<p><strong>1. Resolving power and diffraction</strong></p>

<p>Diffraction happens anywhere there's an edge. <em>"It is defined as the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle."</em> (<a href="https://en.wikipedia.org/wiki/Diffraction" rel="nofollow noreferrer">Wikipedia</a>) So all you need for diffraction is an obstacle of any shape - at the edge of the obstacle, the light will bend a little.</p>

<p><a href="https://i.sstatic.net/q1Ob9.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/q1Ob9.png" alt="diffraction"></a></p>

<p>In telescopes, diffraction takes place all around the edge of the objective, be that a lens or a mirror. With a smaller objective, with a more strongly curved edge, and with opposite sides of the edge closer together, you get stronger diffraction (larger angle of deviation). With a larger objective, more straight edge, opposite sides of the edge further apart, you get less diffraction (smaller angle of deviation).</p>

<p>This is why larger telescopes have smaller <a href="https://en.wikipedia.org/wiki/Airy_disk" rel="nofollow noreferrer">Airy disks</a>. It's similar to how a narrow slit makes a wider diffraction figure, whereas a wider slit makes a tighter figure. In the case of a telescope, the diffraction figure is the Airy disk - instead of having parallel diffraction zones, it's circular. A larger aperture makes a smaller Airy disk.</p>

<p><a href="https://i.sstatic.net/qdYLm.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qdYLm.gif" alt="Airy disk"></a></p>

<p><strong>2. Entrance/exit pupils and apertures</strong></p>

<p>For a lot of modern amateur telescopes, the entrance pupil is actually the aperture of the telescope, or the visible diameter of the objective. In some cases the instrument has an aperture stop that reduces the size of the visible part, but that's rare. In most cases diameter of mirror = entrance pupil.</p>

<p>The exit pupil is simply due to the way the rays of light converge after they exit the eyepiece - they converge until they're bundled in a smallest area (the exit pupil) then diverge again.</p>

<p><a href="https://i.sstatic.net/PHzEC.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/PHzEC.gif" alt="pupils"></a></p>

<p>So the exit pupil is actually smaller than the diameter of the final lens in the eyepiece, because the rays keep converging for a while.</p>

<p>Obviously, you want to keep your eye at the eyepiece so that the exit pupil coincides with the pupil in your eye, so as to capture the greatest amount of light coming out of the scope.</p>

<p>EDIT: The exact place where you keep your eye does not change the size of the image. The size of the virtual image made by the telescope depends only on object size and magnification - nothing else matters. Changing the eye position only changes the amount of light that enters your visual system. (Explaining why this is would probably require a whole 'nother topic on this forum.)</p>

<p><strong>3. Magnification and brightness</strong></p>

<p>The amount of light captured by the scope stays the same. It is given by the size of the aperture.</p>

<p>But with a larger image (higher magnification) that same amount of light coming from the object is spread out over a bigger solid angle. You see the same number of photons, but now they are spread out over a larger apparent surface. Of course it will look more dim.</p>

<p>Same amount of butter spread over a larger slice of bread.</p>

<p><strong>4. Barlow, magnification, and field of view</strong></p>

<p>A Barlow increases magnification. As shown in paragraph #3 above, more magnification = reduced apparent brightness. Same thing would happen without a Barlow, but just using a much stronger eyepiece instead.</p>

<p>It's all about magnification. The light absorption in the glass in the Barlow is negligible.</p>

<p>Also, the object seems to pass more quickly through the field of view because the apparent field of view stays the same, but the whole image is magnified. Because the image is magnified, any motion within it must appear "faster". So the object will seem to pass more quickly through the same field of view.</p>

<p><strong>5. Stars and planets appear the same</strong></p>

<p>They shouldn't. It's a performance issue.</p>

<p>Stars should appear as tiny dots at low to medium magnification. They should appear as Airy disks (but still not large) at high magnification, fluctuating due to seeing (air turbulence).</p>

<p>Most planets (Venus, Mars, Jupiter, Saturn) should exhibit disks that are clearly bigger than the stars. Even Uranus and Neptune (which are far away) should still show disks that, upon close examination, are revealed to be bigger than Airy disks, in an instrument as small as 150 mm aperture (perhaps even smaller).</p>

<p>Could be a number of reasons for this. At 60 mm aperture, your scope should have enough resolving power to distinguish between the largest planetary disks (Jupiter) and any stellar Airy disks, and the rings of Saturn should start to become visible although very tiny. If that doesn't happen, then perhaps:</p>

<ul>
<li>It's a collimation issue. All telescopes except most refractors require periodic collimation, otherwise performance decreases</li>
<li>Quality of optics. Not sure what optics you have, but if they have performance issues then the images will be bloated</li>
</ul>

<p>As long as there are lingering issues with the instrument, higher magnification will not help.</p>

<p>EDIT: Stars are so far away that they are point-like objects to us. So their images in a scope "ought to be" also point-like. But they're not. This is due to diffraction and aberrations.</p>

<p>Diffraction I've explained above. The Airy disk is the smallest possible image of a star you could ever have in a telescope. You cannot make a stellar image smaller than that, due to diffraction.</p>

<p>Then there's aberrations. There are monochromatic aberrations:</p>

<ul>
<li>coma: the further away from the center the image is, the less point-like it becomes</li>
<li>spherical mirrors or lenses DO NOT make perfect point-like images of stars. The point-like image is just an approximation. Spheres make imperfect images by their very design and shape.</li>
<li>astigmatism - like coma, images near the edge of the field of view are distorted. The difference is that astigmatism makes a symmetrical distortion, whereas coma is weird shaped (like a seagull)</li>
<li>field curvature - the focal plane of the primary mirror is not flat, but it's curved. So if the scope is in perfect focus in the center, it's not in perfect focus at the edge, and viceversa.</li>
<li>distortion - a square grid is bloated so it looks like a curved cushion. Doesn't matter much for astronomical instruments.</li>
</ul>

<p>Then there are chromatic aberrations, which happen with refractors because the refraction index of the objective is not the same for all color of light.</p>

<p>It is the hallmark of a good telescope that its aberrations are less than the size of the Airy disk, at least for most of the field of view (perhaps excluding the zone close to the edge). That instrument is said to be "diffraction limited".</p>

<p>If stellar images are very bloated, the instrument is likely not diffraction limited.</p>

<p><a href="https://starizona.com/acb/basics/equip_optics101_aberrations.aspx" rel="nofollow noreferrer">https://starizona.com/acb/basics/equip_optics101_aberrations.aspx</a></p>

<p><a href="http://www.telescope-optics.net/aberrations.htm" rel="nofollow noreferrer">http://www.telescope-optics.net/aberrations.htm</a></p>

<p><strong>6. How to optimize an instrument</strong></p>

<p>You could write entire books on this topic.</p>

<p>A Barlow is not magic. It only gives you more magnification, but magnification is the most misunderstood parameter of a scope. Unless everything else is perfect, more magnification does not help.</p>

<p>There's a lot of reading you could do, and there is a lot of experimentation. Here are some older topics on this stack:</p>

<p><a href="https://astronomy.stackexchange.com/questions/8788/how-much-magnification-is-needed-to-see-planets-of-solar-system">How much magnification is needed to see planets of solar system?</a></p>

<p><a href="https://astronomy.stackexchange.com/questions/2196/first-night-on-a-telescope-questions">First night on a telescope questions</a></p>

<p><a href="https://astronomy.stackexchange.com/questions/2193/best-telescope-for-the-viewing-of-nebulae-stars-and-planets">Best telescope for the viewing of Nebulae, Stars and Planets</a></p>

<p><a href="https://astronomy.stackexchange.com/questions/2223/please-guide-me-to-buy-my-first-telescope">Please Guide me to buy my first Telescope</a></p>

CC BY-SA 3.0
2017-03-31T18:53:04.590Z
Wow what an amazing explanation. Thanks a ton for going through this big question & answering every part of it. An year long wait to get a perfect answer. Just 2 things . I get that we will want to keep our eye at the eyepiece to get all the light but the pupil ( the eye will then be in front of the exit pupil). Also this converging light will converge again in the eye lens. That shall have a serious effect on the size of object we will see through it. Will it not ? Another refraction through the eye has have an effect on size. But that's not the case even in seeing a nearby tree . Why ??.....
Shashaank
CC BY-SA 3.0
2017-03-31T18:59:22.533Z
& for the 5th question . Focus is a point. So we shall see point object in the eyepiece if the rays are all focusing at a point. But that is not what happens. Because the object we see through the eyepiece is on the focal plane. Right ? Because here in your picture the object seen in the eyepiece will be like a point. All rays converging to a point ? & If Stars have to visible as points then "what is that this telescope shows as spheres "
Shashaank
CC BY-SA 3.0
2017-03-31T19:11:18.487Z
I made two edits to my answer addressing your comments.
Florin Andrei
CC BY-SA 3.0
2017-03-31T19:17:49.790Z
So I see the aberration effects . Poor optics.....As it is it's not useful . I will ask another question on how changing position of eye won't change the size of the object or how the "another refraction through the eye won't change what we see. & How Gallieio made his own lenses. I bought Carborundum only to spoil the glass with scratches causing it to be translucent. Hope when I ask you are online or see the questions. Thanks a lot !!!
Shashaank
CC BY-SA 3.0
2017-03-31T19:22:08.933Z
Don't give up. Making your own telescope is doable, although it's a very, very long process - so it takes time and patience. I make my own telescopes. Many other people do too. You can do it too, provided it's agreeable with you to stick to a long term project that may take even one year to complete. Here's a good place to start: http://stellafane.org/tm/
Florin Andrei
CC BY-SA 3.0
2017-03-31T19:29:34.860Z
This looks to be great. I visit IUCAA too. But actually I neither have good resources nor experience. I bought  a plane circular glass 1inch thick 20cm diameter ( the one we use in toilets) and began grinding it with another same glass piece with carborundum grit 60. Instead of seeing the sagitta after a work of 5 hours , I saw only the glass changing from perfect to translucent. Don't know what's that ??
Shashaank
CC BY-SA 3.0
2017-03-31T19:40:20.717Z
By the way if you don't mind I Shall tag you when I ask the question. You can then go through the question . If you wish to answer it , great else you can ignore it. At least you will know that there is such a question on the site . Then if you like it or wish to answer it ,great else you can ignore it !
Shashaank
CC BY-SA 3.0
2017-03-31T20:36:41.097Z
The grinding issue could make an entirely separate question on this forum. And I don't mind being tagged.
Florin Andrei
CC BY-SA 3.0
2017-05-03T11:38:51.603Z
Can you suggest me a basic astronomy book. I mean like how to look at the night sky , all the movements of the earth as well as other bodies like moon ( like the plane of moon intersects the plane if the earth at 2 points , etc) , covering all the details like **When is the sun really overhead**( because we are taught it's at 12 noon but I think it cannot be at 90° ever). And the all techniques of understanding the movement of the night sky ( because I feel it is very hard to study this 3D motion by looking from below). I haven't read anything. I just want to have a beginning...
Shashaank
CC BY-SA 3.0
2017-05-03T18:47:59.873Z
@Shashaank Unfortunately my native language is not English and all amateur astronomy books that I've read (long time ago) are also not in English. So unless you can read Romanian I can't help you. I'm sure there are good introductory books in English, I just don't have any examples right now.
Florin Andrei
CC BY-SA 3.0
2017-03-31T18:30:13.840Z
2017-04-21T17:57:04.453Z
2017-04-21T17:57:04.453Z
Florin Andrei
<p>I'll attempt to answer as many of these questions, to the best of my abilities, as I can at this late hour.</p>

<p>1) Lenses have an index of refraction that is different than vacuum, and air, and they are curved. ie: corners when light is concerned.</p>

<p>2) The entrance and exit pupils are the pupil size required to pass an "extreme" ray through an optic. Meaning that if the light ray from the optical axis was not refracted at all as it passed the optical plane, it would still pass within the pupil (entrance) and if reflected perfectly (exit). The diameters of the objective and the eyepiece are subjective, meaning that you can have, within reason, any size objective and use virtually any eyepiece. The restrictions to this would only be focal lengths. </p>

<p>3) You are looking through a thicker lens and thereby reducing the light incident on your eye. Think about the shape of the lenses, look at them and it might become apparent just how much more curvature the 4mm lens has than the 20mm lens. This added curvature comes at the cost of a higher angle of incident that a light ray has to take, and thus more optic to pass through. </p>

<p>4) Barlow lenses reduce the brightness a lot because you are adding additional optics. With each new addition you decrease the amount of light by a factor relative to the thickness and clarity of the optics. The Barlow lens is increasing the magnification and thereby reducing the field of view.</p>

<p>5) With your specific telescope (of which I have one with the exact same size/focal length) you should be able to see Jupiter as a disk, Mars too should be more than a "dot." Stars are point sources with angular diameters much too small for you to be concerned given the size of your telescope. Also, those planets appear reddish because they are-ish. Many stars will still appear colors other than blue if you can find them.</p>

<p>6) This is up to your discretion and desired results when observing. Please read your owners manual, or check out the telescope mfr website for recommended setups.</p>

CC BY-SA 3.0
2016-10-14T05:56:48.017Z
Thanks for the explanation. I had read somewhere that the entry and exit pupil had something to do with the diameter of the eye pupil. Could you please explain the exit and entry pupil a bit more. And if stars are visible just as dots how are they visible like spheres of the size of mars in my telescope
Shashaank
CC BY-SA 3.0
2016-10-14T12:33:59.767Z
@Shashaank Any time a star appears to have a diameter greater than the diffraction limit of your 'scope (and ALL optics have an aperture, and that aperture controls the Rayleigh limit), it's due either to poor focus, lens aberrations, or atmospheric turbulent aberrations.
Carl Witthoft
CC BY-SA 3.0
2016-10-14T13:24:00.240Z
@CarlWitthoft  So the spherical shape of the star (not a dot ) is because of poor focus or aberrations
Shashaank
CC BY-SA 3.0
2017-03-31T18:33:26.870Z
@LaserYeti - Quite a few mistakes in your answer. Diffraction is not caused by the index of refraction of the lens; diffraction and refraction are very different things. Light absorption is not the main reason why barlows and stronger eyepieces seem to reduce brightness.
Florin Andrei
CC BY-SA 3.0
2016-10-14T05:49:29.760Z
2016-10-14T05:49:29.760Z
LaserYeti
<p>You already have an answer but there are a few interesting Physics things in your question that are linked:</p>

<ul>
<li>1) Diffraction and the fact that telescopes have no "slit".</li>
<li>5) Stars don't appear as a sphere, they appear as dots.</li>
</ul>

<p>The telescope has a round aperture (of say 60mm or so) and this behaves as a slit in some ways. It causes diffraction patterns (an <a href="https://en.wikipedia.org/wiki/Airy_disk" rel="nofollow">Airy disc and concentric circles</a>) when viewing a point source at high magnifications, near the resolution limit of the telescope. Also of slightly out of focus you'll get <a href="https://en.wikipedia.org/wiki/File:Circular_Aperture_Fresnel_Diffraction_high_res.jpg" rel="nofollow">lots of rings</a>.</p>

<p>(By the way this the cause of many "weird" unidentified objects that people photograph or film in the sky; especially a DV camera with odd shaped aperture blades. Surprisingly few people want to hear the explanation though.)</p>

<p>So when viewing stars at high-ish power, on very still nights, they actually look like a small white circle surrounded by a single, very thin, circular ring.</p>

CC BY-SA 3.0
2016-10-14T06:15:16.177Z
Forgot to mention - the Airy disc & single ring can, on my 70mm refractor, be seen at around 80x magnification on very still nights. Just an illustration of size.
Andy
CC BY-SA 3.0
2016-10-14T06:26:28.760Z
Thanks , so the Diffraction is happening at the aperture of the circular objective
Shashaank
CC BY-SA 3.0
2016-10-14T06:36:18.680Z
@Shashaank yes, prove it by observing a star at high power, looking for the diffraction rings if you can see them, then turn the aperture to a square by hanging a bit of cutout cardboard over the objective. :)
Andy
CC BY-SA 3.0
2016-10-14T06:43:08.383Z
I got it. When I will turn the aperture to a square using a cardboard , I'll block the edges and the rings shall go away
Shashaank
CC BY-SA 3.0
2016-10-14T12:35:08.033Z
@Shashaank no, the rings won't go away.  They'll change shape because you changed the shape of the aperture.  You might Google "apodized aperture" for even more info.
Carl Witthoft
CC BY-SA 3.0
2017-03-31T19:31:12.267Z
@Shashaank - as Carl said, the square aperture will simply change the shape of the diffraction figure. It will become a bright cross instead, and quite annoying. But your scope is not diffraction limited, so don't worry about diffraction.
Florin Andrei
CC BY-SA 3.0
2017-03-31T19:35:02.947Z
@FlorinAndrei  If it's not diffraction Limited but still there has to be some. Then there is a lot of aberration in it . That is why I see Stars as spheres. Poor telescope. Always Wanted to see those nebulae those blue n green ( photographical) . Still waiting for that to happen.
Shashaank
CC BY-SA 3.0
2017-03-31T20:47:59.490Z
Even in large amateur telescopes you will not see much color in nebulae, the human eye is just not sensitive enough to see color in low light. Also, what helps with DSOs like nebulae is lots of aperture. But even with 60mm of aperture I feel you should start seeing a bit of the Great Orion Nebula - which is still visible for a while nowadays. Google "how to see the Orion Nebula". Maybe try from outside the city, because urban light pollution makes it hard to see DSOs. It should appear like a small "cloud" in the surrounding blackness.
Florin Andrei
CC BY-SA 3.0
2017-04-03T12:02:57.287Z
@FlorinAndrei I wanna ask. I have seen those pictures of milky way galaxy ....Those spiral ones , rising about a hill , those on the net. From the place where I live only countable 12 stars are visible. I had gone to LeH Ladakh ( it's a very high altitude place in India , the place of highest road , 19000ft) , at a place called Pangong , Hanley ( u might have heard of it).... Most of Indian telescopes are set there. 300 days clear sky....No pollution at all. There above I could see thousands of stars....I could even tell where the Ursa Minor is . Now the question.......
Shashaank
CC BY-SA 3.0
2017-04-03T12:08:08.753Z
@FlorinAndrei  I went to a place supporting the Himalayan Chandra telescope and into those dome shaped cassigrain building like telescopes. In the computer which was taking all the reading , I could just thousands of white dots...The person there tried to explain....Infrared , CCD and all of that which I didn't get. If the best of the telescopes can just see these black n white dots , how do get those mega images as if the telescope has zoomed into the nebula or supernova. And how are those coloured when these where just black n white dots ( no structure at at all)
Shashaank
CC BY-SA 3.0
2017-04-03T12:11:17.903Z
@FlorinAndrei  What is the authenticity of those images . Do they depict any thing real. And about the galaxy. Our eye can see it or figure it out of the thousands of stars. How can those DSLR cameras see it. Is there any way to see it though our eyes
Shashaank
CC BY-SA 3.0
2017-04-03T18:05:10.177Z
@Shashaank The human eye's color perception shuts down in low light; the color receptors need a lot of light to function. Digital cameras do not have this limitation, they can record color at any level of incoming light; even for very faint objects a camera can still distinguish color. Also, cameras can be kept open a long time, even hours, slowly building up a single image even when the object is very faint. Our eyes cannot do that. Finally, our high resolution / fine detail vision also shuts down in low light (same receptors as color). Cameras don't do that, so they see much better than us.
Florin Andrei
CC BY-SA 3.0
2017-04-11T11:44:10.787Z
@FlorinAndrei  Ok thanks ! Was not active ! What about the authenticity of the pictures of nebulae , those big ones. When the best you can see in a best telescope is just  black - white tiny  circles , no structure , no color except black- white
Shashaank
CC BY-SA 3.0
2017-04-11T17:53:53.567Z
@Shashaank It's complicated. If it's UV or IR, clearly the colors are not real. If it's visible light, it depends. Often the colors are real but your eyes are not sensitive enough to see them; if they were thousands of times more sensitive you'd see those colors. Your eyes are not absolute judges of reality, they're just sensors, with good and bad characteristics. Other sensors are better and can see what your "sensors" cannot. So your eyes will claim that the colors are "fake" but the CMOS camera clearly disagrees. If it's red in the picture, it's red in reality, but not the way you see it.
Florin Andrei
CC BY-SA 3.0
2017-04-11T20:48:21.553Z
@FlorinAndrei  ok that's about the colour. But what about the size. Stars visible as dots. The telescope there was also showing only dots on the screen of the computer.  So how are those nebulae picture, etc ( which seem to be as if the telescope has zoomed into it) taken . I know not about the Hubble (which is free of all the pollution on the earth) but certainly it's not possible for it too . Certainly not possible for the non space telescopes​
Shashaank
CC BY-SA 3.0
2017-04-12T00:19:42.757Z
@Shashaank Fast exposure images will only show the bright stuff (planets, stars). For nebulae you need long exposure: a single frame is gradually collected over many minutes or even hours. Many frames (dozens, hundreds) may be stacked together to make a single final image. Then software extracts the useful info out of the huge noise within. This way, even with terrestrial instruments you can capture good images. Use the search function on this site https://www.astrobin.com/ for examples (e.g. M31, M42, etc). Proper capture technique + clever algorithms can remove a lot of obstacles together.
Florin Andrei
CC BY-SA 3.0
2017-04-12T07:52:19.517Z
@FlorinAndrei These pictures  , Are they "real". That's what the structure will look like (to an extent , I know they have been processed by computers , but after that this is what they will look like ). So even thier size n everything has been processed by computers. Because I guess one cannot see these nebulae in the telescope as big as they visible in the pictures.
Shashaank
CC BY-SA 3.0
2017-04-12T23:01:40.440Z
@Shashaank If you define "real" as "the way I can see them with my own eyes in a telescope", then no, they are not "real", your eye cannot possibly match a CMOS or CCD sensor. But these days after postmodernism we should be prepared to accept that what we see is always an interpretation, and always depends on the sensor (eye or camera) and the algorithm (brain or computer) - you get different images from different combinations, the choice is yours. As for that which is truly "real", the thing in itself, both Plato and Kant have told us long ago that it is pretty much unattainable. Not joking.
Florin Andrei
CC BY-SA 3.0
2017-04-15T06:39:53.140Z
@FlorinAndrei Ok I got it , thank you ! I have read a bit of Immanual Kant (through Einstein) , but not Plato !
Shashaank
CC BY-SA 3.0
2016-10-14T06:12:58.450Z
2016-10-14T06:12:58.450Z
Andy
<p>Though I am active on Physics, I am new to Astronomy. I am an amateur astronomer, actually even worse than amateur. I have just started it.
I have a 60 mm objective 700mm focal length refractor which I mainly use with a 20 mm eyepiece.</p>

<p>I have a couple of questions regarding the telescope, observations and explanations behind observations. Since they are all interlinked I decided to ask them together.</p>

<p>1) So, firstly the basics. Regarding the resolution power of a telescope, we know from the Rayleigh criterion that diffraction does happen at the objective. But Diffraction happens when light passes through a narrow slit or bends around a corner (whose dimensions are of the order of the wavelength of light). But there is no slit and probably no bending of light. <strong><em>How come then do we assume a diffraction pattern to have formed when considering the derivation of the resolution of a telescope?</em></strong></p>

<p>2) <strong>How are exit pupil and entry pupil different from the diameter of the eyepiece and objective?</strong></p>

<p>3) <em>Why on increasing the magnification does the brightness of the object reduce</em> (the physics behind it)? <strong>I mean that if I use 4mm eyepiece then, first of all, it becomes very difficult to find the object and secondly its brightness gets reduced when compared to the brightness that I had got using a 20mm eyepiece?</strong></p>

<p>4) <em>Secondly why do the Barlow lenses reduce the brightness a lot.</em> And why on using  Barlow lenses the field of view get smaller and the object passes twice as quickly as without the Barlow?</p>

<p>5) Now the biggest question. <em>How is it possible that every time I point my telescope towards a visible planet or a star (I know it's a star because of the sky map), I see almost the same size, when stars are visible only as small dots.</em> <strong>The stars appear to be bluish and the planets which I have seen (Mars and Jupiter) appear reddish. That's ok but a star can not be visible as a sphere. Stars appear as dots, don't they?</strong></p>

<p>6)How can I make the telescope give the best results i.e which eyepiece should I use and whether I should use Barlow lenses or not?</p>

CC BY-SA 3.0
2016-10-14T12:31:47.527Z
You might want to read Smith, Modern Optical Engineering, to answer many of these questions.  He is a good writer and puts in lots of diagrams to show where the planes of interest are and why they matter.
Carl Witthoft
CC BY-SA 3.0
2017-05-03T11:39:49.270Z
@CarlWitthoft Can you suggest me a basic astronomy book. I mean like how to look at the night sky , all the movements of the earth as well as other bodies like moon ( like the plane of moon intersects the plane if the earth at 2 points , etc) , covering all the details like **When is the sun really overhead**( because we are taught it's at 12 noon but I think it cannot be at 90° ever). And the all techniques of understanding the movement of the night sky ( because I feel it is very hard to study this 3D motion by looking from below). I haven't read anything. I just want to have a beginning...
Shashaank
CC BY-SA 3.0
2017-05-03T11:41:49.627Z
@CarlWitthoft Also is the PDF of the above book available. Also could you suggest a book or article that deals with the exact functioning of a telescope but from exact basics ( ABC of telescope)
Shashaank
CC BY-SA 3.0
2017-05-04T15:08:21.493Z
yes, you can test your Google-fu to find an ebook version.   As to basic astronomy, maybe start with nasa.gov .
Carl Witthoft
CC BY-SA 3.0
2017-05-05T16:43:01.777Z
@CarlWitthoft ok thanks ! Will nasa.gov work or a book ?
Shashaank
<telescope><angular-resolution><refractor-telescope>
Question about Telescopes , observations and explanations of the observations
Shashaank
# Question about Telescopes , observations and explanations of the observations

(Asked by: username_0 on 2016-10-13)

Though I am active on Physics, I am new to Astronomy. I am an amateur astronomer, actually even worse than amateur. I have just started it.
I have a 60 mm objective 700mm focal length refractor which I mainly use with a 20 mm eyepiece.

I have a couple of questions regarding the telescope, observations and explanations behind observations. Since they are all interlinked I decided to ask them together.

1) So, firstly the basics. Regarding the resolution power of a telescope, we know from the Rayleigh criterion that diffraction does happen at the objective. But Diffraction happens when light passes through a narrow slit or bends around a corner (whose dimensions are of the order of the wavelength of light). But there is no slit and probably no bending of light. ***How come then do we assume a diffraction pattern to have formed when considering the derivation of the resolution of a telescope?***

2) **How are exit pupil and entry pupil different from the diameter of the eyepiece and objective?**

3) *Why on increasing the magnification does the brightness of the object reduce* (the physics behind it)? **I mean that if I use 4mm eyepiece then, first of all, it becomes very difficult to find the object and secondly its brightness gets reduced when compared to the brightness that I had got using a 20mm eyepiece?**

4) *Secondly why do the Barlow lenses reduce the brightness a lot.* And why on using Barlow lenses the field of view get smaller and the object passes twice as quickly as without the Barlow?

5) Now the biggest question. *How is it possible that every time I point my telescope towards a visible planet or a star (I know it's a star because of the sky map), I see almost the same size, when stars are visible only as small dots.* **The stars appear to be bluish and the planets which I have seen (Mars and Jupiter) appear reddish. That's ok but a star can not be visible as a sphere. Stars appear as dots, don't they?**

6)How can I make the telescope give the best results i.e which eyepiece should I use and whether I should use Barlow lenses or not?

**Tags:** telescope, angular-resolution, refractor-telescope

> **Comments:**
> **username_1**: You might want to read Smith, Modern Optical Engineering, to answer many of these questions.  He is a good writer and puts in lots of diagrams to show where the planes of interest are and why they matter.  
> **username_0**: @CarlWitthoft Can you suggest me a basic astronomy book. I mean like how to look at the night sky , all the movements of the earth as well as other bodies like moon ( like the plane of moon intersects the plane if the earth at 2 points , etc) , covering all the details like **When is the sun really overhead**( because we are taught it's at 12 noon but I think it cannot be at 90° ever). And the all techniques of understanding the movement of the night sky ( because I feel it is very hard to study this 3D motion by looking from below). I haven't read anything. I just want to have a beginning...  
> **username_0**: @CarlWitthoft Also is the PDF of the above book available. Also could you suggest a book or article that deals with the exact functioning of a telescope but from exact basics ( ABC of telescope)  
> **username_1**: yes, you can test your Google-fu to find an ebook version.   As to basic astronomy, maybe start with nasa.gov .  
> **username_0**: @CarlWitthoft ok thanks ! Will nasa.gov work or a book ?  

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## 3 answers

### Answer by username_2 on 2017-03-31. Score: 3 | Accepted answer

**1. Resolving power and diffraction**

Diffraction happens anywhere there's an edge. *"It is defined as the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle."* ([Wikipedia](https://en.wikipedia.org/wiki/Diffraction)) So all you need for diffraction is an obstacle of any shape - at the edge of the obstacle, the light will bend a little.

[![diffraction](https://i.sstatic.net/q1Ob9.png)](https://i.sstatic.net/q1Ob9.png)

In telescopes, diffraction takes place all around the edge of the objective, be that a lens or a mirror. With a smaller objective, with a more strongly curved edge, and with opposite sides of the edge closer together, you get stronger diffraction (larger angle of deviation). With a larger objective, more straight edge, opposite sides of the edge further apart, you get less diffraction (smaller angle of deviation).

This is why larger telescopes have smaller [Airy disks](https://en.wikipedia.org/wiki/Airy_disk). It's similar to how a narrow slit makes a wider diffraction figure, whereas a wider slit makes a tighter figure. In the case of a telescope, the diffraction figure is the Airy disk - instead of having parallel diffraction zones, it's circular. A larger aperture makes a smaller Airy disk.

[![Airy disk](https://i.sstatic.net/qdYLm.gif)](https://i.sstatic.net/qdYLm.gif)

**2. Entrance/exit pupils and apertures**

For a lot of modern amateur telescopes, the entrance pupil is actually the aperture of the telescope, or the visible diameter of the objective. In some cases the instrument has an aperture stop that reduces the size of the visible part, but that's rare. In most cases diameter of mirror = entrance pupil.

The exit pupil is simply due to the way the rays of light converge after they exit the eyepiece - they converge until they're bundled in a smallest area (the exit pupil) then diverge again.

[![pupils](https://i.sstatic.net/PHzEC.gif)](https://i.sstatic.net/PHzEC.gif)

So the exit pupil is actually smaller than the diameter of the final lens in the eyepiece, because the rays keep converging for a while.

Obviously, you want to keep your eye at the eyepiece so that the exit pupil coincides with the pupil in your eye, so as to capture the greatest amount of light coming out of the scope.

EDIT: The exact place where you keep your eye does not change the size of the image. The size of the virtual image made by the telescope depends only on object size and magnification - nothing else matters. Changing the eye position only changes the amount of light that enters your visual system. (Explaining why this is would probably require a whole 'nother topic on this forum.)

**3. Magnification and brightness**

The amount of light captured by the scope stays the same. It is given by the size of the aperture.

But with a larger image (higher magnification) that same amount of light coming from the object is spread out over a bigger solid angle. You see the same number of photons, but now they are spread out over a larger apparent surface. Of course it will look more dim.

Same amount of butter spread over a larger slice of bread.

**4. Barlow, magnification, and field of view**

A Barlow increases magnification. As shown in paragraph #3 above, more magnification = reduced apparent brightness. Same thing would happen without a Barlow, but just using a much stronger eyepiece instead.

It's all about magnification. The light absorption in the glass in the Barlow is negligible.

Also, the object seems to pass more quickly through the field of view because the apparent field of view stays the same, but the whole image is magnified. Because the image is magnified, any motion within it must appear "faster". So the object will seem to pass more quickly through the same field of view.

**5. Stars and planets appear the same**

They shouldn't. It's a performance issue.

Stars should appear as tiny dots at low to medium magnification. They should appear as Airy disks (but still not large) at high magnification, fluctuating due to seeing (air turbulence).

Most planets (Venus, Mars, Jupiter, Saturn) should exhibit disks that are clearly bigger than the stars. Even Uranus and Neptune (which are far away) should still show disks that, upon close examination, are revealed to be bigger than Airy disks, in an instrument as small as 150 mm aperture (perhaps even smaller).

Could be a number of reasons for this. At 60 mm aperture, your scope should have enough resolving power to distinguish between the largest planetary disks (Jupiter) and any stellar Airy disks, and the rings of Saturn should start to become visible although very tiny. If that doesn't happen, then perhaps:

* It's a collimation issue. All telescopes except most refractors require periodic collimation, otherwise performance decreases
* Quality of optics. Not sure what optics you have, but if they have performance issues then the images will be bloated

As long as there are lingering issues with the instrument, higher magnification will not help.

EDIT: Stars are so far away that they are point-like objects to us. So their images in a scope "ought to be" also point-like. But they're not. This is due to diffraction and aberrations.

Diffraction I've explained above. The Airy disk is the smallest possible image of a star you could ever have in a telescope. You cannot make a stellar image smaller than that, due to diffraction.

Then there's aberrations. There are monochromatic aberrations:

* coma: the further away from the center the image is, the less point-like it becomes
* spherical mirrors or lenses DO NOT make perfect point-like images of stars. The point-like image is just an approximation. Spheres make imperfect images by their very design and shape.
* astigmatism - like coma, images near the edge of the field of view are distorted. The difference is that astigmatism makes a symmetrical distortion, whereas coma is weird shaped (like a seagull)
* field curvature - the focal plane of the primary mirror is not flat, but it's curved. So if the scope is in perfect focus in the center, it's not in perfect focus at the edge, and viceversa.
* distortion - a square grid is bloated so it looks like a curved cushion. Doesn't matter much for astronomical instruments.

Then there are chromatic aberrations, which happen with refractors because the refraction index of the objective is not the same for all color of light.

It is the hallmark of a good telescope that its aberrations are less than the size of the Airy disk, at least for most of the field of view (perhaps excluding the zone close to the edge). That instrument is said to be "diffraction limited".

If stellar images are very bloated, the instrument is likely not diffraction limited.

<https://starizona.com/acb/basics/equip_optics101_aberrations.aspx>

<http://www.telescope-optics.net/aberrations.htm>

**6. How to optimize an instrument**

You could write entire books on this topic.

A Barlow is not magic. It only gives you more magnification, but magnification is the most misunderstood parameter of a scope. Unless everything else is perfect, more magnification does not help.

There's a lot of reading you could do, and there is a lot of experimentation. Here are some older topics on this stack:

[How much magnification is needed to see planets of solar system?](https://astronomy.stackexchange.com/questions/8788/how-much-magnification-is-needed-to-see-planets-of-solar-system)

[First night on a telescope questions](https://astronomy.stackexchange.com/questions/2196/first-night-on-a-telescope-questions)

[Best telescope for the viewing of Nebulae, Stars and Planets](https://astronomy.stackexchange.com/questions/2193/best-telescope-for-the-viewing-of-nebulae-stars-and-planets)

[Please Guide me to buy my first Telescope](https://astronomy.stackexchange.com/questions/2223/please-guide-me-to-buy-my-first-telescope)

> **Comments:**
> **username_0**: Wow what an amazing explanation. Thanks a ton for going through this big question & answering every part of it. An year long wait to get a perfect answer. Just 2 things . I get that we will want to keep our eye at the eyepiece to get all the light but the pupil ( the eye will then be in front of the exit pupil). Also this converging light will converge again in the eye lens. That shall have a serious effect on the size of object we will see through it. Will it not ? Another refraction through the eye has have an effect on size. But that's not the case even in seeing a nearby tree . Why ??.....  
> **username_0**: & for the 5th question . Focus is a point. So we shall see point object in the eyepiece if the rays are all focusing at a point. But that is not what happens. Because the object we see through the eyepiece is on the focal plane. Right ? Because here in your picture the object seen in the eyepiece will be like a point. All rays converging to a point ? & If Stars have to visible as points then "what is that this telescope shows as spheres "  
> **username_2**: I made two edits to my answer addressing your comments.  
> **username_0**: So I see the aberration effects . Poor optics.....As it is it's not useful . I will ask another question on how changing position of eye won't change the size of the object or how the "another refraction through the eye won't change what we see. & How Gallieio made his own lenses. I bought Carborundum only to spoil the glass with scratches causing it to be translucent. Hope when I ask you are online or see the questions. Thanks a lot !!!  
> **username_2**: Don't give up. Making your own telescope is doable, although it's a very, very long process - so it takes time and patience. I make my own telescopes. Many other people do too. You can do it too, provided it's agreeable with you to stick to a long term project that may take even one year to complete. Here's a good place to start: http://stellafane.org/tm/  
> **username_0**: This looks to be great. I visit IUCAA too. But actually I neither have good resources nor experience. I bought  a plane circular glass 1inch thick 20cm diameter ( the one we use in toilets) and began grinding it with another same glass piece with carborundum grit 60. Instead of seeing the sagitta after a work of 5 hours , I saw only the glass changing from perfect to translucent. Don't know what's that ??  
> **username_0**: By the way if you don't mind I Shall tag you when I ask the question. You can then go through the question . If you wish to answer it , great else you can ignore it. At least you will know that there is such a question on the site . Then if you like it or wish to answer it ,great else you can ignore it !  
> **username_2**: The grinding issue could make an entirely separate question on this forum. And I don't mind being tagged.  
> **username_0**: Can you suggest me a basic astronomy book. I mean like how to look at the night sky , all the movements of the earth as well as other bodies like moon ( like the plane of moon intersects the plane if the earth at 2 points , etc) , covering all the details like **When is the sun really overhead**( because we are taught it's at 12 noon but I think it cannot be at 90° ever). And the all techniques of understanding the movement of the night sky ( because I feel it is very hard to study this 3D motion by looking from below). I haven't read anything. I just want to have a beginning...  
> **username_2**: @username_0 Unfortunately my native language is not English and all amateur astronomy books that I've read (long time ago) are also not in English. So unless you can read Romanian I can't help you. I'm sure there are good introductory books in English, I just don't have any examples right now.  

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### Answer by username_3 on 2016-10-14. Score: 3

I'll attempt to answer as many of these questions, to the best of my abilities, as I can at this late hour.

1) Lenses have an index of refraction that is different than vacuum, and air, and they are curved. ie: corners when light is concerned.

2) The entrance and exit pupils are the pupil size required to pass an "extreme" ray through an optic. Meaning that if the light ray from the optical axis was not refracted at all as it passed the optical plane, it would still pass within the pupil (entrance) and if reflected perfectly (exit). The diameters of the objective and the eyepiece are subjective, meaning that you can have, within reason, any size objective and use virtually any eyepiece. The restrictions to this would only be focal lengths.

3) You are looking through a thicker lens and thereby reducing the light incident on your eye. Think about the shape of the lenses, look at them and it might become apparent just how much more curvature the 4mm lens has than the 20mm lens. This added curvature comes at the cost of a higher angle of incident that a light ray has to take, and thus more optic to pass through.

4) Barlow lenses reduce the brightness a lot because you are adding additional optics. With each new addition you decrease the amount of light by a factor relative to the thickness and clarity of the optics. The Barlow lens is increasing the magnification and thereby reducing the field of view.

5) With your specific telescope (of which I have one with the exact same size/focal length) you should be able to see Jupiter as a disk, Mars too should be more than a "dot." Stars are point sources with angular diameters much too small for you to be concerned given the size of your telescope. Also, those planets appear reddish because they are-ish. Many stars will still appear colors other than blue if you can find them.

6) This is up to your discretion and desired results when observing. Please read your owners manual, or check out the telescope mfr website for recommended setups.

> **Comments:**
> **username_0**: Thanks for the explanation. I had read somewhere that the entry and exit pupil had something to do with the diameter of the eye pupil. Could you please explain the exit and entry pupil a bit more. And if stars are visible just as dots how are they visible like spheres of the size of mars in my telescope  
> **username_1**: @username_0 Any time a star appears to have a diameter greater than the diffraction limit of your 'scope (and ALL optics have an aperture, and that aperture controls the Rayleigh limit), it's due either to poor focus, lens aberrations, or atmospheric turbulent aberrations.  
> **username_0**: @CarlWitthoft  So the spherical shape of the star (not a dot ) is because of poor focus or aberrations  
> **username_2**: @username_3 - Quite a few mistakes in your answer. Diffraction is not caused by the index of refraction of the lens; diffraction and refraction are very different things. Light absorption is not the main reason why barlows and stronger eyepieces seem to reduce brightness.  

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### Answer by username_4 on 2016-10-14. Score: 1

You already have an answer but there are a few interesting Physics things in your question that are linked:

* 1) Diffraction and the fact that telescopes have no "slit".
* 5) Stars don't appear as a sphere, they appear as dots.

The telescope has a round aperture (of say 60mm or so) and this behaves as a slit in some ways. It causes diffraction patterns (an [Airy disc and concentric circles](https://en.wikipedia.org/wiki/Airy_disk)) when viewing a point source at high magnifications, near the resolution limit of the telescope. Also of slightly out of focus you'll get [lots of rings](https://en.wikipedia.org/wiki/File:Circular_Aperture_Fresnel_Diffraction_high_res.jpg).

(By the way this the cause of many "weird" unidentified objects that people photograph or film in the sky; especially a DV camera with odd shaped aperture blades. Surprisingly few people want to hear the explanation though.)

So when viewing stars at high-ish power, on very still nights, they actually look like a small white circle surrounded by a single, very thin, circular ring.

> **Comments:**
> **username_4**: Forgot to mention - the Airy disc & single ring can, on my 70mm refractor, be seen at around 80x magnification on very still nights. Just an illustration of size.  
> **username_0**: Thanks , so the Diffraction is happening at the aperture of the circular objective  
> **username_4**: @username_0 yes, prove it by observing a star at high power, looking for the diffraction rings if you can see them, then turn the aperture to a square by hanging a bit of cutout cardboard over the objective. :)  
> **username_0**: I got it. When I will turn the aperture to a square using a cardboard , I'll block the edges and the rings shall go away  
> **username_1**: @username_0 no, the rings won't go away.  They'll change shape because you changed the shape of the aperture.  You might Google "apodized aperture" for even more info.  
> **username_2**: @username_0 - as Carl said, the square aperture will simply change the shape of the diffraction figure. It will become a bright cross instead, and quite annoying. But your scope is not diffraction limited, so don't worry about diffraction.  
> **username_0**: @FlorinAndrei  If it's not diffraction Limited but still there has to be some. Then there is a lot of aberration in it . That is why I see Stars as spheres. Poor telescope. Always Wanted to see those nebulae those blue n green ( photographical) . Still waiting for that to happen.  
> **username_2**: Even in large amateur telescopes you will not see much color in nebulae, the human eye is just not sensitive enough to see color in low light. Also, what helps with DSOs like nebulae is lots of aperture. But even with 60mm of aperture I feel you should start seeing a bit of the Great Orion Nebula - which is still visible for a while nowadays. Google "how to see the Orion Nebula". Maybe try from outside the city, because urban light pollution makes it hard to see DSOs. It should appear like a small "cloud" in the surrounding blackness.  
> **username_0**: @FlorinAndrei I wanna ask. I have seen those pictures of milky way galaxy ....Those spiral ones , rising about a hill , those on the net. From the place where I live only countable 12 stars are visible. I had gone to LeH Ladakh ( it's a very high altitude place in India , the place of highest road , 19000ft) , at a place called Pangong , Hanley ( u might have heard of it).... Most of Indian telescopes are set there. 300 days clear sky....No pollution at all. There above I could see thousands of stars....I could even tell where the Ursa Minor is . Now the question.......  
> **username_0**: @FlorinAndrei  I went to a place supporting the Himalayan Chandra telescope and into those dome shaped cassigrain building like telescopes. In the computer which was taking all the reading , I could just thousands of white dots...The person there tried to explain....Infrared , CCD and all of that which I didn't get. If the best of the telescopes can just see these black n white dots , how do get those mega images as if the telescope has zoomed into the nebula or supernova. And how are those coloured when these where just black n white dots ( no structure at at all)  
> **username_0**: @FlorinAndrei  What is the authenticity of those images . Do they depict any thing real. And about the galaxy. Our eye can see it or figure it out of the thousands of stars. How can those DSLR cameras see it. Is there any way to see it though our eyes  
> **username_2**: @username_0 The human eye's color perception shuts down in low light; the color receptors need a lot of light to function. Digital cameras do not have this limitation, they can record color at any level of incoming light; even for very faint objects a camera can still distinguish color. Also, cameras can be kept open a long time, even hours, slowly building up a single image even when the object is very faint. Our eyes cannot do that. Finally, our high resolution / fine detail vision also shuts down in low light (same receptors as color). Cameras don't do that, so they see much better than us.  
> **username_0**: @FlorinAndrei  Ok thanks ! Was not active ! What about the authenticity of the pictures of nebulae , those big ones. When the best you can see in a best telescope is just  black - white tiny  circles , no structure , no color except black- white  
> **username_2**: @username_0 It's complicated. If it's UV or IR, clearly the colors are not real. If it's visible light, it depends. Often the colors are real but your eyes are not sensitive enough to see them; if they were thousands of times more sensitive you'd see those colors. Your eyes are not absolute judges of reality, they're just sensors, with good and bad characteristics. Other sensors are better and can see what your "sensors" cannot. So your eyes will claim that the colors are "fake" but the CMOS camera clearly disagrees. If it's red in the picture, it's red in reality, but not the way you see it.  
> **username_0**: @FlorinAndrei  ok that's about the colour. But what about the size. Stars visible as dots. The telescope there was also showing only dots on the screen of the computer.  So how are those nebulae picture, etc ( which seem to be as if the telescope has zoomed into it) taken . I know not about the Hubble (which is free of all the pollution on the earth) but certainly it's not possible for it too . Certainly not possible for the non space telescopes​  
> **username_2**: @username_0 Fast exposure images will only show the bright stuff (planets, stars). For nebulae you need long exposure: a single frame is gradually collected over many minutes or even hours. Many frames (dozens, hundreds) may be stacked together to make a single final image. Then software extracts the useful info out of the huge noise within. This way, even with terrestrial instruments you can capture good images. Use the search function on this site https://www.astrobin.com/ for examples (e.g. M31, M42, etc). Proper capture technique + clever algorithms can remove a lot of obstacles together.  
> **username_0**: @FlorinAndrei These pictures  , Are they "real". That's what the structure will look like (to an extent , I know they have been processed by computers , but after that this is what they will look like ). So even thier size n everything has been processed by computers. Because I guess one cannot see these nebulae in the telescope as big as they visible in the pictures.  
> **username_2**: @username_0 If you define "real" as "the way I can see them with my own eyes in a telescope", then no, they are not "real", your eye cannot possibly match a CMOS or CCD sensor. But these days after postmodernism we should be prepared to accept that what we see is always an interpretation, and always depends on the sensor (eye or camera) and the algorithm (brain or computer) - you get different images from different combinations, the choice is yours. As for that which is truly "real", the thing in itself, both Plato and Kant have told us long ago that it is pretty much unattainable. Not joking.  
> **username_0**: @FlorinAndrei Ok I got it , thank you ! I have read a bit of Immanual Kant (through Einstein) , but not Plato !  

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