Special Observing Report Backyard, Boulder Creek: MAC Studies (Perseus and Auriga)
Part I: Best Laid Plans Part II: Under an Day Eight Moon Part III: Under a Day Ten Moon Part IV: Charioteer and Full Moon Part V: The Charioteer Turns the Corner Part VI: The Final Toughies Part VII: The Old College Try Part VIII: A Final Tour of Inspection
Best Laid Plans
Regular readers of these reports may be aware that I am intrigued by certain questions related to deepsky observation. Explicitly: What are the minimum conditions under which a particular deepsky denizen may be reliably found? And under what conditions may a specific study be truly appreciated in all its unique glory?
As you might suspect, neither of these concerns are especially unique to the convoluted turnings of my own mind. Others have contemplated these same imponderables. Among them is the very talented and extremely patient sketch-impressario Jere Kahanpää. Jere himself has published an extensive list of deepsky studies based on the idea of "minimum aperture". It is that list that I hope to explore in the course of this series of observations planned for the next good night in Backyard Boulder Creek...
Meanwhile, this very site (astro.geekjoy.com) includes a list of studies based on the idea of "optimal aperture". That list happens to include an appeal for other amateurs to get involved and help out by validating specific studies based on their own experience. Since I've yet to receive a response from anyone out there in deepsky land, I figured that "turnabout is fair play". Why not make a study of Jere's list and pick a few studies for validation? And of course, along the way check my own?
Knowing that we will soon lose Perseus and that Auriga lies well overhead, I decided to select a studies native to those two well-positioned constellations. Interestingly, a few of these have already been observed from Backyard Boulder Creek. This was done using various apertures and I include those observations here as part of an initial screening (just to get a sense of where Jere's list stands in terms of my own experience...) So let's start with these:
NGC1245, Type: Open Cluster, Magnitude: 7.9, Size: 11' RA: 3:14.7, Dec:+47:15, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 300mm, Threshold Scope Aperture: 104mm To be honest, when I first saw Jere's catalog this one kind of deterred me from venturing into this project. Last winter (2001), I spent a considerable amount of time tracking down this particular cluster under various seeing conditions from Backyard Boulder Creek and China Ridge. At the time, 150mm Argo was the only scope in my possession. When I finally turned up the cluster, filed the following description:
During a trip to China Ridge on Thursday evening February 15, I managed to conclusively track down NGC1245. Having become familiar with (a particular astrophoto), it was fairly easy to identify the keystone of stars that encapsulates it. Under 4.5ULTM conditions, "I noticed a 15 arc-minute sized keystone of stars that looked suspiciously like what I had seen in photos. Near the southern corner of the keystone, scintillation of two or three 11+ magnitude stars appeared. Switching to 120X (15mm Ultrascopic) I could directly acquire perhaps three of the stars. I could also make out the scintillation of perhaps a dozen others (as I flitted my eyes across the FOV)."
Now this doesn't sound much like a 60mm acquisition. However, seeing was less than optimal at the time. Although I've attempted to locate this particular cluster (unsuccessfully) through the 80mm Pup, it should prove interesting to attempt to do so knowing that Jere believes it is possible...
Like NGC1245, I also begin quite doubtful about a 60mm's scopes ability to reveal:
NGC1513, Type: Open Cluster, Magnitude: 8.4, Size: 9' RA: 4:10.0, Dec:+49:31, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 300mm, Threshold Scope Aperture: 116mm Unlike NGC1245, I never did manage to turn up this vagus cluster during the 2001 winter season. The following report from February comes closest:
"The first cluster (NGC1513), proved far too elusive for the regional 4.5 ULTM (Perseus is now positioned within the Boulder Creek lightdome). After checking and double-checking the charts, I abandoned this particular quest in favor of better conditions at a later date. Certainly discretion is the better part of abject bull-headed determination. As it was, I spent half my observing time attempting to turn this one up. With all this said, the reality is that I may have actually caught 1513 as a faint sprinkling of 12 plus magnitude stars about halfway between Lambda and Mu Persi. However, nothing definitive. This particular object is almost half a magnitude dimmer, and slightly larger than, NGC1245. It's magnitude and apparent size gives it a super low average surface brightness. It will probably take a regionally 5.5 night to confirm its position. But even under such conditions, very little of its dense open cluster nature will reveal itself. For now I am content that I've familiarized myself with NGC1513's neighborhood..."
Again seeing conditions were less than optimal. So this may very much be a factor. Yet there is something which gives me pause to doubt. Open cluster's are less impacted by seeing transparency that other deepsky studies. (Certainly galaxies suffer far worse.) Again, I am comforted by the fact that Jere says I should be able to turn this one up in a 60mm refractor. Come now Pup! Surely you can do it!
Now for something delightfully different - a nebula enshrouded cluster! But not just any nebula, one already viewed through both Argo and the Pup:
NGC1931, Type: Open Cluster with Nebulosity, Magnitude: 11.3p, Size: 3' RA: 5:31.4, Dec: +34:15, Minimum Aperture Catalog Scope Size: 150mm, Optimal Scope Size: 250mm, Threshold Scope Aperture: 91mm Let's take a look at Argo's view under 5.0 ULM conditions:
"... This 11.3 magnitude 3 arc-minute bright nebula gives a "globular-like" appearance due to it's "roundness" and the fact that a small cluster of stars may be seen in its midst. Last night Dwight and I could resolve two of the stars at 120x with a third hinted at on the border of perceptibility. Interestingly, Dwight and I both agreed that although the nebula was quite distinct, it lacked the kind of "presence" that would make it an excellent study for a six inch scope. To my eye though, it is a much finer study than M76. Again I am surprised that Monsiour Messier never tracked this one down. It certainly has to be placed on anyone's "Missed Messier's" list of deepsky studies."
Hmmm... Although not posted, I recall having little difficulty turning up this nebula under 5.0 conditions through the 80mm Pup under decent (5.0) conditions. Be that as it may, the trick now is to determine what aperture stop is required to make it undetectable.
Of course there are also studies posted on Jere's site that I have yet to turn up. Among them:
NGC744, Type: Open Cluster, Magnitude: 7.9, Size: 11' RA: 1:58.4, Dec: +55:29, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 250mm, Threshold Scope Aperture: 102mm. Very much like NGC1245. Could prove to be a challenge for Argo to give a half-way decent view...
Moving on we come to:
NGC1857, Type: Open Cluster, Magnitude: 7.0, Size: 6' RA: 5:20.2, Dec: +39:21, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 113mm, Threshold Scope Aperture: 50mm. This one should be pretty straightforward. It's "vitals" remind me of M103 in Cassiopeia - an easy find requiring a bit more aperture than the Pup can muster to show as something other than an asterism...
Then:
NGC1907, Type: Open Cluster, Magnitude: 8.2, Size: 7' RA: 5:28.0, Dec: +35:19, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 234mm, Threshold Scope Aperture: 98mm. Again, very much like NGC1245. A tough catch...
And:
NGC2126, Type: Open Cluster, Magnitude: 10p, Size: 6' RA: 6:03.0, Dec: +49:54, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 331mm, Threshold Scope Aperture: 130mm. A real toughie even for Argo on a good night, I suspect...
Finally:
NGC1893, Type: Open Cluster, Magnitude: 7.5, Size: 11' RA: 5:22.7, Dec: +33:24, Minimum Aperture Catalog Scope Size: 60mm, Optimal Scope Size: 209mm, Threshold Scope Aperture: 86mm. Certainly a nice "acid test" for the Pup at 80mms...
As you can see, Jere has lumped a lot of clusters of very disparate integrated magnitudes and apparent sizes into the "60mm" bucket. Certainly there is some method to his approach. As one who has played the "numbers game" myself, I can tell you that there is a great deal of variance in reported values along these lines. It is also possible that Jere has actually turned up all these studies in a 60mm scope. Questions remain however: Did Jere already know precisely where they were located? Was the seeing exceptional? Does he have "CCD-eyes?
Above chart of proposed studies from Jere's list in Perseus and Auriga. (Note: NGC744 is between Perseus and Cassiopeia.)
To answer these questions for myself I'll need a few nights of decent seeing from Backyard Boulder Creek. My method will be quite straightforward. Sweep the sky with the 67mm aperture stop installed at 40x. If I turn up something that looks suspicious, remove the stop and verify it at 80mms. If confirmed at 80, drop down to 43mm and see if I can make it "go away". If I can't see it at 43mm but can just make it out at 57mm, it'll qualifies as "60mm findable". If not, I'll just have to revisit the region on some later, darker and more stable night...
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Part II: Under an 8-Day Moon
Slowly I accumulate the observations needed to complete the phase two of this report. Can a 6 centimeter achromat truly reveal open clusters of the eight magnitude and beyond? And if so, how many of such cluster's brightest stellar members can be seen? Will such stars require "tricks of the eye"? How much experience is needed at tracking down such "phantoms of the Night Sky"? In this report some preliminary answers!
The night was Thursday, February 21, 2002. An 8 day Moon lay well overhead. The plan was to track down three open clusters cited as 60mm detectable on Jere Kahanpää's Minimum Aperture Catalog. Only three of the eight clusters planned for study were far enough away from the Moon: NGC's744, 1245 and 1513 - all in Perseus. Because of the Lunacy, thought I'd begin with 150mm Argo to "test the waters" first.
Despite the Moon, transparency was better than expected. One reason, the abscence of the typical winter run of high thin clouds. Another, the fine stability (8/10) that gave superb views of Selene and Gas Giants. To get a sense of how good things were, took a look at a much lowered Iota Cassiopeia. At 52x, a neighboring ~12.5 magnitude field star could be seen with moderate aversion. So stars to magnitude 11.8 could be held direct or 13.2 on eye movement. Had the Moon not blinded my eyes with glare I would be holding stars unaided down to magnitude 4.5 or so within the Boulder Creek lightdome (5.0 overhead).
Quickly located NGC744. Turned it up one degree west of 5th magnitude SAO22859. The cluster lay at center of a large "delta-shaped" region of 8 & 9th magnitude stars. Less than a dozen members were visible. Overall shape that of a check mark with its V pointing west. The cluster's brightest member lay at the crotch of the check. The view of the cluster was nothing to write home about. Surely doubling or tripling the magnification would reveal more stars. But more aperture is needed to get the most out of this cluster. A ten incher, under optimal conditions, could easily reveal the two or three dozen sparkling jewelike members needed to show the group to best advantage...
With Cassiopeia settling to the northwest, the next quest was open cluster NGC1245 in a descending Perseus. Surprisingly, the cluster could be definitely identified. Found it as a vague nebulous patch due south and slightly east of a line between Iota and Kappa Persei. The group formed a flat triangle with a pair of nearby 5th and 6th magnitude stars (these about one degree to the north and slightly west) in the finder. Less than a half-dozen members were visible through the main tube at 52x (These under moderate aversion.) In the field of view the cluster formed a small triangle with a pair of 9th magnitude stars. (These oriented northeast to southwest separated by about 10 arc minutes.) The hazy patch lay east of the northwest triangle member. Clearly, open cluster NGC1245 is a more difficult resolve than NGC744 located earrlier. This is true even though it lay in a more advantageous part of the sky.
Made a run on my next study last year. Had no success in turning it up. Not sure why I didn't stay with it. (Usually, I keep at this kind of "lost sheep" until it's either documented as a failure or "stumbled" across systematically...) Amazingly, it was "easily" located. NGC1513 was found to be almost perfectly centered between between Lambda and Mu Persei. The cluster lies in a most memorable region - within the heart of a cochlea or nautilus shell of 8th to 11th magnitude stars. NGC1513 proved to be an easier resolve than NGC1245. Half dozen or so members of this 5 arc minute sized cluster quite visible. Where 1245 needed moderate aversion, 1513 showed its brightest members using the "softeye" technique.
After a satisfying period of Jupiter, Saturn, and Lunar viewing, chanced the Pup on resolving NGC1513. At the time only the eastern arm of Perseus remained well positioned. Had the Moon been out of the picture, or there been better presentation of western Perseus, I would have made a run on the other clusters as well.
Before I tell you the outcome, let me mention how Iota Cassiopeia looked through the Pup at 80x. No sign of Iota-B, but C was direct and obvious. More importantly the 12.5 field star could be held with moderate aversion. Thus at 80x the Pup was doing as well as Argo at 52. All this says - as far as the 80mm Pup was concerned - that the sky was transparent to magnitude 5.5 unaided! I can only conclude that the effect of off-axis Lunar floodlighting on smaller apertures appears to be significantly less than on larger apertures. Effectively, the Pup was almost able to go as deep as Argo! (Although at a somewhat higher magnification.) Of course, all this gives away the results on NGC1513.
No trouble turning up the "nautilus" of stars through the Pup. And of course, there was the cluster! Nope, couldn't be held with soft eyes BUT easily detectable on eye movement with a half dozen stars on slight aversion. Drop down to 67mm: Same half-dozen on moderate aversion. Down to 57mm: A few stars on eye movement. 43mm - vague patch of light - nothing solvable. Only detectable on eye movement.
So what say ye? Say you you already know where to find a certain deepsky study. And can find it using 6 inches of aperture. Then using all the tricks of the eye, you just barely manage to detect its presence at 43mm. Does this mean the study is "findable" at 43mm?
Hmmm...
Reminds me of last springr's study of the Intergalactic Wanderer (in Lynx). Jere's MAC lists the Wanderer (NGC2419) as requiring a six inch. And I could barely locate this distant 10.4 magnitude globular cluster through Argo when first looking for it! Surprisingly, later found I could pick it out as a vague nebulous patch through the 80mm Pup. Of course, I knew exactly where to look and what to look for. Would I have "found" the Wanderer at 80mms? Possibly. But only after years of deepsky observing experience.
So maybe Jere's on track here - with open clusters anyway. 60mms is enough to locate NGC1513! But you got to know exactly where to look...
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Part III: Under a Day Ten Moon
Saturday, February 23rd's sky was kind enough to open a brief window on the heavens. As a result, got about an hour of lunar-afflicted viewing before the anticipated clouds rolled in. As on Thursday, continued my review of Jere Kahanpää's 60mm minmum aperture studies, while also taking time out to ponder Selene and the Gas Giants.
Started things deepsky off with a quick check of Iota Cas. Iota A-B resolved nicely at 140x. 7th magnitude B lay just outside the first diffraction ring of 5th magnitude A. Interestingly, the dimmer C component was not visible until I removed the shorty barlow and dropped down to 80x. At that point 8th magnitude C, was easy and direct. Meanwhile, Iota's 12.5 magnitude field star was only possible on extreme aversion. So telescopic limiting magnitude was in the range of 11.5 and stars to magnitude 5.0 would have been held direct and unaverted had the Moon been absent the sky...
With western Perseus sinking fast, dropped in on NGC744. Had no trouble locating the large delta asterism of stars encompassing the cluster. A little visual casting about in the field turned up a faint sprinking of a half-dozen plus dim members. Two or three could be held direct at 80x using "softeyes". Installing the 67mm aperture stop darkened the field - and everything in it. A half dozen stars sparkled in a small group on eye movement. Only two or three were possible at 57mm.
There was nothing "nebulous" about the rest of the cluster - the Moon was simply too bright. In fact, before tracking the cluster down I found it impossible to locate the Double Planetary M76. (Located near Omicron Persi - a good navigational beacon star five degrees south-southwest of the cluster's locale.)
NGC1245 was next on tap. Blows me away that it was so difficult to locate this small dense, group last winter through 150mm Argo. Tonight found the region dead on using the finder. Located the pair of closish 8th magnitude stars that triangulate with the cluster. The cluster itself, none too obvious. However, a half dozen plus members could be seen under aversion at 80mm. At 67mm about the same number could be seen on eye movement. Two, maybe three members, possible at 57mm - again only on eye movement.
Rechecked NGC1513 as well. It's location in eastern Perseus, makes it a nice baseline check before searching for any dim Aurigaen clusters. Again at 80mm's, half dozen stars direct on softeyes. At 67mm's couple of stars could be held direct. 57mm revealed the same number on eye movement. This was the only cluster of the evening's studies to show the telltail "nebulous haze" of numerous dimmer components. This may have been possible due to the clusters position well overhead...
At this point I had hoped to seque into Auriga. The sky had other plans... Clear and steady skies were replaced by thin clouds. These soon led to thicker ones. I "contented" myself observing Jupiter until things got especially bad then packed it in. Earlier in the day, I had reworked the Pup's lens cell to improve the range of collimatable motion. This gave a visible improvement in image quality - especially when I startested on Capella. During that test, I found that I can now get complete diffraction rings at a location somewhere within the middle half of the 140x field of view. (Unfortunately, not yet in the very center, however...) Last night Jupiter's SEB was an easy resolve into the SEBs and SEBn. And the NTB looked like a high contrast and well defined line. Other belts were seen too - I caught the EB bisecting the EZ without too much effort.
Before skydark I had turned the Pup on Selene. Had no difficulty holding Plato-A direct as an obvious "brightening". Gassendi displayed a "double-cleft" along the crater ridge. (I was alerted to this feature by AstroTalk Member Otto Piechowski.)
One thing that has impact on open cluster susceptibility is scope optics. The ST80 Pup had decent optics to begin with (quarter wave undercorrected). But over the last year I've lavished a great deal of attention on it. The most important goal has been to achieve dead-center field collimation. (Not achieved as of yet due to the scope's lack of precision adjustments and questionable mechanics.) Along the way the doublet has been regapped. (This improving color correction primarily.) How much impact these factors have on resolving faint open clusters is at question. It is possible for a well collimated, diffraction-limited scope to go as much as half a magnitude deeper than similar apertures not so blessed. Should that be the case, then views provided by the Pup at 67mm may be what is possible for "untouched" ST80s under similar conditions.
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Part IV: Charioteer and Full Moon
High thin clouds and near-full Moon made it impossible to do anything other than "round up a few" 60mm MAC study suspects this evening (at 80mm's). Despite conditions, had no trouble turning up two of the Aurigaen Messier Clusters - en passant...
Before getting started, had a view of the rising Moon. Despite a mere 30 or so degrees elevation, seeing stability was passable. This is the time of the "Bright Rays": Tycho, Copernicus, Enche and Aristarchus all dominate the lunascape. The lack of contrast on Selene's full bodied presentation however, failed to engage my attention.
So I switched over to a well-positioned Jupiter. The two main equatorial and North Temperate belts crisply delineated. Both polar regions showed well-defined frontiers. Things missing: South Temperate and Equatorial Belt. Hints of the GRS could be seen as an "erasure" of definition in the SEB near the limb...
Saturn lies well overhead before true skydark. Cassini, cleanly displayed. Passing either Gas Giant through focus revealed a small amount of visible flaring. The lens cell still requires additional rework. (There is just not enough "tilt" or "skew" in the Pup's objective assembly to prefectly center the optical axis in the field.) Based on the quality of Gas Giant views the Pup gives now, I remain commited to seeing this project through...
Made no attempt to characterize sky transparency and stability (referencing Iota Cas). Based on the Gas Giants, stability about 7/10. Based on the excess of high thin clouds to the north, Iota would not have provided much meaningful data. If I had to guess I would say that stars to magnitude 11.0 direct were the limit for the evening - and this at 140x! At no time did the field of view ever appear "black". All was a dingy grey whose density varied only with magnification...
The night's observing schedule included four minimum aperture catalog (MAC) open clusters. One prospect (NGC1903) is said to require 6 inches minimum. That study is in fact the beginnings of a cluster still steeped in a bright reflecting nebula womb. I recall having easily turned this particular study up through the Pup at 80mms previously. I have yet to find where I recorded this observation, and find it easier (and more enjoyable) just to track it down again...
Of the four 60mm clusters (NGC1893, 1907, 1857 and 2126), three were attempted this evening. High thin clouds and that rising moon made it impossible to definitively locate any of the them. (Honestly, I simply forgot about the fourth - NGC2126 - given its wayward locale. But probably would have remembered had the sky been pristine.)
What follows is a simple description of what may turn out to be one or more of the three clusters. Confirmation must await a better night of seeing. The method was to stick with 80mms. (No aperture stopping.) Locate the required star field at 36x. Bump to 48x to see if anything "cluster-like" could be picked out (within the slighty "darker" one degree plus field of view). And examine any prospects at 140x (to see if stellar resolution was possible). Keep in mind that, under the conditions of the night, eye movement may have, at best, been able to reveal stars to magnitude 12.5 (at 140x).
NGC1893: Very vague patch half a 36x field southeast of 19 Auriga. Ran the magnification up to 140x to confirm. Possibly a handful of stars on eye movement. With thin clouds and Moon, nothing inky black about this sky. Contrast quite poor - a nice exercise in picking up faint irregularities in the "fabric" of the "space-time continuum"...
NGC1903: Despite various combinations of nebula filter and magnification, nothing doing. Thoroughly scoured the region one degree west and slightly north of M36. At the time, high thin clouds really started to pile up. Every star in the region sported a vague halo of nebulosity. (Between the halo's and gauzy-sky background it would have been impossible to pick up something "nebular", but I wanted to try anyway.)
In attempting to track down NGC1903, started at M36. Under the circumstances, this fine cluster looked a bit threadbare. Though an obvious "cluster", it had no sense of the finely hewn pyramidal shape accustomed to. Maybe three dozen members possible at 140x. None of the stars were "pin-pointy" due to the conditions - they just kind of "melded" with the background sky.
NGC1907: Located 2 or 3 possible members ensconced in a brightly scattered region of stars one-quarter of a 48x field northeast of Phi Auriga. Bumping the magnification to 140x showed a couple more. Promising, but not definitive.
Visited with nearby Aurigaen Messier cluster M38. M38's view was superior to that of M36. The cluster's cruciform shape well-evident. As a whole, this group showed relatively decent contrast against the "pea soup" of the particular night sky. (It's just possible that an opening in the high thins oiccured while observing this study...)
NGC1857: Of the faint Aurigaens, 1857 was the most remote. Required some star-hopping expertise (and associated double-checks) to zero in on. Found a small bright triangle of 10 plus magnitude stars one-half a 48x field south-southeast of 4.9 magnitude SAO40233. 140x examination turned up a few more. Like NGC1907, the 1857 candidate lies nestled in a large, brightly scattered group of stars that could easily be a cluster in its own right.
So all of the above amounts to a "preliminary expedition" into the Charioteer's group of faint open clusters. This, under the most difficult of conditions. Think of it as a small band of intrepid explorers setting out into the hinterlands seeking landmarks for future reference, recognition and verification. - "To Boldly Go..."
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Part V: The Charioteer Turns the Corner
It had been my hope to wrap up this MAC study series the night of Wednesday, February 27. - but the Moon nixed that. Did manage to squeeze in two clusters and NGC1931, however. By the time I got around to the last two, moonlight made it impossible to detect "cluster-sign". So faint NGC1857 and the most most difficult cluster of the bunch - NGC2126 - await another night.
Started off the evening's quest around 6:30. Seeing stability of the last few nights continued to hold. Jupiter gave very nice edge focus at 140x through the 80mm Pup. Two main belts plus well defined polar regions and NTB. SEB unquestionably rifted. Caught a barely detectable projection (festoon?) into the EB from the SEB. All this with a still slightly misaligned 80mms of achromat. Perhaps Jupiter can be viewed with satisfaction at this aperture after all...
Just before skydark, centered on Iota Cassiopeia. 140x cleanly resolved the A-B pair and showed the 12.5 field star on full aversion. Again, no sight of C until I dropped down to 80x. At that magnification, just able to detect the disparate magnitude 5 and 7 A-B split. B however, was obvious. As the sky darkened further, Iota's dim field star could be held with full aversion. Iota is a fine triple and a good test of small aperture scopes and general seeing conditions...
With Luna yet to clear the mountains, stars to magnitude 5.0 possible overhead. Beginning at Beta Tauri, quickly located 19 Auriga. 19 lies in a region of wide finder scope pairs oriented orthagonally. Cluster NGC1893 could be seen as a faint spray of stars one 80x field east-northeast of 5th magnitude 19. Using softeyes, maybe a half-dozen members scintillated at 80mms. Dropping down to 67mm lost a few. 57mm's left me with a trio of the cluster's very brightest components. Cutting way back to 43mm, these same few stars could only be caught with moderate aversion.
South and east of the cluster - lying well outside its frontier, is a single 9th magnitude star. Trailing that, a fine 8th magnitude matched double of less than 10 arc seconds separation. The pair is oriented north-south and remained solvable at all apertures... Continuing east another two degrees turned up Messier Open Cluster M36. The view was much improved over the last outing. Couldn't decry the "pyramidal shape" seen through 150mm Argo though. M36 does not make for a good 80mm study...
Swinging back due west one 48x field brought me to a region poor in bright-stars. Ensconsed therein was what appeared to be a faint 11th magnitude star embroiled in nebulosity. On eye movement, the nebulosity flared to all directions - so no sense of "frontier" was possible. Increasing magnification to 80 then 140x made it easy to tell that the 80mm Pup was in fact revealing nebula NGC1931. In fact, the nebula could be detected at 67 and 57mm's as well. It only became a serious challenge at 43mm. Throughout the aperture stop series, only a single "central star" could be seen. No sense of emergent central cluster was possible.
My main goal was to verify that the nebula could be located - if not profitably viewed - at something much less than the 150mm's cited on Jere's Minimum Aperture Catalog. (No, not to prove him wrong - just to prove my own recollections on the matter.)
Next stop: Phi Auriga - and north of that - the nicely presented Messier Cluster M38. Between the two (one-half a 48x field south of M38) lay cluster NGC1907. Even at 80mm's, only a pair of members could be held in this densely packed group. All the many other members contributed to a faint "nebular" sheen. This sheen made detection obvious - even at 57mm.
A little later, Luna cleared the eastern range of the Santa Cruz Mountains. Subsequent sky brightening made it impossible to tell the difference between a few faint background stars and bona fide clusters. Runs on NGC1857 (near Lambda Auriga) and NGC2126 (between Beta and Delta) proved abortive.
But of course all this means that I get to continue this series of investigations the next night out.
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Part VI: The Final Toughies
Got a late start last night (round 8:30pm). Auriga already turned the corner and heading northwest. Sky transparency - despite a wonderfully warm and "blue sky" day - only fair (6-/10). Couldn't cleanly resolve Iota Cas. Field star required full aversion - at 80 and 140x.
This failure of magnification to improve magnitudinal reach took away one of the tools needed to verify a particular "faint sprinkling of stars" is in fact a cluster. Another way of verification is to detect a discernable mist (caused by the cumulative luminosity of "on the edge" members). My late start meant that the Moon was again a factor.
The previous outing had already confirmed cluster NGC1907 south of M36. Nebulae 1931 (west of M36) had also been resolved. (This at apertures well short of the the 150mm's minimum scope size accorded by Jere's MAC list.) What remained was to confirm a suspected NGC1893 east of 19 Auriga and to identify candidates for clusters NGC1857 (between Sigma and Lambda Auriga) and NGC2126 (between distant Beta and Delta Auriga). It was my hope to make a start along these lines this evening.
Again, turned up that small patch of a half-dozen very faint stars due east of 19. And again, unable to note any increase in stellar population with magnification. Nor any nebulous mist. This proved to be true even when switching over to 150mm Argo later in the night. Same group at 52x seen with a couple more at 120x. By this time though a lowering Auriga and rising Moon compromised the gains of aperture. "Curses, foiled again!"
Nebula-enshrouded cluster NGC1931 and faint dense open NGC1907 were easily confirmed. These two caused me no real concern.
Spent some time visiting with the two Messier clusters. Both proved to be a bit of an enigma. M36 showed a surprising improvement in sky-contrast over previous nights out. Numerous easily resolved stars could be seen to form a dense "stick-figure" pattern. There was also a large pyramid-shaped halo of faint background members. M38 however, fell down a bit from earlier visits. Members showed less contrast than expected. Also noticed was how much more uniformly luminous M38's components are than those of M36 (which concentrates its brightest members in its central "stick figure"). Again, M38's members could be easily seen to arrange themselves according to the accustomed pattern - that of a crucifix with bent head stalk.
The search for NGC1857 near Lambda Auriga proved futile in both the 80mm Pup and Argo. At best, turned up a bright group of five stars forming a "delphinous-shaped" asterism some 2 degrees east of Lambda. But this is unlikely to be the cluster since NGC1857 lies 1 degree southeast of same according to my charts...
After failing to turn up an NGC2126 candidate through the 80mm Pup on previous nights, made a concerted effort to do so on this evening. In so doing, located an east-west oriented string of 7th and 8th magnitude stars half way between Beta and Delta. Along this string (and slightly southwest of its middle member) lay a faint group of three or four stars caught on aversion. As it turns out, Jere Kahanpää includes an eyepiece impression of NGC2126 on his SkyRover site. Such a neighboring star is annotated to the northeast. Prospects are good for this particular candidate.
I examined the same locale through Argo. At this point Luna was well up and hugely gibbous. Sky stability ebbed to 5/10. Using the 52x 35mm eyepiece, swept due north of Beta roughly 5 fields. Located the string of stars and its middle member. A 120x, easily made out the handful of faint stars seen with the 80mm Pup. - Still no sign of "nebulous mist". If this is the cluster, then I would say that yes, a 9.2 magnitude 6 arc-minute sized open cluster can be detected at 80mms of aperture. However "detection" may mean many things to many observers...
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Part VII: The Old College Try
Looks like I threw myself a curve over the last week or so... Lot's of head scratching revealed that the star thoughtto be 19 Aurigae was actually 16. So my longstanding NGC1893 candidate is no more - and it was the best shot in the region...
Did find a couple of bright asterisms east-southeast of the "real 19" that might have been cluster markers - save for a certain lack of faint stars...
Meanwhile, I have a couple "strong candidates" for NGC1857 and NGC2126. Strong enough that I was willing to sketch them for verification.
The sketch of NGC1857? (at left) was done at 150mm Argo's 120x eyepiece. It shows a sprinkling of faint stars (11 plus magnitude) widely dispersed around a central 7th magnitude sun some 40 arc minutes south-southeast of 5th magnitude Lambda Aurigae. Had I made the drawing at 52 or even 70x, you would have seen numerous stars of similar brightness distributed freely about the field. In isolating this one star, I had to examine each such star closely for faint attendents. The one selected for sketching best matched descriptions of the cluster found on the web: "faint cluster centered around a reddish 7.5 magnitude star". The following is one such description abstracted from a report at
[AZ-Observing] Vekol Observing Report .
" NGC1857 Open Cluster in Auriga (H33). Nice cluster, reasonably rich, smaller, with a nice, bright yellow star in the center which is part of a chain of three stars running north/south making up the center of the cluster. Other chain eliminate from the center star to WSW. Yet another from the southern star that winds it way about 3/4 of the way to the northern star, ending up just to the east. West and South are the richest concentration of stars looking like a smattering of dust. Mag 7.0v, Brightest star is 7.38v, 5', 118x. Authored by Chris Adamson.
The sketch (at right) of NGC2126 was done at 52x. I chose this magnification to illustrate how difficult it is to identify faint clusters in the field of view. Notice, for instance, the concentrations of faint stars west and east of the NGC2126 candidate at center. Is there really that much difference? Obviously, if we had the greater reach of a much larger professional instrument or an astrograph, it would become clear that there really is a difference.
The above NGC2126 candidate is the same one first detected by the 80mm Pup last time out. At that time what alerted me to its presence was a faint smudge detected on eye movement in the vacinity of the 8th magnitude star at sketch center. Under the better 5.3 ULM conditions of this evening and using 150mm Argo the smudge resolved itself into about a dozen very faint stars - some quite close to the primary...
NGC2126 is said to possess a cumulative magnitude of 9.2. Extending over a region of 6 arc-minutes, the cluster has an average surface brightness of about 13.8. Certainly, its brightest members are of the order of magnitude 11.5 - so a few lie within the range of the 80mm Pup. However, for it to be recognized as a cluster probably requires almost all of Argo's 150mm's in aperture. I do believe the sketch gives a sense of just how close this cluster is to simply being a somewhat randomly associated group of faint stars...
Tonight's sky started out quite transparent (for winter in these parts). By 9:30 or so, it took on that "gauzy" appearance I find particularly objectionable. Like last night, stability degraded as evening gave way to night. The air also became quite cold. Having completed the above pair of eyepiece impressions, I visited briefly with the Gas Giants. Bidding them adieu, stashed Argo in the study and called it a night...
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Part VIII: A Final Tour of Inspection
Another fine outing with gorgeous views of pinpointy jewel-like stars set against a velvety, indigo-night sky... And, of course, had the scopes all dressed up with someplace to go. Onward and Upward, Ariba Auriga!
Tonight was the night to pull it all together. Like last week, when wrapping up three faint clusters in Perseus, tonight was the night to accomplish the same in Auriga.
And how did things turn out? Well I have to let you decide that... Somewhere in theeyepiece impression that follows, three-quarters of a degree due east of IQ Aurigae, lies the definitive lesson of things 60mm...
The sketch (at left) was done at the eyepiece of 150mm Argo with Auriga well overhead under 5.3 ZULM and 7/10 stability skies. I figure if you can't locate open cluster NGC1893 at 150mm's, surely it would prove impossible to do so at 60mms!.
NGC1893 was the last remaining "holdout" from Jere Kahanpää's Minimum Aperture Catalog entry for 60mm scopes. The beauty of this cluster is that it isn't. As a result, Jere's approach to assigning minimum apertures is laid bare. And that approach? If you know where a particular study is found. And you can't find it at all with binoculars. And its brightest stars lie within the magnitudinal reach of a 60mm aperture scope. Then 60mm's is the minimum aperture needed to "find" that particular study.
There is nothing "wrong" with this approach - but it makes some assumptions... For instance, you have to already know where the study is located (vis a vis neighboring stars and asterisms). You also have to know what the particular study is as a class. The minimum aperture needed to find it, will not necessarily mean that you will recognize it by type.
For instance, through Argo's 7x35mm finderscope, I had no trouble finding both M36 and M38 this evening. Neither Messier class cluster showed resolved stars. All I saw was a hazy, ill-defined glow in the field of view. That glow could have been a galaxy, nebula, globular cluster, planetary nebula, comet, what have you - but I could "find" something...
Now throughout this investigation, I came across only two instances of true "open clusters" through the Pup. These were M36 and M38. Both showed several dozen stars against the faint glow of unresolved components. No question about it - open clusters!
However, none of the seven faint OC's found on Jere's list of 60mm susceptibles could be classed as "open clusters". Some (such as NGC1907) appeared as a "nebulous haze". The few stellar components that could be seen, required eye movement or strong aversion of the sight. Others, were small groups of stars - typically half a dozen - sprinkled in a more small region of the field of view. These, however, lacked the kind of density and cohesiveness that says: "Eureka, a star cluster!".
BUT, as it turned out - with twp exceptions - just like Argo's 35mm finderscope could reveal the presence of M36 and M38 - Jere's faint clusters could be detected in the field of view as something unique enough to warrant further investigation - through a much larger scope!.
What were those those exceptions? Why NGC1893 for one. That particular cluster just did not distinguish itself to my eye in any way through the Pup - although through 150mm Argo, I had my suspicions...
The other exception was NGC1857 south-southeast of Lambda. Without Internet research, I would have completely missed this particular "cluster" for the same reason. Both cluster's contained a very few, abnormally bright components easily visible (even at 43mms). However, a few bright stars does not an open cluster make...
For the record, here are my notes on the Auriga clusters and the other studies of the evening:
Iota Cassiopeia: Cleanly resolved through Argo at 120x. 12.5 field star direct at 70x. Resolved through the Pup at 140x. 12.5 field star full aversion at 80x. Calculated ULM's in region of Iota: 5.5 Argo, 5.6 80mm Pup. (Good sky stability made this possible! Unaided I could barely see 4.6 magnitude Iota direct - without glasses.)
Theta Aurigae: Hourglass resolution by Pup at 140x. Secondary trailing and to south. (Reminded me of Alnitak but closer...)
Jupiter: Pup showed SEB, EB, NEB, NTB, & NTTB, Faint belt in NPR too. Barge on northern terminus of NEB. All early on. Almost fully resolved the socket of the GRS later in the evening. Argo showed more detail within NEB, SEB along with some textures in the EZ. GRS socket not quite completely resolved.
Saturn: Cassini well defined in Pup and Argo. Better image scale through Argo (360 vs 140x).
NGC1893: 5 stars direct at 80mm / 48x. 3 stars direct at 57mm / 48x. (Hint, hint.)
M36: Three dozen stars at 80mms, two dozen at 57. Obvious cluster at both apertures. Engaging at 80mm. Should be a 3 inch study.) Both apertures resolved the fine matched pair double that lies along the east-west centerline of the cluster to the east. But 57mm stpped Pup needed 80x to make it clear.
NGC1931: At 48x 80mm's easily located the nebula and showed a steady starlike point at center. 57mm revealed the "star" inconsistently. The nebula was a definite but marginal catch. At 80mms many fine dim stars (oriented mostly east-west) surrounding the nebula - very sweet. These looked especially nice through Argo on anight when Argo's superb optics were well met by the Earth's of air...
M38: Obvious cluster at 80mms. Three to four dozen equally luminous dim stars. Many stars at 57mm - but very poor contrast - most members in the magnitude 10.5 - 11 range. definitely not a 57mm study. But not as good a 3 inch study as M36...
NGC1907: Faint spray a light at 48x / 80mms. 2 10th magnitude stars oriented east-west south of the "cluster". At best 2 brightest cluster members on aversion. If I didn't know better I'd a thoght this one a nebula...
NGC1857: Four bright stars oriented roughly north-south forming a check or J-pattern in the midst of a region of bright stars. Bright stellar triangle just north of the "cluster" a 1957 candidate earlier during these exploits. Some slight "haze" and a few dimmer members detected. (80x helped to stabilize the few additional scintillating components.) Color of brightest member? "Pinkish" - but I wouldn't have noticed anythin unusal had not others already remarked on the unusual color of this star. 57mm showed the same basic four stars and that's it. Nothing to suggest a cluster whatsoever.
NGC2126: Large sparse group of less than a dozen 12 plus magnitude stars. Require full aversion at 48x / 80mm. No sense of "nebular glow". Clusterlike only because visible members all share a similarity of brightness. The cluster looks rather "triangular" with a bright 8th magnitude (line of sighter) at the peak towards the east. 80x makes it possible to hold the entire bunch using "softeyes". Only 3 or 4 stars at same magnification stopped down to 57mm. All sense of "cluster" lost...
Note: Research on the Internet confirmed that previous eyepiece impressions done using 150mm Argo have in fact captured views of both NGC1857 (south-southeast of Lambda Aurigae) and 2126 (between Beta and Delta Aurigae).
One last thought: None of what has been learned during this project really matters much in the scheme of things. Jere Kahanpää's Minimum Aperture Catalog remains a useful resource for developing observing plans based on scope sizes. Astro.Geekjoy's Deepsky Susceptibility Calculator remain's of some small value for calculating limiting threshold apertures needed to recognize a particular deepsky study by type. BUT, the fact that such tools and references exist on the web does conduce to one thing: "Get thee hither with chart and scope. Explore that night sky!"
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