Re: [ATM] [atm_free] RE: data and musings on thin mirror

["Jerry" <wa4guu@verizon.net>]

It is f# of the light cone from the mirror and line density that determines
the number of lines seen in Ronchi for a given distance COC to grating.

I guess slit width would matter if you used a large slit. I don't think we
are using slits so large we should have to take that into account.


Testing at COC the f# is twice the f# at the mirror's focus.  And then for
consistency everyone needs to take into account whether the light source
moves with the grating or if only the grating is moved. If using a tester
with both moving, finding the position where the grating is at the COC of
the center zone and then backing the grating .1 inch moves the grating .1
inch back and the COC .1 inch forward making a .2 inch difference COC to
grating.from the from the COC.  I have found examples where an image is
labeled as being at some distance from COC and I can tell they were off by a
factor of 2. And then it is the COC of the center zone of the mirror that
should be referenced if one states the distance of the grating from the COC.
I think some use the 70% zone, and some just somewhere between the inner and
outer zone.

Those are some of the factors I believe may have led people not to realize
the proper relation represented by the number of lines seen. If one person
posts a 100 lpi Ronchigram .1 inch from COC of an f6 mirror and he used a
moving source tester, and another post a picture 100 lpi Ronchigram .1 inch
from COC of an f6 and his tester has a fixed source tester... well you can
see how something could go wrong here and one shows twice as many lines.
Those other factors I mentioned also offer opportunities for drawing the
wrong conclusion . Many of us have been "trained" to believe that we must be
informed of the distance COC to grating. 



Jerry  





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