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Reading optical measurement data
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Reading optical measurement data
These days, interferometers become cheaper and therefore more common every day. This is an asset for amateur astronomy because this allows far better optics than in the days of only Foucault testing. In this short article, based on actual measurements of a secondary mirror, I will very briefly explain how to interpret this data.
In my daily life I work as a optomechanical engineer. Since this topic does not review any equipment, it is needless to declare any bias since this is irrelevant for the article.
Below one can find the reflectance data from a secondary mirror measured.
On the horizontal axis one can see the wavelength, the color of the light measured. 400nm is violet and 700nm is red light. 550nm is green light. On the vertical axis one can see the reflectance. For aluminium the reflectance is high for all wavelengths, preferably above 90% at an angle of incidence (AOI) of 0⁰. However, this mirror has been measured at 45⁰ thus values are slightly different. Above 85% for the entire visual spectrum which is enough. If you get this report, check the following:
- AOI, angle of incidence. Should be the AOI which the mirror is used.
- %R, percentage of reflectance. Should be the about 90% for normal aluminium and about 93% for enhanced aluminium.
This is a report from MetroPro, the standard datasheet that comes out of a Zygo interferometer.
- The surface wavefront error map. This is a height map of all the wavefront errors of the mirror. Compare this to a normal map: these highest values are the mountains whereas the loweast values are the valleys. The colors in itself mean nothing apart that it gives an indication of the roughness and zones.
- This is the same mirror, now in helicopter view. Nothing can be seen from this apart from that it gives a better idea of the surface shape.
- This is an interferogram of the mirror. For a flat mirror the lines should be straight, parallel to eachother and on equal distances. If the surface is spherical or parabolical, either extra optics are needed to correct for this, or one should do special techniques to get the surface data from this. Often a combination is done for optical surfaces.
- This is a crossection of the surface at the black line you see in (1). The PV and rms values are measured the same way as explained in 5 but measured only over the cross section.
- This is the personal data from the supplier. This should be correct.
- The PV value is the “depth of the valley” + “height of the mountain”. According to the Rayleigh criterion, this value should not be more than 0.25 waves or lambda/4. However, these days an optical is often made much better than this, less than 0.125 waves or lambda/8 is more common.
The RMS value is the average height difference. For a good mirror, this should be lambda/14=0.071 or less. (Marechal criterion)
The surface power is the inverse of the radius of curvature of the surface. This is zero for a flat mirror but for other mirrors, the curvature can be measured this way very accurately.
All the other data on the data sheets is irrelevant. Most other outputs from other programs work roughly similar.
- buckeyestargazer, Alfredo Beltran, artem2 and 3 others like this