during the night. Why it is, I don’t
Now, I do have neighbors behind
my backyard. To see if one of them
has a bright infrared light source in
their backyard (as part of a security
system, perhaps) the experiment was
run again, but this time with the
infrared photometer sitting on my
front porch. Figure 7 shows the chart I
got from that run.
Whoa! What’s going on here?
Again, the sky is very bright in infrared
at night. Then, just as the sun appears
over the porch, the infrared intensity
nose dives only to climb again as the
sun comes into full view. Then, just as
the sun sets, the same thing repeats.
There are only two houses across the
street and I can’t imagine that they’re
infrared bright. Therefore, I’m left with
something of an infrared mystery here.
A comparison of red and ultraviolet photometers gives the chart in
In this chart, the ultraviolet doesn’t really peak until just after 9:00 PM.
What’s also surprising about the ultraviolet LED is that it produces a background current when in the dark.
From measurements made to create
the table in this month’s column, I
realized that the ultraviolet LEDs don’t
produce any significant current.
Neither do the blue LEDs. This leads
me to believe that blue and ultraviolet
LEDs do not made good photometers.
This may be due to their construction
■ FIGURE 8
since they don’t contain a silicon die.
This summer, one of my near
space missions carried three LED
photometers. After the interesting
data I collected on the ground, I was
excited to see what would happen in
near space. The chart from that
mission is shown in Figure 9.
The ultraviolet intensity
decreased slightly until 60,000 feet
(see the notch). From there, it held
mostly steady (there’s a slight
increase). Based on the voltage measured across blue and ultraviolet LED
photometers on the ground, I suspect
this has to do more with changes in
temperature. Notice that the yellow
and red photometers are still sensitive
to the pointing direction. That’s why
THE ONSET PENDANT
■ FIGURE A
■ FIGURE 9
Last year, OnSet Computing (www.
onsetcomp.com) introduced me to their
Pendant G Logger. Upon hearing about
this three-axis acceleration datalogger,
I was excited to try it out. Like the company’s other dataloggers, it’s programmed
for a mission then sent on its way. The
keyword here though is programmed. I
had one opportunity last year to fly the
logger, but I forgot to program it before
the mission. Finally this year, I did it right
and have flown it on several near space
missions. Needless to say, the results are
The Pendant G logger is a small,
self-contained device. As you can see
from the photo, it’s not much larger than
an old-fashioned box of matches at 0.9”
x 0.5” x 0.4”. Since air temperatures in
near space can drop to - 60 degrees or
lower, the Pendant G Logger must be
kept inside the near spacecraft module
rather than outside. The Pendant G
logger’s 3G maximum acceleration
works well for near space missions.
The sensor inside the logger can
detect differences as small as 7% of
a g or a 1.4 degree tip of the logger.
You can tell if the recorded change in
November 2007 79