Forest, Capitol Reef, Black Canyon of the Gunnison) and, for me, some special units of the park system (Chaco...). Likewise, in the winter sky, a similar grand circle of not-to-be-missed sights includes M42, the Horsehead, the Rosette, the Pleiades, and the Hyades as well as "lesser" sights (the Crab, IC 443, S240). Many of these come and go, too, in the "standard folio" of astrophotographers' enthusiasms.

Then an insight got in the way. Here are some OSC images from recent nights, followed by reprocessed, post-epiphany versions of the same or similar data:

Sharpless 240 (2025/02/27)
400mm F2.8
62x180s, gain 300, -15C, Duoband

M42 (2025/02/28)
400mm F.8, ASI1600MC
53x10s, g139 + 32x60s, g300 (only Nikkor filtration)

I don't believe orange has a place in the Orion Nebula, but I've so far been unable to remove it. I wonder if some extra IR slipped through, if the Nikkor's internal UV/IR cut filter is, in fact, merely or mostly a UV cut filter. Try that again with the ZWO in place.

Williamina's Pony (2025/02/28)
400mm F2.8, ASI1600MC
15x180s (45 min), g300, -15c, Duoband, starless remix

I wanted the Horsehead photo to run about two hours, but some pine trees unexpectedly stepped into the way 45 minutes in. It's satisfyingly red because there were really few if any other colors to try to preserve in there and I could go ape adjusting hue and saturation. A greenish flare from Alnitak made a fine appearance as an unknown 15th magnitude comet. It had a plausible amount of trail in the finished photo; alas, it had the same amount of trail in each of the subframes, and it was exactly symmetrical with Alnitak relative to the center of the field of view. That means it was definitely an internal reflection, but it made for an exciting few minutes.

2025/03/08. Take two. The one-shot-color camera makes imaging very convenient, and processing its take can be quick and easy, but I've been leaving much on the table by not attending to the imagery channel by channel rather than all together. When deBayering the data, one of PixInsight's options is to extract the individual channels; this is often worth doing.

I reshot M42 using the Duoband filter, then used the red channel of M42 to bring up faint hydrogen in the nebula's surroundings and layered in data from the blue channel of some short exposures which is predominantly white-light and O-iii for the brightest bits:

400mm F2.8, 30x60s (red) 30x10s (blue)
ASI1600MC, g300, -15c, Svbony Duoband filter

Here's Williamina Fleming's Pony with emphasis on the shenanigans of hydrogen while the mostly-noise blue and green channels are ignored:

400mm F2.8, 15x180s
ASI1600MC, g300, -15c, Svbony Duoband filter

And what the hell? Because I can... the color image from several days ago informing the monochrome rendition just above:

Here's the Spaghetti Nebula --same data as at top of this page-- also with the red channel boosted and used for a luminance layer then (over)saturated to match. Is it overcooked? Well, maybe. I keep tweaking it. Adding in even some weak O-iii signal helped (see below). I'm just figuring out this channel-by-channel workflow:

400mm F2.8, 63x180s
ASI1600MC, g300, -15c, Svbony Duoband filter
red channel used for both color and luminance
green channel aggressively processed and used in place of the blue channel

All this was suggested by my best efforts to bring out the Boogeyman Nebula (this week: "Li'l X's Booger"), LDN 1622, under moony skies. A quarter moon was about 30 degrees above and a little left of this field. During processing, it became clear that the blue and green channels contributed mostly noise. So forget 'em (they help with the stars), and make the most of the available signal in the red channel:

400mm F2.8, 51x180s
ASI1600MC, g300, -15c, Svbony Duoband filter

Now, one thing I need to keep in mind: the Bayer matrix in the ASI1600MC means that only one pixel in four is collecting H-a light. If I used the ASI1600MM for hydrogen-rich targets, would I get 4x the signal? Likewise, O-iii is collected only by the blue pixels, so that signal, too would be much easier to collect in the monochrome camera. If the O-iii signal tickles the green pixels, so I can put that to work, too. But I routinely discard excess green early in the OSC workflow, so, that signal, too, is weaker than it needs to be. For the Spaghetti Nebula, the Sharpless 240 image, I've expressly isolated the green channel, treated it as aggressively as the red, and replaced the blue channel with it. This didn't make a world of difference, but it did make an appreciable difference in this subject. I'm going to need to try some of these same targets with the monochrome camera and the same exposure times using dedicated filters then compare the subs.

Don't get me wrong: the OSC is great for the right targets, especially for bright or moving ones (like comets and next week's lunar eclipse where stars are problematic for a monochrome camera and filterwheel). Just be aware that there have to be some trade-offs.

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