Starship Asterisk*

by **Nitpicker** » Tue May 09, 2017 5:16 am

Ah, that makes more sense. If the "eye's entrance pupil times magnification" is larger than the physical aperture, the surface brightness is reduced compared with the naked eye brightness.

In most cases, smaller telescopes operating at recommended magnifications (and higher) give exit pupils that are significantly smaller than the typical night-time entrance pupil of the eye (i.e. the pupil diameter). This does cause some vignetting at the edges, but it normally isn't too significant and it doesn't noticeably affect the central portion of the image. Push the magnification to extremes and even the centralised image of a bright planet is noticeably degraded. If you don't push it too far, the scope is still better for small and bright things like planets (compared with a lower magnification and a perfectly matched exit/entrance pupil), though not so worthwhile for small and dim extended objects. One should not expect to see a small and dim extended object, highly magnified through a small telescope. That seems intuitive.

by **Chris Peterson** » Tue May 09, 2017 4:20 am

Nitpicker wrote:I understand the exit pupil as the physical aperture divided by magnification. I don't understand how the exit pupil times magnification could be larger than the physical aperture.

Not the exit pupil, the entrance pupil of your eye.

by **Nitpicker** » Tue May 09, 2017 1:22 am

I understand the exit pupil as the physical aperture divided by magnification. I don't understand how the exit pupil times magnification could be larger than the physical aperture.

...

Re brightness as total signal, versus signal per unit area, perhaps it is best to say that telescopes make objects appear brighter without increasing their surface brightness. One can certainly see dimmer objects with a typical telescope, than one can without (until the magnification is increased to the point that the exit pupil is too small in relation to the pupil of the eyeball).

by **Chris Peterson** » Tue May 09, 2017 12:42 am

Nitpicker wrote:Back to telescopes with eyepieces ... Assuming a perfect instrument, I would consider the ratio of the area of the telescope aperture, to the area of the eyepiece exit pupil (especially if well matched to the area of the observer's pupil) to be a good measure of how much brighter objects appear in the telescope, compared with the unaided eye. Simply a measure of the increase in the number of photons per unit time, reaching the retina. So, Chris, where have I gone wrong?

You haven't. We're just talking about different kinds of "brightness". I'm considering surface brightness- photons per unit area. A telescope can't increase that (although it frequently decreases it). You're talking about the total signal- the photon count. As I noted, we can perceive a magnified image with the same surface brightness as apparently brighter because we involve more of the retina. We're getting more signal even though the surface brightness hasn't increased.

(The surface brightness of the retinal image decreases when the aperture and exit pupil aren't properly matched. No matter how large the aperture, the eye only sees photons collected by a diameter equal to the exit pupil times the magnification. If the exit pupil is larger than the eye, the full aperture isn't being utilized. If the projected aperture (exit pupil times magnification) is larger than the physical aperture, the surface brightness is reduced compared with the naked eye brightness. This is why you can't make a unit magnification telescope that gives you an unmagnified but brighter view. The largest aperture a unit magnification telescope can have is one which is the same size as your eye's pupil. It's also why the Moon will never damage your eye, no matter how large an aperture your telescope has.)

by **Nitpicker** » Tue May 09, 2017 12:07 am

I was going to mention that a telescope used with a camera isn't technically a telescope, but most people seem to have moved past that point, as all the giant professional ones are called telescopes. It hardly seems worth mentioning anymore.

Back to telescopes with eyepieces ... Assuming a perfect instrument, I would consider the ratio of the area of the telescope aperture, to the area of the eyepiece exit pupil (especially if well matched to the area of the observer's pupil) to be a good measure of how much brighter objects appear in the telescope, compared with the unaided eye. Simply a measure of the increase in the number of photons per unit time, reaching the retina. So, Chris, where have I gone wrong?

by **Chris Peterson** » Mon May 08, 2017 11:44 pm

Nitpicker wrote:However, it is not quite right to say that telescopes don't make dim objects any brighter. They do, but not necessarily bright enough to see them "live" in the normal way that our eyes work. The larger the aperture of the telescope, the brighter the image (reducing required exposure times).

To be clear, I'm talking about using a telescope visually. Technically, a telescope used with a camera isn't even a telescope! But visually, a telescope can never make an extended object brighter than we see it with the naked eye. Only larger. In some cases, a larger object with the same surface brightness is more easily seen, since more retinal cells are involved. But the surface brightness never exceeds the naked eye view. And often it is less. Aperture doesn't matter in this regard, except to the extent that it defines the maximum magnification you can use before the surface brightness decreases from what we see without the telescope.

Photographically, increasing the aperture increases the number of photons collected, and therefore decreases the exposure time required for any given S/N.

by **Nitpicker** » Mon May 08, 2017 11:37 pm

Chris is right that giant, professional telescopes don't have eyepieces. They have camera sensors, which can (in theory) display a live video feed to a computer monitor, in lieu of an eyepiece. But a dim object like this would still show a very dim image this way on the monitor (if not entirely black). You need to record long exposures to see anything resembling this APOD.

However, it is not quite right to say that telescopes don't make dim objects any brighter. They do, but not necessarily bright enough to see them "live" in the normal way that our eyes work. The larger the aperture of the telescope, the brighter the image (reducing required exposure times). The longer the focal length of the telescope, the larger the image (which also reduces the brightness of the image for the same aperture).

...

I jumped straight into astronomy as a hobby a few years ago and I bought a small, motorised, 6 inch aperture telescope and a collection of eyepieces. I used it to see a lot of bright stuff in the sky. But it wasn't until I started attaching cameras to the telescope, that I was able to observe things much dimmer (though not as dim as Abell 370). I barely use my eyepieces for visual observation now (unless showing things to others). Even for bright objects, I see the same or more detail through the use of a camera and a computer monitor (significantly more, following a very rudimentary level of image processing).

by **Chris Peterson** » Mon May 08, 2017 2:21 pm

K1NS wrote:I know very little about the actual business of using astronomical telescopes.

Would I be able to walk up to a giant telescope somewhere on Earth, put my eye up to it, and see an extraordinary image like this? Or is this a very faint image that must be laboriously processed to come into view?

No, you would see nothing like this in a telescope (and most large telescopes don't even have eyepieces). Telescopes don't make dim objects any brighter, just bigger. If they're too dim to see with your eye (as almost all astronomical objects are), the telescope won't help.

What makes images like this possible isn't so much the telescope (although it's still required!) but the camera. A camera is able to integrate the light it collects. The longer the exposure, the more photons you record and the brighter the image gets. It doesn't necessarily require significant processing at all (although such processing is typically applied to reduce artifacts, adjust color, and generally make the image look good to our eyes).

by **K1NS** » Mon May 08, 2017 2:14 pm

I know very little about the actual business of using astronomical telescopes.

Would I be able to walk up to a giant telescope somewhere on Earth, put my eye up to it, and see an extraordinary image like this? Or is this a very faint image that must be laboriously processed to come into view?

by **Nitpicker** » Sun May 07, 2017 11:49 am

Sure, no sense being idle. I just thought it odd that each cluster was presented as a diptych in the "fast facts". Took me a while to figure out what it was showing.

by **geckzilla** » Sun May 07, 2017 11:38 am

Nitpicker wrote:The "parallel" frames that accompany each of the galaxy cluster images in this Hubble project, are very close to the clusters. Do they serve a purpose beyond showing 2x2 arcmin patches of the sky nearby?

They are there for astronomers to "do a science" on if they can think of something. Cosmologists can never have enough samples to characterize the Universe, for instance. All Hubble time is hyper optimized to make maximum use of the telescope, so we can't have one camera idling for hours at a time while another is doing the primary observation. So you could say that the purpose is to make use of Hubble.

by **Nitpicker** » Sun May 07, 2017 11:33 am

The "parallel" frames that accompany each of the galaxy cluster images in this Hubble project, are very close to the clusters. Do they serve a purpose beyond showing 2x2 arcmin patches of the sky nearby?

by **Ann** » Sun May 07, 2017 6:53 am

: Click to view full size image

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609.
Image credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli
Institute for Cosmological Physics, Univ of Chicago), and M. Gladders
and E. Wuyts (Univ of Chicago)

: Click to view full size image

NGC 3310. NASA/ESA and The Hubble Heritage Team (STScI/AURA)

I still can't get over how the reconstructed image of RCSGA 032727-132609 is systematically less blue than the lensed versions of it inside elliptical frames, in the image at left. I think that perhaps the people who made the reconstruction were inspired by the iconic, but false-color, Hubble picture of extremely ultraviolet nearby spiral galaxy NGC 3310.

Because NGC 3310 really is so ultraviolet, the galaxy was photographed through one violet and two ultraviolet filters, as well as the Hubble standard 814 nm infrared filter. The purpose was to separate the ultraviolet populations by age. Therefore, in the Hubble image, only the most ultraviolet clusters look blue at all. By contrast, the center of NGC 3310 looks downright orange, a very unrealistic color.

As I said, I think that the people who reconstructed the image of RCSGA 032727-132609, the lensed blue galaxy in the core of Core of RCS2 032727-132623, were perhaps inspired by the "intentionally redshifted Hubble image of NGC 3310" and imparted a somewhat similar "intentional redshift" to RCSGA 032727-132609.

Ann

by **Boomer12k** » Sun May 07, 2017 5:05 am

MarkBour wrote:
Boomer12k wrote:I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *
EE "Doc" Smith would have loved it.

Exactly, I loved the Lensman Series...

:---[===] *

by **MarkBour** » Sun May 07, 2017 2:28 am

As I look at the many (apparent) galaxies forming a broad upper arc in this image, I was thinking, Ann, that if you can get really good, detailed imagery, color comparison can be used to help determine which items in an image such as today's are actually the same galaxy. In spite of distortion, I realize that shape can be a major clue, but I would think that color analysis could be very important supporting evidence. Surely astronomers use this in the process. Although the lens changes the path, I would think that in most cases, the various ghost images would all be roughly the same path-length from source to us, so their red-shifts should not differ by much (IMHO).

by **MarkBour** » Sun May 07, 2017 2:09 am

Boomer12k wrote:I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *

EE "Doc" Smith would have loved it.

by **distefanom** » Sat May 06, 2017 9:28 pm

Maybe it's exactly what I meant. I was paying attention to the very same galaxy, for it's peculiar characteristics, like the "blue tail" and "more than one" bright dots in it. For the very same reason my attention was captured also by the almost single-line arcs nearer to the center of the image; for the bright dots over the blue arc, which appear also in the reconstructed image... MAYBE are ghost images of the same one.
An interesting feature of this galaxy is (what seems to me) those white bright dots, also near the center of the galaxy itself; which resemble me as more-than-one nucleus galaxy; expecially in the reconstructed image.
Anybody agree?

by **Ann** » Sat May 06, 2017 7:07 pm

Chris Peterson wrote:
Ann wrote:As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?
In the image, red is the combined light of F160W, F125W, and F098M; red thus covers 0.9-1.7 micrometers. Green is the combined light of F814W and F606W; green thus covers 460-950 nm. Blue is the light from F390W; blue thus covers 330-440 nm. However, the lensed galaxy has a redshift of z=1.7. So in terms of the emitted wavelengths, red = 630-333 nm, green = 352-170 nm, blue = 163-122 nm. So what we're seeing is UV in the green and blue channels and most of the visible spectrum mapped to red.

Thanks, Chris, really.

Make of that what you will in assessing the appearance.

: Click to view full size image

NGC 4449. Photo:
Robert Gendler.

My real question was, is the color mapping different in the rectangular box than in the elliptical outlines, given that the rectangular box supposedly shows a "more realistic" portrait of the galaxy?

My implied criticism is that whoever constructed the "more realistic" portrait of the galaxy perhaps wanted the galaxy to look "normal", at least in terms of color and stellar populations. My implied criticism is that this galaxy may have been quite different from most galaxies in the nearby universe, and therefore we should, perhaps, not try to give it "normal colors" or force it to appear to have "normal stellar populations".

However, it is possible that the lensed, blue-looking galaxy had a yellow population that was redshifted all the way out of the sensitivity of the filters used for this image. So yes, it is possible that a "reconstructed" portrait of the galaxy's "real appearance" should show it as having a yellow population.

My guess, for what it's worth, is that the lensed galaxy was relatively similar in its stellar populations to a starbursting present-day dwarf galaxy like NGC 4449. NGC 4449 certainly contains a yellow population of stars, but the yellow population is clearly not very bright and absolutely not the dominant component of of the galaxy.

By the way, and if you are interested, here is a splendid 1 MB portrait of the central part of NHC 4449.

Ann

by **Chris Peterson** » Sat May 06, 2017 6:07 pm

Ann wrote:As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?

In the image, red is the combined light of F160W, F125W, and F098M; red thus covers 0.9-1.7 micrometers. Green is the combined light of F814W and F606W; green thus covers 460-950 nm. Blue is the light from F390W; blue thus covers 330-440 nm. However, the lensed galaxy has a redshift of z=1.7. So in terms of the emitted wavelengths, red = 630-333 nm, green = 352-170 nm, blue = 163-122 nm. So what we're seeing is UV in the green and blue channels and most of the visible spectrum mapped to red.

Make of that what you will in assessing the appearance.

by **Ann** » Sat May 06, 2017 5:25 pm

bystander wrote:
Hubble Zooms in on a Magnified Galaxy
NASA | STScI | HubbleSite | 2012 Feb 02

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609

This graphic shows a reconstruction (at lower left) of the brightest galaxy whose image has been distorted by the gravity of a distant galaxy cluster. The small rectangle in the center shows the location of the background galaxy on the sky if the intervening galaxy cluster were not there. The rounded outlines show distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster's mass distribution derived from studying the distorted galaxy images.

Illustration Credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli Institute for Cosmological Physics, Univ of Chicago), and M. Gladders and E. Wuyts (Univ of Chicago)

Source-plane Reconstruction of the Bright Lensed Galaxy RCSGA 032727-132609 - Keren Sharon et al
Astrophysical Journal 746(2):161 (2012 Feb 20) DOI: 10.1088/0004-637X/746/2/161
arXiv.org > astro-ph > arXiv:1202.0539 > 02 Feb 2012

As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?

Or have I misunderstood something?

Ann

by **Chris Peterson** » Sat May 06, 2017 4:52 pm

bystander wrote:
distefanom wrote:But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

This has been done (at least once, probably more).

Maybe not more than once. The conditions have to be just exactly right. In this case, it was possible to correlate structure between different lensed components, which allowed for a lens model to be built. Given the lens model, raytracing allowed for a possible reconstruction of the source galaxy. (I say possible, because gravitational lenses don't produce normal images, and the reconstruction is necessarily a statistical one, although in this case apparently robust.)

by **bystander** » Sat May 06, 2017 4:06 pm

distefanom wrote:But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

This has been done (at least once, probably more).

Hubble Zooms in on a Magnified Galaxy
NASA | STScI | HubbleSite | 2012 Feb 02

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609

This graphic shows a reconstruction (at lower left) of the brightest galaxy whose image has been distorted by the gravity of a distant galaxy cluster. The small rectangle in the center shows the location of the background galaxy on the sky if the intervening galaxy cluster were not there. The rounded outlines show distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster's mass distribution derived from studying the distorted galaxy images.

Illustration Credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli Institute for Cosmological Physics, Univ of Chicago), and M. Gladders and E. Wuyts (Univ of Chicago)

Source-plane Reconstruction of the Bright Lensed Galaxy RCSGA 032727-132609 - Keren Sharon et al

Astrophysical Journal 746(2):161 (2012 Feb 20) DOI: 10.1088/0004-637X/746/2/161
arXiv.org > astro-ph > arXiv:1202.0539 > 02 Feb 2012

by **Chris Peterson** » Sat May 06, 2017 3:25 pm

distefanom wrote:But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

No. In order to do that, it would be necessary to know the exact characteristics of the lens, which we don't know because we don't know the precise matter distribution (most of which is dark matter). In actual practice, what is done is to treat the background galaxies as point sources (which is a reasonable approximation) and then use the shape of the lensing to infer the mass distribution. That is, we use the lensed image to understand the lens, not the lens to understand the background sources. (And that is reasonable, since we don't really care what the background galaxies look like... they are galaxies, and we understand their morphology and have countless thousands that we can study using ordinary images.)

by **distefanom** » Sat May 06, 2017 3:10 pm

But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?
I think it would be interesting, since one could "image" the "shape" of the dark matter around all this?

by **Boomer12k** » Sat May 06, 2017 7:29 am

I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *

Starship Asterisk*

APOD: Galaxy Cluster Abell 370 and Beyond (2017 May 06)

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