Re: [ATM] Why Lurie-Houghton telescope very rare in the ATM

[Kevin Whitefire <chaosopher23@yahoo.com>]

I took a look at the numbers and tossed them through a
C++ compiler.  I built a rudimentary calculator that
determines the optimum curvature based on the
formulae, and it looks like a do-able instrument, if I
can afford the BK-7 or similarly clear optical glass.

I'll say that the LH should stay under 16 inches,
which will allow it to see anything in the solar
system called a planet.

The advantages are a nice, wide field with lots of
correction, but low primary magnification requiring
Barlow lenses and really nice eyepieces for that
planetary close-up that we like when looking at
Jupiter.

I'll build a Cass with a f/D of about 18 or 20, and
use a wire spider to minimize diffraction.  There's my
planetary scope at low cost.  Even the eyepieces can
be inexpensive.

That LH, however, seems like it will be ideal on an
extended target such as those galaxy clusters and
nebulae that make some of the prettiest pictures I've
ever seen.  Few sights are more beautiful than the
birthplace of stars, and I can't see a better
telescope configuration for looking at the stellar
nursery than the LH.

I'll need both soon.

Kevin

--- Bob May <bobmay@nethere.com> wrote:

> Paul, the LH design is not hard to buiild.  It's
> problem is that
> the cost of refractor glass goes up as the size of
> the scope by
> more than the size by a fair margin.  The curves on
> the corrector
> are spherical and complementary so there really are
> no tools
> needed to grind them.  Unfortunately, the volume of
> the glass
> goes up fast.
> When it comes to focal ratio, the basic Newtonian
> works well at
> the slower focal ratios so there is no need to build
> a bit more
> difficult scope which has 4 glass to air interfaces
> to reduce the
> light input.  Without good anti-reflection coatings,
> you lose
> about 20% of the light in the corrector.
> The corrector is going to have gentler curves with a
> longer focal
> ratio so that can become a bit of a problem as one
> pair of
> surfaces will have a long ROC which can be a bit
> difficult to
> test if you don't have the space to do the test.
> Bob May
> bobmay@nethere.com
> http://nav.to/bobmay
> http://bobmay.astronomy.net
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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