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NGC 6118 Herschel 400 - most difficult object in the list??


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I have tried for several years to view this very faint DSO in Serpens Caput but without success. I attach two detailed star charts showing its position using Stellarium. My largest scope is my 10" Dob and I will try again, hopefully tonight, to view it with the 21mm Ethos and 13mm and 8mm Ethos.

The general location is easy to find using Yed Prior and Yed Posterior in Ophiuchus as your guide stars.

If anyone is out tonight I would appreciate if you could have a look to see if you can view it. I would be particularly interested in your scope and dark sky condition.

Mark

post-1628-0-10715400-1375696114_thumb.jp

post-1628-0-53942800-1375696131_thumb.jp

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I viewed NGC 6118 in mid-May at my dark site (SQ 21.18 mag/sq-arcsec), using my 12" f4.9 Flextube and a Baader Hyperion 8-24mm zoom. The object was approximately 31.5 degrees above my horizon, i.e. very close to the meridian. Here's what I wrote:

"Difficult. Very faint, pretty large, extended, very little brighter in the middle. Almost uniform, very faint patch, best seen at just below 8mm [x187.5], i.e. at about 10-12mm [x125-150]. At 8mm it almost completely disappears."

The NGC description is: "very faint, considerably large, considerably extended with approximate position angle 45 degrees, resolvable [i.e. mottled]". The NGC/IC Project gives the surface brightness as 13.9 mag/sq-deg (22.8 mag/sq-arcsec). Herschel designated it Class II, a "faint nebula" rather than a "very faint" one. Herschel judged visibility by size as well as brightness; for him, the large size of this object (4.7x2.0') would have outweighed its low surface brightness, which would not have been such a challenge in his very dark skies.

So the summary for this one would be that given its size it should be visible in a 10" or even smaller, but a dark sky is essential, and it should be viewed when it is as high as possible in a sky that is as dark as possible. If at your latitude you're currently able to get it at the meridian in a fully dark sky then go for it, but at my latitude the best chance was 1a.m. in May. I don't have my planisphere to hand to check when it currently culminates but just check with one of those and then check astronomical twilight time to see if your sky will be fully dark.

On the same night I viewed some other objects in the vicinity, and my ratings for those give a comparison for the visibility of NGC 6118. The Box Nebula (NGC 6309) was easily spotted at low power (though probably because of a close star making it looking fuzzy) and at 8mm with UHC filter was bright, very small, extended. M9 and NGC 6356 were both spectacular and clearly resolved. Planetary nebula NGC 6439 was stellar but clearly visible at 8mm with UHC.

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Acey an excellent post as usual. I believe I will have astronomical twilight at 22.39 UT tonight so its going to be well past the Meridan and checking its position at that time is going to be at 22 degrees above the horizon.

I have a dark site which is roughly the same location at the Lucksall site used by the forum for its star parties so I will have a go again.

Some interesting objects that you have listed. I viewed M9 and NGC 6356 in June although the others I have not observed.

Mark

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Good luck with it, Mark. Forecast is looking promising for tonight, transparency may be a lot better than it has been of late, in which case you stand a decent chance. I know what it's like with those stubborn objects that you keep trying for. Very nice when you finally get them.

Do bear in mind, though, that viewing it at 30 degrees above horizon means looking through about 2 airmass, while 20 degrees is about 3 airmass (from sec(z) where z is zenith angle). If you don't manage it tonight then you may want to wait for a time when you'll be able to view it at the meridian.

Fingers crossed for you!

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This thread makes a good test bed for calculations. In my development calculator I just punched in some data, it will be very much touch and go in a 10 inch. Good skies needed for sure. I'll be watching with interest. In fact according to my calcs you will not see it unless you are hitting a dark skies NELM 6.8 - 7.0 at zenith, given the drop you will get at 25 - 30 degrees above the horizon, but that assumes a even distribution of brightness across the object, which in this case may not be such a bad one, In reality it will be better than that though. I've not really honed the calculations for non stellar targets well, but I can approximate it assuming a uniform distribution (and that is the crux , because that can easily make or break the calculation in a case with a fine line situation such as this ). Still pleased to see that Acey's description heading fairly closely with what the calcs predict to some degree.

You wouldn't have an SQM-L meter Mark and do some measurement would you ? wishful thinking I know, enjoy the hunt :)

Edited by AlexB67
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  • 2 years later...

I have been trying to see this galaxy for many years as this thread will show. A few months back I purchased a 12" Revelation and I am very pleased with it. Last night I was viewing objects from Steve O'Meara's book 'The Secret Deep' and I was viewing faint galaxies pretty well.

I suddenly remembered NGC 6118 and thought I would give it a try. Some time back I had produced a more detailed star map of the area so I went looking for the "Blinking Galaxy'. Well it was difficult and really, really faint but I caught a glimpse using averted vision with the 14mm E.S.82. So pleased that I have nearly completed the Herschel 400.

I attach my working star map if someone wants a go.

ngc6118001.pdf

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So last night I had a go with the 16 inch at 6118. Very good skies for me. I saw down to mag 15.1 stars overhead. So pointed the scope low down at an angle of 25 deg at midnight at NGC 6118.

Lots of stars in the field down to mag 14.7.

NGC 6118 was very very tough. Easily the hardest object of the night. Why? Well it has a very low surface brightness across its oval shape. I could see the glow with averted vision at times just below two mag 14.7 stars.

It was one of those I would want to go back and confirm as I am not 100% sure the glow was the object and not just a brighter part of the FOV!

MUCH harder than NGC 6023 I was looking at before it which is a mag 14.2 object 500 million light years away and a candidate for lensing background galaxies.

So yes 6118 maybe the toughest of the Herschel 400.

I suspect this one like M101 etc would respond well to a dark sky. I bet if you had mag 6.5 skies a 10 inch would bag it.

It looks amazing in photos though!

Mark

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Mark your post confirms how difficult this galaxy is to observe. I tried for so long with the 10" and never got a glimpse. I will return to observe this galaxy as it moves higher - I am only glad that I produced the star map which allowed me to pinpoint its exact position.

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Very interesting 

a couple of years ago i posted

been doing the Herschel 400 for a few years and as the list gets shorter it becomes more difficult, I've seen 613 very low and 6118 very dim, but can not seem to be able to see 3621, I would like to see all the 400 from the UK, just wondering if anyone has seen it?

I really struggled seeing NGC3621, in books it looks easy being mag 8.5 but it a quite large galaxy and at -32 49' very low in the sky. finally managed to see it on the 24/4/2014 after many attempts.

Mark

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13 hours ago, messiermark said:

Very interesting 

a couple of years ago i posted

been doing the Herschel 400 for a few years and as the list gets shorter it becomes more difficult, I've seen 613 very low and 6118 very dim, but can not seem to be able to see 3621, I would like to see all the 400 from the UK, just wondering if anyone has seen it?

I really struggled seeing NGC3621, in books it looks easy being mag 8.5 but it a quite large galaxy and at -32 49' very low in the sky. finally managed to see it on the 24/4/2014 after many attempts.

Mark

Thanks Mark for this information. I am 3 short of the 400 - the ones that are outstanding are NGC613, NGC3621 and NGC2185. I think NGC3621 is just too low for my latitude. I suppose the best time to view NGC613 is November/December/January - although I had a 10" to see most of the Herschel 400 I did get the new 12" until February this year so did not attempt to see this galaxy this year. Finally, try as I might I cannot see NGC2185 - I know its a small reflection nebula and I have located the stars but cannot see nebulosity.

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1 hour ago, Mark at Beaufort said:

Thanks Mark for this information. I am 3 short of the 400 - the ones that are outstanding are NGC613, NGC3621 and NGC2185. I think NGC3621 is just too low for my latitude. I suppose the best time to view NGC613 is November/December/January - although I had a 10" to see most of the Herschel 400 I did get the new 12" until February this year so did not attempt to see this galaxy this year. Finally, try as I might I cannot see NGC2185 - I know its a small reflection nebula and I have located the stars but cannot see nebulosity.

Herschel discovered all the “400”, plus more than 2000 others, from observing sites in the Windsor area. So they can certainly all be seen from UK. The issue is not latitude but light pollution: Herschel had the benefit of unpolluted horizons, we don’t.

I don’t know about hardest in the 400 because I haven’t attempted that selection (which was made by some U.S. astronomers in about 1980). More interesting for me was James Mullaney’s original suggestion of going for all but the H-III objects (“very faint nebulae”) in Herschel’s original catalogues, which leaves about 600 objects. In the end, though, when I moved from 8-inch to 12-inch, I found I could see any Herschel object at my dark site in Northumberland if conditions were good enough and it was high enough in my sky, so now I’m just working my way (very, very slowly) through Herschel’s full catalogue. Certainly among the H-IIIs there are some very tough objects.

I haven’t yet tried for 3621 but I’ve observed two other objects that have been mentioned: 613 and 2185.

I’ve observed NGC 2185 twice, with an 8” f6, on 21 January 2004 and 30 November 2008. It is part of a nebulous region that includes NGC 2170, 2182, 2183-85. I wrote:

“2170 is nebulous patch round a star, another star of equal brightness nearby possibly shares the nebula. UHC filter does little. Easily spotted in 32mm [x37.5] as a hazy “star”. 2182 is less obvious, round a brighter star. Could easily be taken for a steamed eyepiece. 2183-85 picked up with 20mm [x60] plus UHC. Four stars involved in nebulosity. Largest and faintest nebula of the group, but still pretty small. Also most obvious because the involved stars are faint.”

The complex is a reflection nebula, so the effect of the filter in this case was merely to dim everything, but by dimming the stars it seemed to help visibility of 2185.

On 12-13 Oct 2012, with my 12” f4.9 Flextube, I observed NGC 908 and 613 between midnight and 1 a.m. Both were extremely low in my sky (I was on my knees at the eyepiece). For NGC 613 I wrote:

“Amazed to see it at all – didn’t expect to. Viewed due south, only about 10 degrees above horizon. Very dim but distinctly seen in good moments as a round or possibly oval patch in 8mm [x187.5], well detached from neighbouring star. Best in high power which brings out brighter inner region. Easier than NGC 908 though just as poorly placed.”

 

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Thanks for that excellent summary. I bought James Mullaney's book several years ago and was aware that he had set out 615 targets. Also of interest was the fact that he observed these objects with a 5" SCT. I must find a lower southern horizon to view NGC3621. I have no light pollution in that direction but I have a railway line which really limits me to -30 Dec.

The information on NGC2183-85 is really helpful especially your comment on using a UHC - must give it a try when next visible.

Finally, your post made me decide to go for the 615 objects as specified by Mullaney - so thanks again.

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On 2016-05-06 at 13:55, Mark at Beaufort said:

I bought James Mullaney's book several years ago and was aware that he had set out 615 targets. Also of interest was the fact that he observed these objects with a 5" SCT.

Very interesting indeed. Another practical example about importance of sky darkness, just like  Acey demonstrated in his paper

http://arxiv.org/pdf/1405.4209v1.pdf

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  • 3 weeks later...

Sorry if I've come in a bit late to this conversation. I made an observation of NGC 6118 last year in conditions I described as 'better than average'. Under average skies for my location, I doubt I would have seen it. My notes for the evening state that NGC 6118 was quite large and elongated but very, very faint and no structure could be discerned.

Here's the drawing I made - please note that the brightness of the galaxy is vastly exaggerated!

 

NGC 6118 - 15134.jpg

Edited by DeepSkyBagger
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