| article: Interferogramms and Foucault-Gramms for Synta Refractor 150 mm / 1200 mm |
On the astro market there have been telescopes made by Synta (for example: Sky-Watcher, Celestron) for some time now.
Following features predetermine them for amateur astronomers:
- + Frauenhofer Objective (2 lenses)
- + very reasonable price for such a big lens-opening
- + short focal distance
- + most of the telescopes are once more tested before being sold
- - relatively high chromatic error
In the world of astronomy Mr. Wolfgang Rohr is very well-known for his verifications of optical features of telescopes.
Therefore, he is reachable every year at the biggest international meeting in Vogelsberg, where he´ll be impatiently awaited again this year.
With his mobile Optic-bank he´ll be checking some telescopes, whether their features- as they are presented by a producer or in an advert- are correct.
Now the measuring device in each case is against a λ/33 PV Zeiss plane mirror with double precision at 670 nm laserdiode for red,
resp. C-Linie with 656.3 nm and green laserdiode with 532 nm or for e-line 546.1 nm.
An objective before treatment at 670 nm: |
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PV-wave
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RMS-wave
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Strehl
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0.08
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0.016
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0.99
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After the treatment: no value traced: |
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PV-wave
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RMS-wave
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Strehl
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--
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--
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--
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Before the treatment with 532 nm: |
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PV-wave
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RMS-wave
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Strehl
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0.31
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0.071
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0.82
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After the treatment: |
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PV-wave
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RMS-wave
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Strehl
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0.17
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0.033
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0.96
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The Foucault pictures are selected over the continuous difraction line grating appr. 600 lp/mm or more for each individual color:
Foucault tests for different colors after the treatment: |
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for blue-green
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obvious spherical aberration
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Foucault tests for different colors after the treatment: |
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for yellow-green
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approx. λ/5.5 PV wave
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Foucault tests for different colors after the treatment: |
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for red-orange
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at close to 600nm
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According to the Foucault test there are obvious zonal errors in the objective, which are appr. lower than λ/10 PV wave.
The chromatic error between the wavelength of green and red lies between + 0.5 mm, for blue with appr. 1.5 mm under the green.
The blue-violet part can be eliminated with a yellow filter.
Before that the objective should be collimated accurately to the tube with a Chesire eyepiece.
The decentralization of the lens, caused by too big loop clearance of appr. 0.2 mm, should be reduced slightly.
This could be achieved with tape of 0.04 mm thickness.
With this adjustment it´s possible to observe Cassini separation on Saturn quite sharp with 200-times magnification.
With 300-times magnification a lamp 1 km away was still visible very sharp.
I don´t know though, whether each objective has this wrong lens distance from 0.2 mm, which I have increased to 0.7 mm.
In lab it´s possible to use the optics with comfortable 480-times magnification on artificial stars.
(Pinhole with 0.012 mm by Melles Griot on 0.01 mm steelfoil, perfectly round und sharp).
Should be used an interference filter in addition, that passes everything from blue-green to red-orange and eliminates the rest, than it´s possible to get perfect refractor objective for very reasonable price.
(text and pictures are strictly copyrighted by the author Wolfgang Rohr and published here with his explicit allowence.)
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