I purchased a 90x refractor telescope with below configuration.

  • Objective: 50mm diameter with 360mm focal length
  • 2 Eye pieces: 6mm, 20mm (10-1/ 8" scope)
  • 1.5x erecting eye piece

It measures 13- 5/8” long x 2-1/2" diameter.

I was trying to see a building half Km far from my balcony. But I can't see anything with any eyepiece other than white lighting circle. Moreover when I tried an eyepiece of toy binocular, I could see something, not much larger but at least visible.

Is it something minimum distance of view for a telescope? Or do I need to focus it? (I don't know how)

  • $\begingroup$ You'll generally need to focus... $\endgroup$ Feb 14, 2013 at 4:24
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    $\begingroup$ Do you have a fixed focal plane instrument, or are you going to be peering through the eyepiece (or sometimes one of the several eyepieces)? Are you going to let other people look? $\endgroup$ Feb 14, 2013 at 4:31
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    $\begingroup$ @articlestack I gave an answer, but now come to think of it I'm not too sure what your setup is. Where did the eyepieces come from? Is that a different 10-1/8" scope? It also might help if you gave the exact model you are using. $\endgroup$
    – user10851
    Feb 14, 2013 at 5:43

2 Answers 2


Any optical assembly, whether a telescope or a camera or whatever, will generally have a minimum distance at which it can focus. That said, the scale for distances is set by the focal lengths of the optics. As you point out, they are $360~\mathrm{mm}$ and either $6~\mathrm{mm}$ or $20~\mathrm{mm}$. (The $360~\mathrm{mm}$ is the really important one.) Distances long compared to these are all much the same to your telescope, so the building half a kilometer away and the stars thousands of light years away are all "at infinity" as far as it is concerned.

The $360~\mathrm{mm}$ actually means that parallel rays "from infinity" need to travel $360~\mathrm{mm}$ beyond the lens to converge to a point. Diverging rays from "closer than infinity" objects need to travel further. Thus the minimum distance at which you can focus is set by how much you can extend the optical path, and the maximum distance you can focus is set by how short the path can be made. Any telescope, whether for terrestrial or astronomical use, should be able to focus at infinity - that is after all where its targets of interest are.

Note though that I mentioned changing the optical path. In order to focus, assuming you don't have some multimillion dollar deformable optics on hand, you need to turn some knob somewhere to move something. While there are systems built that are fixed to focus at a single distance, I'm guessing that's not what you have, since it seems designed for changing eyepieces (doing so requires refocusing) and personal use (while contacts are fine, people who wear glasses are best served by removing them and changing the focus to correct for nearsightedness, since you generally need to get your eye close to the eyepiece).

  • $\begingroup$ Thanks @Chris, You game me an idea how to focus. Problem was with telescope tube. I have updated another answer with my experience based on your answer. $\endgroup$ Feb 15, 2013 at 3:20

This problem was arising due to the length of telescope tube.

First I scared when I become able to focus distance object like this;

I can't stablize the view like this. And image was also inverted.

enter image description here

But then I relaxed when mirror increased the length like this;

enter image description here

However I was getting mirror inverted image.

So I used eye erecting piece who gave me correct image, not inverted not mirrored;

enter image description here

I used toy camera eye piece for targeting the object. However it is not a good diea. Because every time I need to separate eyepieces, mirror, and eye erecting piece. And need to again joint all.

Well!! now I need to do something so telescope will not wiggle on my light touch, I'll not see my eyelash in the lens/mirror, some easy way to target the object, some way to photograph image, and using telescope without harming, itching, and uncomfortable eyes.


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