Вот ссылка на использование баса в автоколлимации
http://www.trivalleystargazers.org/gert/rc10_M1_V3/RC10_M1_V3.htmlРор говорит, что басом в автоколлимации при расстоянии между лучами в 5мм позволяет мерять до Л/10.
http://rohr.aiax.de/hp_new/interferometertest.phpВот хороший анализ преимуществв-недостатков Баса
http://astrochonum.com/index.php?option=com_content&view=article&id=122&lang=enBath interferometry
Mirrors
Mirrors are tested at their radius of curvature which can be either the center of a spherical mirror, or the local center (twice the focal length) of a parabolic mirror.
In this case (parabolic), the test introduces a spherical aberration due to the fact we are not working on a flat wevefront. This SA can be calculated this the radius, the ROC and the conic constant of the mirror, and then removed from the Zernike Polynome. Nevertheless of the mirror is large and/or very short, this SA becomes important regarding the precision we are looking for (some L vs some L/100s). In that case, the test should be performed in autocollimation or this a Null lens. That limitations is true for ALL testing methods (Fizeau, bath, Wavefront analysis...).
The astigmatism induced the the off axis reference beam is to be taken into account as well. It is negligeable though with average mirrors with not to short F/D ratios. Open fringe can calculate this error. Subtracting it directly from the X 3rd order term of the polynome is tricky though as we must be sure of the igram alignment on the X axis.
Summary :
+ very goog precision
+ Zone defaults can be seen
+ Zernike interpolation
+ Tests are very reproductable
+ Fringe tracing and change of operator doesn't change much results.
+ PTV, RMS, FTM and PSF
- Induced astigmatism can be an issue on large/short F/D mirrors.
- Spherical aberration when testing asphericals at the ROC.
Refractors
Mirror are autocollimated on themselves, but in the case of refractor the collimation is done on the flat mirror. So we are never sure to be on the optical axis of the instrument !
That means that when lookink at the astigmatism, we have three components :
- From the instrument : lens alignment, lens defaults and mecanical constraints. This is this part we would like to measure.
- Induced by the beam separation of the bath setup, the reference beam being off axis. It can be calculated (185nm here), but it's difficult to remove it directly due to the lack of precision in the DSLR alignment on the X axis.
- The fact that we are never sure to be on the optical axis with this double pass setup.
So astigmatism may become important on small and/or short refractors (below 100mm).
At the contrary of parabolic mirror, we don't have any SA when testing refractors as they generate spherical wavefront.
+ Very good precision excellente for 5th order and more, and on 3rd order spherical.
+ Infinite-focus combinaison
+ Zernike interpolation
+ Tests are very reproductable
+ Fringe tracing and change of operator doesn't change much results.
+ PTV, RMS, FTM and PSF
+ possibility to analyse chromatism and sphérochromatism.
- Impossible to separate instrument astigmatism from test induced astigmatism.
- need a good flat mirror
- difficult to know when on optical axis
Conclusions
Bath is a very good metrology method for the amateur. It's cheap, easy to set up and it shows much more than the Foucault. It's reproductable and shows few sensitivity to the operator.
Nevertheless some limitations have to be taken into account such as the induced astigmatism for short F/D, or the ROC limitation when testing an aspherical mirror and therefore it cannot replace professional tests.
