Starship Asterisk*

by **Chris Peterson** » Sun Nov 30, 2008 5:11 am

Pete wrote:As Chris pointed out, a telescope is afocal, so rays emerging from the eyepiece are parallel. The telescope aperture diameter could be any size in principle. Increasing the aperture diameter (and adjusting magnification to ensure that the eyepiece collimates all gathered light into a pupil-sized ray bundle) must increase the wattage gathered and concentrated through the eyepiece. Given a large enough telescope, shouldn't you be able to catch enough moonlight to boil water?

Yes, you could. But that would be with the telescope operating as a focal system. If you allow the field to be focused on the retina, the energy density is still no higher than it is on the Moon's surface. It might help to consider that my comment about parallel in/parallel out is a bit of a simplification. In practice, you are looking at an object with some angular size, so not all the rays enter the telescope parallel to the optical axis. And the rays that leave the telescope are not parallel to those that enter; their angle is changed by the magnification factor. That's why there is such a thing as eyepiece relief, and a narrow distance you can be from the eyepiece before you no longer get all the light. It isn't actually a cylinder of light coming out the end. And the higher the magnification, the less cylinder-like it is.

Is it possible to start a fire at the eyepiece by pointing a telescope at the Sun? I've never tried. My guess would be "yes" because (a) solar observatories need to address serious heat issues at the observation point, and (b) a big concave parabolic mirror (array) can melt metals by focusing sunlight, and I don't see how that's any different energetically from a reflecting telescope.

It is possible. But again, the image of the Sun on the retina will be no brighter than it is without using a telescope. But the total energy will be quite high, which is one reason that observing the Sun with a telescope is so dangerous. BTW, solar observatories don't technically use telescopes, they use focal systems, so this doesn't really apply. Heating usually isn't a big problem; the objectives have long focal lengths, and the resultant image sizes don't produce particularly high energy densities. I worked at Big Bear Solar Observatory, and our concerns with heat were limited to how the optical path was distorted, not about heat at the focal plane causing damage.

Finally, is this webpage incorrect?
they wrote:In fact, you can easily burn your eyes if you attempt to look at the full moon through the telescope without using any filters.

The page is generally incorrect. No telescope on Earth is big enough to cause any problems with the Moon. In theory, a big enough scope could cause corneal burns, but no such scope exists, and if it did I doubt there would be an eyepiece on it. I've viewed the Moon through the 60" scope at Mt. Wilson, and the image was no brighter than what you can see with binoculars (actually, it's no brighter than what you see with the naked eye, but having the image spread across the entire retina does make it seem so- just like in binoculars. Some lunar observers like using a filter to reduce brightness, but in reality there's no more light hitting the eye than you get off a fresh asphalt parking lot at noon. In the end, most observers prefer not using filters because reducing the total light also reduces acuity.

by **Pete** » Sun Nov 30, 2008 4:12 am

Sorry to divert back to optics, but NoelC and Chris's discussion made me call into doubt what little I thought I knew about telescopes. Chris's 1:1 magnification thought experiment helped. Correct me if I'm wrong, but the only way to fit all the light gathered by a large telescope through your pupil is to boost the magnification, which necessarily enlarges the image, thus keeping the object's surface brightness (watts per unit solid angle) unchanged in theory, and reduced in practice.

That makes sense to me. However, so does the following, and I'd be grateful if someone could help reconcile it with the above.

As Chris pointed out, a telescope is afocal, so rays emerging from the eyepiece are parallel. The telescope aperture diameter could be any size in principle. Increasing the aperture diameter (and adjusting magnification to ensure that the eyepiece collimates all gathered light into a pupil-sized ray bundle) must increase the wattage gathered and concentrated through the eyepiece. Given a large enough telescope, shouldn't you be able to catch enough moonlight to boil water?

Is it possible to start a fire at the eyepiece by pointing a telescope at the Sun? I've never tried. My guess would be "yes" because (a) solar observatories need to address serious heat issues at the observation point, and (b) a big concave parabolic mirror (array) can melt metals by focusing sunlight, and I don't see how that's any different energetically from a reflecting telescope.

Finally, is this webpage incorrect?

they wrote:In fact, you can easily burn your eyes if you attempt to look at the full moon through the telescope without using any filters.

Thanks in advance!

by **Phil G** » Sat Nov 29, 2008 2:14 am

Just from looking at a few photographs of this phenomenon, I get the impression that the cone-shaped "nebula" is nothing at all. By that, I mean it looks like the trailing "void" left behind a star plowing into and through the massive dust cloud. If it were a nebula, then it would be in front [from our POV] of the dust cloud. But the apparent interaction of the leading star with the dust cloud seems to rule out this possibility. But that's only my uneducated guess.
Peace to you all, Phil

by **Chris Peterson** » Mon Nov 24, 2008 10:01 pm

NoelC wrote:I'm done arguing here. You are free to believe what you want.

You may want to read up on the concept of minimum magnification. There are quite a few pages out there with good discussions, as well as good images (which can help with understanding what's going on when the exit pupil of a telescope is larger than the entrance pupil of the eye).

Also, if you're into optics at all, you might be interested in trying to work out a design for a 1:1 magnification telescope that can get more light in the eye than no optical system at all. I tried to do that when I first encountered this problem, in the context of why it is that there is no risk in viewing the Moon even with enormous telescopes. For me, it all made sense when I realized that the entrance and exit pupils of a telescope are related to each other by the magnification. In a unit magnification design, the two must be the same, so your eye can only use the light coming from the central 7mm (or so) of the telescope, regardless of the actual aperture. You can sketch the raytrace for that pretty easily.

by **NoelC** » Mon Nov 24, 2008 9:01 pm

I'm done arguing here. You are free to believe what you want.

-Noel

by **Chris Peterson** » Mon Nov 24, 2008 7:58 pm

NoelC wrote:People see brighter images in binoculars than they could see with the unaided eye all the time.

No, they don't. When you spread a small object across more retina, you see it better. There's even a sense sometimes of increased brightness. But it is physically impossible using optics to make an image on the back of your eye that is brighter than the unaided image.

The key here is that magnification and light gathering are two separate things. The optics could be designed to present 1:1 magnification (or even less; consider wide angle camera lenses).

A 1:1 optics system with a large aperture would produce an image on your eye indistinguishable from using no optics at all. I know it's not particularly intuitive, but it's true. Consider a 1 meter aperture telescope configured for 1:1 magnification. This telescope will produce an output beam that is one meter in diameter (exactly what you would have if you looked through a 1 meter diameter hole in a wall). You could put your head in this beam, and your eye will still intercept only a 7mm diameter spot out of it. You'll have exactly the same light entering your eye as if you used no scope at all.

The larger the aperture, the more light is gathered. Certainly you can't be convinced that your telescope, with its multi-inch aperture, is gathering only as much light as your iris allows through unaided.

The bigger scope absolutely captures more light. And in an imaging context, where the system is used focally (that is, with just the objective and film or a sensor, not a true telescope), that matters. You clearly place more photons in a given area with the larger aperture. But a telescope is an afocal device- it takes parallel rays in, and outputs parallel rays. Once the exit pupil of the telescope exceeds the entrance pupil of your eye, you don't get any more light. In terms of intensity per unit area on the retina, you can never exceed what the unaided eye can produce.

Imagine plugging in a hypothetical eyepiece that has the same focal length as your scope. You would then have 1:1 (i.e., no) magnification and a very bright image.

See the above analysis for this case. Consider as well that neither the Sun nor Moon ever look brighter, no matter how big of optics you use to view them (not a good idea to try this with the Sun, of course!) A 20" scope collects 5000 times more light than your eye; yet you can comfortably view the Moon through this scope without fear of damage- because it's still no brighter than it appears to your eye without the telescope.

by **NoelC** » Mon Nov 24, 2008 7:23 pm

It is well known that telescopes do not yield brighter images, only larger images. No telescope is possible that will allow the eye to see more light, or more color, than it could see unaided (except to the extent that more of the retina is involved, giving us a better image overall).

Ah, herein lies our problem. I respectfully ask that you go research this further, as I believe you have it wrong.

People see brighter images in binoculars than they could see with the unaided eye all the time. Most telescopes (and binoculars) add magnification, so we can see more small/distant detail as well as brighter images, but it is not necessary for this to be the case.

The key here is that magnification and light gathering are two separate things. The optics could be designed to present 1:1 magnification (or even less; consider wide angle camera lenses).

The larger the aperture, the more light is gathered. Certainly you can't be convinced that your telescope, with its multi-inch aperture, is gathering only as much light as your iris allows through unaided. Imagine plugging in a hypothetical eyepiece that has the same focal length as your scope. You would then have 1:1 (i.e., no) magnification and a very bright image.

-Noel

by **Chris Peterson** » Mon Nov 24, 2008 7:00 pm

NoelC wrote:That may be true when looking visually through a small telescope. However, we see these APOD and other digital images greatly brightened through the magic of imaging equipment. Why should we not show the object in its true visual color?

Like I said, I have no problem with this, as long as we recognize that we never can show these objects with a "true visual color", we simply can make a choice as to how we want to represent a complex spectral space as "color". For instance, the emission wavelength of Ha is what we might call a saturated red if we could see it (which we can't), so it is reasonable to represent it this way. But our eyes will never see an emission nebula with much color, so this really is just a representation. FWIW, I use gas tubes to calibrate a spectrograph, and I have a hydrogen tube. It is capable of outputting a much more intense light than any emission nebula (by several orders of magnitude), and appears pink to the eye.

In another place you made the (incorrect, or at least incomplete) statement that a large telescope does not brighten the objects that we see. That's bunk. If it were possible to see a wide field image visually through a very large telescope, in which the light from Ha emissions was brightened enough to trigger the cones in our eyes, it would indeed look very deep red.

It is well known that telescopes do not yield brighter images, only larger images. No telescope is possible that will allow the eye to see more light, or more color, than it could see unaided (except to the extent that more of the retina is involved, giving us a better image overall).

Most of the images I prepare have been taken through an RGB Bayer-pattern camera that, after the data is processed, represents things taken in bright daylight in colors in more or less their true visual colors. And I'm telling you that Ha objects show up as deep red.

No argument. But "color" only applies to the eye, not to images. So like I said, the choice of colors in the representation makes perfect sense, but doesn't reflect what the object actually looks like. In that sense, accurate color has little meaning.

Frankly, the number of bright colors visible in the universe is quite astounding, and represents a beauty that is lost in a "false color" image.

A perfectly reasonable opinion, but a subjective aesthetic one. I don't usually take color images with my telescope because I simply don't like them. I find a good B&W image far more pleasing. And when it comes to color, I prefer narrowband false color images- aesthetically- to RBG.

I agree that it is useful - and often desirable scientifically - to show images in "false color" simply because we are translating things the eye CANNOT see into light so they can be visualized. However, it does a disservice to those folks who have no concept of "false color" to show them an image of something and leave them thinking that's the way it "really looks". Imagine the number of people who think the cone nebula is orange.

True, the nebula is actually gray, so this might mislead some people <g>. I do think the caption should describe the filters used to make the image, in order to avoid confusion.

by **NoelC** » Mon Nov 24, 2008 6:32 pm

We may have to agree to disagree, then.

in emission nebulas the intensity is never enough for our eyes to see that as deep red. We see it as nearly completely desaturated.

That may be true when looking visually through a small telescope. However, we see these APOD and other digital images greatly brightened through the magic of imaging equipment. Why should we not show the object in its true visual color?

In another place you made the (incorrect, or at least incomplete) statement that a large telescope does not brighten the objects that we see. That's bunk. If it were possible to see a wide field image visually through a very large telescope, in which the light from Ha emissions was brightened enough to trigger the cones in our eyes, it would indeed look very deep red. I see no reason to represent an (admittedly brightened) image of a celestial object as anything other than in its actual colors, just because you can't see it well through your telescope visually.

Most of the images I prepare have been taken through an RGB Bayer-pattern camera that, after the data is processed, represents things taken in bright daylight in colors in more or less their true visual colors. And I'm telling you that Ha objects show up as deep red. Frankly, the number of bright colors visible in the universe is quite astounding, and represents a beauty that is lost in a "false color" image.

I agree that it is useful - and often desirable scientifically - to show images in "false color" simply because we are translating things the eye CANNOT see into light so they can be visualized. However, it does a disservice to those folks who have no concept of "false color" to show them an image of something and leave them thinking that's the way it "really looks". Imagine the number of people who think the cone nebula is orange.

-Noel

by **Chris Peterson** » Mon Nov 24, 2008 6:19 pm

NoelC wrote:And to be fair, Hydrogen glows a deep, deep red. It is a narrowband emission and so it's an extremely saturated color. An image you see with Ha emissions that are not deep red represents someone making artistic choices to bring out the detail more at the expense of visual accuracy.

We could start an entire thread on this subject. "Color" is not a physical concept, but a physiological and psychological one. In fact, the true "color" of any of these Ha objects is somewhere between pale gray and gray with just a hint of pink. That's what our eye is capable of seeing. Sure, the Ha line has a specific wavelength, but in emission nebulas the intensity is never enough for our eyes to see that as deep red. We see it as nearly completely desaturated. This is further confounded by the fact that other emission lines, as well as continuum sources are present, further reducing saturation and shifting the apparent color.

Even when imaging with good quality RGB filters, the choice of colors is largely arbitrary, and based on aesthetics. There really is no such thing as a true color representation of extended deep sky objects. Personally, I have no problem with that. An image should be colored to enhance some scientific detail (as in the image under discussion), or for aesthetic merit. I only get concerned when people start getting too worried about how "accurate" the color representation is. Because it's never accurate, and it doesn't matter anyway.

by **NoelC** » Mon Nov 24, 2008 6:00 pm

Astronomers typically prefer more understated, subtle imagery while the general public usually prefers more flashy stuff. Consider Rob Gendler's success.

And to be fair, Hydrogen glows a deep, deep red. It is a narrowband emission and so it's an extremely saturated color. An image you see with Ha emissions that are not deep red represents someone making artistic choices to bring out the detail more at the expense of visual accuracy. Or the use of non-visual color mapping, as in the APOD in question.

However, I aim to please, so here Chris, this is for you:

-Noel

by **Chris Peterson** » Mon Nov 24, 2008 3:41 pm

NoelC wrote:My complaint about the APOD of 11/23 is its color... While pretty, it's nowhere near naturally colored in the APOD image; it's being shown as orange and purple, where the real thing is an even more beautiful deep red with stunning blue-white reflection regions and multicolored stars.

Well, the image was made in Ha, OIII, and SII, so there was no intent to represent "real" color (whatever that is). The complaint, if anything, should be that the APOD caption doesn't identify this as a narrowband image. (I'd have to say, personally, that I prefer the aesthetics of the latest image; I find the typically oversaturated RGB images like the one you reference quite unattractive.)

by **NoelC** » Mon Nov 24, 2008 3:23 pm

Oops, looks as though Art has caught the APOD staff napping a bit.

Thanks for sharing that info, Art. I hadn't realized they'd found the star blowing the cone into shape.

My complaint about the APOD of 11/23 is its color... While pretty, it's nowhere near naturally colored in the APOD image; it's being shown as orange and purple, where the real thing is an even more beautiful deep red with stunning blue-white reflection regions and multicolored stars. It's also a flipped mirror image, though that's a minor nit.

To give you an idea what I'm talking about, here's a version in visual color:

-Noel

by **neufer** » Mon Nov 24, 2008 2:04 am

DavidACaruso wrote:In the APOD today, November 23, it says how the cone nebula formed remains a mystery. It would appear logical to me to assume that in the middle of the tip of the cone is a bright star, and that the gases in the larger portion of the nebula are flowing towards the outside. The winds produced by the star would cause part of the gas to bunch up and accumulate, which, like putting a rock in a flowing creek, causes the flowing material to produce an empty region that widens the further away from the "rock" (i.e. star) you get. The brightness of the tip of the cone may also be explained possibly by the ionization of the gas in the vicinity of the star.

What IS still a mystery about this 5 year old APOD:
http://antwrp.gsfc.nasa.gov/apod/ap030512.html
is that the apparent source of that wind
is neither the nearby bright B3 giant HD 47887
nor the more distance supergiant S Mon

but rather a totally obscured new star: NGC 2264 IRS

-------------------------------------------------
<<Strange shapes and textures can be found in the neighborhood of the Cone Nebula. These patterns result from the tumultuous unrest that accompanies the formation of the open cluster of stars known as NGC 2264, the Snowflake cluster. To better understand this process, a detailed image of this region was taken in two colors of infrared light by the orbiting Spitzer Space Telescope. Bright stars from the Snowflake cluster dot the field. These stars soon heat up and destroy the gas and dust mountains in which they formed. One such dust mountain is the famous Cone Nebula, visible in the above image on the left, pointing toward a bright star near the center of the field. The entire NGC 2264 region is located about 2,500 light years away toward the constellation of the Unicorn (Monoceros). >>
http://antwrp.gsfc.nasa.gov/apod/ap070509.html
-------------------------------------------------
<<One hypothesis holds that the Cone Nebula is formed by wind particles from an energetic source blowing past the Bok Globule at the head of the cone.>>
http://antwrp.gsfc.nasa.gov/apod/ap051225.html
-------------------------------------------------
<<The massive star NGC 2264 IRS, seen by Hubble's infrared camera in 1997, is the likely source of the wind sculpting the Cone Nebula and lies off the top of the image. The Cone Nebula's reddish veil is produced by glowing hydrogen gas.>>
http://antwrp.gsfc.nasa.gov/apod/ap040529.html
-------------------------------------------------
<<The Hubble telescope's infrared camera has peered into the Cone Nebula, revealing a stunning picture of six babies, Sun-like stars surrounding their mother, a bright, massive star. Known as NGC 2264 IRS, the massive star triggered the creation of these baby stars by releasing high-speed particles of dust and gas during its formative years. The image on the left, taken in visible light by a terrestrial telescope, shows the Cone Nebula, located 2,500 light-years away in the constellation Monoceros. The white box pinpoints the location of the star nursery, which cannot be seen in this image because dust and gas obscure it. The infrared image on the right shows the massive star ? the brightest source in the region ? and the stars formed by its outflow.>>
http://hubblesite.org/newscenter/archiv ... es/1997/16
-------------------------------------------------
<<The star cataloged as NGC2264 IRS is normally hidden from the inquiring gaze of optical telescopes. It resides in the midst of the obscuring gas and dust of a nearby star forming region popularly known as the Cone Nebula. Imaged in penetrating infrared light by the Hubble Space Telescope's NICMOS instrument, this young and massive star was found to be surrounded by six "baby" sun-like stars - all within less than a tenth of a light-year of their "big brother". Astronomers believe that the high speed winds generated by the massive star compressed nearby material causing the formation of the smaller stars in a text book example of triggered star formation. The young suns appear to lie along an otherwise invisible boundary where the high speed gas has collided with the wall of a denser molecular cloud. NGC2264 IRS also seems to be the source of the outflow which created the striking cone shape of the optical nebula.>>
http://antwrp.gsfc.nasa.gov/apod/ap000219.html
-------------------------------------------------
http://www.ipac.caltech.edu/2mass/galle ... 1atlas.jpg

by **DavidACaruso** » Mon Nov 24, 2008 12:29 am

In the APOD today, November 23, it says how the cone nebula formed remains a mystery. It would appear logical to me to assume that in the middle of the tip of the cone is a bright star, and that the gases in the larger portion of the nebula are flowing towards the outside. The winds produced by the star would cause part of the gas to bunch up and accumulate, which, like putting a rock in a flowing creek, causes the flowing material to produce an empty region that widens the further away from the "rock" (i.e. star) you get. The brightness of the tip of the cone may also be explained possibly by the ionization of the gas in the vicinity of the star.

http://apod.nasa.gov/apod/ap081123.html

Starship Asterisk*

Mysterious Cone Nebula (2008 Nov 23)

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