Eagle Nebula (M16)

[Bugdozer]

I am not sure if I am doing something wrong here, or just misinformed. 

A few weeks ago I had a look at the Eagle Nebula for the first time. I had read that it is supposedly easy to see the nebulosity. I was in a location with pretty good dark skies, from which I have seen other diffuse objects that are supposedly more challenging. However, I couldn't see ANY nebulosity at all. Not even a hint. I knew I was in the right place because I could see the stars within it quite clearly. 

Obviously I wasn't expecting to see the pillars of creation but surely I should have been able to see something? Is it just that the nebula itself isn't as easy to see as I thought? 

[michael.h.f.wilkinson]

What magnification were you using. I can pick it out quite readily with 16x80 from a dark site, so it isn't a very hard object. I do notice using a UHC filter in my C8 at 65.5x (with the Nagler 31T5) helps a lot

[Bugdozer]

11 hours ago, michael.h.f.wilkinson said:

What magnification were you using. I can pick it out quite readily with 16x80 from a dark site, so it isn't a very hard object. I do notice using a UHC filter in my C8 at 65.5x (with the Nagler 31T5) helps a lot

I was using my 25mm eyepiece so probably about 50x. I don't know much about UHC filters, I asked a guy selling them at the Herstmonceux Astronomy Festival a few weeks ago how useful they were, and he said they didn't make much difference... which I thought was odd coming from someone actually selling filters! 

[Nik271]

The sky transparency has been very poor over southern UK recently and the Eagle nebula is not very high over the horizon, only 25 degrees. Did you see the nearby Swan nebula, it's a bit brighter? It may be just the haze and humidity in the air.

[michael.h.f.wilkinson]

16 hours ago, Bugdozer said:

I was using my 25mm eyepiece so probably about 50x. I don't know much about UHC filters, I asked a guy selling them at the Herstmonceux Astronomy Festival a few weeks ago how useful they were, and he said they didn't make much difference... which I thought was odd coming from someone actually selling filters! 

UHC filters only work well for emission nebulae, not galaxies or reflection nebulae, so they certainly have the ir limitations. For most emission nebulae, they work very well

[Bugdozer]

17 hours ago, Nik271 said:

The sky transparency has been very poor over southern UK recently and the Eagle nebula is not very high over the horizon, only 25 degrees. Did you see the nearby Swan nebula, it's a bit brighter? It may be just the haze and humidity in the air.

I wasn't aware of the Swan Nebula so I didn't try it. I will probably have to wait until next year now, but that's OK.

Maybe a UHC filter would be a good investment. Do planetary nebulae and supernova remnants count as emission nebulae? I am hoping to observe the Crab this winter, it's been on my bucket list for decades but somehow I have just never got round to it.

[Nik271]

 

9 minutes ago, Bugdozer said:

Do planetary nebulae and supernova remnants count as emission nebulae?

Yes they are, so UHC or Oiii filters help with them. The Crab (M1) is a bit fainter, bit with the advantage that is much higher from the UK. With experience I can see it in 20x80 binoculars in my Bortle 5/6 sky without filters. Don't expect much though, it is just a faint smudge in small instruments.

[bomberbaz]

what scope were you using to observe the nebula? The focal ratio/length can have a big bearing on nebula, especially in smaller scopes.

As mentioned above you also need good transparency for viewing nebula, and a larger exit pupil is preferred especially on more diffuse and dim objects. 

Can't understand what the guy was on about telling you UHC filters don't really help. They provide a huge difference on very many objects, see here : Filter Performance

The crab without a filter is little more than a fuzzy blob. With one you will see more structure to a point but the best all round thing for nebula viewing is dark skies (with a good filter).

[Bugdozer]

1 hour ago, bomberbaz said:

what scope were you using to observe the nebula? The focal ratio/length can have a big bearing on nebula, especially in smaller scopes.

As mentioned above you also need good transparency for viewing nebula, and a larger exit pupil is preferred especially on more diffuse and dim objects. 

Can't understand what the guy was on about telling you UHC filters don't really help. They provide a huge difference on very many objects, see here : Filter Performance

The crab without a filter is little more than a fuzzy blob. With one you will see more structure to a point but the best all round thing for nebula viewing is dark skies (with a good filter).

The scope is a Celestron Nexstar 5SE, f/10 I believe, using 25mm eyepiece. What has confused me is how I have seen other objects that are supposedly harder to see than the Eagle, in comparable conditions. 

Just out of interest, how does the focal ratio of the scope affect the appearance of nebulae, apart from their brightness which will also be related to aperture? 

[John]

Here is a sketch of the Eagle Nebula as observed with a 200mm scope under a dark sky by an experienced observer. He used an eyepiece +barlow lens and a UHC filter combination which resulted in 80x with his scope.

Messier 16 (Eagle Nebula) - Deep Sky Watch

 

Edited October 2, 2023 by John

[bomberbaz]

On 03/10/2023 at 00:15, Bugdozer said:

The scope is a Celestron Nexstar 5SE, f/10 I believe, using 25mm eyepiece. What has confused me is how I have seen other objects that are supposedly harder to see than the Eagle, in comparable conditions. 

Just out of interest, how does the focal ratio of the scope affect the appearance of nebulae, apart from their brightness which will also be related to aperture? 

Ok so here in lies part of the issue with your scope. F10 / 25mm ( your eyepiece) is giving you an exit pupil of 2.5mm, if it was F5 you would have 5mm. The former gives you an area coverage of your retina of 4.9mm sq, the latter 19.6mm sq. So you get 4 times the level of retinal neurons activated by the faster F5 ratio meaning dimmer objects are brighter from your perspective. 

To get around this small exit pupil you could A, get a larger focal length eyepiece such as a 40mm plossl, which is the largest available 1.25" eyepiece to you and would give you a 4mm exit pupil / 12.5mm sq retinal covereage and this would make a significant difference to dimmer objects.

Option B you could get a UHC filter (get a decent one if you take this option) as this would isolate the OIII & HB lines in the nebula and make it stand out more against the now darkened night sky. However the effectivness of  a UHC is variable but an exit pupil of 3-4mm is generally optimal on diffuse emmision nebula.

Another option is to get a 0.5 reducer as this effectively gives you an F5 scope which as we have seen increases your exit pupil and is also the cheapest option I believe (£19 with flo). It might produce some aberrations to the view as a result but you would have a greater chance to see the eagle easier than at F10.

The best option imho is a combination of UHC and either reducer or longer focal length eyepiece.

 

Edited October 4, 2023 by bomberbaz

[ollypenrice]

Even in a 20 inch at our exceptionally dark site (and a good deal south of the UK) I have always found the Eagle disappointing, visually. There is certainly nebulosity to be seen but I never found much to link the EP view to the photographic. (Obviously I wasn't hoping for the tiny Pillars of Creation either.) On the other hand the Swan is spectacular, visually, and the Swan shape is clear and insistent.

Olly

[Richard]

On 01/10/2023 at 00:04, Bugdozer said:

I was using my 25mm eyepiece so probably about 50x. I don't know much about UHC filters, I asked a guy selling them at the Herstmonceux Astronomy Festival a few weeks ago how useful they were, and he said they didn't make much difference... which I thought was odd coming from someone actually selling filters! 

Going a bit off-track, but my first view of the Veil, through a 16” Dob and Lumicon UHC, was the first time I’d seen something that looked something like the observatory images available then (~25 years ago). Even from pretty dark skies (Bortle 2/3), the UHC filter made a big difference. Without it the Veil was clearly visible, but with it I could see quite a lot of detailed structure.  

[ollypenrice]

13 hours ago, Richard said:

Going a bit off-track, but my first view of the Veil, through a 16” Dob and Lumicon UHC, was the first time I’d seen something that looked something like the observatory images available then (~25 years ago). Even from pretty dark skies (Bortle 2/3), the UHC filter made a big difference. Without it the Veil was clearly visible, but with it I could see quite a lot of detailed structure.  

My experience, precisely.

Olly

[Bugdozer]

Thanks for the info everyone. I have a UHC filter now so hopefully that will make a difference, but from the observations with the sketch it sounds like the nebulosity is simply just quite difficult to see without a really contrasty sky.

From reading up about the Bortle scale and various other things I am starting to form the impression that contrast between the sky and the object is more important than the actual dark adaptation of your eye - if you can see the circle of the inside of the sky within the eyepiece as anything other than pitch black, then your eye is already dark adapted well enough to detect that level of light, therefore the barrier to seeing the object must logically be the low contrast difference.

[bomberbaz]

On 05/10/2023 at 09:26, Bugdozer said:

Thanks for the info everyone. I have a UHC filter now so hopefully that will make a difference, but from the observations with the sketch it sounds like the nebulosity is simply just quite difficult to see without a really contrasty sky.

What make of UHC did you get out of curiosity?

 

[Bugdozer]

19 hours ago, bomberbaz said:

What make of UHC did you get out of curiosity?

 

I got the Astronomik one, because it seemed to have overall positive reviews. I have only had a couple of opportunities to use it since it arrived but not on the Eagle yet. It definitely improves contrast on the things I have looked at so far, although I find focusing with it in a little tricky because you can actually notice the slightly different focal points between the green and red wavelengths. But as they so famously didn't say in Star Trek, you cannae change the laws of physics! 

[bomberbaz]

4 hours ago, Bugdozer said:

I got the Astronomik one, because it seemed to have overall positive reviews. I have only had a couple of opportunities to use it since it arrived but not on the Eagle yet. It definitely improves contrast on the things I have looked at so far, although I find focusing with it in a little tricky because you can actually notice the slightly different focal points between the green and red wavelengths. But as they so famously didn't say in Star Trek, you cannae change the laws of physics! 

Same as mine, it is a very good filter indeed. I still think you would benefit a lot with a longer focal length eyepiece to increase your exit pupil but see how you get on. 

[Bugdozer]

On 09/10/2023 at 22:06, bomberbaz said:

Same as mine, it is a very good filter indeed. I still think you would benefit a lot with a longer focal length eyepiece to increase your exit pupil but see how you get on. 

Just out of interest, do you have a particular 32mm eyepiece you would recommend? As well as the increased exit pupil I am thinking of getting something with a wider field of view, if I can.

[bomberbaz]

3 minutes ago, Bugdozer said:

Just out of interest, do you have a particular 32mm eyepiece you would recommend? As well as the increased exit pupil I am thinking of getting something with a wider field of view, if I can.

I was thinking of a plain 32mm plossl, although this still only gives you a 3.2mm exit pupil, that said it gives 70% larger exit pupil.

Personally I would go for a 40mm plossl as this gives you the same fov as a 32mm but a 4mm exit pupil which is considerable 130% larger than what the 25mm offers (and costs less than 30 quid). Astro Essentials Super Plossl Eyepiece | First Light Optics It also still gives a very good x31 magnification due to the long focal length of your OTA. Don't worry about magnification either, x 31 is plenty, I only get x19 with my large bins and it is easily seen with them. A lot of people get caught up in magnification but exit pupil size is the most important factor to consider.

If you do get one in time to revisit the eagle, go to the omega/swan (M17) nebula first, try with and without filter to get your eyes used to seeing obvious differences. It should really pop right out at you, and I mean considerable difference. Then with a short hop goto the eagle with dark adapted eye and you will see it. This is one of those that the longer you look, the more you see. The blinking technique often helps with these fainter nebula in showing up more detail.

Please do let me know how you get on, it would be interesting to know which way you go.

[bomberbaz]

21 minutes ago, Bugdozer said:

Just out of interest, do you have a particular 32mm eyepiece you would recommend? As well as the increased exit pupil I am thinking of getting something with a wider field of view, if I can.

Forgot about this. Due to the limitations of the 1.25" system you are at the limits of fov with the 40mm plossl. However it is still giving you a fov of 1.3 degrees which is big enough for the vast majority of DSO.

[Bugdozer]

On 09/10/2023 at 22:06, bomberbaz said:

Same as mine, it is a very good filter indeed. I still think you would benefit a lot with a longer focal length eyepiece to increase your exit pupil but see how you get on. 

Finally managed to JUST see the nebulosity very faintly tonight with the aid of the UHC filter. It really is pretty dim. Then I took the filter off and it just vanished. When summer rolls around and it's in a better position for me I reckon I will be able to see it a little better. And hopefully will have a longer length eyepiece by then.

[bomberbaz]

9 hours ago, Bugdozer said:

Finally managed to JUST see the nebulosity very faintly tonight with the aid of the UHC filter. It really is pretty dim. Then I took the filter off and it just vanished. 

The longer focal length eyepiece will really help. Trust me on that