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Telescope resolution - What is limiting factor?



 
 
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  #1  
Old September 8th 03, 10:30 PM
John Honan
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Default Telescope resolution - What is limiting factor?

Newbie question. What is the main limitation of telescopes that causes the
image to lose contrast/sharpness as the magnification is increased? Is it
mainly due to the quality of the optics which causes aberrations to become
more noticable. Or is it mainly down to the amount of light gathered (the
aperture), or does the f/ ratio have anything to do with it? In other words,
the more light that is gathered, the more the eyepiece has to 'work with'?

If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?

Am I correct in assuming the optics in the objective lens are more important
in the quality of the final image than the optics in the eyepiece?

Hypothetically, if I had a 10" refractor, with near perfect optics. I would
assume that I could take the magnification to levels which would allow me to
see very precise detail and contrast/colour on objects? Not that I could
ever afford a 10" refractor... :-)

From reading websites and FAQs my understanding is that the eyepiece acts
like a 'microscope' on the focal point produced by the objective lens. What
would happen if I had a telescope with just the objective lens (no
eyepiece), but ensured that the image (i.e. focal point) was actually on my
retina. What would I see?



  #2  
Old September 9th 03, 04:01 AM
Gregory Phillips
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The atmosphere is the main limiting factor, The column of air you are
looking up through is moving and causes the image to distort to varying
degrees. That is why, for very bright objects (say a nearby planet) a
"smaller" refractor may allow viewing more detail than a larger reflector.
It can be looking through a thinner column of air with less distortion.

However, size matters, to see faint objects a larger aperture is needed.
A mirror has zero color distortion, unlike the color distortion in even the
best refractors. All telescopes have some diffraction effects due to
the limited aperture and a possible central obstruction (secondary mirrors).
The diffraction effects can lessen the contrast so the refractor can do better
in this area. However a simple solution is to "stop down" the large reflector
when viewing bright objects by using a primary lens cover with an off center
smaller circle cut out of it. This can, for example, turn a 10" reflector into a 4"
higher contrast telescope for planetary work. (with NO color fringes!)

The "f-ratio" is a measure of how short your telescope is. The f-ratio is
focal length divided by aperture. Short, fat telescopes have lower F numbers
which also means the light needs to be bent more to reach focus in a shorter
distance. It is more difficult to bent light further with the same amount of
accuracy. Another way to look at this is to note that for same focal length a
telescope with a lower f-ratio will gather more light (since the aperture is bigger)

All the optics are equally important (weakest link in the chain and all that).
You want the best optics you can afford, low cost optics are normally not
worth wasting your time with. Look through a good scope with a televue
nagler on it some dark night and you will never be satisfied with cheap
optics again.

I own a 10" LX-200 (better than average quality). The highest magnification
I find useful down around sea level is about 266x. Maybe on a very rare night
the air will be clear and still enough to go to twice that.

Finally... don't fight it, use an eyepiece.

On Mon, 8 Sep 2003 22:30:50 +0100, "John Honan" wrote:

Newbie question. What is the main limitation of telescopes that causes the
image to lose contrast/sharpness as the magnification is increased? Is it
mainly due to the quality of the optics which causes aberrations to become
more noticable. Or is it mainly down to the amount of light gathered (the
aperture), or does the f/ ratio have anything to do with it? In other words,
the more light that is gathered, the more the eyepiece has to 'work with'?

If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?

Am I correct in assuming the optics in the objective lens are more important
in the quality of the final image than the optics in the eyepiece?

Hypothetically, if I had a 10" refractor, with near perfect optics. I would
assume that I could take the magnification to levels which would allow me to
see very precise detail and contrast/colour on objects? Not that I could
ever afford a 10" refractor... :-)

From reading websites and FAQs my understanding is that the eyepiece acts
like a 'microscope' on the focal point produced by the objective lens. What
would happen if I had a telescope with just the objective lens (no
eyepiece), but ensured that the image (i.e. focal point) was actually on my
retina. What would I see?



---
Gregory Phillips Seattle, Washington, USA
  #3  
Old September 9th 03, 06:12 AM
Llanzlan Klazmon The 15th
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"John Honan" wrote in
:

Newbie question. What is the main limitation of telescopes that causes
the image to lose contrast/sharpness as the magnification is
increased?


The main limitation is the aperture of the telescope. The in focus image of
a point source in a telescope with a circular aperture is a diffraction
pattern where the central illuminated disk is referred to as the Airy disk.
The disk will be surrounded by ever fainter diffraction rings. The angular
size of the Airy disk is directly related to the inverse of the telescope
aperture. I.e the greater the telescope aperture, the smaller the angular
size of the Airy disk. This means that a greater aperture allows the
telescope to be able to separate point sources that have smaller angular
separations in the sky. In other words, higher resolution.

On earth, we come unstuck when the atmospheric conditions make it
impossible to achieve higher resolution for any size telescope. That's one
of the reasons the Hubble telescope was put into orbit. These days there
are some tricky techniques (adaptive optics) that can circumvent this
problem to some degree.


Is it mainly due to the quality of the optics which causes
aberrations to become more noticable. Or is it mainly down to the
amount of light gathered (the aperture), or does the f/ ratio have
anything to do with it? In other words, the more light that is
gathered, the more the eyepiece has to 'work with'?


Quality of optics is definitely a factor. My previous remarks about
aperture are assuming that the quality of the telescopes is at least
diffraction limited.

The f ratio changes the image scale. If you have two telescopes of the same
aperture but different focal lengths, they will have equal ability at
resolution but the image scale of the long focal length scope will be
greater and dimmer than the short focal length. The proviso is that it gets
harder to achieve good optical quality as the focal ratio decreases.


If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?


The resolution of the reflector will be related to (D-d) where D is the
aperture and d is the diameter of the central obstruction. The reflector
will also have a smaller fully illuminated field than the refractor and
will also suffer from the off axis aberation known as coma. For visual use,
a well made and well baffled 6" newt should equal or better the refractor
on axis. It's a different story for wide field photography.

Some people just prefer the view through a refractor - no diffraction
spikes for one thing. For visual DSO's, go with the big Dob :-).


Am I correct in assuming the optics in the objective lens are more
important in the quality of the final image than the optics in the
eyepiece?


That is always the case. Bad optics give bad results in any type of scope.


Hypothetically, if I had a 10" refractor, with near perfect optics. I
would assume that I could take the magnification to levels which would
allow me to see very precise detail and contrast/colour on objects?
Not that I could ever afford a 10" refractor... :-)


I'm sure that Thomas Back will be happy to take your order for a 254mm f9
APO :-) a mere $39,990.00 for the OTA. I hope you have a decent mount to
put it on!

http://www.tmboptical.com


From reading websites and FAQs my understanding is that the eyepiece
acts like a 'microscope' on the focal point produced by the objective
lens. What would happen if I had a telescope with just the objective
lens (no eyepiece), but ensured that the image (i.e. focal point) was
actually on my retina. What would I see?


You can do that, I have heard of people who have viewed directly at the
prime focus of the 100" Mt Wilson scope. Awesome.

Llanzlan.







  #4  
Old September 9th 03, 06:59 AM
David Knisely
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Hi there. You posted:

Newbie question. What is the main limitation of telescopes that causes the
image to lose contrast/sharpness as the magnification is increased? Is it
mainly due to the quality of the optics which causes aberrations to become
more noticable. Or is it mainly down to the amount of light gathered (the
aperture), or does the f/ ratio have anything to do with it? In other words,
the more light that is gathered, the more the eyepiece has to 'work with'?


There are two factors which limit the performance of a telescope. One is the
stability of the Earth's atmosphere (known as "seeing") which can disturb the
incoming light and blur the image (one reason we have the Hubble Space
Telescope in orbit). The other is aperture (ie: the size of the main lens or
mirror, whichever is used). The light gathered by the telescope is focused
down to a region where the light's wave properties become important. It
creates a pattern at the focal point known as "the diffraction pattern". At
high power when looking at a point-source like a star, this pattern looks like
a tiny disk with several faint rings around it. The angular size of this
pattern directly depends on the aperture of the telescope. The larger the
aperture, the smaller the diffraction pattern is, and the finer the detail
that the telescope can resolve will be. A larger aperture will also gather
more light as well. Increasing magnification will increase the scale and will
show more detail up to a point. That point is where the diffraction pattern
becomes *easily* visible, which is around 35x to 50x per inch of aperture
depending on the acutity of your eye. Using really high power well beyond
this 50x per inch level just makes the diffraction pattern bigger and reveals
no more detail, since the diffraction effects will prevent smaller details
from being seen. From that point on up, the image just gets fuzzier and
fuzzier (we call it "empty magnification").

If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?


It may or may not produce a better image. Optical quality is important, and
some of the improvment comes from some refractors being figured to a higher
standard of optical quality. A reflector also has a secondary mirror which
will obstruct some of the light and cause minor diffraction effects. This can
reduce the contrast of some low-contrast detail, but the effect is often
overblown. A good 6 inch reflector should equal the performance of a
high-quality 4 inch refractor (or in some cases, exceed it, as in the case of
double-star resolution).

Am I correct in assuming the optics in the objective lens are more important
in the quality of the final image than the optics in the eyepiece?


They are both factors, but the first one to consider is the size of the
objective (lens or mirror). We have a saying in amateur astronomy: APERTURE
WINS. You want as big an aperture as you can afford. That having been said,
optical quality is also very important, both in the objective lens and in the
eyepiece.

Hypothetically, if I had a 10" refractor, with near perfect optics. I would
assume that I could take the magnification to levels which would allow me to
see very precise detail and contrast/colour on objects? Not that I could
ever afford a 10" refractor... :-)


You could also take a 10 inch *reflector* to powers high enough to see very
fine detail and good color contrast as well. However, a 10 inch refractor is
probably well-beyond the price range of most amateur astronomers.

From reading websites and FAQs my understanding is that the eyepiece acts
like a 'microscope' on the focal point produced by the objective lens. What
would happen if I had a telescope with just the objective lens (no
eyepiece), but ensured that the image (i.e. focal point) was actually on my
retina. What would I see?


Well, just calling the eyepiece a simple magnifier is a bit of an
oversimplification. However, without the eyepiece, you would see a much
smaller image formed by the objective lens if you stood back from the focuser
a certain distance. I have done this for the moon, but its just too small an
image to be very useful. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club:
http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************



  #5  
Old September 9th 03, 11:20 AM
Roger Hamlett
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"John Honan" wrote in message
...
Newbie question. What is the main limitation of telescopes that causes the
image to lose contrast/sharpness as the magnification is increased? Is it
mainly due to the quality of the optics which causes aberrations to become
more noticable. Or is it mainly down to the amount of light gathered (the
aperture), or does the f/ ratio have anything to do with it? In other

words,
the more light that is gathered, the more the eyepiece has to 'work with'?

There is a bit of 'history' here. When telescope optics were first made, the
people making them found that the better they made them, the better the
images were. It seemed obvious that if they could produce 'perfect' optics,
everything would be visible. So over the next decades/century or so, there
are examples of scopes with absolutely 'superb' optics being produced.
Unfortunately it didn't work. Eventually they worked out what was going on,
and the final solution, is the 'Airy disk'. This is named after Sir George
Airy, who worked out how light from perfect optics would behave, and the
fact that the 'point' stars would still get spread into a disk shaped
pattern, rather like waves on a pond (with most energy in the centre wave).
The size of this disk decreases as aperture increases, and presents an
absolute 'limit' on the resolving ability of the system, in perfect
conditions.

If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?

No.
Generally aperture always wins. There are however some limits, and
exceptions. The first is that in the case of a reflector with a 'central
obstruction', this obstruction, changes the shape of the Airy distribution,
moving light from the central peak outwards. This is the basis of the 'lower
contrast' statements about SCT's especially. However (like all things), this
is not as simple as it may appear. _small_ central obstructions (up to
perhaps 20%), can actually result in sharper edges to the main 'peak' in the
Airy curve, moving light from the very centre, to the edges of the peak, and
sharpening the edges in doing so. Beyond this, the contrast does start to
degrade, but not but that much. A scope with a 10" aperture, will produce an
Airy peak, that is less than half the size of the one produced by a 4"
scope, and even with a large central obstruction (say 34%, a typical SCT),
though now a significant amount of light has moved into the first 'ring',
this entire ring, fits _inside_ the central peak from the 4" scope. The
rules commonly used a
1) Light gathering/deep space performance - simply measure the area loss to
the CO. A typical 10" scope will match perhaps a 9" unobstructed scope on
this basis.
2) Ultimate resolution. Here the CO doesn't change things, and a 10" scope
will potentially be 2.5* 'better' than a 4" scope.
3) Contrast performance. The 'rule of thumb' (generally quite good, but open
to error), is to take the _linear_ scope diameter, less the linear diameter
of the CO. Hence a typical 10" SCT, will match about a 6.5 ot 7"
unobstructed scope.

Now, there is one other factor. Generally, though the above gives 'optical'
limits, the sky for most low altitude observers, has a much larger effect on
the final resolution. The atmosphere introduces a whole 'set' of blurring
conditions. The first is the obvious 'haze'. Then there is a variety of
'turbulence' effects. The largest of these is normally in the lower
atmosphere (from heat radiating off buildings, mountains etc.). This tends
to decline in the course of the night. Linked to this, are 'bubbles' in the
atmosphere, which shift the focus as well. These 'bubbles', have typical
diameters around 8". Since they are moving, when observing planetary detail
with a scope smaller than this sort of diameter, if you watch long enough,
there will be instants, when you look centrally through one 'bubble', and
the view momentarily improves. This only applies when looking at very bright
objects (planets), since for deep space observation, your eyes are not
'quick' enough to see this effect, and cameras too, have to take longer
exposures, and 'miss' this effect. This is why very high quality scopes
around perhaps 6" in aperture, are often seen as the 'best' planetary
performers. However if you (say) take an 12" SCT, with 'perfect' optics,
shift it to a really high altitude observing site, and get ideal conditions,
it will resolve finer detail than the smaller scope.

Am I correct in assuming the optics in the objective lens are more

important
in the quality of the final image than the optics in the eyepiece?

In part.
Certain aberrations will get worse, the further they have to take effect. So
chromatic aberration in the main objective will be more visible than that
from the eyepiece. However 'basic' features (such as the clarity of the
glass), will have the same effect at either point.

Hypothetically, if I had a 10" refractor, with near perfect optics. I

would
assume that I could take the magnification to levels which would allow me

to
see very precise detail and contrast/colour on objects? Not that I could
ever afford a 10" refractor... :-)

It would depend on your viewing conditions. Unfortunately you also have to
understand, that a refractor, inherently introduces the possibility of
chromatic aberration. To reduce this to acceptable levels, would require the
use of expensive glasses (read flourite as the 'best', if built in the right
combination). The cost of such glasses, will inherently rise as the cube of
the scopes diameter, and may be 'worse' than this (since the mechanical
limits may require the larger lenses to be more than proportionately
'thicker'). Unfortunately a 10" apochromatic objective, would probably run
to perhaps $500000...
This type of combination, has become rare, because there are other ways of
getting as good results for less money. A 'Schiefspiegler' reflector,
retains the same unobstructed views, intruduces the possibility of slight
surface scattering (reflectors exhibit this), but removes the chromatic
aberration, and costs a lot less. There are also (in the same family), 'off
axis' Newtonian designs.

From reading websites and FAQs my understanding is that the eyepiece acts
like a 'microscope' on the focal point produced by the objective lens.

What
would happen if I had a telescope with just the objective lens (no
eyepiece), but ensured that the image (i.e. focal point) was actually on

my
retina. What would I see?

Unfortunately you can't do it, without removing the lens from your eye...
Basically the lens in your eye, is designed to allow you to focus on objects
between perhaps 4" in front of the eye, out to infinity (as you get older
the 'close focus' tends to decay). In each case, the 'cone' of light rays,
is tapering inwards to a point in front of you. The basic cone from a
telescope, is tapering _outwards_ as it approaches the focal point, and your
eye lens doesn't have the adjustment range to deal with this. You can view
through a telescope like this, by allowing the light to come to a focus, and
then viewing this from the other side (as the light is now tapering out).
Like this, you get very low magnification. You can also use a Barlow lens,
to
'flatten' the cone, and refocus the scope so that the result is a parallel
beam of light, or simply use an eyepiece!...

Best Wishes



  #6  
Old September 9th 03, 08:07 PM
John Honan
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Thanks for all the excellent answers. Gives me a lot of reading to do! :-)


  #7  
Old September 10th 03, 12:53 PM
Ray
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Figure about 50x max. for each inch of refractor aperature.


"John Honan" wrote in message
...
Newbie question. What is the main limitation of telescopes that causes the
image to lose contrast/sharpness as the magnification is increased? Is it
mainly due to the quality of the optics which causes aberrations to become
more noticable. Or is it mainly down to the amount of light gathered (the
aperture), or does the f/ ratio have anything to do with it? In other

words,
the more light that is gathered, the more the eyepiece has to 'work with'?

If aperture is the main factor, how can a 4" Takahashi refractor (for
example) produce better images than a larger aperture reflector? Is it
because a reflector loses some light in the mirrors?

Am I correct in assuming the optics in the objective lens are more

important
in the quality of the final image than the optics in the eyepiece?

Hypothetically, if I had a 10" refractor, with near perfect optics. I

would
assume that I could take the magnification to levels which would allow me

to
see very precise detail and contrast/colour on objects? Not that I could
ever afford a 10" refractor... :-)

From reading websites and FAQs my understanding is that the eyepiece acts
like a 'microscope' on the focal point produced by the objective lens.

What
would happen if I had a telescope with just the objective lens (no
eyepiece), but ensured that the image (i.e. focal point) was actually on

my
retina. What would I see?





 




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