APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

Comments and questions about the APOD on the main view screen.
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johnnydeep
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Re: APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

Post by johnnydeep » Wed Jul 15, 2020 3:28 pm

ddeval wrote:
Wed Jul 15, 2020 10:56 am
Hello all,

I took this image, the 1500mm focal length can easily be explained by me pressing the 0 one too many times. It's 150mm focal length.

If anyone wants to see where it was taken from, it was the south side of the A303 south and slightly west of Stonehenge.
Thanks for that clarification on the focal length! Indeed, it is now obvious once you have pointed it out :ssmile:
"To Boldly Go......Beyond The Fields We Know."

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johnnydeep
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Re: APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

Post by johnnydeep » Wed Jul 15, 2020 3:41 pm

Chris Peterson wrote:
Tue Jul 14, 2020 9:54 pm

Telescopes are afocal. They have no focal plane- you can't hold a screen behind the eyepiece and see an image. What is coming out of the eyepiece is a fairly collimated bundle of rays. This enters your eye, and the lens in your eye brings the image to a focus on your retina. Afocal photography works the same way, with the camera lens being a required element of the optics to create a focus at the film or imager. There should be no focusing involved, except to the extent that the eye or the camera is focused at infinity- just the same focus they'd be using if you pulled the telescope out of the way. And from this, the concept of magnification makes sense. Divide the size of the object with the telescope by the size without it, and you have the magnification. You can't do that with a focal system, however, like the usual optics used for astrophotography or for ordinary photography.

FWIW: the image scale of the full-sized picture is 25.2 arcsec/pixel. Given that the E-W diameter of Stonehenge is about 35 m, this means the image was made 75 m south of the center of the circle, which is right on the big semicircular path that is around the stones. It is also consistent with a lens focal length of 150 mm.
Thanks. So what is the focusing knob on a telescope eyepiece doing? Is it just there to enable you to make sure the output light rays are parallel to each other?
"To Boldly Go......Beyond The Fields We Know."

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johnnydeep
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Re: APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

Post by johnnydeep » Wed Jul 15, 2020 3:43 pm

neufer wrote:
Tue Jul 14, 2020 10:36 pm
johnnydeep wrote:
Tue Jul 14, 2020 9:37 pm

With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I really need to read more about optics, cameras, telescopes and photography...)
An objective lens of focal length F turns a scene of angular width θ into an image of physical width Δ:

Δ = (θ x F)

An eyepiece lens of focal length f turns an image of physical width Δ into a scene of angular width ϴ:

ϴ = Δ / f = (θ x F)/f

Hence, an angular magnification M of (F/f)

Images often provide a scale Δ = so many degrees/minutes/seconds of the original scene

but "magnification" (in the common sense) depends upon how closely you, yourself, observe that image

(e.g., are you right up front in the movie theater or sitting way at the back of the balcony?)

so as to provide a scale Δ = so many degrees/minutes/seconds of the viewers experience.
.................................................................................
P.S., My 65" TV screen often shows people more or less "life sized"
...if you wish to define that as a "magnification of one" then that's a whole other matter.
Thanks. Still struggling, but I now have another way to think about it.
"To Boldly Go......Beyond The Fields We Know."

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Chris Peterson
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Re: APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

Post by Chris Peterson » Wed Jul 15, 2020 3:46 pm

johnnydeep wrote:
Wed Jul 15, 2020 3:41 pm
Chris Peterson wrote:
Tue Jul 14, 2020 9:54 pm

Telescopes are afocal. They have no focal plane- you can't hold a screen behind the eyepiece and see an image. What is coming out of the eyepiece is a fairly collimated bundle of rays. This enters your eye, and the lens in your eye brings the image to a focus on your retina. Afocal photography works the same way, with the camera lens being a required element of the optics to create a focus at the film or imager. There should be no focusing involved, except to the extent that the eye or the camera is focused at infinity- just the same focus they'd be using if you pulled the telescope out of the way. And from this, the concept of magnification makes sense. Divide the size of the object with the telescope by the size without it, and you have the magnification. You can't do that with a focal system, however, like the usual optics used for astrophotography or for ordinary photography.

FWIW: the image scale of the full-sized picture is 25.2 arcsec/pixel. Given that the E-W diameter of Stonehenge is about 35 m, this means the image was made 75 m south of the center of the circle, which is right on the big semicircular path that is around the stones. It is also consistent with a lens focal length of 150 mm.
Thanks. So what is the focusing knob on a telescope eyepiece doing? Is it just there to enable you to make sure the output light rays are parallel to each other?
Exactly. When a telescope is focused, your eye's accommodation is at infinity. You should be able to move back and forth between the eyepiece and the sky without changing your eye's focus.
Chris

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