Building telescopes...

[Wing Nut]

I want to build a refractor and have several convex lenses.  Anyone know how to tell what the focal length on a lens is?

Kevin

[Smoke2much]

Simple:
Thick lenses can be handled with thin lens type equations if the distances are measured from hypothetical principal planes. The power of a lens with respect to the second principal plane H2 is given by Gullstrand's Equation:



The focal length f with respect to that second principal plane is given by:



Forms of Gullstrand's Equation are applicable to both separated lenses and to single thick lenses. Since Gullstrand powers and focal lengths are measured with respect to a hypothetical plane, it is often more useful to deal with front and back

OR

Lens-Maker's Formula
For a thin lens, the power is approximately the sum of the surface powers.




The radii of curvature here are measured according to the Cartesian sign convention. For a double convex lens the radius R1 is positive since it is measured from the front surface and extends right to the center of curvature. The radius R2 is negative since it extends left from the second surface.




For a lens in air which has index of refraction n =  
with surface radii R1 =  cm and R2 =  cm
the thin lens approximation of the power is P =  diopters
which corresponds to focal length f =  cm .  

If the lens is in another medium, such as water, its lens strength will be diminished.

If the lens is in a medium of index n =  
then the lens power will be  diopters  
and the corresponding focal length f =  cm.  

The above calculation is a single purpose calculation which returns the powers and focal lengths based on the values of the radii and indices of refraction. Substitute a number in any field and the calculation will be initiated. Default values will be entered for any unspecified parameter, but the index and radius values may be changed.

The second part of the calculation above shows the diminshed power of a lens in a medium other than air. This gives important insight into the role of the two lenses of the human eye. The cornea has the greatest power because light enters it from the air. The inner crystalline lens has much less refractive power because it is suspended in a water-like medium.



I haven't a clue what that all means but you might!

http://hyperphysics.phy-astr.gsu.edu/hb ... oclen.html

Will

[Fly2e]

I am sure that Alrot can help you Kevin.
From the looks of his signature, he is surrounded by them!

Dave

[Wing Nut]

That was so far above my head an SR-71 couldn't have reached it...  :)

I thought it was something simple like diameter times radius or something like that, but I guess not...

Kevin

[eno]

Alrot builds telescopes ....... he's your man.

[Fly2e]

http://sfsidewalkastronomers.org/telescopes/building/

http://www.jlc.net/~force5/Astro/ATM/atm.html

[alrot]

To made an refracting unacromatic lenses you'll need a special lab,im sure you know what is cromatic aberration is the effect of discomposesing the light by prism effect ..to make a refractor telescope you should need two differerent types of glass of a difrerent density a flint glass and crown glass each one of them to make their sum of both aberrations and their diferent type of desity and presition curves and thik,

[Fly2e]

Yeah, exactly what Alrot said  ;D

[Jared]

wow! I remember taking physics and learning a little bit about this stuff, but not that detailed, good luck mate. ;-)

[alrot]

thanks you guys,im embarased...  ...lol  ;D

[Wing Nut]

I already have the lenses, so I am pretty much committed to a refractor.  I live in an apartment, so a Dob is out anyway, just because of space.  My SCT and cases took up a lot of room too, and it was smaller than even an eight inch Dob would be...

At any rate, I still need to figure out the focal length of these.

Kevin

[Smoke2much]

I seem to remember at school shining light through lenses and moving a piece of paper along a meter ruler until the focal point was found.  Would this give you the focal length?

[Fozzer]

[quote]I seem to remember at school shining light through lenses and moving a piece of paper along a meter ruler until the focal point was found.

[alrot]

[quote]I already have the lenses, so I am pretty much committed to a refractor.

[Wing Nut]

Thanks Alrot, I appreciate any advice you can give.  I have been into astronomy for a while now, but have never tried building a scope from scratch before.  To be perfectly honest, I am not even sure if the lenses are suitable.  I am only doing it for a learning experience.  I'll snap a couple of pics and post them too...

Kevin