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perseus and fornax clusters; LM for an 18?

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#1 george golitzin

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Posted 12 February 2013 - 09:00 PM

I had a nice night on Saturday, with a bunch of guys at the local site (about mag 6), one with a 28-inch, three 18s, and some imagers, etc. I decided to go after the Fornax and perseus clusters with my 18-inch. I had a really nice view of ngc 1365, given its low altitude: the bar and both arms were easily seen, the northern arm brighter, and the core was brilliant. I then started to pick off about 18 members of the cluster in my 10mm XW, which was a little tough, bent over the eyepiece, with the cluster at about 12 degrees altitude.

After some nice eye candy (Thor's helmet, Flame nebula, ngc 891) I went over to Perseus for ngc 1275 et al. It had been a while since I last visited this rich field, but this time I had a good map printed from megastar. The field is so crowded--I identified 36 galaxies inside a 50 arcminute field--that I went to the 7mm just to get enough separation between targets. This put me at about 275X, as I had left the paracorr out of the focuser for this exercise. As it turned out, the field of the 7xw coincided nicely with the thumbnail image that megastar produces, about 15 arcmin across, and I was pretty much seeing everything in that little image that had a designation of any kind. In particular, I was able to pick out tiny ngc 1279 and 5ZW339, both of which were pinpricks in averted vision. Megastar lists 1279 at mag 17.6 (I don't know what filter, and this number seems off the chart and therefore suspect) and 5zw339 at mag 16.3; NED gives the same mag for the ZW galaxy, but 15.5 for ngc 1279, which I'm more inclined to believe, although it seems a bit low, as I was seeing down to 15.5 consistently and more easily on other targets in the cluster.

So...was I seeing down to the limit for my scope and my 54-year-old eyes? Thing is, conditions were only so-so compared to darker sites I've been to, and I think I could go a good bit deeper at a darker location. Just wondering, because I'm not quite sure how detailed I need to make my maps for this region!

-geo.

#2 David Knisely

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Posted 12 February 2013 - 10:06 PM

The Fornax Cluster is just too easy, as quite a number of its galaxies are visible in rather small scopes with some showing detail in my 10 inch (like the giant barred spiral NGC 1365). I consider it to be the second easiest galaxy cluster to observe after the Virgo Cluster. The Perseus cluster is considerably more difficult, although the main two NGC objects (NGC 1275 and 1272) are at least detectable in smaller scopes. In my 10 inch f/5.6 Newtonian, at my dark sky site, I can see around 15 to 18 galaxies in the central part of the Perseus Galaxy Cluster (Abell 426) centered on NGC 1272, although most are almost stellar and require a lot of study to bring out (and I am 57). I can track down a number of others, but they don't show any detail other than being very faint fuzzy star-like objects. In my 14 inch Newtonian, that number jumps considerably (probably in excess of 35 to 40 galaxies), although again, many are so small and faint that it requires a lot of scanning and reference to my Megastar plots to get most of them. The NASA Extragalactic Database (NED) lists NGC 1279 as having a magnitude of 15.5, which is probably a blue magnitude. Its true visual magnitude is probably somewhere in the 14.7 to 15.1 range. Clear skies to you.

#3 hbanich

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Posted 13 February 2013 - 12:47 AM

So...was I seeing down to the limit for my scope and my 54-year-old eyes? Thing is, conditions were only so-so compared to darker sites I've been to, and I think I could go a good bit deeper at a darker location. Just wondering, because I'm not quite sure how detailed I need to make my maps for this region!
-geo.

Hi George, I think you could have gone a bit deeper if the sky had been darker but you did quite well with the sky you had. Published magnitudes are suspect to some degree – most sources disagree with each other to some degree and many times I find surface brightness is the more telling descriptor anyway. So whether you saw down to 17.6 or 15.5 hardly matters except that you pushed your scope and your skill as far as could that night. Way to go!

#4 IVM

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Posted 13 February 2013 - 09:59 PM

Not sure, George; sounds deep enough to me! I was observing the Perseus Cluster not long ago with my 16" from Cherry Springs and found the field fascinating visually but a little confusing as far as identifications. This may involve the case of NGC 1279 - could this account for the discrepancy with magnitudes? I recorded:

NGC 1278, 1293, and 1294 are the 3 Herschel members of Abell 426, the Perseus Cluster. 1278 is E pec, 1293 E0, and 1294 is SA0-?. 1278 and -94 have the same size, 113 kly, at similar distances (276 and 299 Mly). 1293 is a foreground object if the distance 191 Mly is accepted, and it then has diameter 50 kly. Bratton notices that Herschel's position for II.603 is closer to 1975 than to 1278, and that it is therefore possible that Herschel actually discovered the cluster's brightest member, 1275 (Caldwell 24, Perseus A), although it is not credited to him. NGC 1293 and -94 are III.574 and 575 respectively, and none of the three is in Herschel 400.

The cluster is a curdled misty patch almost 1/3 of the field of the Pentax [XW40] at 45x [effective aperture 12"]. NGC 1275 stands out. At 225x, NGC 1275, 1272, and 1278 are the immediately obvious triangle in the core of the cluster, framed by the Ethos. NGC 1273 completes the parallelogram with the three. On the N side of this figure is NGC 1274. N of it is IC 1907. Next to NGC 1278 is 1277, which is smaller but has a concentrated core with a starlike nucleus. Uranometria places NGC 1276 S of this pair. On DSS there is a very small lenticular there, which is not seen. Uranometria does not plot a much larger and brighter galaxy that lies on the same line as -77 from -78 (NW) but 3 times farther. It is labeled NGC 1276 on DSS by Bratton and I would accept that designation. NGC 1281 is farther NE, and E of a star.

E of the parallelogram is the triangle of NGC 1267, 1268, and 1270. Of these, -67 appears brighter because of its compactness; it may have a starlike nucleus. NGC 1271 considerably farther to the S forms an equilateral triangle with -72 and -67. On the same parallel with -71 is the pair of 1282 and 1283. The two are rather faint and, despite their proximity, almost difficult to hold simultaneously.

SE of 1275 is a small elongated galaxy. O'Meara labels it NGC 1279. It is not in Uranometria, and it is not labeled by Bratton. O'Meara mislabels two small galaxies to the E as 1293 and 1294 (the other 2 Herschels). I see the southern one; the N one is close to a star and cannot be seen reliably. The pair of larger and brighter objects farther E is outside O'Meara's DSS field. Their positions match the positions of NGC 1293 ad -94 in Uranometria.

To "grok" the cluster, I return to the central core. The parallelogram, the galaxy in its N side, the IC, the 1279, -77, and -76 (meaning the galaxy to the NW) are framed by the Ethos [225x]. At least the parallelogram can be held simultaneously without any difficulty, and for me signifies the Perseus Cluster.

#5 GlennLeDrew

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Posted 14 February 2013 - 07:34 AM

Holy smokes! Who cares about the precise limiting magnitude for the detection of galaxies at given aperture? So much depends upon transparency and sky darkness, object size and surface brightness, and observer acuity and experience, that differences in faintness of detection come down to essentially bragging rights. Boooring.

#6 IVM

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Posted 14 February 2013 - 03:41 PM

Glenn, I see the logic leading to the conclusion that discussing the limits achieved is bragging. Professional observatories have no qualms about it... I don't find it boring, however. This is how we know, measure, and perhaps improve to the humanity's ability to peer into the cosmos. (And it is not all determined by the professionals' capabilities since they gave up visual observing.)

#7 GlennLeDrew

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Posted 14 February 2013 - 06:51 PM

To elaborate on my previous, strident missive.

The faintness to which an extended object is seen under given conditions depends on the combination of surface brightness and size.

Consider an extreme example. A nebula of 1 deg^2 and having a surface brightness of 25 MPSAS has an integrated brightness of 7.4 magnitude. Yet it poses a challenge for observers with pretty big scopes. (It's worth pointing out that that dim nebula appears rather brighter due to the contribution of foreground sky glow. If the sky has SB = 22 MPSAS, the 7.4m nebula will apear as 0.06m brighter than the 4.4m/deg^2 sky, or 4.34m.)

At the other extreme are the quasars, which have very high surface brightness and minuscule size, appearing essentially as dim stars and hence are seen to the same limit as applies for stars.

And in between--but nearer to quasars than ghostly nebulae--are the galaxies. Those having the highest surface brightness in (primarily) their core regions are the faintest in integrated light which will be seen. Galaxies which are larger but have lower surface brightness may be invisible, in spite of a somewhat brighter integrated magnitude.

#8 george golitzin

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Posted 14 February 2013 - 09:54 PM

Thank you, Howard and IVM, for your encouragement.

Glenn and David, the point of my question was to ask experienced observers with apertures near 18 and up as to whether I was doing reasonably well for my sky conditions, and whether I should try to go much deeper on a target like Perseus, should I have the opportunity in much darker skies. It's a serious question, calling for comment from someone who has studied the target in question (rather than a theoretical missive), because to go much deeper requires taking one's research to a whole new level, such as one finds at Albert Highe's site. Not sure I want to do that much homework if I'm already into vanishing returns visually. My gut feeling is I could manage something like another half magnitude in a pristine sky...perhaps Howard was suggesting something of that order.

IVM, glad you mentioned that parallelogram--that figure helped immensely to reorient myself in the field repeatedly. The field indeed is so rich as to be quite daunting--my first reaction was that I was too tired to tackle individual identifications! But cranking up the power helped to separate the targets, and having that parallelogram for orientation, I was able to go at it. A good map is everything, because one needs to know exactly where to look in order to pick up those tiny guys at the limits of visibility. But of course you know that!

-geo.

#9 GlennLeDrew

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Posted 14 February 2013 - 11:55 PM

Has there been some collation of observations to determine if a fairly reliable relationship exists for the integrated magnitude limit of compact galaxies with respect to that for stars. It's probably safe to say that galaxies which are 2 magnitudes brighter than the faintest visible stars can be seen. Could that magnitude delta be pushed to as small as one?

#10 azure1961p

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Posted 15 February 2013 - 12:15 AM

Wow - what a difference subtracting 10" from the aperture makes...

I have seen 1275 and 1272 but the others were work. The field(s) never appeared crowded though many were there simply because they were s faint it took concentrated focus - and a fair use of mediu magnifications. I was amazed at the fainter members tininess. One pair looked like an unresolved miniature version of M51 and NGC5185. If they weren't so apparently faint it'd been one rich field.

Pete

#11 george golitzin

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Posted 15 February 2013 - 12:23 AM

Well Pete, that will sure change when you pick up that 16 you were talking about in the other thread!

-geo

#12 george golitzin

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Posted 15 February 2013 - 12:30 AM

I think that delta mag is near 0: the smallest members in these clusters are well under a half arcminute. The faintest I saw is listed at 16.3; I don't think I was seeing stars much fainter than that, which is already past the usual computed LM for stellar sources in an 18 under the conditions I've stated.

-geo

#13 hbanich

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Posted 15 February 2013 - 01:20 AM

George, you may find that returning to the Perseus cluster every night the sky looks good and it’s well placed will allow you to see a bit deeper even if you're not lucky enough to see it under a significantly darker and more transparent sky. Gaining familiarity with this galaxy cluster by itself will gain you a deeper look over time. Challenging objects become a little easier as you become used to the effort needed to see them well.

A high quality photo can do the same thing over a shorter period of time as it shows exactly where each member is located while providing a reasonable idea of their size and relative brightness. Knowing exactly where to look and what to expect is a marvelous way to see deeper, although you do have to be careful with your preconceptions. I enjoy drawing what I see and adding more details as I see them, which helps a great deal in identifying all the fainter stuff. Plus the process of drawing forces your attention on the object for a longer, higher quality period of time, also helping you see more.

I've found that the better I know the visual appearance of a special object like the Perseus Cluster the more curious I become about its physical nature and I start doing a bit of reading just to understand a little more about what I've seen. This invariably leads me to observe more, do more reading and so on – becoming a virtuous cycle that doesn't have to have an ending.

However, trying to predict how much fainter you can see seems impossible to predict quantitatively simply because of all the variables, so the only way to find out is to keep looking. But then that’s the fun and the challenge, isn’t it?

#14 george golitzin

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Posted 15 February 2013 - 03:39 AM

Thanks Howard--that's good advice, and a sound observing philosophy.

-George

#15 azure1961p

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Posted 15 February 2013 - 07:55 AM

Heh - looks like the 16" is iced for now. How fleeting are budgets. I also learned the AGENA 16" mirrors can often be horrendous. Between the money redirect and the poor QC on *cheap* big glass its shelved for now.
It's not as sad as it sounds but I was starting to like the idea of detail in ngc galaxies.

I was going to try for Copelands Septet in Leo with the 8" at one time, how's that rate compared to the Perseus group?

Pete

#16 IVM

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Posted 15 February 2013 - 11:33 AM

Pete, as far as galaxy detection and structure with 16 inches I doubt you can go wrong with a Chinese mirror (I have an f/4.5 from a Meade scope, must be the same as sold by Agena). Their suitability for other uses is more debatable - but thankfully ;) that suitability would also be outside the scope of this forum.

Howard, you said it very well about the "virtuous cycle". I wish I could pursue this approach more often.

Glenn and George, the "delta mag" is near 0 in my experience also. Of course for stars these are "visual" magnitudes and for galaxies they are "photographic", and the difference between these may be 1 as found for brighter galaxies. The detectability drops (we may call this the limit) for both galaxies and stars with my 16" under dark (21.7 mpsas) skies around 16.0. The data for galaxies were based on the Hercules Cluster (A2151) and were collected fairly systematically, and for stars the observations were incomparably more sporadic.

#17 george golitzin

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Posted 15 February 2013 - 01:07 PM

Heh - looks like the 16" is iced for now. How fleeting are budgets. I also learned the AGENA 16" mirrors can often be horrendous. Between the money redirect and the poor QC on *cheap* big glass its shelved for now.
It's not as sad as it sounds but I was starting to like the idea of detail in ngc galaxies.

I was going to try for Copelands Septet in Leo with the 8" at one time, how's that rate compared to the Perseus group?

Pete


Hoo boy, Hickson 57. The bright light in that group is ngc 3753, blue mag 14.5, but NED says about mag 13 visual. This is a tough group--although I managed seven of the eight under a dark sky last June, I found even 3753 pretty faint in my 18. Several members of the Perseus cluster are much brighter.

Sorry to hear about the budget, Pete, but I wouldn't worry about Chinese mirrors. However, nor would I spend the money on a new one--too many great deals out there on the used market. Build your own from used components, and you'll save a bundle, or find a deal on an old light bucket at craigslist...one doesn't need a high-end planetary scope for satisfying DSO encounters.

#18 azure1961p

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Posted 15 February 2013 - 07:19 PM

Thanks George.

Ill probably start building it regardless and hash out the mirror later. I AM psyched about getting my mirrors in my 8 recoated - going for the 98% reflectivity from Spectrum down in Florida. I'm so deepsky starved lol. I used to do a lot but since the coating are do old and I kno they are not even the 93% they were when new its kinda sucked the enthusiasm out of the prospect of heading to the dark hills . I should have em back in time for spring galaxies. Gosh I'm looking forward to it so much.

Pete

#19 GlennLeDrew

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Posted 16 February 2013 - 12:22 AM

When finding a limiting magnitude for compact galaxies as being similar to that for stars, has the visual magnitude for the galaxies been consulted?

All the light for elliptical types, and much of it for spirals having a dominant bulge, is yellowish and hence a blue magnitude can be up to a magnitude fainter than visual.

#20 IVM

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Posted 16 February 2013 - 03:04 PM

Exactly, Glenn. I did not consult the visual magnitudes. The only source I know that systematically gives visual magnitudes for faint galaxies is Deep Sky Field Guide, the catalog accompanying Uranometria. But even they stop at 15.0 (not always in practice, but it was their intent). So it would not be useful even for the 16" at the limit.






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