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?

  • $\begingroup$ 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. $\endgroup$ Commented Oct 14, 2016 at 12:31
  • $\begingroup$ @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... $\endgroup$
    – Shashaank
    Commented May 3, 2017 at 11:39
  • $\begingroup$ @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) $\endgroup$
    – Shashaank
    Commented May 3, 2017 at 11:41
  • $\begingroup$ yes, you can test your Google-fu to find an ebook version. As to basic astronomy, maybe start with nasa.gov . $\endgroup$ Commented May 4, 2017 at 15:08
  • $\begingroup$ @CarlWitthoft ok thanks ! Will nasa.gov work or a book ? $\endgroup$
    – Shashaank
    Commented May 5, 2017 at 16:43

3 Answers 3


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) So all you need for diffraction is an obstacle of any shape - at the edge of the obstacle, the light will bend a little.


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. 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

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.


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.



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?

First night on a telescope questions

Best telescope for the viewing of Nebulae, Stars and Planets

Please Guide me to buy my first Telescope

  • $\begingroup$ 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 ??..... $\endgroup$
    – Shashaank
    Commented Mar 31, 2017 at 18:53
  • $\begingroup$ & 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 " $\endgroup$
    – Shashaank
    Commented Mar 31, 2017 at 18:59
  • $\begingroup$ I made two edits to my answer addressing your comments. $\endgroup$ Commented Mar 31, 2017 at 19:11
  • $\begingroup$ 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 !!! $\endgroup$
    – Shashaank
    Commented Mar 31, 2017 at 19:17
  • $\begingroup$ 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: stellafane.org/tm $\endgroup$ Commented Mar 31, 2017 at 19:22

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.

  • $\begingroup$ 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 $\endgroup$
    – Shashaank
    Commented Oct 14, 2016 at 5:56
  • $\begingroup$ @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. $\endgroup$ Commented Oct 14, 2016 at 12:33
  • $\begingroup$ @CarlWitthoft So the spherical shape of the star (not a dot ) is because of poor focus or aberrations $\endgroup$
    – Shashaank
    Commented Oct 14, 2016 at 13:24
  • $\begingroup$ @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. $\endgroup$ Commented Mar 31, 2017 at 18:33

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) 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.

(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.

  • $\begingroup$ 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. $\endgroup$
    – Andy
    Commented Oct 14, 2016 at 6:15
  • 1
    $\begingroup$ Thanks , so the Diffraction is happening at the aperture of the circular objective $\endgroup$
    – Shashaank
    Commented Oct 14, 2016 at 6:26
  • $\begingroup$ @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. :) $\endgroup$
    – Andy
    Commented Oct 14, 2016 at 6:36
  • $\begingroup$ 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 $\endgroup$
    – Shashaank
    Commented Oct 14, 2016 at 6:43
  • 2
    $\begingroup$ @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. $\endgroup$ Commented Oct 14, 2016 at 12:35

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