NGC/IC Project Restoration Effort

===== NGC 7088, along with M 102 (see NGC 5457 and NGC 5866), may well be one of the most controversial deep sky "objects" in the early catalogues. While clearly non-existent, there have been many "observations" of it over the years. Here is what I originally wrote about it. NGC 7088 does not exist, even though around a dozen sightings have been reported of it in the literature, including one by Dreyer himself in the NGC Notes. The nominal position, by Dreyer from Baxendell's description, is about half a degree north of M 2 (NGC 7089), but there is nothing there but faint field stars. Baxendell describes it like this: "It has an irregular oval form, its longer axis lying in a nearly east and west direction. A mean of several estimations gives its length about 1 1/4 [degrees], and its breadth about 52 [arcmin]." Baxendell does not tell us which telescope he used for his observations, but he is known to have owned a 6-inch F15 Cook refractor. His usual targets were variable stars and solar system objects. About half a degree on to the northwest of the NGC position is a 1.5 degree long streak of interstellar "cirrus", dust well above the plane of the Galaxy reflecting the light of the Galactic disk back to us (see IC 336 for some of these dust clouds that are definitely in the old catalogues). The cirrus is most easily visible in the IRAS 100-micron ISSA images of the area, though it also shows up in the 12-, 25-, and 60-micron images, and on the POSS1 prints, AND on the IIIa-J film copies of the latest optical surveys from Palomar and Siding Spring. While I suppose it is just vaguely possible that this may be Baxendell's object, his clear description of a southern boundary just 7 arcmin north of M 2, and of an oval shape, clearly rules this out. The IRAS 100-micron images in particular show a "hole" in the dust north of M 2, just the opposite of what we'd expect if the nebula were real. My own feeling about this object is that it may have been a reflection of some other object (perhaps even M 2) within Baxendell's telescope or eyepiece, and that later observations are similar illusions simply "wished" into existence (see NGC 2529 and NGC 2531 for a discussion of two other such objects). There is also the possibility that some transient atmospheric phenomenon led to Baxendell's belief that he had found a large, faint nebula. Or, following an idea floated to me recently (Feb 2007) by Dave Riddle, could this have something to do with the gegenschein or other zodiacal dust scattering phenomenon? Regarding this, I think it's worth noting that the nominal position for NGC 7088 is roughly as far north of the ecliptic as that of NGC 56 (see that for more on the gegenschein hypothesis). Also see NGC 1990 where a similarly elusive nebula has occasionally been seen around Epsilon Orionis. Added July 2013: Steve Waldee, one of the several modern observers who has seen NGC 7088 repeatedly, has an extensive article on it at http://freescruz.com/~4cygni/haggisizing/astro/invisible.htm (Steve's web site was unfortunately shut down when he moved to Utah recently, but the article apparently can still be dug out of the "WayBack Machine" web archive.) In it, he quotes (from a note to me in 2012) some speculation on how the eye interprets threshold stimuli. There is apparently quite a literature on this going back to the early 1940s (I would not be surprised to find even earlier work). In any event, here are the paragraphs from Steve's note that I think are particularly relevant to this discussion: Now, I understand that perception of faintest stars is certainly unreliable. Over-simplifying all that I've read about this, from Hecht's 1942 paper forward, it seems to me that the eye/brain has quantizing error down at the threshold of detection of a single receptor stimulus. Below a certain threshold, the receptor registers NOTHING. The only thing you get is random optical noise that, with your eyes closed, you cannot localize at one apparent "place" in what would, with eyes open, have been the field of vision. Just a jumble of erratic speckles like the picture on an analogue TV receiver turned to a channel without a TV station carrier. [Note that about one percent of these particular speckles are due to our "old" TV's responding to 13.7-billion-year-old photons from the microwave background radiation!] ... So with, say, my 10 inch scope, no matter how long and hard I stare, I can never see the 17th-19th mag individual stars that are so clearly registered on the POSS plates. It is as though they simply do not exist. A lot of amateurs have used faulty reasoning to assume that these faint stars form a "bias", a background noise that is detectable IF you pull back and see them across a sufficiently wide field: the wider you go, the more they add up to a sheen of glow. I believe this is wrong. The quantizing effect of the visual threshold cut off ignores them and they might as well not exist at all, close or wide view. So, it's not that I was detecting something "too faint to be seen"; no, whatever it is, I *could* see it. I think that it's simply a matter of stars that are faint and close together so that at a larger scale the individual, virtually identical, stimuli are perceived on more than one receptor, and they are so similar in magnitude that the brain says, "Aha: a continuum over this region! The light I perceive is all the same brightness, from here to here." And of course, the astronomically-trained mind equates that with gaseous nebulosity...and Baxendell's "nebula" is thus the conception that the cognitive mind creates, to explain what was experienced. While I remain skeptical about this as an explanation for NGC 7088 -- as I do for all the explanations so far put forward -- this is nevertheless a step toward a reasonable approach to understanding Baxendell's (and all subsequent) visual observations of this non-existent nebulosity. By the way, one of those subsequent observers of NGC 7088 is Bigourdan. He searched for the object on three nights (5 and 27 October 1888, and 2 August 1889). He records only "Non vue" (not seen) for the 1889 attempt, but has these notes, translated by me, from his observations in 1888: 5 October: Occasionally, I believe I suspected, at the [NGC position], a few traces of nebulosity whose existence is quite dubious. 27 October: I suspected extremely faint traces of nebulosity in several places in the region eight minutes south of the GC position. This object would require a better sky and a more powerful instrument. These notes make it clear that Bigourdan did not see much, if anything, of the extremely large nebulosity that Baxendell describes. Finally, on 12 August 2013, Steve Waldee sent another observation of NGC 7088 which he made around 2AM on that day. With his permission, I quote this in full as it usefully expands on what he has said previously: By the way: at 2 am, the seeing and transparency were just awesomely good, and the fog [below] had cut out ALL light pollution. I was able to pick out stars fainter than 7th mag. with my mediocre Wal-Mart plastic lens spectacles. I have no idea how a youngster with good distant vision and no astigmatism would do; maybe > 7.6 or so. My glasses cause a loss of something around half to two thirds' magnitude. Since the elevation is 3400 feet, well above the inversion layer and fog, much better than the distant coastal site I used a couple of months ago to study Baxendell['s NGC object], I put the 10" Newtonian on M 2 and looked around with a wide field eyepiece. INSTANTLY the Baxendell "effect" just jumped out at me--unmistakable! I had been looking earlier at some nebular objects in Sagittarius, including NGC 6559 and three associated IC nebulae in one large continuous patch, and Sh[arpless] 2-35. The apparent "nebulosity" of Baxendell['s object], in a FOV of about 70', was very distinctly brighter than the background away from the center; had about the same sense of character and brightness as the Sharpless nebula, if not being quite as bright as the best parts of N6559; and had a sort of "sheen" that was contributed by the six or seven stars across the "face" of it, as Steve Coe remarked. Of all the previous attempts to see what this might look like, THIS was the most convincing! I tried numerous eyepieces and kept moving about 1/2 to 1.5 degrees away from the apparent center; and always I could "return" to it. I did not use a chart; it just leaped out with no ambiguity! The most effective magnifications, with a 10" f/4.7 reflector, were from 80x to 109x (exit pupils of 2 to 3 mm.) I tried an anti-skyglow filter that works quite well if there are traces of sodium vapor streetlight glow; that had a degrading effect, causing the "nebulosity" effect to disappear completely, but revealing a vague, lumpy background texture as if the sky had very faint darker, broken lanes or streaks. A narrow passband filter centered on the H-Beta and ionized oxygen lines just dimmed everything to a dead black. I think it's reasonable to conclude that this was not "imaginary" and that, furthermore, the "confirmations" were not due to argument from authority and delusion. You have some even-brightness faint stars that cause the brain to "connect the dots"; and "underneath" them might be some VERY slight concentration of dust or cirrus. Those stars tend to focus the attention on that very pale "difference" in background illumination level, and the eye/brain system logically concludes that there's something there in a vague continuum. It can be duplicated and repeated with different scopes--I've now seen it in FOUR different instruments from 4.7 to 11 inches' aperture. It is definite enough that you don't need to plot or triangulate carefully from known stars; just put M 2 at the edge of the FOV and nudge the scope a bit north: and there it is! I guesstimated that it was, on the long axis, about 1/3 the FOV of my 15 mm (49' true field) view at 80x, or about 16+ arcminutes; with a size ratio of around 3 to 1. This is much smaller than Baxendell's original report; and it was further N from M 2 than his starting boundary at 7' distance. The globular was so bright that it was best to get it entirely out of the field of view to see the "nebulosity" to best effect. ... So, conceivably, an acute observer like a young Baxendell, in a sky without electric lights affecting it anywhere, MIGHT have hit some nights with better clarity and at least as good registration of dimmest stars; therefore he could seem to sense that the glow started closer to M 2... Now that I've had this last "sighting" of whatever-it-is, I really cannot fault Dreyer et al. for confirming it. If one had NEVER seen a photograph of unmistakable nebulosity, and had experienced only dim averted-vision glimpses without bandpass filters, then this effect is JUST like, say, a rather bright and distinct Sharpless nebula! I even had something of the same sensation of "sheen" from Sh 2-35, as there are numerous stars in the spot, some being distinct enough to look like a notable asterism or cluster. They seemed to make that real nebula have lanes or bright zones: purely a psychological effect...JUST like Baxendell['s object]! My conclusion from years ago remains the same: NGC 7088 does not exist and is nothing more than a visual illusion. What has changed is my understanding of the possible cause of the illusion. Thanks to Steve Waldee's work, I can accept that the distribution of stars at the threshold of perception, combined with the known neural effects within our eyes/brains, can actually lead to the repeatable observation of "nebulosity" that really does not exist on the sky. If so, there must be many other areas of the sky that will have the same sort of effect on a careful observer's visual system. It will be interesting to follow this thread over the years to see if others can pinpoint other patches of sky similar to that which Baxendell found over a century ago. This same effect may also lead to similar explanations for those of William Herschel's fifty-two large areas of sky "affected by nebulosity" that have not been found by anyone else in subsequent investigations (but see Arndt Latusseck's article in The Journal of Astronomical Data 14, 4, 2008 for other possible explanations).