Re: ATM ? about interpreting thickness

["PETER JOHN SMITH" <pjifl@bigpond.com>]

Jorge wrote,

> I
>      SEE IT, THE THICKNESSES IN THE TABLE OF THE HOUGHTON TELESCOPE (AND
> 
>      MANY OTHER ARTICLES IN THE JOURNAL) DO NOT REPRESENT THE PHYSICAL,
>      TANGIBLE THICKNESS OF THE GLASS. I GUESS -AND THAT IS MY QUESTION-
>      THAT THE NUMBER -"D"- IN MILLIMETERS CALLED THICKNESS MAY HAVE
>      SOMETHING TO DO WITH BOTH THE DISTANCES BETWEEN OPTICAL COMPONENTS
> AND
>      THEIR OWN THICKNESS, BUT I DON'T KNOW HOW TO READ THEM.
> 
>      THE HOUGHTON EXAMPLE:
> 
>                 RADII OF CURVATURE              THICKNESSES
> 
>                 R1  1286.81                     D1   16
>                 R2 -4810.34                     D2    3
>                 R3 -1286.81                     D2   12
>                 R4  4810.34                     D4  617.67
>                 R5 -1590.97                     D5 -793.84
> 
>      THERE MUST BE FORMULAS INVOLVING RADII, RELATIVE DISTANCES AND REAL
> 
>      THICKNESSES TO GET THESE "D" NUMBERS. RELATIONSHIP I FRANKLY DO NOT
> 
>      KNOW AND TRY TO FIND OUT.


The usual system these days is for d  (usually called d instead of D which
is often reserved for diameter) to represent the distance from this surface
to the next one. The material following each surface must also be stated.
It was probably mentioned in the context of the article but it is a good
idea to always include it in this table in a third column. :   (If you see
a refractive index of -1 this is another way of saying the naterial is a
mirror.) 

To work it out, you do need the refractive index of the material following
each surface but familiarity with the layout of a Houghton dictates 

In this design,

between R1 and R2 there is glass.

between R2 and R3 there is air.

between R3 and R4 there is glass

between R4 and R5 there is air.  (R5 is actually a mirror) 

Finally from R5 to R6 (which is the image) there is again air.

So, depending on the material, the d values represent either the thickness
of elements or the spacing to the next material.

Hope this helps.

Peter Smith.