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Explanation of features on reports
To assist customers and prospective customers in understanding a typical Zygo GPI report, we have put together descriptions of the main displayed features of the reports. Many other features are available but, due to a limited amount of space, we have decided on the current display. Please scroll down for more . . .
Before going into detail it is important to know what parameters
are used in testing our optics.
1. When testing paraboloids we test at the radius of curvature. This ensures that no astigmatism can be introduced into the optical path. This is a problem sometimes encountered when testing a paraboloid on a double pass set up with a perforated flat either the same size or larger than the mirror under test. Astigmatism is often introduced with this set up due to mis alignment of the optical componenets and incredible care has to be taken in correct alignment. It is totally incorrect in testing a mirror to remove astigmatism from the results electronically or otherwise on the assumption that all the measured astigmatism is in the mis-alignment and not the mirror under test or, the mirror sagging on the test bench. (If its sagging on the test bench, it will sag in your telescope.) Doing this falsifies the test as it also removes all the astigmatism from the test mirror, giving an incorrect reading. Any mirror you buy which has had astigmatism removed from the test results, is totally an incorrect value, no matter what the supplier says, its wrong, completely. Its an easy way to make a bad mirror look good by removing errors which cause a lower PV value. steer well clear.
2. To test a mirror on a Zygo there is a need to slightly tilt the mirror to see fringes which are measured and also the mirror has to be perfectly focused (power). Flat surfaces are tested in a similar way but with different aspects of the test removed. Here are the features we remove to achieve a correctly designed test;
Paraboloids, spheres and telescopes . . . Piston, Tilt, Power and Coma are removed
Flats . . . Piston and Tilt only are removed only.
We will discuss any aspect of our GPI Zygo and its capabilities however, we cannot, due to technical development procedures, discuss the actual optical set up of the testing methods we use which were developed in conjunction with Zygo and the National Physical Laboratory in London.
If anyone wishes to have further details of any feature on the report or, more information on what other features are available, please feel free to email us using this link . . .
Windows in report:
Before explaining the 'windows' on the report a word first about scaling. when an optic is being tested the Zygo's optics are zoomed to have the majority of the test area filled with the optic under test. This amount of zoom is not important however, it does show as if two identical optics being tested are not the same size due to the number of pixels being used in the display. The zoom factor on two identical optics can range from about 1mm per pixel down to 3mm per pixel. On average, 1mm per pixel is about normal.
There is no definitive scale of mm-pixels due to this variable and arbitrary zoom feature. Keeping the zoom level to a scale where the optic fills the 'window' ensures a more realistic RMS value is achieved due to the highest possible resolution being applied by the Zygo's camera.
1. Measurement Controls.
This window shows the customers name, the optic type, the optic's identifying number and the Zygo's wavelength.
2. Profile Plot.
This plot displays the section of the mirror across the section shown in the window below it. The section has a small triangle on each end of the section line. This can be drawn anywhere if needed and is extremely useful to the optician when studying where small areas of the mirror need to be worked to achieve specification. The scale of the irregularities is shown on the left hand edge of the plot. This plot is also automatically scaled to suit the PV errors, as such, a very good PV mirror will show very small errors here and tend to look a little rough. Mirrors of the highest quality show quite rough plots here but, the actual size of the errors are incredibly small but magnified due to the auto scaling.
3. Oblique Plot.
The wavefront of the mirror as seen from an angle showing a 3D picture of the wavefront errors. Here again, the better the mirror, the smaller the scale but the very small amount of roughness is magnified. The maximum and minimum PV values are shown on the right hand vertical axis of the plot.
4. Plan view
This shows the 3D plot as viewed directly from above. The red colours show the high areas and the blue areas, the low sections. A valuable tool for the optician who can immediately see what areas are involved in the overall PV of the wavefront. From this plot he can choose very accurately which areas (red) need further hand work to achieve a better PV overall.
5.Synthetic Fringes.
When a mirror passes through its final test on the Zygo, it is measured often over a hundred times and often 1024 times, the Zygo computer averages all the readings to arrive at its final values. It then transfers the data gathered from typically 30,000 plotted points and produces a set of fringes from the data. These fringes are the result of up to 1024 measurements of around 30,000 points and as such give a very accurate representation of the wavefront under analysis. Any turbulence or vibration is almost totally removed in this averaging process.
6. Data under the Synthetic fringes.
Time. The time when the test was carried out
Strehl. The Strehl value of the optic. On paraboloids we electronically remove the central section of the mirror on final test, having first tested it in full aperture with no central section removed. This is to simulate the mirror in use in an actual telescope. The size of the 'obstruction' is the typical size of the secondary mirror in the telescope in use. In the example above, the customer requested a minimum sized flat to achieve maximum optical performance in visual use.
P.V. wavefront. The PV wavefront of the optic under test. The example above shows a wavefront error of slightly better than lambda/6
RMS. The root mean square of the errors on the wavefront. A more paractical figure than PV wavefront as it is a measurement of the whole surface errors and is not just a measure of the highest and lowest points as in PV.
AstMag. This figure shows the amount of astigmatism present in the mirrors wavefront.
We ask any German customer interested in buying any of our optics individually or as an OTA to contact Wolfgang Ransburg at Teleskop-Service in Munich.
As mentioned earlier, we can discuss any aspect of Zygo functions and displays with existing or prospective customers, unfortunately we are not a liberty to discuss actual individual set up procedures for testing. Anyone wishing to enter a little deeper into typical Zygo set ups for optical testing will find a wealth of information at Zygo's web site on typical optical measuring methods.
I hope anyone who reads this finds it useful. It is not intended as an essay on Interferometry testing, it is merely a descriptive account of what occurs on our Zygo reports and a little background into each 'window' report displayed.
B. Pemberton, Orion Optics
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