Reply: ATM refractor question

[" Gianluca Carinci" <g.carinci@iol.it>]

-----Messaggio originale-----
Da: Mike Coslo <mjc5@psu.edu>
A: atm@shore.net <atm@shore.net>
Data: sabato 10 luglio 1999 17.26
Oggetto: ATM refractor question
>
> I notice they sell both air-spaced and oil spaced kits with a
>significant price difference, the air spaced being more expensive. (100
>dollars more expensive for a 6-inch lens)
Are there performance differences between oil and air-spaced acromats?
>Difficulties in fabrication?
> - Mike -



Hi Mike

>From a few being connected to the net, this is the first time I'm writing
to the ATM list
    Having a little experience as an optical amateur designer and maker, I
wish to say that:
1) air spaced designs have different ROC (radius of curvature) on each
surface, id est, four different tools are necessary to generate the
four optical surfaces of the achromat.
On the contrary, cemented or oil doublets kits have only two tools,
these being necessary to work the two more external surfaces of the
doublet;
the two internal surfaces (the two bein cemented or "oiled" together) are
to
be worked one on the other as  one is concave and the other convex and, more
important, in cemented designs both have the same ROC so that each is the
tool for the other.This implies that  you work two surfaces at a time and
you need only two tools in your kit.
    Newport can supply also kits without tools: also in this case, the two
blanks for the air spaced design are more expensive of the two for the
cemented one, even if the f/14.1 cemented and the f/15 air spaced kits
comprise either blanks of the same BK7 and F4 types: price differences
maybe
due to the fact that glasses for the air spaced needs a longer
grinding
time in Newport facility, or in this kit there is an higer grit
quantity...(?)

2) Reguarding the performance differences between the two design, I can't
say nothing more that the air spaced kit is based on a design due to James
G. Baker appeared on the feb 1963 issue, vol. 2  n.2, of Applied Optics.
    Having one more degree of freedom (four ROCs, an air gap), an air
spaced
(generally referred as "Fraunhofer's"), if optimized, is better than a
Clark's (three ROCs, an air gap). A cemented or oiled type is a simplified
version of the latter, the air gap being zero and the two surfaces of same
ROC bein in contact via Canada's balsam or a suitable mineral oil (wich may
help in adjusting little imperfections of the two surfaces).
    A degree of freedom is the possibility of choise between parameters
such
as a different ROC, the variation of the distance between two optical
surfaces (two lenses or also a lens thickness), or the choise of another
glass type; for this, the Fraunhofer air spaced design, being the type with
the higher number of degree of freedom (we are not constrained to stay with
two surfaces having the same ROC, neither we must put them in contact, but
we can choise a free combination of four ROC and lenses' separation) is
the type that allows the optimization with a general choise of a pair of
glasses' type, even if a Clark's design, for a particular glass choise, can
perform very well.
    Fraunhofer's designs have convex surfaces to be tested on a mating
concave good quality reference, while the Clark's and Cemented types have
the two internal surfaces that are also each the reference for the other
(the concave one, tested by Foucault's, is in particular the reference for
the convex one). On the other side, working two different lenses one on the
other, may be difficult when we have to consider possible wearing rate
differences when the lenses' thicknesses are to be well controlled. At
last, it isn't particularly difficult to make a doublet of classic design,
but there are more parameters to be evaluated and narrower tolerances in
ROCs and thicknesses (respectively 0.05% and +/- 0.2mm, as the Newport
reports for its kit designs).
    Hoping of having been a little helpful.
    Excuse me for my awful English. Let me know and
    Clear Sky to you all
                                                        Gianluca Carinci