Re: Star Testing using large disc

On Nov 2, 11:45 am, Tenifer <tensorsur...@xxxxxxxxx> wrote:
Many years ago whenever I star test. I used any bright star
or even street light many miles away. By dofocusing it, I
thought I could see the diffraction pattern and I matched
the Suiter patterns and concluded my scope was 1/8 wave
undercorrected. But I realized yesterday that I was not
seeing the airy disc and the diffraction patterns. What I
was doing was simply defocusing it. But this is
the puzzle. How come I was able to see a 1/8 wave
undercorrected pattern in any defocussed image of any
light including street lamp miles away. Is this a rough
means to star test? Now if the result of this quick star
test produces 1/8 wave undercorrected. What would happen
if I successfully made a true star test using a real airy
disc and diffraction patterns (I no longer owned the scope
so can't do it and the atmospheric condition is seldom stable.
I wonder if there a relationship between the rough
defocussed image and the true star test pattern,
for example. Whenever a rough defocussing using street
lamp produces 1/8 wave undercorrected. There should be
1/3 wave undercorrected when a lone airy disc with
diffraction patterns is used (?) Pls. try your 8" SCT on
street lamp, defocussed it and see if you also come
up with 1/8 wave undercorrected or whether the result
varies. Also shouldn't the image of this street lamp star test
produces identical image at both focussed directions?
Why the 1/8 wave undercorrected pattern? Thanks.

Teni


Hi
Some how I think you were defocusing an uncontrolled amount.
The test as noted by others, needs an effective point source
and specific amount of defocusing.
With a large amount of defocusing, you are using a smaller
portion of the primary to create each part of the diffraction
image. This will make the mirror look better and better as
one defocuses.
I suspect this was your problem. Not rocket science.
Dwight

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Relevant Pages

  • Re: Star Testing using large disc
    ... seeing the airy disc and the diffraction patterns. ... if I successfully made a true star test using a real airy ... Some how I think you were defocusing an uncontrolled amount. ...
    (sci.astro.amateur)
  • Star Testing using large disc
    ... Many years ago whenever I star test. ... seeing the airy disc and the diffraction patterns. ... was doing was simply defocusing it. ... light including street lamp miles away. ...
    (sci.astro.amateur)
  • Re: Star Testing using large disc
    ... seeing the airy disc and the diffraction patterns. ... was doing was simply defocusing it. ... if I successfully made a true star test using a real airy ...
    (sci.astro.amateur)