PART 1: Update on Happy Birthday Dr. Tombaugh!
PART 2: Designing the Neptune imaging sequence
Many schools are engaged in preparing birthday cards for Dr. Clyde Tombaugh, who turns 90 in a few days. Dr. Tombaugh discovered Pluto in 1930. February 9 is the deadline for submitting your creations for a presentation to Dr. Tombaugh. For details about this activity, see the first updates-hst message. Several wonderful student-created cards are already online on the Web in the area called Kids' Corner. Please have a look if you are Web capable.
Also, to correct something from the activity description: We provided an incorrect address to the commentary entitled "Clyde Tombaugh's Blinking Persistence". The correct address is http://www.jpl.nasa.gov/pluto/vol1a.htm (that is vol one-a)
Please accept our apologies for any inconvenience this mistake caused.
Heidi B. Hammel
January 23, 1996
The votes are in and the students have spoken. After
tallying up all the counts and discussing the options, it
was decided that two of the "Live From HST" (LHST) orbits
will be used to image Neptune. Now my job was to decide
exactly how to use the time in those two orbits.
Using the Hubble Space Telescope (HST) is sort of like using a very complicated camera. In fact, the instrument we are going to use for LHST is called the "Wide-Field Planetary Camera." This camera is actually four cameras together, but we are going to use just the part called the "Planetary Camera," because it has the best resolution (it will show us the best details of the clouds on Neptune).
Unlike the cameras that most people have, which use color film, the Hubble Space Telescope takes pictures with electronic cameras, which are black and white. To get colors with Hubble, we take pictures through different filters. Filters are pieces of glass that are specially treated to let just certain colors though the glass. So you can get a "blue" picture through a "blue" filter or a "red" picture through a "red" filter, depending on which filter you choose. (By the way, we can use a computer to COMBINE pictures taken through different filters to make a "true-color" picture to show what Neptune would look like if a person could actually look THROUGH the Hubble Space telescope - but remember, no people physically look though Hubble - it is in outer space, and we use computers to "look" through it).
Neptune looks very different depending on which color you look at it. (That's not unusual: most things do - you can check this yourself with plastic wrap of different colors.) On Neptune, the Great Dark Spots are mostly visible in the blue, but the bright spots are most visible in the red. You have to be very careful about exactly which filters you pick. I picked five different filters that range from blue to very red. That way, we should be able to see bright spots, dark spots, and anything else interesting.
Once we decide what filters (or colors) we will be using, we have to decide the exposure time. This means, how long will we need to keep the camera's shutter open to make a picture. Most regular cameras figure that out automatically using some kind of light sensor. With Hubble (and actually with any fancy camera) you must figure this out yourself. For fancy cameras, this is because the photographer can then make the picture look exactly like she wants. In the case of Hubble, it is both that, and also that the targets are very faint, so standard sensors wouldn't work. Anyway, we are in luck, because I have used Hubble in the past to look at Neptune already, so I know what the right exposure times should be, and they range from 14 seconds in one filter up to 400 seconds (almost 7 minutes) for the red filters.
We have talked about what filters we will use, and how long the exposures will be. But we still need to think about the fact that we have two orbits. My plan is to use the same set of filters for each of the two orbits, and to time the two orbits so that we see as much of Neptune as possible. If you were going to make a map of the whole Earth from outer space, you'd take one picture, then wait 12 hours and take another picture, since you know that the rotation period of the Earth is 24 hours. Why can't we do the same kind of thing for Neptune? The problem is that on Neptune all we see are the tops of the clouds, and the winds that move these clouds have different speeds at different parts of the planet. So there is not just "one" rotation period. What should we use, then? We will use the rotation period from the part of the planet where we see the most clouds. Since the rotation period there is about 16 hours, we will space our two orbits 8 hours apart.
When exactly should we take the data? Which two PARTICULAR Hubble orbits do we want? Well, we cannot say for sure exactly which we will get, because there are a lot of observations that need to be made with Hubble Space Telescope. What we can do is request to have our data taken within a certain time (within a certain week, for example). Then the scientists at the Space Telescope Science Institute compare our request with the requests from everyone else who wants to use Hubble around that time, and they use a computer program to put all of them together in the most efficient way possible.
On March 14, we will be having a live TV broadcast about the LHST data, so to make this program as exciting as possible, I will request that the first orbit be taken 9 hours before the live broadcast, and that the second orbit be taken just before the broadcast, so that the data are sent down from the spacecraft DURING the broadcast! But remember, we may not get this exactly - it really depends on what else the Space Telescope has to look at on that day. In any case, if all goes well our pictures will be taken sometime in the day or so before the live broadcast. They will be very fresh and brand-new pictures of Neptune! I hope you are excited about this as I am!
Heidi Hammel
NOTE: Heidi Hammel has prepared a document which includes the technical details of the planning discussed above. To review those details of our Neptune observations, take a look on the Web here.
(If written, this URL should be on one line, without any space or breaks)
