Re: Lens cost?

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 09:31:24 +0100

"Nav" <optica@novationz.com> wrote in message
news:40c907aa$1@news.auckland.ac.nz...
>
>
> Roger Hamlett wrote:
>
> > "Farr1220" <m.cannell@auckland.ac.nz> wrote in message
> > news:336e84ab.0406091734.6f2ac447@posting.google.com...
> >
> >>Can anyone give a rough guidline as to how much it would cost to have
> >>an oiled triplet made which would include an ED glass? I'm wondering
> >>about the feasiblity of constructing a short lightweight portable of
> >>7" diameter. Recomendations for a company that could do this would
> >>also be a help.
> >
> > You need a lot more data to get any sort of sensible answer. A big part
of
> > the cost, will depend on the final accuracy required, and depending on
the
> > shapes required on the surfaces, the testing involved in checking this.
The
> > cheapest way, will be to grind it yourself, and design a test sequence
using
> > something like a zone diagram with masks and a foucault tester, working
> > through the 'back' of the lens as if it was a mirror. On lenses where
both
> > faces are convex, and non-spherical, a complementary concave test
element
> > will have to be made and figured first. This will all take a lot of
time,
> > but if you are prepared to do it, not much actual 'money'. Remember
though
> > that you would also need to find a company prepared to multi-coat the
> > elements. If the shapes of the element surfaces can be kept simple
> > (spherical), yet achieve your required objective, the cost plummets
> > relatively.
> > A 7" lens assembly, is not going to be 'lightweight', especially if the
> > focal length is short (since the lenses will have to be thicker), and
the
> > mounting cell will also require careful design.
> > I'd suspect, based on some optics I had made a while ago, that the 'one
off'
> > cost, from an optical company, would probably be in the order of perhaps
> > $50000, if you wanted this finished to the sort of quality associated
with a
> > good refractor, and the shapes required were not too complex. This may
> > actually be 'low', since the cost of a single element just slightly
larger
> > than this, as the 'reference' for a interferometer, was itself over
> > $30000...
> > Building it yourself, or buying a pre-made lens assembly in a cell, will
be
> > the cheapest routes. The TMB optical 175 f/8 lens cell assembly, gives
an
> > idea of how much this would cost. If you want to go much 'shorter' than
> > this, the optical problems get larger...
> >
> > Best Wishes
> >
>
> Thanks for the thoughts. It's an oiled triplet which has only 4
> different spherical radii. The glasses are all standard -and as I read
> glass prices, the glass is nothing compared to labor costs. The fact
> that it's oiled means only 2 surfaces need high polish and coating.
> Would that help you give me a better idea -at $50,000 I could never
> afford it but the TMB 178 f/8 is $13,000 including tube etc. I was
> hoping to make a lens cell for less than that -I';ve already sourced
> carbon fiber bits...
OK. You are talking something that is a lot easier to test, and which a well
equipped ATM could manage. As you say, the glass price itself should be
fairly acceptable. I have seen a couple of posters on some of the ATM lists,
who have made elements like this, and might be prepared to make such
elements for a easonable amount. Also you should browse some of the optical
supply house catalogues, since if (for instance), you can buy bi-convex
lenses in BK7, and SF11, 'off the shelf', and if the design could be
modified to use two standard lenses, and only one custom item, the price
plummets. I suggest you ask on one of the ATM lists whether anyone can
suggest a person or company, that will make such lenses.
The lens assembly in cell from TMB, is $8990 for the 175 f/8. Unless you can
find a friendly ATM, who is prepared to make the parts 'at cost', I think
you will be hard pressed to beat this.

Best Wishes

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