In order to explore more of the large gas structures of our Milky Way, I have started to use the Finkbeiner H-alpha all-sky map as my atlas. I've printed out sections of it by constellation so I can refer to it as I explore all the wisps and tendrils which connect all the familiar (and not so familiar) nebula with my night vision device. One area which caught my attention a couple months ago was a cloud floating above the plane of the Milky Way somewhere to the northwest of Scorpio. It's pointed out here and looks like an eye with an eyebrow. It is considerably fainter than Sivan 10, which is in turn considerably fainter than Sh2-27, which is a difficult object visually but which many a night vision device user has discovered by panning around the sky.
While on the Finkbeiner Gas line layer in Aladin, I navigated to the spot but at first there was no nebula seen. I needed to adjust Aladin's pixel mapping (like adjusting the contrast on a TV) to make it appear. To my shock I found the nebula is more or less centered on Spica, Alpha Virginis. This is what it looks like:
It is enormous, roughly 20-degrees W-E by 18-degrees N-S--larger than the main star grouping of Corvus. In the Finkbeiner image it appears to be either lit from below (from the main plane of the Milky Way) or perhaps even illuminated by Spica itself? I have no idea of its distance and don't find a designation for it. Curiously, there is a noticeable lack of galaxies plotted in atlases in this region of Virgo -- I wonder if they are being obscured by this cloud?
I first tried to view this during the May new moon from a site in San Benito County. I only used 1x (ie. no magnification, just looking through the device) and a 7nm Ha filter. I used a mirror mount to hold the device steady. I made a sketch of what I perceived as a very excessively faint wash of nebula south of Spica. Mark Wagner, who was with me, sketched nearly the same view, but he was more confident in his observation than I was of mine. I needed to try again to be sure.
Monday night I went up to Williams Hill and found a turnout on the road to the tower. It was very windy and cold, mid- to low- 40s. Fortunately, I came dressed for it, but it was disconcerting to observe in winter-like conditions during the summer. I brought a NP-101 (4-inch refractor, 4-degree FOV), a 60mm f/5 refractor (8-degree FOV), and my mirror mount and a monopod to view 1x (40-degree FOV). I also printed a finder chart of the nebula from Aladin with the coordinate grid included, so I could use my alt/az mount's DSC to locate specific parts of the nebula. During the hour I observed it my SQML read between 21.2-21.4 -- transparency was not as good as expected.
Once dark I pointed my 4-inch to what looks like the brightest part, at approximately RA 13:40 Dec -15:00. I set the device's gain quite low (which darkens the sky and stars but gives more contrast for the nebula). As I flipped my filter wheel from unfiltered to a 3nm Ha, I immediately saw an expanse of very faint mottled nebulosity. It was like blinking an OIII filter to see a planetary nebula: at first, it's not there and then--add the filter, and there it is. The nebula faded from view, so I had to flip back and forth between unfiltered and filtered to get used to seeing it -- eventually I could pick it out against the sky background regularly. In this area, the north was weaker of nebulosity, but it filled the eastern, western, and southern parts of the field, and beyond the edge of the field as I panned. This is as should be expected.
I tried some other areas, such as the NW corner centered at RA 13:00 Dec -10:00. Just as before, the nebula appeared when flipping from unfiltered to filtered. Weak nebula to the north, less faint to the west, and stronger east and south. Next I switched to the 60mm, and brought it back to the brightest section in the SE. The nebula was quite strong in this entire quadrant, and I followed a loop to the east back up to Spica. Both the Ha+OIII and OIII filters showed different aspects of the nebula. I also looked at the "Eyebrow" portion centered RA 14:14 Dec -5:23 and found a very faint E-W wisp and a stronger N-S stream in the east of the field. All as expected.
Finally, I mounted the device on the monopod and scanned the area. This time I had less difficulty seeing the southern portion of the nebula (I could not see the eyebrow), and more confidently sketched it. I scanned over to Sh2-27 and judged its brightness against Siv 10, and then panned over to the Alpha Virginis nebula. While the entire extent of the nebula is fainter than Siv 10, the brightest pocket to the SE of Spica is actually not very difficult.
Sivan 8: After taking a break and basking in the triumph I continued with more. I've been really fascinated by Sivan 8, at the base of Scorpio's tail. With the 60mm and Ha I could see the entire "wheel" shape with IC4628 shining brightly, along with the column of nebula which divides the wheel N-S near the center. In Finkbeiner's image there is a very faint parenthesis of wispy nebula beyond the southeastern curve of Siv 8, and I had the strong sense I was picking it up. Williams Hill itself was disrupting the view, so I will need to come back to this area. There's also a "pillar" in the western arc of Siv 8, but I was disorientated a little and did not see it.
Cassiopeia "C": Later in the night I tried for another feature I noticed, a large "C" shaped nebulosity which flows down from IC59/63 to the east south toward NGC281 (aka "Pacman"). It was faint but once seen I could make out some wispy detail. I tried hard for the opposite parenthesis stream flowing along the west, but could not make it out.
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