Heidi Hammel - January 6, 1996
Planet Advocate for Neptune
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"A Detailed Plan for Neptune" Heidi Hammel - January 6, 1996 Planet Advocate for Neptune |
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Here is the plan for the "Live from HST" Neptune observations. At the top of each
section is a summary, followed by more detailed discussion.
I've tried to be pretty straightforward - if there is major jargon or more explanation
needed about some aspects, let me know.
I hope this is helpful.
The three key filters for studying the cloud structure on Neptune
are FQCH4P15, F467M, and F547M. The two secondary filters are
FQCH4N15 and F410M.
Why a methane band filter? First and most important is the
889-nanometer methane-band filter, named FQCH4P15. This is one of
the quandrants in the methane quad filter, rotated at an angle of
plus 15 degrees, hence the name FQCH4P15 (F= filter, Q = quad, CH4
= methane, P15 is rotated plus 15 degrees). This filter shows the
bright cloud structure, which is not only interesting in its own
right, but seems to be a good tracer of Great Dark Spots. This is
a narrow-band filter centered on a strong methane absorption band
at 889 nanometers (889 nm = 8890 angstroms: near infrared, redder
than the typical human eye can see). The methane molecules in
Neptune's atmosphere absorb almost all the photons of sunlight
with this particular wavelength. Therefore, Neptune is typically
very dark at this wavelength. Howeever, if high-altitude clouds
are present, they scatter the sunlight back towards Earth (and
Hubble) before they have a chance to be absorbed. Thus, clouds
are very bright against a very dark background. We have the best
contrast here.
Why blue and green filters? Great Dark Spots themselves are
not visible in the methane filter. Thus, the other key filters
are one in the blue and one in the green. All Great Dark Spots
seem to have maximum contrast in the blue. I would prefer the
F410M, but the exposure times for that are long, so my initial
choice is to go with the F467M (F = filter, 467 = 467 nanometers
central wavelength, M = medium bandwidth filter). The green
filter, F547M, gives an intermediate measurement, which is
sensitive to the darkest Great Dark Spots and the brightest
methane clouds.
Any other filters? In addition to the FQCHP15 889-nm
filter, it is also important to take an image through the FQCH4N15
619-nm filter. As you can tell from the filter name, this is also
a methane quad filter. But the filter rotation here is negative
15 degrees. That puts the image through a different wavelength:
the 619-nm filter. This is also a methane absoprtion band, but a
weaker one. Not quite as many photons are absorbed by methane at
this wavelengths. That means that light penetrates deeper into
Neptune's atmosphere. So the clouds we see at this wavelength are
deeper than the ones we see in the 889-nm FQCH4P15 filter. This
allows us to study the vertical structure of the clouds, or how
they vary with height. It is a neat way to get three-dimensional
data out of a a series of two-dimnesional data. If there is time,
I would also add the F410M filter, since this gives an even better
handle on the contrast of any Great Dark Spot further out in the
blue. It will also allow us to make a color composite image using
the "Blue" (F410M), the "Green" (F547M), and a "Red" (either one
of the methane filters.
Why are there two FQCH4P15 exposures? These exposures are so
long that cosmic rays are a significant problem. Cosmic rays are
high-energy particles that pervade outer space. When one happens
to strike the CCD (the detector in the camera) it causes a bright
patch to occur. Not only do these add noise and look bad, but
they can sometimes be mistaken for small bright clouds! By taking
two pictures, we can decide what is real cloud structure and what
is just a cosmic ray hit. This is less of a problem for the other
wavelengths, since the planet is a lot bright at those
wavelengths, and there are a lot less clouds with high contrast.
Why does this only total 1354 seconds = 23 minutes, if there
are about 33 minutes available to take data? There is overhead
(extra time) required for each picture, which must be factored in
when setting up a program. It takes a full minute to read out the
CCD. It also takes a full minute to change filters. That adds
two minutes minimum to each picture. It also takes time to rotate
the filter wheel to the proper positions for the two methane quad
filters, so that means another minute for each of those. You can
see that these extra minutes start to add up quickly!
What if there is extra time after these five images are
scheduled? It is not likely there will be much extra time, but if
there is we can always use more integration time on the 619-nm
filter. In fact, it would be best if that could be split into two
exposures, to avoid the cosmic ray problems. But in the past,
there just hasn't been enough time (given the overhead mentioned
above).
==================================================================
The second orbit should be 5 orbits (8 hours) after the first one.
The problem is, that on Neptune all we see are the tops of the
uppermost clouds, not a solid surface, and the winds that move
these clouds have different speeds at different latitudes.
Neptune's rotation period near the equator, for example, is about
18.5 hours. But near the poles, its rotation period is only 13
hours. We "define" the rotation period as 16.11 hours for text
books, since that is what the Voyager spacecraft measured for the
magnetic field. But we are interested in the clouds, since that's
what we can see with Hubble. So what do we do - 9 hours? 7
hours?
We pick the period that is most likely to show the clouds we are
interested in. Those are the periods for latitudes where clouds
are likely: from latitude -50 degrees through the equator up to
latitude +40 degrees. In that range, the periods are 16 hours to
18.5 hours, thus our sample time should be 8 hours to 9.25 hours.
Since HST orbits the Earth every 1.6 hours (96 minutes), that
means the two orbits should be 5 orbits apart = 8 hours. The
alternative 6 orbits (9.6 hours), is a little long and 4 orbits
(6.4 hours) is definitely too short.
For historical reasons, the quad filters are not quite the right
size and shape for the camera (this was caused by budget cuts when
the camera was being built; the quads were designed for an earlier
version with a bigger camera). Therefore, we have a problem with
these filters called "vignetting" (vin NYET ing). The bottom line
is that part of the light coming onto the camera through these
filters is blocked, so that the normal "best" position is not
good. You have to repoint the telescope to put Neptune in the
unblocked part of the detector. So that's why there are different
apertures for the two different methane filters - those positions
are optimized for those particular filters.
==================================================================
The start time of the first orbit should be at around 1 am Eastern
on 14 March 1996, and absolutely no later than 4 am. The real
window is anytime within a 24-hour period ending at 1 am. If
forced by the SAA, go earlier, not later.
The second orbit should be taken such that the data are dumped in
the normal mode (from tape recorder, *not* real-time) as close as
possible to 1 pm Eastern on 14 March 1996, the time of the live
broadcast. Assuming that there's about 4 hours between when the
data are taken and when it is dumped (and I just made up that
number, based on my experiences with the comet crash data dumps!),
the second orbit should be taken about 9 am on the 14th. Since
the first orbit is five orbits (8 hours) earlier, it should be
taken at around 1 am. If we can't be as tight with the schedule,
then we should err on the side of caution and get the data
earlier, so that we will definitely have something for the live
broadcast. Thus, to put a window on it, data taken any time
within the 24-hour period ending at 1 am Eastern.
The big caveat here is that I don't know when the SAA hits are,
which are a major constraint for these observations. SAA is the
South Atlantic Anomaly, a region of space where the cosmic ray
density is much higher than normal (due to the Earth's magnetic
field). We cannot take data when Hubble is near the SAA. We will
just have to try to do as well as we can, and adapt our observing
to the telescope when it comes time to formally schedule these
observations.
Abstract: Neptune's atmosphere is extremely dynamic. With a
series of images, we will determine a relaxation time for changes
in vertical aerosol structure by observing Neptune at the same
wavelengths as was done in Cycle 4 and Cycle 5. Specifically, we
propose to obtain images of Neptune with the Planetary Camera at
wavelengths from 410 to 889 nanometers. We will model the aerosol
structure by measuring both the general center-to-limb behavior as
a function of latitude, and the wavelength-dependent reflectivity
of discrete features. If Neptune's clouds have changed yet again,
we will study the new structure. The data are part of an
educational program called Live from Hubble Space Telescope, and
that participation drive s some of the timing of the observations.
Questions
Observing_Description:
We request 4 (6 if time) exposures of Neptune with the WFPC2 in
Planetary Camera mode in each of 2 orbits. The exposures in each
orbit are (F410M if time), F467M, F547M, (FQCH4N15 if time), and
two each at FQCH4P15. The start times of the two orbits should be
separated in time by 8 hours to cover all Neptune longitudes.
Real_Time_Justification:
Special Scheduling Requirement for timing: We request time near
13 March 1996 to coordinate the observations with a live broadcast
on 14 March 1996 showing the return of the data from the second
orbit. This will maximize the educational aspects of the
observations.
Calibration_Justification:
Additional_Comments:
Fixed_Targets
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