I made my telescope, with two convex lens of focal length 1000mm and eyepiece (plano-convex) with 25mm focal length. But I can see only a very small field of view, with inverted image. Do I need to increase the diameter lenses for more area capture, and can I use concave as eyepiece for upright image?

Or does some different solution exit?

I can read all information related to telescopes and binoculars, ie. how lenses work, which lenses combination are for which purposes (terrestrial and astronomical), and increase or decrease area and magnification.

Please note here I am talking about only refracting telescope, and binoculars.

note: This is a follow-up question based on comments below this answer to my previous question.

  • $\begingroup$ Wikipedia for a start. Your magnification is $1000/25 = 40$, which implies small field of view. $\endgroup$ Commented Jun 26, 2020 at 4:46
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    $\begingroup$ Welcome on the Astronomy SE! I edited your post into a better comprehensible form, but I am not 100% sure if it is correct. Please fix it if not. $\endgroup$
    – peterh
    Commented Jun 26, 2020 at 12:14
  • $\begingroup$ Refracting or reflecting makes no difference when it comes to field of view. The more magnification, the less FoV . TINSTAAFL, after all. I highly recommend Warren Smith's Modern Optical Engineering (any edition is fine) for a detailed presentation of simple, thick-lens, and complicated optical systems. $\endgroup$ Commented Jun 26, 2020 at 15:17
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    $\begingroup$ Folks please do not close this question! Here you have a new user asking a follow-up question based on my recommendation in order to better understand how to build a telescope. Closing simply blocks everybody from being able to post an answer, which seems like the worst result in this case. $\endgroup$
    – uhoh
    Commented Jun 27, 2020 at 3:42
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    $\begingroup$ @KeithMcClary why not help this new amateur telescope maker and post a short answer explaining that? Is there a second lens that can be added to the eyepiece to address this somehow? en.wikipedia.org/wiki/Eyepiece#Field_of_view $\endgroup$
    – uhoh
    Commented Jun 27, 2020 at 3:45

1 Answer 1


There basically are three main properties which influence what you see with your telescope:

  • The aperture defines how much light you get, thus how sensitive your setup will get. The bigger, the more light you collect, thus the fainter objects you will be able to detect in principle. There's the somewhat sense-free saying that you can compensate (clear) aperture only by clear aperture.

  • The focal length is the primary factor which drives your field-of-view. You can somewhat vary it by the choice of your eye piece. The magnification is the ratio of the two focal length, of your primary optics and the eye piece: $m = \frac{f_{tube}}{f_{eye}}$. Thus the bigger this magnification is, the more details you can see, the easier you resolve things which are close to eachother. However, phyics strike here and the size of the aperture limit how much magnification makes sense due to the diffraction ocuring at it. The so-called useful magnification (also see here, thus maximum magnification) is by rule-of-thumb about twice the aperture in mm. You can increase or decrease your focal length on an existing telescope with either a Barlow lens or focal reducer respectively. Both, of course, result in some optical degradation, as any optical system does.

  • The actual field-of-view is also determined by the eye piece properties and its opening angle. This can vary in a limited range only and is usually around 50°. See also this telescope calculator by Sky & Telescope.

Taken aperture, focal length of tube and focal length of eye piece, you derive one additional property which immediately affects what you see: the so-called opening ratio or f-ratio which is the ratio of focal length of your tube and the clear aperture. This determines how bright you will perceive a certain object as that defines how much light from an object is projected per unit area. Telescopes with identical f-ratio will show identical objects at the same brightness - independent of their actual field-of-view. Thus this number really defines how well you can see faint objects like nebula or faint / distant stars.

If I should put it in three sentences, this is it in short: your choice of focal length determines how big an area you see of the sky. The longer, the smaller your FOV. The ratio of aperture and focal length defines how faint objects you see.

As to the inverted image: astronomical telescopes usually have that, it keeps the system simpler. There are terrestrial telescope which employ an additional lens. Taking the words of uhoh's comment: The nature of real images is that they are upside-down and reversed left-right. It is possible to use a 1:1 relay lens to re-image your focal plane again but it will make your system more complicated to build and to hold everything in the right place. You can also use a prism to flip it in one direction, or two prisms to make an image inverter. If you don't want to ask a new question about using the relay lens, you can just read about it in wikipedia.

  • $\begingroup$ Thanks, and about inverted image, how to get upright image without losing magnification? Can I put concave lens between two convex(objective and eyepiece) for upright image. $\endgroup$
    – Binder
    Commented Jun 27, 2020 at 10:58
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    $\begingroup$ Assuming your permission I added your comment to this answer @uhoh $\endgroup$ Commented Jun 27, 2020 at 13:28
  • $\begingroup$ @planetmaker excellent! $\endgroup$
    – uhoh
    Commented Jun 27, 2020 at 14:49

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