So I was very curious when reading a post on Cloudy Nights which mentioned a nebula designation I had not seen before, "Siv," which is for J.P. Sivan of France. In 1974 Sivan published a paper called "A New Look at the Interstellar Hydrogen Through a Very-Wide-Field Photographic H-alpha Survey of the Whole Milky Way." Sivan used telescopes with a 60-degree field diameter and filters centered on Ha passband of 10 angstroms, with the purpose of visualizing potentially larger nebula structure than prior catalogs, like Sharpless, could see because they had smaller fields. Sivan lists 11 such new nebulae in his paper, all of which are in SIMBAD. I thought this would make a fun observing project for my wide field devices.
On Saturday night I observed on the Central Coast. The sky was ok but transparency was a bit off, 3/5. I pre-printed images of the nebula using the Feinbeiner layer on Aladin, among other sources. Sivan 7 & 8, which are at the bottom of Scorpio's tail, were too low and blocked by a nearby ridge. But I had a successful go at the others, plus a few other very interesting objects. I used my NP-101 at 10x and 4° TFOV with Ha 3nm, Ha+OIII 5nm, OIII 5nm, and 642 longpass filters in a filter wheel, and the PVS-14 at 1x with a Ha 7nm filter, with a a 40° FOV. The objects are not in order of RA, so the observations were made a different times through the night. Not much is written about these objects.
Siv 1: In Sagittarius, centered 18 17 12.0 -21 30 00; size: 150' x 60' (2° x 1°). Sivan notes this stream connects Sh2-34 to Sh2-35. With 4-inch and Ha, it was bright, easy, a large oval within an even larger "C" shaped nebulsity linking the two brighter Sharpless clouds. The inside of the "C" was filled with gauzy nebulosity, and there was a large wisp of nebula wafting from the back of the "C." Nearby is the Lagoon Nebula, which was spectacular.
Siv 2: In Cassiopeia, centered 00 14 36.0 +50 45 00; size: 330' x 180' (5.5° x 3°). Distance: 1380 pc. Sivan noted the exciting star is 9th magnitude AO Cas. Also known as the “Manta Nebula,” there is a beautiful photo by RBA on NASA's website framing it with M31. Another great shot notes the prevalance of OIII emission and the author speculates it might be a planetary nebula. I found this object extremely difficult using the 4-inch. I only noticed a large, ragged, dome-shaped glow when changing filters from unfiltered to Ha, and noticing the "relatively" bright dome. Very tenuous long filaments flowed from the dome to the south, making the nebula look very much like a rag mop. (The Finkbeiner survey coverage is incomplete, so there's a blank spot in Aladin).
Siv 3: In Camelopardalis, centered 04 57 30.0 +65 29 00, 450 x 240’ (7.5° x 4°), distance 830 pc. Exciting star: alpha Cam (09.5 Ia). For these very large nebula, I can only frame them with the widest field possible, otherwise a narrow field would simply make them part of the background sky. Using a monopod to keep the PVS-14 steady, at 1x, I was able to observe a large, extremely faint trapeziod cloud spreading through the constellation NE-SW. One aspect of the filters is they are brighter in the center of the field than the edges, so as one pans from side to side, a nebula at the edge will brighten when it moves to the middle. After looking directly at Camelopardalis for a while and not seeing anything, I slowly panned down to some brighter nebula (such as Heart & Soul), then panned back to central Camelopardalis, and was able to detect the glow. There's a good write-up of the area here. In the 4-inch I tried pretty hard to see the bow shock in front of Alpha Cam, but did not see it. There's a good write up about this feature here.
Siv 4: In Perseus, centered 03 52 24.0 +32 36 00, 540' x 420' (9° x 7°). Sivan notes: "Diffuse region of extremely faint emission, connected to NGC 1499. Barely detected in the mosaic in Figure 1. Unambiguously revealed by a longer exposure Ha photograph….This new HII region is found to be associated with an expanding shell of neutral hydrogen discovered by Sancisi (1973), connected with the II Per association." Also with 1x, and also only seen after panning the device through the field. NGC 1499 is very bright and pops into view. Filaments stretch out from the eastern side of NGC 1499 and hook to the south, fading as they go. But by following that curve it leads directly toward the star Atik, which is close to the center of Siv 4, and I could see the very large faint glow in this area fairly well. In this image NGC 1499 is to the upper left. The bright nebula on the right are associated with Barnard 3, of which more following below.
Siv 5 & 6: In Monoceros, centered 07 01 30.0 -02 43 00, Siv 5 is 2° x 1° in size, and Siv 6 is 1.5° x 1°. Sivan notes: "Possibly associated with the drapery-like region of faint emission which extends from the outer edge of the Barnard Loop." This was the last object viewed for the night, since I needed to wait for it to rise sufficently high. I saw it both 1x and through the 4-inch with Ha. In both cases I located the very bright Rosette Nebula, then from there "nebula hopped" down some bright Sharpless islands southward (Sh2-280, -282, -284) until I was centered in the region. These were faint but easily seen both 1x and in the 4-inch, with Sivan 5 a large, curved-backed glow with mottling to the south, of which Siv 6 looks like a broken-off puff. I hope one day, in a good enough sky, to be able to trace the wispy edge of the large nebulous bubble from this area back to the bottom of Orion, as shown in the Finkbeiner image below.
Siv 8: In Scorpius, centered at 16 43 24.6 -41 23 45, size: 420'x420' (7° x 7°), distance 2,000 pc. Sivan notes: "Wheel-shaped nebula, possibly connected to RCW 114 by faint, outlying filaments. Contains RCW 113 and IC 4628. Connected with the Sco OB1 association. The derived linear diameter is 244 pc." I was really disappointed that this one was too low to observe. In the image (with the nebula outlined in the bright yellow circle) the large ring is very apparent and there are even elephant trunk-like features. The Sco OB1 is close to us, so the HII clouds around them appear huge to us.
Siv 9: In Scorpius, centered 16 38 36.0 -28 46 00, size 600'x420' (10° x 7°), distance 182 pc. Sivan notes: "RCW 129 containing the star Tau Scorpii, and fainter extensions." This one was low but still doable. Using 1x with the night vision device on a monopod, I scanned the sky to move the nebula through the center of the field and watch for brightening. I needed to wait for complete astronomical darkness before I could make out the form. I first noticed the glow after "nebula hopping" from Sagittarius (Lagoon etc.) down above Scorpio's stinger stars (Cat's Paw, War & Peace nebula, etc. -- all of which were pretty bright) and then over toward Tau Scorpii. The cloud was very large, roughly triangular shaped, considerably faint, with a brighter rim toward Antares but very diffuse in the east.
Siv 10: In Scorpius, centered 16 01 00.0 -20 19 00, size 840'x720’ (14° x 12°), distance 174 pc. Sivan notes: "Almost circular emission region containing two concentrations, Sh2-1 and Sh2-7 surrounding the stars Pi Sco and Delta Sco respectively. Appears to be associated with an expanding shell of neutral hydrogen connected with the II Sco association (Sancisi 1973)." Like Siv 9, I only saw this after astronomical dark and with panning the device 1x with Ha filter on a monopod. This was very excessivly faint, a vague, round, very large glow, about a quarter of which was setting behind a nearby ridge. I did not see any detail or separation of the eastern outer rim with the rest of the nebula. I need to come back next year on a better night. Nearby Sh2-27 was very bright by comparison. The below color image is from Finkbeiner's Ha Map: north is roughly left, and it shows the very bright Sh2-27, next to it to the right Siv 10, and Siv 9 is below that. The ring shape of Siv 8 can be seen in the lower right. These are all close to us, so the HII regions appear very large compared with the Milky Way, which runs along the bottom of the image.
Berkeley 59 / Sh2-171: Cassiopeia, 00 04 40.0 +67 09 24, size 180'. Also NGC7822 + Ced214. This rather ordinary open cluster was transformed with the 4-inch with Ha filter. It is large, bright, and round, but broken up into two large pieces: to the north a curved "C" shape with the outer rim brighter and with a sharp edge, the inner rim diffuse. To the south a brighter, smaller also curved arc (like parenthesis to the larger), but its inner edge is frayed with many dark nebula intrusions. Vogel notes: "This expanding shell of gas and dust was created by the original star cluster at the heart of the Cep OB4 association, which has now dispersed. It is now lit and ionized by the young star cluster Berkeley 59, at its south end (Cederblad 214) and it is expanding into the dark nebula NGC 7822 at its north end. Berkeley 59 is surrounded by another dark region of disturbed gas and dust containing the radio source W1."
Barnard 3: in Perseus, centered 03h 39.93m +31° 55.55’, 1.6° x 1.8° degrees. "The Wreath Nebula." A three-for-one object, combining dark nebula, emission nebula, and and HII region, it is 1000 light years distant and 22 light years across, in the Perseus Molecular Cloud complex. With the 4-inch and Ha it appeared as a fairly bright, very diffuse oval glow, with many subtle dust lanes and a bright star in the middle.
Sh2-175: 00 27 18.6 +64 42 13, 2'. Very small, non-stellar round haze around a star; did not have enough magnification to show any detail. This is This is in Cas OB5, and LBN 596.
G53.6-2.2: Supernova remnant. 4-inch and Ha+OIII. Very faint streak, gently curved to the north.
Right Ascension: 19 38 50
Declination: +17 14
Size (/arcmin): 33×28
Type: S
Flux density at 1 GHz (/Jy): 8
Spectral Index: 0.50
Notes: Has been called G53.7−2.2.
Radio: Ring of emission, with extension to NW.
Optical: Filaments and diffuse emission.
X-ray: Centrally brightened, offset to NW.
Distance: Association with HI gives 2.8 kpc.
G54.4-03: Supernova remnant. 4-inch and Ha+OIII. Very vague, ring nebula not quite forming a circle, suspected rather than seen, centered on a star
Right Ascension: 19 33 20
Declination: +18 56
Size (/arcmin): 40
Type: S
Flux density at 1 GHz (/Jy): 28
Spectral Index: 0.5
Notes: Has been called G54.5−0.3.
Radio: Shell, in complex region.
Optical: Faint filaments.
Point sources: Pulsar outside NW rim.
Distance: HI and CO observations suggest 6.6 kpc.
G55.7-3.4: Supernova remnant. 4-inch and Ha+OIII. Diffuse ragged glow, thicker rim with diffuse edges, and the center filled with gausy nebulosity, pretty large.
Right Ascension: 19 21 20
Declination: +21 44
Size (/arcmin): 23
Type: S
Flux density at 1 GHz (/Jy): 1?
Spectral Index: 0.3?
Radio: Incomplete shell.
Point sources: Old pulsar within the boundary of the remnant.
Sagitta Bubble: A large circular feature covering most of the constellation, imaged by the MDW Sky Survey. At 1x and Ha, I did see a very large round glow in the correct area, but to my eye the nebula joined a larger flow of nebulosity to the north. This is captured in their image.
I should mention, during twilight there was a Starlink train which passed overhead. It was a tight line of satellites as they had not separated very much yet. They were moving pretty quickly, and for a moment they glinted in the sun and glittered dramatically, fading a little until they caught another ray of sunlight. I've seen individual tumbling satellites light up like this, but a whole short string of 60 or so was something else.
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