Re: M42 in color

On Sat, 18 Mar 2006 16:30:43 -0600, starburst <nope@xxxxxxxxxx> wrote:

I'm really trying to get my head around what your saying, and I just
don't see how it is correct. I'm not being argumentative, I'm just
trying to understand.

I'd suggest you start by taking a look at Brian Tung's well written
explanation of why you can't blind yourself looking at the Moon through
a telescope: http://astro.isi.edu/reference/faq.html#moonlight .

Here is how I would put it. First, consider what a telescope is. Two
components are required: an objective and an ocular. A collimated bundle
of rays passing the objective is converted to another collimated bundle
of rays at the output of the ocular; that is, the system is afocal.
Consider the case where the telescope has a magnification of one (the
objective and ocular have the same focal length). The largest bundle of
rays on the output that can enter your eye is determined by the size of
your pupil. You can map your pupil back through the system and you'll
see that all the light that enters your eye is coming from a subaperture
of the same size; it doesn't matter how big the actual aperture is.
Pretty clearly, what you'll see is the same size and brightness as you
would see naked eye (ignoring losses in the optics). Now lets consider
what happens when the magnification is two. Your pupil now translates
back to a subaperture twice as large, so you get four times as much
light. But the image is twice as large on your retina, so the flux
density (brightness) is just the same. As you continue to increase the
magnification, you utilize a larger and larger subaperture, until
finally you reach the full aperture of the telescope. Up to this point,
the image brightness has remained the same: just equal to the brightness
of the object itself. As you now continue to increase the magnification,
the image gets dimmer. That's because you are collecting all the light
possible with the full aperture, but spreading it out over a larger and
larger area of the retina. The flux density is decreasing. This is a
major reason why we like large apertures: you can reach a higher
magnification before the image starts getting less bright.


If I place a piece of paper at the focus of my
telescope, it will burst into flames. If the amount of energy (light)
hitting the piece of paper is the same as it is hitting my body, why
doesn't my body also burst into flames in sunlight?

Now you are talking about a focal system (not a telescope). A simple
objective _does_ concentrate light into a smaller area.


Naked eye my
limiting magnitude is around 6 on a dark night, yet with my telescope I
can see down to around 13th. Are you saying that it is simply
magnification that makes me able to see the stars? How can this be if
they are apparently point sources?

Stars are point sources. As such, they behave differently than extended
sources. The apparent angular size of the star is determined by the
aperture, as is the brightness. As the aperture increases, the star
appears smaller, and the number of photons in it increases. So you get
brighter stars, and you can naturally see more of them.


But there is a difference between focal length and focal ratio. A given
focal length will yield a particular magnification of image scale, a
faster telescope with the same image scale will produce a brighter image
at the same duration of exposure, all other things being equal, right?

No. The focal ratio makes no difference to image brightness. An f/10
scope and an f/5 scope of the same aperture, operated at the same
magnification, will produce identical images (ignoring minor performance
issues with eyepieces).


If I shoot a frame through my 50mm lens at f/2.8, it will give a much
brighter picture than one taken at f/16. Then why should this not be the
case with my eyes as the collector instead of a piece of film?

Again, you are considering a focal imaging system here. Optically, this
is not a telescope you are imaging with, even though we conventionally
call the piece of equipment that.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

  • Re: where is the red stuff?
    ... >telescope collects photons across the area occupied by its objective and ... Keep in mind that the actual number of photons you can get onto your ... retina may be less than the aperture collects. ... you need to increase the magnification of the telescope. ...
    (sci.astro.amateur)
  • Re: Comparisons for Beginners
    ... eyepiece view of M51 simulating, separately, increasing aperture and ... Matt Weir developed an eyepiece telescope simulator, ... rapidly varying aperture and magnification, ...
    (sci.astro.amateur)
  • Re: Searching for M104
    ... The first is what level of background surface brightness I am dealing ... All stars forming the "Little Dipper" were visible to the unaided eye, ... stars would likely be visible with your telescope). ... magnification until you get your own 'best' view. ...
    (sci.astro.amateur)
  • Re: Newbie asks about xmas scope...
    ... >My wife purchased a telescope for me for xmas from a site called ... It's a Bushnell Northstar reflector, 76mm aperture, ... >The scope came with two eye pieces and a barlow lens that looks like it ... that magnification on very steady nights so I would think that the 6, ...
    (uk.sci.astronomy)
  • Image Luminosity vs magnification
    ... I think I recall several people saying that assuming same aperture and ... same magnification, visual image brightness remains the same. ... the ratio will be ...
    (sci.astro.amateur)