TEST DATA

A test report should include the following information.

1) An actual interferogram photo
2) RMS data
3) P - V data
4) Strehl data
5) and sometimes astigmatism

Most reports will also include other information like data points, Wavelength tested, scale, serial number of optic/telescope tested, etc.

I would like to describe in a bit more detail what you should look for in your test report, this applies only to results from testing by interferometry.

1) INTERFEROGRAM PHOTO (not to be confused with a Ronchigram).

Even without a computer generated data analysis sheet one can come to reason ,within a certain degree of accuracy, a quality assessment of the optic or optical system under test. I can easily detect an excellent optic by just looking at the interferogram.

It can be very difficult to capture interferograms. The interferometeris usually a very expensive system so cost is one hurtle and then one has to deal with a very large OPD (optical path difference) which has it’s own difficulties, namely vibration and unwanted air movement. Telescope mirrors are tested at focus or at Radius, depending on the interferometer and test set up, and the separationmbetween the two can be many feet.

2) RMS DATA

In theory this needs to be 0.075 or lower to be “diffraction limited”. However this is just one of the values that need to be taken into consideration. Well - corrected optics/telescopes will usually have an RMS value in the 0.080 to 0.095 range. Please remember that large and/or “fast” optics become increasingly difficult to figure to the same “tight” high tolerances as smaller optics and you will see that the RMS values are affected.

3) P-V DATA

This seems to be the one value that everyone looks at first and I’m as guilty as you are, however it is just one of the values to be taken into consideration in evaluating optics or telescopes. The “old” school of thought is that an optic
or system needs to be 1/4 wave P - V or better to be “diffraction limited”, but
that is not necessarily the case.

You like to see 0.250 P - V or less and you usually do on small to moderately sized optics (not easy but very attainable), but you may not get that value on large “fast” optics and the reason is that the software data is using fringe centers or points numbering in the hundreds or even thousands and the P- V is the lowest point and the highest point out of these hundreds or thousands of points. So you can see with the difficulties of figuring a fast optic, vibration and unwanted air movement, this value could get easily distorted which is why it should not be considered as the most important value.

4) STREHL DATA

This value is of great importance in optical evaluation. It is also one of the values that can get grossly exaggerated.

Theoretically this value should be 0.80 or higher, you will typically see 0.90 - 0.95. This value becomes much more difficult to attain as you work larger and “faster” optics. Please be a bit skeptical of Strehl numbers that, for example, are posted as 0.998 or 0,999. In those cases, Interferometry probably was not to make that determination.

5) and ASTIGMATISM, sometimes........

Real astigmatism can be difficult to see or detect (unless gross).
The interferometer can detect very small trace amounts of it and the difficulty with this is that sometimes even the weight of the optic being tested and resting in a vertical position can create astigmatism. Air currents or vibration can also induce false astigmatism. So what is real and what isn’t?

Fortunately one can rotate the optic and determine whether the optic is astigmatic or not. We also subject every optic to a high power/artificial star test, usually 400X to 1000X, this allows us to mimic a star but under controlled shop conditions without the turbulent atmosphere interfering. The “roundness” of revolution can then be examined in close detail.

You will usually see astigmatism subtracted from test data if it is small enough not to be seen on a high powered pinhole but is still picked up by the software analysis program in trace amounts.

A high powered laser is used to illuminated a very small pinhole to
check the surface of revolution (astigmatism).