Re: ATM Invisible spider vanes....

[kevin@aquilagroup.com (Kevin Ferguson)]

At 03:08 PM 5/1/97 -0400, Anderson, Jay wrote:
>Just wondering if anyone has experimented with a window to suspend the
>secondary holder on a Newt?  I have read some things that indicate that
>optical glass must be used and its surface must me near perfect.

Been done by lots of others, and I started down that path, but have
set it aside for a while, owing to dropping the mirror intended for 
that telescope.

Tex' goes into some detail in second edition over just how near perfect
surfaces have to be, and the answer is "not so perfect", the required
tolerances on wedge, and allowable curvature are easilly measurable
by mechanical means, trivial to achieve for a good optician, and
even pretty easy for me! The window is then null figured in front of 
a spherical mirror.  In addition to removing zonal imperfections in surface,
there may be some compensation for inhomogineity (spelling?) of
the index of refraction of the window material.  

Don't forget that the mirror was left on test stand when moving it the 
next morning before you put your glasses on. (voice of very
painful experience, that)

>  What
>about 3/8" or 1/2" plexiglass?  It is 10X clearer than optical glass (I
>understand). 

Look at a piece of plexiglass through crossed polarizers and see if you
want that in front of your mirror. Further, though it may well be
amazingly clear, the index of refraction is rather high, meaning you 
have increased reflection at the two surfaces.  Dunno if there is a 
workable AR coating material (and process to apply it) for plexiglass.  
Really, for the thickness in question, attenuation within the material 
is irrelivent...doesn't much matter if you lose .01 or .001 percent of 
the light inside the window when you lose ~1% due to surface reflection. 
(and lots more if uncoated)

> Maybe I am missing something but seems to me that having a
>window will remove all vane obstuction and diffraction caused by the
>vanes and so improve image contrast. 

You are missing nothing, except that making a  optically neutral window
is rather non-trivial.

> Also, the telescope tube could be
>a closed unit and so not suseptible to thermals, dust on optics, dew,
>etc.  Geeze, you could even pump out the air and fill the tube with
>Nitrogen or something.  Just a thought.

Well since air is mostly nitrogen, and is available cheaply, you might use
that instead. <g> For those who DO go so far off this edge,
helium is prefered due to high thermal conductivity (helps keep temp uniform)
and low sensitivity of index of refraction WRT temperature.

Tip the window at least 1/2 the max field of view angle to avoid glare
problems and you
have all the pieces.

If you want to try it, read that chapter in Texereau at least a dozon times.
You will
need to make a spherometer with very high resolution, though _accuracy_ is not
a requirement for flat work, since it compares rather than measures.  A
US$10 chinese 
micrometer will tell you the story on wedge, which again is a comparison,
rather that
a measurement.  

You will need a spherical mirror of about 
1" larger aperture than the window for null testing.  ~f/10 is preferable,
but you can 
make do with faster.  You can tip the window in front of a smaller mirror,
as I did, but
I wasn't thrilled with this, and have resolved to make a bigger test sphere
when I 
again take up this task.

Several have told me that float plate is a sutable material, but I took
Bratislav's
advice that for all the effort put into it you might just as well use best
available 
material.  A 10" x 1/2" BK7 blank cost around US$65 from Newport a year ago.
They
slice them from a "log" so you can get most any thickness you request.