How to make a telescope for viewing planets, moon and DSOs using a convex lens of aperture 100 mm and focal length 200 mm and other lenses at home?

Question

I wanted to make a telescope with DIY things lying around in home. I read up that the aperture was a very important aspect and thus bought a convex lens with 100mm aperture and 200mm focal length, to use it as an objective lens. Apart from that, I have the following lenses lying around:-

1. 1 convex lens - focal length 20 cm aperture - 10 cm
2. 2 convex lenses - focal length 17 cm aperture - 5 cm
3. 1 convex lens - focal length 2.2 cm aperture - 2.5 cm
4. 1 concave lens - focal length 8.33 cm aperture - 5.5 cm
5. 1 convex lens - focal length 200 cm aperture - 5.5 cm

I can also buy lenses of range of Power(In Dioptres) from (-14D) - (+14D) of aperture range of 5.5 cm to 7 cm, if suggested.

I made a telescope using lens 1(as objective) and lens 3(as eyepiece) which on calculation, gave a magnification of 9x, but according to stellafane.org and http://skyandtelescope.com the minimum magnification is 14x and that I was 'wasting' the light gathered by the objective lens. This telescope showed me a bright Mars, Jupiter and Saturn, but as dots. Upon seeing the Moon, I could just see it a little bigger than what would be seen with the naked eye.

I would like to know the way to make the telescope, which I would like to use to observe planets and DSOs if possible. I am ready to make separate telescopes also, if suggested.

I would like to know which lenses should I use, or buy and the way to use it. I live in a suburb and it is not very lighted up.

Is there any way I could use the lens 1 in a telescope which I could use to view the planets?

[Edit]

I have accepted @JamesScreech 's answer and am on my way to making it. Apart from that, I would also like to use the big aperture of lens 1, for viewing DSOs, But, it has a very short focal length(just 200mm), and therefore, I am finding it hard to get any eyepiece lens which can "completely accommodate the exit pupil of my scope"(as said on skyandtelescope )

Any way to get such a lens, or make an eyepiece barrell sort of, using a combination of lenses mentioned above or buying. And how to use it?

Thanks.

Answer 1

You would be better off using lens 5 and 3. Though don't expect much, a simple single lens for the objective and for the eyepiece will give a lot of chromatic aberration (false colour). Refracting telescopes usually use at least two lens of different glass types (crown & flint often in a doublet) and eyepieces usually have 4+ lens.

You should be able to see craters on the Moon, the 4 Galilean Satellites of Jupiter, a couple of belts on Jupiter and Saturn's rings. You wont be able to reduce chromatic aberration as simply as that, the type of glass (it's refractive index) and curvature of the lens all need to be matched. You would be much better off buying a ready built objective and eyepieces if you want to make a refractor yourself, though it would probably be cheaper to buy a ready made telescope.

Answer 2

I generally agree with the answer above, but have a couple more insights which might help you if you decide to proceed with trying to make your own scope... The lens pairs that James mentioned (crown and flint) are known as a doublet. Glass has two key properties in play here - its index of refraction (how much it bends light) and its dispersion (how much that bending changes over color). The lens pair balances a strongly convex crown (low index and low dispersion) with a weakly concave flint (high index and high dispersion). The dispersions are designed to cancel out, while you want the curvature of the convex crown to overpower the concave flint in terms of index, so it still has some ability to focus. The design also inherently lends itself toward long focal lengths which are desirable in telescope objectives.
Eyepieces, due to their short-desirable focal lengths necessitate more lenses which allow you to balance the chromatic aberration, and also address other optical aberrations which come into play with such a short focal length (distortion, astigmatism, coma, and spherical aberration being the main concerns). There are well-established design forms which are often used for making well-corrected eyepieces, some of which can be found right on wikipedia: https://en.wikipedia.org/wiki/Eyepiece#Eyepiece_designs

One thing to consider, which I don't think has been mentioned is selection of focal lengths and apertures. A 5cm aperture is plenty sufficient to view the Galilean moons, and probably some bright DSOs if you're well-corrected, and if your focal lengths are well-chosen. The system magnification is the ratio of focal lengths between the objective lens and eyelens/eyepiece. (200cm/2.2cm = 90.9x). This means that something like the Galilean moons, which have a max extent of about 1/8 degree, would be magnified to have an apparent extent of 11 degrees (much easier to resolve). Your aperture selection (particularly of your objective) will determine the light-gathering ability. But the magnification applies here too, so if you have a 5cm objective at 91x, your "exit pupil" will only be 0.55mm diameter, which is tiny compared to your eye's aperture. You'd still be able to see the object, but your eyes will easily accommodate up to a 30cm objective aperture (3mm exit pupil). Keep in mind, there is a tradeoff between aperture and aberrations, so unless you're designing a very well-aligned 2- or 3-element objective, you may want to stick with a max objective aperture of 50-75mm.

In terms of alignment, don't just set the lenses a certain distance apart and expect to see an image. You will need to allow for some adjustment, which is probably easier looking at a distant object during the day. After you form an image, you may need to adjust the centration and tilt of the eyelens to form the sharpest image to optimize your alignment.

All that said, a small aperture, high-end refractive (glass) telescope can perform better than a reflective telescope of the same aperture. But as aperture increases, the cost of the materials and impact of aberrations makes refractive telescopes vastly inferior to reflective (mirror) telescopes. Since the design for these only necessitates 1 powered mirror and an off-the-shelf eyepiece for $50 or less, the best bang for your buck will definitely be a reflective telescope. Sorry if that's not what you're hoping to hear, but it's why most telescopes on the market today are reflective.