Interferometric
Protocol |
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Laserinterferometric
Wavefront Test with Baader Astro Films
The following stringent test has been performed by an independent
optical Institute in Germany which tests amateur telescope optics. |
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1.
Test Method
In order to simulate a telescope in the test arrangement, the uncoated
films were mounted between two reference optical windows of highest
accurracy and measured in transmission in a parallel beam set up.
The influence of the wave front deformation caused by the film was
then registered by a Zygo-Laserinterferometer at a wavelength of
lambda = 632.8 nm.
Aside
of the Interferogram itselfe (first row), the wavefront deformation
is presented in relief maps (four graphs).
As
an addition, the point spread function (PSF - second row of images)
has been recalculated for each measured film. These images represent
the Airy disc of a star as it would be seen at the focus of a telescope
system.
The
registered wave front deformations also served to verify the relevant
quality criteria, such as P-V and RMS-values, as well as the Strehl
ratio, given below the Airy disc renditions. |
2.
Rating the films as "optical Windows"
Three film samples and the readings for the reference system w/o film
are presented in the table No.1 below. Three substrates have been
tested: |
- Plain,
uncoated AstroSolar™ film (annealed). The film had been
mounted carefully between two rings of cardboard.
- Plain
TurboFilm™ (annealed), mounted frameless between the reference-plates.
- Original
Mylar of 12ym thickness (48 gauge), mounted frameless between
the reference-plates.
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substrates from Baader Planetarium tested out to be diffraction limited
under all aspects of the measurements across the full testfield of
100 mm diameter. In terms of RMS-values and strehl-ratio both materials
surely outperform many Amateur-Telescopes. In the contrary, in spite
of frameless mounting - free from stress and ripples, the Mylar-sample
deteriorated the laser wave front given by the Zygo Interferometer.
The wavefront deformation was notably stronger than it would be tolerable
for a diffraction limited optics. The test-results give evidence that
the Films from Baader Planetarium are very well apt for optical applications.
Most probably they outperform by far a good amount of the so called
"affordable Glass-filters" on the market.
The test results also attest to the fact that interferometric
values are strongly influenced by the way in which the films are
mounted or "framed". When producing an AstroSolar™
filter, care should be given to avoid the film to be folded or put
under tension. Any stress on the annealed film would deteriorate
the transmitted wavefront, notably reducing the image quality.
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Reference
system
w/o film
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annealed
AstroSolar™
non-metallized
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annealed
TurboFilm™
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original
Mylar™
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Airy
disc of refernce system
w/o film, Strehl ratio 99.9%
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Airy
disc with AstroSolar™
Strehl ratio 94.1%
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Airy
disc with TurboFilm™
Strehl ratio 96.8%
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Airy
disc with Mylar™
Strehl ratio 57.2%
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Reference
w/o film
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AstroSolar™
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Turbofilm™
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Mylar™
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