I want to make an astronomical telescope

[sanphy]

I want to make an astronomical telescope, can anybody help. I need specifications for the lenses.

 

[mgb_phys]

Depends what you want the specifications of the telescope to be!
You can build a keplerian telescope with any two convex (magnifying) lenses

You can easily measure the focal lengths with just the sun and a ruler so any two random lenses will work.

For a more detailed description see: http://www.pacifier.com/~tpope/Building_A_Galilean_Telescope.htm

 

[sanphy]

how much magnifying power can I get, I want to a > 60X

 

[mgb_phys]

No you don't
Firstly if you are using this with your eye, then to capture all the light form a 60x telescope the diameter of the front lens would have to be 300mm, otherwise you are just wasting light.
Secondly you would need a much more complex optical system to magnify 60x and not be dominated by optical aberations.
Finally all you will see at 60x is a bigger atmopsheric blur.

 

[russ_watters]

I'm not following most of that, mgb - 60x is not much magnification to be talking about such problems.

But especially the first thing - where are you getting that and why does it matter?

 

[mgb_phys]

Magnification of 60x means the exit pupil is 1/60 diameter of the input aperture.
If your dark adapted pupil is 5mm then you need the input to be 300mm (5mm*60)

60x is probably a bit optomistic for a homemade 2 lens telescope.

 

[sanphy]

So may be i would go for a reflecting telescope,

 

[HallsofIvy]

Why do you want to get "magnification> 60X"? I could understand for a terrestral telescope but you posted this under "astronomy". In astronomy, magnification is not very important. "Light gathering capacity" (which is proportional to the surface area of the primary lens or mirror [radius squared]) and "resolution" (which is proportional to the radius of the lens or mirror).

Slightly less important, in my opinion, than radius of the primary is focal length. You can always change the magnification by using different secondary lenses. The magnification is the focal length of the primary divided by the focal length of the secondary. The longer the focal length of the primary, the larger the focal length of the secondary can be for a given magnification which makes it a little easier to focus.

 

[alseth]

sanphy, the type of telescope you need depends on what you want to observe. If you want to do planets then you should aim for the highest useful(!-cannot emphasize enough) magnification possible. Then a refracting telescope is better for you but this type of telescope(with sufficient performance) is difficult to build for amateurs. If you want to build one on your own I think you should go for a reflecting telescope. You may even attend a workshop where you can build your own telescope under the supervision of more experienced folks -in my opinion this is the best solution for you

 

[Waveform]

What do you mean ‘make’ a telescope? Do you mean buy lenses and assemble an optical tube, or obtain a mirror blank and grind a mirror. I suspect you do not mean purchase optical grade glass and attempt grind lenses.

Useful magnification ranges ≈0.2D to 2D (D being the diameter of the objective lens or mirror in mm)
The low end limit of 0.2D almost assures all the light will reach the retina.

As already stated here if you were to look at magnification of 60X at 0.2 D then the objective lens would have to be in the order of 300mm.

As an example a 330mm objective would yield a magnification of 66X at 0.2D The same 330mm lens at 2D would have a magnification of 660X.

Objective diameter, focal length, light grasping, are all interchangeable, one thing is a trade off against another.

Making a Newtonian telescope is not all that difficult of a job for most people, however, it is labor intensive, but if you take your time and are careful, one can make quite a respectable instrument.

 

[sanphy]

HallsofIvy, actually i mean making an astronomical telescope(gazing planets and other constellations). Alseth advices for a reflective telescope, and I am very much acuainted with the Newtonian reflective telescopes , but the concave mirrors are not readily available here in INdia. I m thinking of the advices of Waveform .