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Hi i have made a telescope with 1100 mm focal length objective lense and 21mm plano convex as eyepiece. Both these lenses are not achromatic. But when is looked through it , the images were completely blurry. Is this because I have not used achromatic lense.?? Can it be resolved if I used any coated eyepiece.? The diameter of objective is 7.5 cm and eyepiece is 1cm. Is this issue is because the size of eyepiece is so small.

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    $\begingroup$ Did you adjust the optics to focus on your object of interest (i.e. adjusting the lens separation slightly) so that your image becomes sharp? Like with manual photo lenses you have to manually set to focus on what you look at. $\endgroup$ – planetmaker Mar 11 at 16:46
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    $\begingroup$ Yes. I tried it but still kind of blurry... $\endgroup$ – sanoj Mar 11 at 18:39
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    $\begingroup$ How do you make sure they are axially well aligned? $\endgroup$ – planetmaker Mar 11 at 18:56
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    $\begingroup$ Actually I didn't. I am kind of new to telescope making. This is just a first try... Can u guide..? $\endgroup$ – sanoj Mar 12 at 19:12
  • $\begingroup$ The two focal lengths are f1 = 1100 mm and f2 = 21 mm, in principle the two lenses should be placed f1+f2 = 1121 mm apart. If not, you may see a blurry image, together with the chromatic aberration. Also, the diameter of your eyepiece is too large in that only beam radius of 21/1100*7.5 = 0.14 cm can travel through, much smaller than the 1 cm aperture size -- which is okay and does not relate to a blurry image. $\endgroup$ – WDC Mar 13 at 5:19
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Is this because I have not used achromatic lens?

An easy way to check for the effects of chromatic aberration will be to look at a narrow wavelength source. These days that's easy because the spread in wavelengths from single color LEDs is only 10 to 20 nm1 depending on the specific LED. If you can find something with red, green or yellow LEDs (not the white ones) can you check to see if those can be focused more clearly? All you need is one small LED to do this, maybe across a room. If your focal length is 1.1 meters and the LED is 10 meters away, you'll have to pull your eyepiece out by another 10 cm farther than you would for focusing at infinity.

1random example

Note also that chromatic aberration is stronger in the blue than in the red.

You can also estimate the effect with a bright white object with a sharp edge (like a white piece of paper) on a black or dark background. If the edges are blurry with strong color effects (blue and red in different places) then this may indeed be the problem.

If so, you might have to invest in an achromatic objective to get rid of this problem, or look only at things that are monochromatic (and in Astronomy there aren't many of those!)

In addition to making sure the eyepiece lens is centered on the objective lens' axis, you also have to make sure the eyepiece lens isn't tilted. Both of those will cause problems and the point-like LED spot will show this. It might look like a fuzzy comet coma aberration.

Can it be resolved if I used any coated eyepiece?

No, don't worry about that for now. Adding antireflection coatings to lenses reduces reflections which are a big problem when there are multiple lens elements with air gaps between them within the eyepiece or the objective. Those cause lens flare usually only a big problem if there are very bright, point-like objects in the field of view. But in your case each is a single element, so not an issue.

Is this issue is because the size of eyepiece is so small.

No, I think it's because of some combination of the eyepiece not being positioned properly and chromatic aberration, and a test on an LED might be one way to separate the two effects for diagnostic purposes. If the blurriness has a lot of color associated with it, especially around white objects on black backgrounds, this is your primary culprit I think.

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