Re: ATM Adding a Prism Diagoanal

["Frank" <thewards@mindspring.com>]

You have paraphrased me correctly.  I use prisms in my 10"
and in my 5" refractor.  All you have to remember is that the
hypotenuse is a diagonal (akin to a secondary mirror) however,
the hypotenuse need not be silvered because the rays are
reflected off the inside surface and out the next side to the eyepiece.

That means the axal rays meet the center of the hypotenuse on a
prism leaving atleast an inch of glass and its housing to be 
accounted for when you are designing the placement.

Frank
-----Original Message-----
From: lifedata@vol.com <lifedata@vol.com>
To: ATM@shore.net <ATM@shore.net>
Date: Sunday, July 04, 1999 10:56 AM
Subject: Re: ATM Adding a Prism Diagoanal


>
>"Frank" <thewards@mindspring.com> said:
>
>>A prism acts exactly like a secondary mirror......it is a surface at
>>45 degrees to the axis.  You won't have to change the length of the
>>tube because of the actions in the focal path.  You will have to
>>change it to accommodate the prism.  You can't just stick a prism
>>diagonal into a focuser and add an eyepiece.  The body of the prism
>>will take up enough room necessitating shortening the tube.
>
>Perhaps the elementary points are being left unsaid here, but what
>I've seen so far on this thread seems rather unclear.  So I'll post
>what I think is what has not been said.
>
>The point I haven't seen specifically covered is whether the
>questioner is considering the difference between a mirror and a prism
>or the difference between a diagonal and no diagonal.  Maybe Frank is
>referring to this in his last two sentences, but if so the word
>"prism" could be exchanged for "mirror" could it not?
>
>That is, "You can't just stick a [?] diagonal into a focuser and add
>an eyepiece.  The body of the [diagonal] will take up..."
>
>Frank, correct me if I'm wrong.
>
>Jim L
>Lawbreakers will all suddenly obey the law if you write more gun
>laws!
>