TEST DATA

What does it all mean? A test report should include the following information.

1) An actual interferogram photo
2) Synthetic interferogram photo (optional but
never in place of an actual interferogram photo).
3) RMS data
4) P - V data
5) Strehl data
6) 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 little more detail what you
should look for in your test report, whether you buy from
us or anybody else.

1) INTERFEROGRAM PHOTO (see examples below).

This is what I’d consider as the “Holy Grail” of optics. 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. If you are
provided with a synthetic interferogram it should match very
closely to the actual interferogram. For your own protection
you should not accept a synthetic interferogram as your only
“proof” on your optic or optical system.

It can be difficult to capture interferograms. The interferometer
is 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 usually tested at focus
or at radius, depending on the interferometer, and the separation
between the two can be many feet.

The examples above are from very accurate optics that test at or
in excess of 1/10 wave.

2) SYNTHETIC INTERFEROGRAM

I do not place to much value in the synthetic interferogram.
It does have it’s place in a report but it can be distorted and
manipulated which is why you always want to see the
actual interferogram.

3) RMS DATA

Rather than trying to impress you as to how the software
creates these values I’d like to tell you what numbers you
should look for. 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.

4) 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 be considered as the third most important value to
look at, after Strehl and RMS.

5) STREHL DATA

This value is of great importance in optical evaluation. It is
also one of the values most grossly exaggerated by “other”
optical manufactures who incorporate testing other than
interferometry. Theoretically this value should be 0.80 or
higher, you will typically see 0.90 - 0.95. This value
becomes more difficult to attain as you work larger and
“faster” optics. Please be skeptical of Strehl numbers that,
for example, are posted as 0.998. I can assure you that
whomever posted a number like that did not use interferometry
to attain the data.

6) and ASTIGMATISM, sometimes........

Astigmatism is difficult to see or detect unless it is gross
(greater than 1/4 wave). The interferometer can detect it
and it certainly is detected with the computer generated
software. The difficulty with this is that sometimes even
the weight of the optic being tested and resting in a vertical
position can create 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 600X to 1000X, this allows us
to mimic a star but under controlled shop conditions without the
turbulent atmosphere interfering.

The optic must not show any trace of astigmatism with this test,
if it does it is reworked until it is removed. You will usually see
astigmatism subtracted from test data if it is small enough not to
be seen but is picked up by the software analysis program in
trace amounts. Once again, the actual interferogram would
certainly show astigmatism if it were large enough to affect the
optical performance.

The data provided below is actual data supplied to a customer.
The optic is a 15" f/5 mirror and four sets of interferograms were
averaged together.

In the example above please note the similarity to the interferograms
and the synthetic interferogram. In the example below the interferogram
is included along with the fringe centers, this is how we normally supply
the data sheet.