Re: aperture and magnification gain puzzle

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.

.

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 ... > best contrast between the background night sky and object by going to ... > or the Andromeda Galaxy through binoculars or a telescope. ...
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  • Re: aperture and magnification gain puzzle
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