Starship Asterisk*

Pillars and Jets in the Pelican Nebula

Explanation: What dark structures arise from the Pelican Nebula? Visible as a bird-shaped nebula toward the constellation of a bird (Cygnus, the Swan), the Pelican Nebula is a place dotted with newly formed stars but fouled with dark dust. These smoke-sized dust grains formed in the cool atmospheres of young stars and were dispersed by stellar winds and explosions. Impressive Herbig-Haro jets are seen emitted by a star on the right that is helping to destroy the light year-long dust pillar that contains it. The featured image was scientifically-colored to emphasize light emitted by small amounts of ionized nitrogen, oxygen, and sulfur in the nebula made predominantly of hydrogen and helium. The Pelican Nebula (IC 5067 and IC 5070) is about 2,000 light-years away and can be found with a small telescope to the northeast of the bright star Deneb.

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Awesome image .. but isn't it a bit ambitious to suggest its visible in a small telescope? This is an emission isn't it, like the N.America nebula, so that's not gonna be visible in my 4inch f/14 for example... correct me if I'm wrong ...

sOnIc wrote:Awesome image .. but isn't it a bit ambitious to suggest its visible in a small telescope? This is an emission isn't it, like the N.America nebula, so that's not gonna be visible in my 4inch f/14 for example... correct me if I'm wrong ...

You've changed "can be found" to "visible" and arbitrarily defined "small" to 4 inches. We can argue about the meanings of these words if we want.

sOnIc wrote:Awesome image .. but isn't it a bit ambitious to suggest its visible in a small telescope? This is an emission isn't it, like the N.America nebula, so that's not gonna be visible in my 4inch f/14 for example... correct me if I'm wrong ...

A bigger telescope doesn't make something brighter, it just lets you get more magnification before it gets dimmer than the naked eye view. The focal ratio doesn't matter. Your 4-inch scope will give the same view as any other scope up to about 14X, which isn't an unreasonable magnification to use for this nearly Moon-sized field of view. For higher magnifications, a larger scope will certainly be better. Still, this object is plenty bright enough to be seen with a 4-inch scope, and possibly even with binoculars.

I'm pretty sure there are people at this forum who consider anything < 1m to be small, anyway.

geckzilla wrote:I'm pretty sure there are people at this forum who consider anything < 1m to be small, anyway. ;)

Yup. But I think most amateur astronomers these days probably consider about 8 inches to be the boundary between small and medium aperture.

Nice HH jet. But the Potoo isn't a fowl (nor do I find it foul). I suppose a Potoo could foul things with dust, though it lowers the tone somewhat.

Chris Peterson wrote:

sOnIc wrote:Awesome image .. but isn't it a bit ambitious to suggest its visible in a small telescope? This is an emission isn't it, like the N.America nebula, so that's not gonna be visible in my 4inch f/14 for example... correct me if I'm wrong ...

A bigger telescope doesn't make something brighter, it just lets you get more magnification before it gets dimmer than the naked eye view. The focal ratio doesn't matter. Your 4-inch scope will give the same view as any other scope up to about 14X, which isn't an unreasonable magnification to use for this nearly Moon-sized field of view. For higher magnifications, a larger scope will certainly be better. Still, this object is plenty bright enough to be seen with a 4-inch scope, and possibly even with binoculars.

For the same magnification, a larger aperture telescope makes something easier to see than a smaller aperture telescope.

Two scopes: 6"-f/10 and 12"-f/5, both have the same focal length of ~1500 mm, so both offer the same magnification and field of view, when used with identical eyepieces. But the exit pupil will be twice the diameter through the larger scope, and so long as the exit pupil is not larger than your dark-adapted pupil, more photons will reach your retina through the larger scope, and so it will appear brighter.

I cannot believe that this image is coloured as indicated by the link. The (in my view rather silly) so-called Hubble palette would imply that the major emission here is from sulphur. I think that most unlikely. Surely the major emission is from hydrogen and making that red shows the colours more nearly what one would see. If hydrogen had been coloured green, the whole thing would have been green - most unnatural.

phlloydl wrote:I cannot believe that this image is coloured as indicated by the link. The (in my view rather silly) so-called Hubble palette would imply that the major emission here is from sulphur. I think that most unlikely. Surely the major emission is from hydrogen and making that red shows the colours more nearly what one would see. If hydrogen had been coloured green, the whole thing would have been green - most unnatural.

According to this page,

The color for this image comes from Nii, Oii, and Sii narrowband emission filters colorized to a modified CHFT (Canadian Hawaii French Telescope) color pallet.

I think the APOD just links to the Hubble site to explain what "scientifically colored" means in general, not necessarily how it pertains to this image. Here, the red would apparently represent singly ionized nitrogen. I personally find "Hubble" or whichever palettes to be so nuanced from person to person that they mean virtually nothing anyway.

True color? I remember hearing about a tourist, on a cave tour, complaining to the guide about the UV fluorescence demonstration that it "wasn't the true color" of the rocks. The guide said, "OK, here's the true color," and turned off all lights.

This area is also sometimes referred to as the Mountains in the Mist. An interesting scientific paper about the Herbig Haro objects in this region was published more than 10 years ago. Also in the APOD image, north is to the right.

Sorry but a little 101 question here- I don't seem to see how this Nebula got its name the Pelican Nebula? Can anyone offer any light on this?

On another note- I think the APOD of late have been really really good!!

CURRAHEE CHRIS wrote:Sorry but a little 101 question here- I don't seem to see how this Nebula got its name the Pelican Nebula? Can anyone offer any light on this?

It's because some observers in some telescopes thought they saw a Pelican-like profile. "A remarkable bird is the Pelican: its beak holds more than its belly can!"

Nitpicker wrote:For the same magnification, a larger aperture telescope makes something easier to see than a smaller aperture telescope.

Not necessarily. At 14X, your eye will only be receiving the photons from four inches of aperture, so it doesn't matter how much larger the aperture gets. We have to consider not just aperture, but magnification as well.

CURRAHEE CHRIS wrote:Sorry but a little 101 question here- I don't seem to see how this Nebula got its name the Pelican Nebula? Can anyone offer any light on this?

Google images of this nebula. In many, it strongly resembles a pelican.

CURRAHEE CHRIS wrote:Sorry but a little 101 question here- I don't seem to see how this Nebula got its name the Pelican Nebula? Can anyone offer any light on this?

You’re not looking at the whole nebula. The second Pelican Nebula link in the description links to other views of this nebula. Today's APOD is of a small section at the back of the pelican’s “head,” with up towards the east. The dark “eye” of the pelican straddles the top of the frame, towards the right.

Chris Peterson wrote:

sOnIc wrote:Awesome image .. but isn't it a bit ambitious to suggest its visible in a small telescope? This is an emission isn't it, like the N.America nebula, so that's not gonna be visible in my 4inch f/14 for example... correct me if I'm wrong ...

A bigger telescope doesn't make something brighter, it just lets you get more magnification before it gets dimmer than the naked eye view. The focal ratio doesn't matter. Your 4-inch scope will give the same view as any other scope up to about 14X, which isn't an unreasonable magnification to use for this nearly Moon-sized field of view. For higher magnifications, a larger scope will certainly be better. Still, this object is plenty bright enough to be seen with a 4-inch scope, and possibly even with binoculars.

The North America nebula is pretty easy to see with binoculars from a dark site, the Pelican is more difficult.

Extremely large, diffuse nebulae like these can be difficult in larger scopes, because as Chris points out, you have to use a higher magnification else the exit pupil gets too large (thus negating the benefit of extra aperature.) I've observed both of these nebulae with a 12" f/5 and 20" f/3.5, and the 12" gives the better overall view. This APOD image is only a small part of the Pelican - no way can I see those dust pillars.

Ok, thanks for the replies .. interesting that some people are suggesting you can see the N.America nebula with bins under dark skies, I've seen most of the Messier catalogue with my ETX-105 but wouldn't even try for an emission nebula assuming only reflection nebula were visible .. perhaps with the exception of the M8 lagoon which is particularly bright. I know the f/14 doesn't help me. Also note I'm a massive fan of bins on a tripod, particularly for large objects like the Andromeda galaxy which is very disappointing in a 4 inch scope since you can only see the core. Anyway I'll have a try for the N.America nebula if I get to some darker skies sometime ..

Chris Peterson wrote:

Nitpicker wrote:For the same magnification, a larger aperture telescope makes something easier to see than a smaller aperture telescope.

Not necessarily. At 14X, your eye will only be receiving the photons from four inches of aperture, so it doesn't matter how much larger the aperture gets. We have to consider not just aperture, but magnification as well.

Well, I already mentioned the exit pupil limitation, but you clipped it out:

Nitpicker wrote:Two scopes: 6"-f/10 and 12"-f/5, both have the same focal length of ~1500 mm, so both offer the same magnification and field of view, when used with identical eyepieces. But the exit pupil will be twice the diameter through the larger scope, and so long as the exit pupil is not larger than your dark-adapted pupil, more photons will reach your retina through the larger scope, and so it will appear brighter.

I don't know why you keep mentioning a magnification of 14X. To see the full extent of the Pelican Nebula, you need a field of view of about two moons, or one degree, or 60 arcmin. For sOnIc's 4", f/14 scope, that would require a 28 mm eyepiece with an apparent FOV of 50&deg;, giving 50X magnification and an exit pupil of only 2 mm. But if we increase the aperture to 14" and keep the focal length at 1400 mm, thereby speeding it up to f/4, the same eyepiece will yield the same magnification and actual FOV, but with an exit pupil of 7 mm, which will appear a lot (more than ten times) brighter to the eye. (A healthy, dark-adapted, human pupil is typically said to have a diameter of 7 mm.)

Here is a table I've just whipped up, showing the numbers (and formulae) for a range of telescope apertures, all with a focal length of 1500 mm (like my 6", f/10 SCT), and setup to achieve an actual FOV of 60 arcmin. It shows that only scopes bigger than 14", give an exit pupil bigger than 7 mm. The effect of using a scope too large for this target, is to reduce the effective aperture and slow down the focal ratio, so as to limit the exit pupil to 7 mm. (It is also in line with Visual_Astronomer's observations comparing the 20" and 12" scopes.)

Ultimately, the point I'm trying to make is that, whilst aperture is the most important factor, other factors are important too. And for the same magnification, bigger apertures do make things brighter to the eye (up to the exit pupil limitation).

Actually, Chris, I think I just figured out why you keep mentioning a 14X magnification. It is the aperture divided by the dark-adapted pupil size (100/7 for a 4" scope). This is the minimum magnification theoretically achievable for a 4" scope, without wasting light on the eye. But it would require a very expensive eyepiece, methinks. (Note that in my table above, both J = F/E and J = B/H are true.)

When I first purchased my telescope a few years ago, I didn't have a brilliant understanding of how it worked. What little optics I had studied in high school had seemingly been archived in an inaccessible part of my brain and I struggled to apply it to my shiny new Schmidt Cassegrain, which physically, is only about a quarter as long as its focal length of 1.5 metres (which was a selling point for me). So, I started reading up on how telescopes work, which wasn't really that difficult. But along the way, I became increasingly dissatisfied with many of the ray tracings I'd seen documented, which purported to explain various things about telescopes and lenses in general. So, I ended up drawing my own ray tracing of my own telescope, drawn to scale, with labels for some of the terms I've already mentioned in earlier posts in this topic. If you don't have two monitors, you might need to scroll right to view the full extent of this diagram at 1:1 pixel scale:

geckzilla wrote:

phlloydl wrote:I cannot believe that this image is coloured as indicated by the link. The (in my view rather silly) so-called Hubble palette would imply that the major emission here is from sulphur. I think that most unlikely. Surely the major emission is from hydrogen and making that red shows the colours more nearly what one would see. If hydrogen had been coloured green, the whole thing would have been green - most unnatural.

According to this page,

The color for this image comes from Nii, Oii, and Sii narrowband emission filters colorized to a modified CHFT (Canadian Hawaii French Telescope) color pallet.

I think the APOD just links to the Hubble site to explain what "scientifically colored" means in general, not necessarily how it pertains to this image. Here, the red would apparently represent singly ionized nitrogen. I personally find "Hubble" or whichever palettes to be so nuanced from person to person that they mean virtually nothing anyway.

A definition of the CFHT colour palette may have been in the CFHT website but I could not readily find it. However, in some information that I found elsewhere it stated of "the famous Canada-France-Hawaii Telescope" (CFHT) palette. It assigns Ha to the red channel, OIII to the green, and SII to the blue". Does this therefore mean that any red in the APOD image would be Ha, green would be OIII and SII would blue, or does channels not necessarily mean that the colours seen in an image will correspond? Thanks for any help.

DavidLeodis wrote:A definition of the CFHT colour palette may have been in the CFHT website but I could not readily find it. However, in some information that I found elsewhere it stated of "the famous Canada-France-Hawaii Telescope" (CFHT) palette. It assigns Ha to the red channel, OIII to the green, and SII to the blue". Does this therefore mean that any red in the APOD image would be Ha, green would be OIII and SII would blue, or does channels not necessarily mean that the colours seen in an image will correspond? Thanks for any help.

There is no Ha channel in this data, assuming that the NII filter is very narrow (NII at 658 nm and Ha at 656 nm are usually captured together except with very narrow filters). Also, the data for this image says that one of the channels captured OII, but that's a near-UV wavelength. I suspect they meant OIII (501 nm).

I guess that in this image NII (naturally red) was mapped to red, OIII (naturally green) was mapped to green, and SII (naturally red) was mapped to blue.

Thanks for your help Chris.

I was not going to mention it but I thought I would that in the information with the image in the LVVASTRO website it states "Date: July/Aug 2015". An image from the future eh!