Re: ATM 90mm f-11 APO

[Brian Boothman <brian@boothman.net>]

  I am still in the contemplation stage of this project.
I was thinking that at only 90mm the glass could not be
all that expensive. Perhaps I was wrong.
 I was thinking of a refractor because I am limited to
a small space and the mess associated with a large mirror
is not an option in my location.
 I understand that a lens can be more difficult in many ways
than a mirror. But easier in some. Wedge and thickness are
a factor with a lens but the surfaces are spherical
 Testing is more difficult, the surfaces are smaller and
more numerous.

 How much could 3 pieces of 90mm glass cost?


Bob May wrote:

> Unfortunately, refractor designs have a big variable in the actual glasses
> used - even to the point where different melts of the glass type change the
> ROCs of the various surfaces.  You need to actually calculate out the curves
> you need AFTER you get the glasses.  Another reason why high performance
> optics is so expensive.  Additionally, you will be paying absolutely silly
> prices for the glass itself!  FWIW, I have a pair of glasses for a 6"
> refractor that I am happy to have paid only $600 for them - a price way
> below the price that was originally paid for them 20 years ago.
> Are you sure you really want to do a refractor?
> Bob May
> http://nav.to/bobmay
> bobmay@nethere.com