Re: [ATM] Slit and source arrangment

["Bob May" <rmay@nethere.com>]

This is probably a good time for me to ask a question that has
been on my mind for a while.

How do you figure the Ecentricity, Schwartzchild Constant and
other such values and how do you express an oblate elipsoid (the
long side of the elipse) with each of the numbers.  They all seem
to have a sphere as a "0" and a parabola as a "1" with or without
the negative sign in front of the number.  It looks like you can
describe the oblate with an Ecentricy number but the rest seem
not to be able to describe it.

The reason for this question is that you can then describe in
real numbers what the error is going to be for a sphere when the
source and destination of the light is for a particular offset of
the KE from the light.

Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Jerry <wa4guu@verizon.net>
To: 'ATM list' <atm@atmlist.net>
Sent: Saturday, April 26, 2008 10:12 AM
Subject: Re: [ATM] Slit and source arrangment


>
> On the original (I think) concern about the lateral offset
knife and slit
> showing an oblate ellipse as a sphere (null), I agree that it
is trivial, at
> least for most mirrors. Maybe with big and/or fast mirrors it
would become
> significant. I'll let those more competent with the math work
that out. In
> any case I choose to have my knife and slit at "reasonably"
minimum
> separation. Reasonable meaning that I do not go to excruciating
engineering
> and construction effort to get them as close as possible, but I
do not
> ignore that getting them close should not be ignored. 1/2 inch
is pretty
> easy to do so I wouldn't separate them by inches.
>
> Another problem that I will leave for the mathematicians is
that I am not so
> convinced that the vertical separation as in a slitless tester
does not
> affect the knife readings just as much as lateral separation.
I'm reasonably
> sure the effect on knife readings would be trivial.
>
> Concerning very wide separation or diagonal separation, and why
anyone would
> do it that way? Because they do not know it might matter at
some extreme. I
> think diagonal separation of source and image would also have
to be large to
> matter much.
>
> I have seen testers that minimum separation would be 3 or 4
inches. That
> would probably still not alter knife readings significantly for
most
> mirrors, but I wouldn't build that much separation into a
tester, just
> because it is easy to get the separation down to 1/2 inch or
less. And even
> though I don't think a small diagonal separation will matter,
it is easy to
> have none.
>
> I'm not a rocket scientist. I can be as dumb as box of rocks
sometimes. I
> know... you are thinking to yourself "What do you mean
'sometimes'?"  Well
> you see that me just knowing you were thinking that shows that
it is only
> sometimes.
>
> Someone out there may be building a Foucault tester right now
and wondering
> (or not) how close they need to get the source and its image.
They could be
> reading this thread right now and reconsidering their design.
Maybe all the
> concern over tester astigmatism will scare them slitless.
>
> Jerry
>
>
> -----Original Message-----
> From: David Harbour
>
> Jerry,
>
> Thank you. I already had a pretty good idea of what you meant;
but what I
> would like to know is, why would anyone do this; also, I just
replied to a
> post by Dale about a K.E. and slit seperation of 16". This is
crazy to me.
> Are we trying to make telescopes, or write inanity here? No
offense
> intended. I do understand what you are saying; I am just
wondering why we
> are continuing this thread, inasmuch as everyone knows the
right thing to
> do. We are all rocket scientists here, who have made very good
optics and
> mountings; it seems to me that we are continuing unproductive
threads. What
> is this pursuit really about- inasmuch as we already understand
these
> applications, and their principles?
>
> Thank you, Jerry-
>
> R-101
>
>
>
>
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>

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