Re: aperture and magnification gain puzzle


canopus56 wrote:
> chemstudcou@xxxxxxxxx wrote:
> > It is mentioned in Rutten Telescope Optics that as contrast
> > get lowered in an object, one must increase the magnification
> > to see more details. Well. Increasing the magnification
> > can further lower the contrast so how could you see more
> > details[?]
>
> This is what Clark's book and animated gifs at his website are about,
> referenced previously in this thread.
>
> http://www.clarkvision.com/vis­astro/index.html
> http://www.clarkvision.com/vis­astro/m51-mag/index.html
> http://www.clarkvision.com/vis­astro/m51-apert/index.html
>
> It means that when you observe in the best dark sky, you try to get
the
> best contrast between the background night sky and object by going to
> the best dark sky site. Once the selection of the best contrast is
> made, then you try various levels of magnification to get the optimal
> level contrast and detail between the object and the background sky.
In
> my experience, this boils down to choosing between two or three
lenses.
>
>
> Sometimes you increase magnification to get the best view; sometimes
> you decrease magnification to get the best image. The best
> magnification depends on the brightness of each individual extended
> object and the level of light pollution on any given night. If the
> Rutton page you are reading really does say that you only increase
> magnification to get the best image, it may be an ambiguous choice of
> words and that has misdirected your thinking.
>
> What is the specific page in Rutten are you reading? I have a copy.
>
> Your ability to see an extended object depends on the level of
contrast
> between the object and the background light polluted sky. Even from
a
> light polluted suburban sky, you still will see part of M37 in Auriga
> or the Andromeda Galaxy through binoculars or a telescope.
> http://www.seds.org/messier/m/m037.html
> But if you really want to see the details in these objects, you have
to
> go a dark sky site. Try watching a TV picture with the screen pointed
> towards an open sunny window. Then watch it with the window blinds
> closed.
>
> Magnifying an image of an object that has a low contrast to its
> background mostly magnifies the washed-out contrast. There is a
minor
> improvement in contrast as you increase magnification because as more
> magnification is applied, the background night sky gets dimmer faster
> than the extended object itself. This effect (the background sky
> darkening faster than an object in the background sky) isn't the
result
> of optics. This effect is the result of how the chemical processes
> within the human eye respond to different light levels.
>
> You can easily experience the telescopic change in darkness of the
> background sky with a camera with that has a manual zoom or with a
pair
> of zoom binoculars. Or you can use various lenses in any small
> aperature telescope. On a clear night in a heavily light polluted
> urban sky, point the telescope and focus on a "blank" piece of sky at
> the lowest possible magnification. You will not see any stars; you
will
> see just a blank light polluted sky in the field of view. As you
> increase magnification, you will notice that blank light polluted
field
> in the eyepeice will appear darker. With a zoom lens, it is easy to
see
> this change in background brightness as you rapidlly ramp the zoom
> magnification in and out.
>
> Depending on the level of light pollution and the magnitudes of the
> stars in the scope's field of view, you may get lucky and see
> background stars "pop" out of the blank field. This occurs when
> darkness of the telescopically-magnified dark-background-sky becomes
> darker than the stars in the field of view.
>
> The same contrast principle applies at sunset. As the sky darkens,
> usually you can see the brightest stars and planets first. These
bright
> stars and planets have the greatest contrast between themselves and
the
> twilight sky. The number of visible stars increases as the sky gets
> darker. This is because as the contrast between the star and night
sky
> increases, we can see more detail.
>
> But the minor improvement in contrast as telescopic magnification is
> increased is not enough to "fix" a washed out light polluted sky.
>
> When you want to improve a telescopic image of an extended object
like
> galaxies and nebula seen in your eyepiece, your choices are usually
> are:
>
> - 1) go to a better darker sky to improve the contrast between the
> background night sky,
> - 2) use more magnification to see more details in the object, but
> losing some detail from the reduced brightness of the image.
> - 3) add more aperature ( get a bigger scope ) to make the image
> brighter.
> - 4) add both aperature and magnifaction. This gives you a brighter
> image and more detail.
>
> With respect to aperature and magnification alone (and omitting the
> contrast factor discussed above), there is a good table in Sidgwick
> (referenced above) at p. 31 that summarizes aperature and
> magnification. As Sidgwick states it:
>
> "It can readily be seen that a large aperature gives a brighter
> extended image than a smaller one using the same magnification, and
> that the large instrument can use a higher magnification than the
> smaller in production of an equally bright image."
>
> You can see this principle in practice by looking at an extended
> object, for example, the planetary nebula NGC 3242 we discussed in
> another thread, through various scopes. Currently, M37 and M36 in
> Auriga are favorably positioned, as are galaxies M101, M81 and M82 in
> Ursa Major. Go to a star party at your local astronomy club and look
at
> these objects through various scopes. If you own a small cheap
> refractor and larger DOB, you can use your own equipment of varying
> aperature. Take a range of your lenses, like a 10mm, 15mm and 20mm
> with you and look at the extended object through each scope and each
> series of lenses. It also helps to look at objects in the "good" dark
> part of the sky and also other extended objects on the fringe of the
> light polluted part of your local sky.
>
> If you hand a copy of this usenet post to a local club member at a
> party, they quickly will understand what you want to see and probably
> will happily walk you through a few observations.
>
> This practical experience will fix the brightness relationships we
have
> been discussing firmly in your mind.
>
> > Thanks for the clarifications.
>
> You are welcome. It helps me to have to force myself to think through
> this stuff periodically.
>
> - Canopus56
>
> Clark, R.N. Visual Astronomy of the Deep Sky. Cambridge Univ.
Press.
> 1990.


Thanks for the assistance. You must be a professor or a one
with great potential for that.

It is in Rutten book page 217

It says something like this. The eye has resolution of 60
arcseconds. A telescope has 120/D arcsecond... so the
optimum magnification is 60/(120/D) or 0.5D. However our
eyes 60 arcseconds is for object with great contrast. For
lower contrast, our eye resolving power gets poorer. This
means the magnification has to increase in objects with
poor contrast. Quoting him "For lunar and planetary objects
the contrast is generally much lower than 100%, so that a
higher magnification is necessary to resolve the detail".
Since he uses charts for the testing. Let's not use sky
objects but earthbound charts samples.

When contrast is poor, you increase the magnification to
resolve more details. Now there must a a formula where
when you increase magnification, more details get resolve
yet contrast got poorer. So the rate of contrast degradation
must be lower than image magnification clarity, or something
like that. Because if contrast got worse exponentially, you
can't resolve better in higher magnification because the
image is smeared already. Let's not use sky objects to
complicate it but just charts in laboratory testing.
Wonder if there is a formula or just common sense.


ch

.

Relevant Pages

  • Re: aperture and magnification gain puzzle
    ... > It is mentioned in Rutten Telescope Optics that as contrast ... > get lowered in an object, one must increase the magnification ... It means that when you observe in the best dark sky, ... or the Andromeda Galaxy through binoculars or a telescope. ...
    (sci.astro.amateur)
  • [OT] Her in the Sky
    ... , have always looked out for Her in the sky. ... My Full Moon ritual is barely a ritual, but I've shared it with the Yowlet, ... Its a crappy telescope, I admit, barely ... I finally got my first ever glimpse of Her under magnification. ...
    (rec.pets.cats.anecdotes)
  • Re: Her in the Sky
    ... (and Antares, my favourite star), have always looked out for Her in the sky. ... Its a crappy telescope, I admit, barely ... I finally got my first ever glimpse of Her under magnification. ...
    (rec.pets.cats.anecdotes)
  • Re: Please accept my apology
    ... >eyepiece in both, the magnification will be different. ... >f/16 telescope, with the same eyepiece, you are absolutely right - it ... Given that the starry sky has a fractal structure, ...
    (sci.astro.amateur)
  • Re: aperture and magnification gain puzzle
    ... > magnification, more details get resolve yet contrast got poorer. ... Mel Bartel's implementation of Clark's ODM algorithm: ... Deep Sky" webpage ... Another related Clark term is the Minimum Detection Magnification ...
    (sci.astro.amateur)
Loading