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The vanishing Eagle and the indomitable Swan


wasteland

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Hello there! I just wanted to hear some comments on this interesting fact I've noticed in the past few weeks of observing the M16-M17 pair.

Throughout the summer I've observed those amazing nebulae in moderate-to-heavy light polluted sky (limiting visual magnitude somewhere between 4 and 4.3) using my 15x70 binoculars. In the best night of the early summer season (May 28th) both nebulae were extremely well defined and noticeable without using averted vision. Under the less ideal and by far more common sky conditions, the Swan nebula appeared to be the more challenging of the duo, the most willing to vanish in the humidity-induced haze. The Eagle, on the contrary, would remain a fairly easy direct vision target regardless of how much the sky was compromised by humidity.

Fast forward to the second half of August. I finally got myself a 8'' f/5.9 dob with a handful of eyepieces and a narrowband filter (Orion Ultrablock) to facilitate the hunt for the deep sky goodies in my hopelessly polluted area. 

The thing is, even using the narrowband filter under the best condition in which I was able to operate my telescope (Milky Way + M8 were visible to the naked eye) I've been unable so far to see any nebulosity surrounding the star cluster in M16 beyond what might have very well been a product of my "averted imagination", even in the lowest power accessible to me (35mm). 

On the other hand M17 has turned out to be a very straightforward and easy target, visible in its swan-like shape even from my balcony of my urban house without the narrowband filter. Using the filter M17 remained easy to spot without averted vision even with the 3/4 full moon 6° away. I was quite surprised by that, really, I was expecting the proximity of the moon to wash out all the Sagittarius region nebulae completely.

So, how do you explain this seemingly opposite behaviour of the Eagle and Swan nebulae with respect to the observing instrument? I know it's most likely about the different surface brightness profile of the two nebulae, but I'd like to hear what observers much more experienced than I am think of this.

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Interesting, I must have observed this a hundred times and not thought anything of it.

Some objects the telescope seems to see straight through, especially if there is no sharp defining edge. Whilst the Binos with their big field of view can make out the slightly brighter patch of sky. M33 Triangulum is another one which exhibits this phenomenon.

Paul

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Ok, here's my take on it.

I think that with the binoculars, you are mainly seeing the star clusters rather than the nebulae. Whilst they are both of equal brightness, M16's cluster is 6" across vs 25" for M17. This will make for less concentrated light in M17 (lower surface brightness) and so make it harder in the binocs.

With the scope, you are actually getting to see the nebulosity rather than just the cluster. Whilst they are not too dissimilar in terms of brightness and size, M17's nebulosity is much brighter in the 'Swan' section which is the part you were seeing more easily. M16's brightness is more evenly spread out, meaning there are no higher surface brightness areas and it is harder to see.

I certainly found M17 much easier than M16 in the 8" SCT recently, but observing the wider nebulosity was probably similarly difficult. I hope that makes sense.

These may help.

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I saw M33 in both a widefield scope and the SCT in the same session recently. Finding it was definitely easier in the widefield, but once found it was surprisingly easy in the narrow field of the SCT ~1 degree. As usual, dark sky is the key.

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Absolutely, it does make a lot of sense! The only thing I'm not sure about is whether what I see through the binoculars is the star cluster or the actual nebula. It seemed to me that the shape of M16 as seen through the binos was pretty similar to what is usually depicted as the central region of the nebula.

It was somewhat like this, just way more fuzzier, clearly. http://www.ellenrooneydesign.com/gsproduction/sites/default/files/images/M16.jpg

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The brighter portions of gas in M16 do seem to roughly follow the star clusters shape. This makes it very easy to confuse the two when using bins or low power in a small scope.

By the time you've used a mid sized scope the realisation kinda hits you (as you've just found out) that this is, one tough object to see. :)

It requires a dark sky to observe the nebula in M16. Mid sized and above apertures help due to getting a good image scale at the brighter exit pupils. Your 8" should manage it fine IF the sky is nice and dark enough. 

Filters will help on this too.

Good hunting and clear skies :)

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Thanks a lot, I'm sure I'll be much more experienced by the time Summer 2016 arrives! (as a long-distance-from-home student, I won't be able to use my scope until next October, which I'm quite sure will be too late for the sagittarius region to be observed properly)

Onto the Autumn sky, then!

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The brighter portions of gas in M16 do seem to roughly follow the star clusters shape. This makes it very easy to confuse the two when using bins or low power in a small scope.

By the time you've used a mid sized scope the realisation kinda hits you (as you've just found out) that this is, one tough object to see. :)

It requires a dark sky to observe the nebula in M16. Mid sized and above apertures help due to getting a good image scale at the brighter exit pupils. Your 8" should manage it fine IF the sky is nice and dark enough.

Filters will help on this too.

Good hunting and clear skies :)

Thanks for that Steve, it certainly backs up my experience of the two. M16 was tricky in the 8"

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I observed M17 and M16 back on 8 August with my 12" dob. On M16 my notes say that the nebula required my O-III or NBP filter to bring it out with any clarity, just being "suggested" amongst the stars of the associated cluster when a filter was not in use. With the filters I thought the nebulosty looked a little like a smaller, dimmer version of M42 in form.

M17 was clear and distinctive with the 12" and no filter but adding a filter bought out excellent details and additional elements.

I'd suggest that with M16 in 70mm binoculars, presumably unfiltered, what you saw was the star cluster and the "haze" from the fainter stars in it. I found the nebulosity a challenge with 12" of aperture (no filter) and it's not really "in your face" when the filters are used so it's possible that your 8" dob even with the Ultrablock filter was not quite enough.

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M17 has been absolutely gorgeous from here. We're at the edge of town and I didn't expect to see such a bright glow of nebulosity and dark lanes . A simple SW UHC filter brought out the finest details and glow.

Well worth seeking out,

Nick.

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