RE: [ATM] Re: considering new design FW: Re: website down

[Woodchuck <djv@bedford.net>]

On Sun, 9 Jan 2005, Jerry wrote:

> 	I don't think there would be a much smaller obstruction particularly
> in smaller apertures. The mirror is at an angle and you have to allow room
> for glass thickness and a cell of some sort and a focuser. Then it would be
> nice to have some head and body heat clearance from the mirror and the light
> path. Lets be fair and not compare the obstruction for the "new" design
> planetary scope obstruction with a Newtonian designed for deep sky. If you
> only want a narrow field you can reduce the diagonal on a Newtonian to about
> the same size as the hole you would need in the flat.

For the 6", the hole size corresponds to a maximum backfocus of 4" to
avoid making the hole the system's stop, and ignoring vignetting of the
field.  This can be achieved with a Newtonian as well, using an 7.5 or 8"
tube and the same sort of low-profile focusser as the proposed design,
with the same central obstruction and diffraction due to the obstruction,
as was revealed to the Old Ones.

The larger sizes offered by Custom Optical seem to scale linearly.
They're all f:8, with holes and back-focus to scale.

> 	It would be an expensive and heavy way to avoid spider diffraction

... which is the Newtonian's only disadvantage, once we agree to
compare oranges and oranges.  The visual spikes of a standard spider
can be banished to the diffraction rings by using a circular spider.
Given the small flat's size and weight, and the acceptability of a
narrow field, a "single circle" spider of some significant width
might be OK, i.e.  rigid enough and with negligible diffraction.
In my mind's eye I see a 4"OD by 4" length of very thin titanium
tubing for a 6" Newtonian, perhaps damped in some way.  We only have
to support a .5" minor OD diagonal mirror.

In the small sizes, the Newtonian has the eyepiece position advantage,
but in the large sizes, the new design requires a large mount to
get its eyepiece out of the dirt -- not that big an objection for
a fixed viewing site.  Make an English mount for either system, if
we're to restrain viewing to the plane of the ecliptic.  I assume
an alt-az is not so convenient for high-mag planetary viewing.

As an amateur optical (as opposed to woodworking) *project*, the
new design is very interesting, capable of null-testing throughout.
First make primary to a sphere, null test with Foucault. (A snap).
Chemically silver it, then use it to figure the flat, again null
testing. (Flat in beam at 45 deg).  Then coat the flat permanently
and use it to test the refiguring of the primary to a paraboloid,
now null-testing the primary in autocollimation.  Given the moderate
F ratios, only the flat is hard to do.  One might want to make the
central hole of the flat into a cone, say of about 20 deg, to
facilitate autocollimation.  Of course, once one has a good-sized
flat in the shop, all kinds of fun can happen.

Dave

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