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

["Greg Jones" <gregj888@comcast.net>]

I'm not sure if mine was first or second to see star light but was at OSP in
1995 and Table Mountain in '96 (ATMJ #10).  I think there are several
reasons you don't see too many.

First, it is a fairly new design, and the design was presented not the
scope.  It took a leap of faith to build one.

Second, I think the lenses scare a lot of people and really shouldn't, they
are fairly easy, just a lot of surfaces.  I've always thought I would
re-figure mine (it was the first scope I completed from blanks) but haven't
yet.  Once re-figured I'll send the lenses out for an AR coat which I also
think is a must for optomum performance.

Cost is a factor but again, less than many think.  The lens glass can be
ordered Plano-plano fine ground rather than polished; at quite a savings.

One pro made a LH an claimed it had "too much glass."  OK, I'm not a pro but
question that analysis.  The first set of lenses for my 8" were from .5"
blanks.  The second set (a little problem with a door) are from .75" blanks.
If I did it again I would use either .5" or .625" blanks.  Compared to an 8"
refractor, the Hubble "eye glasses" et al, I don't see the glass quantity as
an issue.  For what you get, the design is easy to realize.  BTW, the second
set took less than 2 weeks start to finish.

My 8", if made correctly has a 1" dia. diffraction limited field and 2" with
film resolution.  My CCD is 1" square (and not finished.  My cookbook is,
ummm smaller:-).  Film?  Oh yea, I could use film I guess...  OK, film isn't
dead, but for CCD work the LH is overkill.

I haven't found cooling to be worse than any other closed tube system.
Again, the lenses are fairly thin and with zero power...  It does probably
take longer than a Newtonian, but has not been noticed as a problem.

Last, like any full aperture corrector, the LH has a "sweet spot."  IMHO-- 
8" to 10" is about it.  6" is OK, but kind of small for the effort.  Above
10", I think some of the Hyperbolic designs with sub-aperture correctors
make a lot more sense.

With all that said, if you want an astrograph and have a Dob budget, the LH
is a great project that is very likely to be completed. No difficult surface
figures and performance is very good even if you mess up a little.  Note
every element is null tested or interference tested to a nulled surface.
I've seen several claims a knife edge null on a spherical mirror is better
that 1/50th wave...

Hope that helps,


Greg Jones
http://home.comcast.net/~dac20/dac_001.htm

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