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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
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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 |
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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. |
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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 * ********************************************** |
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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 |
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Thanks for all the excellent answers. Gives me a lot of reading to do! :-)
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#7
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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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