Re: Field of View and Magnfication calculations

wsnell01@xxxxxxxxxxx wrote:

The telescope could be either a refractor with a focal length of 850
mm or a small Mak-Cass with the same focal length, but with a tube
length only a fraction of the focal length. Use the thin-lens
formula to figure out what is actually happening

1/object distance + 1/image distance = 1/focal length

and then solve for the unknown quantity.

Right... but in his scenario, if we measure only to the objective,
then you've two unknowns--the object distance and the focal length.

Or am I missing something?

The object distance is the distance from the object to the objective
lens, known to be 5700mm in this example.

The image distance is the distance from the object to the image formed
by the objective lens.

Since we know the magnification, we can calculate that the image
distance is 15mm (FL of eyepiece) multiplied by the magnification of
66.7 = 1000mm. Plug these two numbers, 5700mm and 1000mm, into the
thin lens formula to get the focal that the lens would have if the
object were located at infinity, instead of at 5700mm.

Optics is not my strong suit. I never understood this stuff as well
as I'd liked.

It seems that if what you say is true, then the distance between the
objective and the eyepiece doesn't matter when using the thin lens
approximation. How can that be?

Granted, for astronomy where everything is at infinity, who cares.

But if the physical distance between the objective and the eyepiece is
a non-trivial fraction of the distance from the eyepiece to the
object--in this case, perhaps 15%--then discounting the size of the
scope would artifically decrease the calculated magnification, which
would then skew the subsequent focal length calculation.

For example, in his scenario, if the eyepiece were 1.2 meters behind
the objective, then the magnification is 80 instead of 67. That makes
the image distance 1200mm, making the objective lens focal length
1022mm instead of 850mm.

Or am I still missing something?

....heaven forbid I draw a diagram or something...
--
Dave
.

Relevant Pages

  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... 15mm plossl with magnification of 37X? ... D2 is the distance from the lens to the eyepiece. ...
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  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
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  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: Stupid question about magnification
    ... And that view is at least somewhat useful - the magnification ... meaningless when the imaged object is at infinitive distance. ... magnification of the simple single lens at terresterial distances is ...
    (sci.astro.amateur)
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