ISAC, The Hubble Telescope, and TWO quizzes
Roederer at Darwin.Stanford.EDU
Wed Mar 18 11:50:41 EST 1998
(Preface: Joe Gray commissioned me to report on this incident to the EMail
list. Please direct all complaints to him)
Dr. Ed Weiler gave a very interesting lecture at ISAC about the Hubble
telescope. Part of his talk was devoted to the correction for the misshapen
main reflector. In demonstrating the pictures "before" and "after" the
corrective optics were put in, it was rather obvious how much more detailed the
resolution had become. In the high-resolution pictures, the stars showed up as
poinpoints surrounded by a "starburst pattern"-i.e., 4 bright lines of
dispersion diverging from the pinpoint at right angles. Dr. Weiler claimed that
this pattern was proof of the sphericity of the optics.
Later that night, over dinner (OK, wine was involved), Dick Stovel asked a
question of our table: why, if the optics are now spherical, do we get
non-spherical aberrations (the star pattern, which has four fold symmetry rather
than axial symmetry)? Since Dick, the reigning optics expert at our table, was
posing the question, it seemed nearly impossible that we might actually contrive
to come up with the correct answer. Therefore, Dick and I went on a mission.
We figured that in such an erudite crowd of FlowJocks and ImageJocks we would
find the answer!
We posed the question to expert after expert. Below is the torrid tale of our
mission--the names of the Experts have been removed to protect nearly everyone
We started our mission with Experts #1 and #2. Expert #1 tried and tried, but
failed to come up with any good answer. From his vantage point, Expert #2
suggested that the star pattern was caused by a diffraction around the support
wires for the secondary reflector mirror (this "spider" of guy wires would sit
in the way of the main reflector). Expert #1, however, shot down this concept
by noting that these support wires would be completely out of the focal plane.
As they argued, Dick and I moved on to Expert #3, who gave us a song and dance,
but ultimately also proposed the idea of the support wires causing the problem.
I was still not convinced--after all, why would they use 4 support wires (there
were four lines of dispersion); if you wanted a minimum number of supports, you
would use 3. Expert #1 interposed and pointed out that the support wires could
be made transparent; however, Expert #3 pointed out that this would only make
things worse (refraction). We left Experts #1 and #3 fighting it out; last I
heard, Expert #3 was beginning to change his tune.
Undaunted and unsatisfied, Dick and I continued our pursuit of optical
excellence. Expert #4, at this late hour, was unable to suggest any answers,
putting him squarely in the middle of this elite crowd. Nearby, we found Expert
#5, who summoned an old excuse: an aperture-induced diffraction (the camera is
rectangular, leading to asymmetry). Even as we stood there, however, this young
theory died an untimely death as Expert #5 shot it down himself... but still we
wondered. On to Expert #6, who editorialized at length about different
possibilities, but never said anything concrete.
Expert #7 made a a rather presidential declaration: "It's due to
rasterization." Now here was an outstanding possibility (at least, at first
glance). Rasterization is inherently a rectangular process--that could
definitely lead to 4-fold asymmetry! However, at this point my Socratic method
of argument was well honed, and Expert #7 could not defend his theory against
even the mildest objections. Expert #8, involved in this little discussion,
proposed that it was due to the filters employed. There was general laughter...
but he stuck to his theory. Apparently, his hypothesis was hewn in stone.
We were still at a loss. However, we had one last possibility: Expert #9, who
also happens to be an amateur astronomer. In a stroke of brilliance, that could
only be described as an avalanche of prescience, Expert #9 left before we could
find him. We were left only with Expert #10--a tall, handsome, extremely
charming man, who, I must admit, is me--who proposed that the pattern is
actually a public relations artefact induced by NASA. After all, 85% of the
public believes that stars should twinkle and have a "starburst pattern", and
NASA is damn well going to give them starburst patterns if that's what they
We left the ISAC meeting feeling empty.
Later, we caught up with Expert #9, who also proffered (is that a bad word these
days?) the support wire theory.
Thus, let's recap with our First Quiz:
What causes the starburst pattern in the Hubble images of stars?
(1) Diffraction around the supports for the secondary reflector (Experts 2, 3,
(2) Asymmetric aperture (Expert #5)
(3) Rasterization artefact (Expert #7)
(4) Filter effects (Expert #8)
(5) PR gimmick (Expert #10)
(6) Unknown (Experts 1, 4, 6)
Answer before reading on!
Yesterday, I sent an EMail to Dr. Weiler, who was kind enough to respond
quickly. Here is his response, verbatim:
> The "star" pattern is due to the starlight being
> diffracted around the 4 spider arms (the support legs
> for the secondary mirror). A nearly perfect mirror and
> secondary will produce a diffraction limited image with
> several "airy" rings and a diffraction pattern due to
> the spider supports. When the mirror was uncorrected,
> instead of seeing exactly the four spikes expected, the
> stellar images had many more "spikes".
> Four supports are ideal as each one will produce its
> "ghost" image on top of the one 180 degrees on the other
> side. In other words, 4 supports produce 8 spikes, but
> 4 of the spikes fall on top of 4 others. 3 supports
> will produce 6 spikes, all 60 degrees apart.
> Hope this helps, you might find more details within the
> HST home page at <http://www.stsci.edu>.
The second Quiz: Can you identify the 10 experts above? The first person to
identify all 10 correctly will get a free drink coupon at the next ISAC
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