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

Comments and questions about the APOD on the main view screen.
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geckzilla
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Re: APOD: Deep Magellanic Clouds Image... (2016 Jul 25)

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

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

Post 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.

Image

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.

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

Post 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:

Image

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:

Image

(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.)

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

Post 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.
Chris

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

Post 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:

Image

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

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

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

Post 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.

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

Post 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. :ssmile:
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. :D
https://en.wikipedia.org/wiki/The_Sleep ... s_Monsters
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