Re: [ATM] design question

[Woodchuck <djv@bedford.net>]

On Tue, 13 Sep 2005, Kevin Williams wrote:

> I have built a 6" f5

Congrats!

> I calculated the distance from primary to secondary based on a formula given
> in Richard berry's  book

Next time, work it out yourself, using a 1:1 engineering drawing
(not a sketch) fortified with algebra.  It will clear up what you
need to do.  Measurements may be taken directly from a good drawing.
(It's a dead art, though).  Until you are an "adept", drawings
should be made with great care, on a drafting board, etc etc.  This
is for discipline, one that was beaten into the engineers of
yesterday, who slaved for years perched on high stools at large
tilted tables in drafty lofts, like Dan Cratchit, going slowly
blind, their young hands bent prematurely with the cramp, while
cruel, merciless "checkers" (the next job up the employment ladder)
went over their work.  Like a lot of things, it's a lot more fun
when you do it as a hobby.

> back of mirror location  = focal length - (1/2 tube diameter )- (height of
> focuser(racked in)) - (1" for travel in focuser ) - (mirror thickness)
>
> x = 30 - 7.75/2 - 2 - 1 - .625
> x=22.5

I think there is an error in the formula.  The intent of the formula is
to bring the image plane 1" outside of (beyond, above) the top of the
focusser, when the focusser is fully "in".  (You might want more if
traditional camera bodies are to be used for astrophotography, less if a
Barlow is to be used all the time, etc etc).

x should be "distance from back of mirror to center of hole (optical
axis) for focusser".  Then "x" should get larger as the mirror gets
thicker, i.e. the sign of "mirror thickess" is wrong.  I keep scratching
my head, but I don't believe that I can accept the formula with a minus
sign on mirror thickness.  I don't have Mr Berry's book, so cannot
check his formula, i.e. you may have inadvertantly changed the sign
in transcription.  If I am wrong, somebody please yell at me.

This is almost the same amount that you are "off".  UNFORTUNATELY,
the error is in the *other direction*, the error makes the tube too
short, whereas your tube is too long.

Maybe 7.75 is the inside tube diameter; one should use the outside
diameter.  For thick sonotube, this might account for the error,
but the mirror thickness sign error should mitigate it.

I'm left with the conclusion that the focal length is not 30", but
possibly 31", only a 3% error.  Or that the focusser in more than
2" from base to top of draw tube *with adapter*.  Or you are using
a very unusual eyepiece.

> so I cut the hole for the focuser,  mounted the secondary mirror, then
> mounted the primary mirror
>
> when I used the scope to look at the stars, I could not focus
>
> the scope has a 2" focuser, and I use an adapter to use my 1 1/4 eyepieces,
> I took off the adapter, and moved the eyepiece into the hole by holding it,
> and I could focus, I estimate I had to hold the eyepiece 1/2" lower then the
> focuser top (fully racked in) get it to focus
>
> 2 questions
> what did I do wrong?

Hmmm.  An error in measurement?  An error in going from "x" to "location
of focusser mounting holes"?

> how much and which way do I need to move the  primary mirror? or something
> else?

Obviously, you need to shorten the tube, i.e. move the mirror towards
the front.  Depending on the material etc, you may want to drill
new holes, plug the others, repaint the tube, or file slots (not
recommended, you want the mounting to be "definite").  Moving the
mirror 1" forward moves the image plane 1" outward.  This is fretful
and embarassing, and yet it is much less loathsome than having to
*lengthen* the tube, which means redrilling a whole new focusser
site (and spider holes, if spider adjustment is used up) and enduring
the shame of the old hole, the trouble of patching and disguising
it with autobody techniques, or lying about it. ("That's for the
spectroscope mounting.")

An alternative is to turn an adapter that is recessed.  This might be
unwieldy, though. (nose clearance).  This is silly, probably. It
should be reserved for government clients.  Bill 'em $1800 for the
custom adapter.

Something not mentioned is the location of the object plane of the
eyepiece.  For some focal lengths and designs, (long ones), this
may be beyond the bottom of the eyepiece.  For other designs
(Huyghens, not encountered anymore in new, serious production) and
focal lengths (short) the object plane may well be up the eyepiece's
tube.  Also, the location of ridge on the eyepiece is arbitrary
(manufacturer's whim, although I believe there is a standard for
microscopes, there probably is none for telescopes).

Hmmmmmmmm.  Are you by any chance unusually *nearsighted* and not
using your glasses?

Dave
-- 
        In each of us, there burns a soul of a woodchuck.
        In every generation a few are chosen to prove it.

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