Re: ATM - Help finding materials for cassegrain project.??

[GregJ888@aol.com]

>Date: Mon, 13 Dec 1999 19:59:21 -0500
>From: "Frank Ward" <thewards@mindspring.com>

>I personally have never heard of this design.  What are it's advantages
>other than the spherical figures for  null testing?

These are Houghton derivatives.  ATMJ has a couple of very good articles on 
the Houghton Cassegrains that you will find interesting.  The software that 
comes with Ruttan will design one for you.  The designs are generally used 
for astrographs, wide flat, well corrected fields.  

In general the designs are very forgiving.  BK-7 is a joy to work with.  The 
only issue with the optics are all the surfaces to polish.  I have build a 
couple of gringing/polishing machines since.  Would suggest you consider one 
while you collect you materials.

Mine is an 8" f/4 Laurie (Newtonian), It needs some tweaking, mostly to the 
mount.  Even though it has a few optical problems, they are minor, I'm very 
pleased with the outcome.  


>From: Ken.Noesgaard@siemens.ca,

> Most places I've tracked down are wholesale
> > suppliers, and Newport
> > > hasn't even provided me with a price.
> > >
> > > I need 2 pieces of BK-7 roughly 10.5" x 1" circular would be vastly
> > > preferable, although trepanning is an option.
> > >
 I got my 8" blanks from Newport.  They can be a pain to deal with but I have 
had good luck with their products.  Their catalog has some lens making info 
that I also found very useful.  The test jig and using spray paint (rustolium 
sp?) to protect the side you are not working on -- both work well.

You can save quite a bit of money by ordering  the blanks, Plano-Plano fine 
ground, instead of polished.  Since you have to grind both sides anyway it 
causes no hardship. 

IMHO, you should also factor in AR coatings for the lenses.  


Greg Jones
<A HREF="http://hometown.aol.com/gregj888/DAC/DAC.htm">Deschutes Astronomical 
Center</A>