Re: [ATM] Null tests and astigmatism

["Bob May" <rmay@nethere.com>]

Astigmatism can be seen in repeated tests with the Foucault test
if you rotate the mirror between tests.  The center zone will
provide a reference between the repeated tests so you measure the
outer zones and, if you get different results, you have
astigmatism.
The Ronchi test near the ROC also does a good job of seeing
astigmatism as the center line takes on a S shape with
astigmatism.
Note that when testing for astigmatism, it is possible with the
Foucaullt test to obtain two points that are 90 degrees relative
to each other where the results are equal. those points being at
a 45 degree angle to the horizontal so don't test there and think
that you gon't hav astigmatism.  The 45 degree angle is the best
tho for the Ronchi test as the li9nes will be straight  )or
apparently properly curved) for the Ronchi.
The sure  check for astigmatism getting into the null lens vs.
the tested objective is to turn the lens and see if th9ings
change.  If they do, you kkow where the astigmatism is comong
from!
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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