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

["Paul Bradley" <paul@paul-bradley.com>]

What a curious coincidence that I should see this just when I'm reading
Rutten and van Venrooij and getting seduced by the LH's spot diagrams.

I'd be grateful if you could answer a few questions:

Why do you feel that above 10" hyperbolic designs make more sense - is it 
because
of the difficulty of figuring full aperture correctors above this size, cost 
or some other reason?

If it is remotely practical to attempt a 14" LH, would it be doable slower, 
say f/5 or f/6, or is
the corrector much harder to make for a slower primary?

You said cost is a factor, would I be looking at roughly 3x the cost in 
glass of a similar aperture
newtonian?

Also, you say a LH is overkill for CCD work, if I were to buy a ccd camera 
with around a 3/4"
chip would I really not see a worthwhile difference?

Thanks.

Paul Bradley

----- Original Message ----- 
From: "Greg Jones" <gregj888@comcast.net>
To: <atm@atmlist.net>
Sent: Thursday, August 11, 2005 6:53 AM
Subject: Re: [ATM] Why Lurie-Houghton telescope very rare in the ATM


> 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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> ATM mailing list http://www.atmlist.net/
>
> 

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