Starship Asterisk*

by **MarkBour** » Wed Jul 27, 2016 8:06 pm

Fred the Cat wrote:Ps – And thanks for pointing it out. I might have woken up tonight in a cold sweat thinking, "Did those diffraction spikes have six or eight points?" and had a hard time getting back to sleep.

You're welcome, Fred! I find it's bad enough to wake up from any night mare about black spiders, without having to add any math anxiety into the equation.

https://en.wikipedia.org/wiki/The_Sleep ... s_Monsters

by **starsurfer** » Wed Jul 27, 2016 3:11 pm

For this confusing chapter of Starship Asterisk, a slightly confusing and a loosely related song:

Click to play embedded YouTube video.

by **Chris Peterson** » Tue Jul 26, 2016 10:59 pm

Knight of Clear Skies wrote:I guess we're past the point of useful discussion if you're disputing the dictionary definition of telescope.

It is a simplistic and limited definition you have provided. In a forum like this, I would generally recognize two reasonable definitions for "telescope". One is the name commonly used for any optical system intended to visually or photographically enhance astronomical objects. So we call the Keck a telescope. We call HST a telescope. And we call a camera lens a telescope when it's used on an astronomical target. The second definition is that of an actual optical telescope, which is a focal device which has a magnification (said magnification can be less than one or greater). Optically, none of the above devices identified as telescopes actually are such.

by **Knight of Clear Skies** » Tue Jul 26, 2016 3:44 pm

I guess we're past the point of useful discussion if you're disputing the dictionary definition of telescope. I doubt that many people would be persuaded that referring to a system like this as a small telescope is descriptive:

My comments were meant constructively, apologies if they didn't come across that way.

(Must run as I need to take my vehicle into the municipality.

)

by **Chris Peterson** » Tue Jul 26, 2016 1:04 pm

Knight of Clear Skies wrote:
Chris Peterson wrote:Nevertheless, there is no real distinction that needs to be drawn in an astronomical image made with an objective designed to attach to a camera and an objective designed to have an eyepiece. All that is really relevant is the aperture and the focal length- the key parameters for any imaging optical system. The rest is details. Nice to know, but not essential.
A short telephoto lens is very similar to a small telescope, I agree.

The focal length is entirely irrelevant. In terms of optical function, there is no difference between a camera lens and the instrument we call an astronomical telescope.

However, not all camera lenses can be usefully described as telescopes (an optical device designed to magnify distant objects).

If you use that definition of telescope, than no camera lens qualifies. Neither does any astronomical telescope used as a focal imager. Optically, telescopes are afocal. They require both an objective and and ocular, and when we image we lack the latter. Imaging systems are focal. They do not magnify.

To take an extreme example when using a fisheye lens with a field of view of 180 degrees the photographer has to be careful to keep their feet out of the shot.

Which is as much an imaging telescope as the Keck or the HST.

50mm focal length is about the point where describing a camera lens as a telescope becomes misleading I'm afraid.

Not at all. In an imaging system, it is trivial to construct a camera with a 50mm lens that provides a higher resolution image of a target than a different camera with a 1000mm lens.

by **Knight of Clear Skies** » Tue Jul 26, 2016 11:38 am

I should add, I find it quite an inspiring APOD - makes me wonder what i could achieve with my own imaging rig. I also own a 50mm f1.4 lens, it looks like this on a tracking mount:

I won't be able to go as deep as the researchers' image with my DSLR but the IFN should be within reach. I got this on Orion with a 30 second exposure at f1.4, using an Ha filter:

(Posted small as distortion is quite extreme with the lens wide open, these lenses typically need to be stopped down to about f4 to produce an acceptably flat field.)

by **Knight of Clear Skies** » Tue Jul 26, 2016 10:42 am

Chris Peterson wrote:Nevertheless, there is no real distinction that needs to be drawn in an astronomical image made with an objective designed to attach to a camera and an objective designed to have an eyepiece. All that is really relevant is the aperture and the focal length- the key parameters for any imaging optical system. The rest is details. Nice to know, but not essential.

A short telephoto lens is very similar to a small telescope, I agree. However, not all camera lenses can be usefully described as telescopes (an optical device designed to magnify distant objects). To take an extreme example when using a fisheye lens with a field of view of 180 degrees the photographer has to be careful to keep their feet out of the shot.

50mm focal length is about the point where describing a camera lens as a telescope becomes misleading I'm afraid. A simple experiment, if you put a 50mm lens on a camera body and look through the viewfinder the image scale does not noticeably change compared to what can be seen with the naked eye. Such a combination does not function as a makeshift telescope, a longer lens is required.

by **alter-ego** » Tue Jul 26, 2016 4:57 am

geckzilla wrote:
BMAONE23 wrote:I would say the Hubble is a fantastic camera with the world's best telescopic lens
I personally don't think camera does it justice. It's an entire suite of instruments. Even telescope simplifies it, but that is often done for many observatories. It is, as they say, one of NASA's great observatories.

On the other hand, the planned LSST is a camera!
That's how I think of it anyway. In my opinion, it's purpose as a dedicated 8-meter class objective with a 3.5° FoV makes it appear and function more as a camera, albeit a phenomenal one with it's sub-arcsecond resolution and 15 terabytes of data per night! However, let's call it a telescope as that's what it also is. It's capability is equally phenomenal and it resides in a very special class of telescopes.

by **geckzilla** » Tue Jul 26, 2016 4:14 am

BMAONE23 wrote:I would say the Hubble is a fantastic camera with the world's best telescopic lens

I personally don't think camera does it justice. It's an entire suite of instruments. Even telescope simplifies it, but that is often done for many observatories. It is, as they say, one of NASA's great observatories.

by **Boomer12k** » Tue Jul 26, 2016 4:12 am

Show me the path of the LMC, SMC.....

Also... distortion from other satellites, as well as the Milky Way itself, I should think... was there not an APOD like last year or so, that showed star streams from other satellites and the MW???

I am sure they are all playing off of each other... also, the Andromeda Galaxy at only 2.2 mly.... and closing...

Packing like a banshee for my move, I close on this house next Monday... I have started taking loads to the new home.... sigh...
:---[===] *

by **BMAONE23** » Tue Jul 26, 2016 3:22 am

I would say the Hubble is a fantastic camera with the world's best telescopic lens

by **Chris Peterson** » Mon Jul 25, 2016 10:49 pm

Tszabeau wrote:
Chris Peterson wrote:
Tszabeau wrote:Where is the center of the Milky Way in relation to the point-of-view in this image?
A long way away. The image is oriented approximately north up, east left. The galactic center is about 75° away to the southwest (far, far out of the frame), which actually means you cross near the south celestial pole and head back north again.
So... if I was looking up at the SMC and LMC, from the same vantage-point, would the center of the Milky Way be behind me and to my left "under" the horizon?

It need not be below the horizon. It depends on the location of the pole with respect to your location, too. And your own orientation. At the right place and time you could easily see them both in the same view, 75° apart (even parallel to the horizon).

by **Tszabeau** » Mon Jul 25, 2016 10:19 pm

Chris Peterson wrote:
Tszabeau wrote:Where is the center of the Milky Way in relation to the point-of-view in this image?
A long way away. The image is oriented approximately north up, east left. The galactic center is about 75° away to the southwest (far, far out of the frame), which actually means you cross near the south celestial pole and head back north again.

So... if I was looking up at the SMC and LMC, from the same vantage-point, would the center of the Milky Way be behind me and to my left "under" the horizon?

by **Chris Peterson** » Mon Jul 25, 2016 10:00 pm

Tszabeau wrote:Where is the center of the Milky Way in relation to the point-of-view in this image?

A long way away. The image is oriented approximately north up, east left. The galactic center is about 75° away to the southwest (far, far out of the frame), which actually means you cross near the south celestial pole and head back north again.

by **Tszabeau** » Mon Jul 25, 2016 9:42 pm

Where is the center of the Milky Way in relation to the point-of-view in this image?

by **Chris Peterson** » Mon Jul 25, 2016 9:02 pm

geckzilla wrote:I think Chris once called Hubble Space Telescope a camera. I can see his point, but I'm not sure a lot of people agree with his use of language. ;)

Depending on how technical or precise you want to be, the HST can be either a telescope or a camera. What would be incorrect would be to say it isn't a camera. Likewise for today's APOD. It's reasonable to call the optics either a telescope or a camera lens, but it's simply wrong to say it isn't a telescope because it's also a camera lens.

by **geckzilla** » Mon Jul 25, 2016 8:56 pm

I think Chris once called Hubble Space Telescope a camera. I can see his point, but I'm not sure a lot of people agree with his use of language.

by **RJN** » Mon Jul 25, 2016 6:10 pm

In the interest of clarity and correctness, I have now changed the words "star streams" to "filaments" in the APOD text. Initially, in my first writing, I had thought that all of the faint filaments were star streams. The lead author of the paper that contains this image corrected me -- many of the filaments are actually Galactic Cirrus dust clouds. I thanked her, made a change in one place of the APOD text, but did not realize that another part of the text then became lacking. My hope is that this is now corrected. I apologize for the oversight. - RJN

by **Fred the Cat** » Mon Jul 25, 2016 5:26 pm

MarkBour wrote: All of the "spiders" I see in this image appear to have the proper number of 8 legs ...

I'd say that two of them are fangs but it's really that I didn't count.

Ps – And thanks for pointing it out. I might have woken up tonight in a cold sweat thinking, "Did those diffraction spikes have six or eight points?" and had a hard time getting back to sleep.

by **Chris Peterson** » Mon Jul 25, 2016 4:21 pm

Guest wrote:
Chris Peterson wrote:
Knight of Clear Skies wrote:One minor error I noticed in the description is that it was taken with a camera lens, not small telescopes.
A camera lens is a small telescope.
Not really, no, the 50mm lens in question doesn't provide much in the way of magnification. ;)

Yes, really. A telescope objective doesn't provide any magnification, either.

(Optically speaking, the design of a camera lens is very different from a refractor, which are simple designs optimised to produce a flat field at infinity.

Optically, neither a camera lens or a "telescope" used for imaging is a telescope. But both are optically equivalent- objectives. It is only in terms of engineering that a typical camera lens and a typical astronomical refractor differ- not surprising given different design goals.

Nevertheless, there is no real distinction that needs to be drawn in an astronomical image made with an objective designed to attach to a camera and an objective designed to have an eyepiece. All that is really relevant is the aperture and the focal length- the key parameters for any imaging optical system. The rest is details. Nice to know, but not essential.

by **MarkBour** » Mon Jul 25, 2016 4:16 pm

Fred the Cat wrote:
rj rl wrote:A little unrelated, but does the inversion itself highlight some features somehow? Are we better at seeing black against white than white against black?
No but it helps us see the six-legged spiders more easily!

All of the "spiders" I see in this image appear to have the proper number of 8 legs ...

Why didn't they just leave the whole image in the gray-scale? Why did they insert distinct "pretty" images of the two galaxies?
The main image would have been nearly black there, I assume, but I'd like to see it. Do we have a version?

Just looking at this image, really, I don't get the point of its evidence for a collision or strong interaction. I see a few shape features that could be evidence of their gravitational interaction ... or not.

by **Knight of Clear Skies** » Mon Jul 25, 2016 3:51 pm

Sorry, looks like I wasn't logged in when posting the above. ^

by **Knight of Clear Skies** » Mon Jul 25, 2016 3:49 pm

Chris Peterson wrote:
Knight of Clear Skies wrote:One minor error I noticed in the description is that it was taken with a camera lens, not small telescopes.
A camera lens is a small telescope.

Not really, no, the 50mm lens in question doesn't provide much in the way of magnification.

(Optically speaking, the design of a camera lens is very different from a refractor, which are simple designs optimised to produce a flat field at infinity. Lenses need to produce a colour corrected image at a range of focal distances, here's a diagram of the lens in question:

Lenses do a good job for astrophotography but typically need to be stopped down a little to produce a flat field, as this often isn't a major design consideration. I find myself wondering if a specialist design would do a better job for widefield imaging, but given the economies of scale involved in current camera lens production it's probably a moot point for astrophotographers.)

by **Fred the Cat** » Mon Jul 25, 2016 2:12 pm

rj rl wrote:A little unrelated, but does the inversion itself highlight some features somehow? Are we better at seeing black against white than white against black?

No but it helps us see the six-legged spiders more easily!

by **Chris Peterson** » Mon Jul 25, 2016 2:10 pm

Knight of Clear Skies wrote:One minor error I noticed in the description is that it was taken with a camera lens, not small telescopes.

A camera lens is a small telescope.

Starship Asterisk*

APOD: Deep Magellanic Clouds Image... (2016 Jul 25)

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