QUESTION: 
How long does it take for the Hubble to get an image of Pluto and
Neptune.  How does the image from the Hubble get back to Earth

ANSWER from Bill Hathaway on April 29, 1996: 
Well, this has several answers, depending on the interpretation of 
"how long".  There is the time scale of how long does it take for someone 
to develop the idea for observing these planets, make a proposal to the 
Space Telescope Science Institute, get it accepted, prepare the detailed 
observing scheme, have it transformed into a format the observing ground 
system and scheduling systems can handle, wait for a precise time for 
the observing with the HST to actually occur and get the image.  If 
you've been able to follow the progress of the Live From Hubble Space 
Telescope project from the beginning, or read its history on the World 
Wide Web, you can see the whole process can take quite a few months, 
and in general, observations with HST can easily take more than a year 
from start to finish.  (For the many astronomers who started trying 
to get observations before the telescope was launched in 1990, their 
wait could have been many years.  The astronomer who first proposed 
orbiting such a large telescope had to wait over forty years to see 
his dream in operation.)  

As for making the actual observations and getting them to the 
ground, the time scale is of course much less.  I believe all the 
observations for Neptune and Pluto for the LHST project were actually 
done by the HST in a total of three "orbits".  An orbit being one 
revolution of the telescope around the earth, each taking approximately 
ninety-six minutes.  During a typical orbit, a target such as Neptune 
would only be visible to the telescope for about half that time, with 
the earth moving in the way for the rest of the time.  

To see exactly how long each of the steps to make an observation take 
to occur, here is a copy of the actual Calendar display file that we used 
in scheduling the Neptune observations on March 8, 1996 (on the 68th day 
of 1996).  This shows the activities that occurred for the observation 
to succeed and the times they started and finished.  Add up all the 
durations and you will have the time it takes to do the observation.  

Following this display I will translate some of the odd "words" so you 
can follow the progress.  We use this display in scheduling routinely so 
here you are seeing the kinds of things that we schedulers use every week.  

 
                              CALENDAR TIMELINE
 
   C&C List ID:   960647D1;1                   Generated On:  26-APR-1996 21:06
    Orbit File:   OR96036WA;1                 Baseline Name:  GLOBAL  
     TDRS East:   TDRSTDE                         TDRS West:  TDRSTDW
Displayed From:   1996.063:11:57:47          Calendar Start:  1996.064:00:00:00
            To:   1996.072:00:00:00                     End:  1996.071:00:00:00
--------------------------------------------------------------------------------
     START         END           DESCRIPTION                 PROP-PROG:OB:AL VER 
--------------------------------------------------------------------------------

068:00:39:00  068:01:19:39      FGS_AVD    (ENT,L=  15.8)       06846:02:01 01
068:00:59:47  068:01:12:16   M  Slew (AN= 74,RA=299,DE=-20,PA= 79,O=   0,SN= 51)
068:01:12:16  068:01:17:46   M  FHST Updte (FULL ,MAN,E= 118,2,3, )             

068:01:17:46 ********        Main SU 0684602     Link Set L0684602        ******

068:01:17:46  068:01:26:46   M  PCS AQ(FGS ,E= 54,BASE1   ) 06846-34U:02:01(  3)
068:01:19:39  068:02:15:30      FGS_AVD    (EXT,L=   7.7)       06846:02:01 01
068:01:26:19  068:01:27:19   SI UP   WFII      READY        06846-34U:02:01 01  
068:01:26:46  068:01:47:51   M  TRK51  (ANGLE=   1.3308",RATE=0.0009,MODE=FGS ) 
068:01:27:19  068:01:27:20   SI UP   WFII      OPER         06846-34U:02:01 01  
068:01:27:20  068:01:50:20   M  Sci  WFII UFQCH4N15      1  06846-34U:02:01 01  
068:01:47:51  068:01:48:18   M  SAM    (ANGLE=   5.0786",ROLL=   0.00)          
068:01:50:20  068:02:10:25   M  TRK51  (ANGLE=   1.2640",RATE=0.0008,MODE=FGS ) 
068:01:50:54  068:02:12:54   M  Sci  WFII UFQCH4P15      2  06846-34U:02:02 01  
068:02:10:56  068:02:24:50   M  Slew (AN= 93,RA=193,DE=-30,PA=143,O=   1,SN=138)
068:02:12:54  068:02:13:54   SI DOWN WFII      READY        06846-34U:02:02 01  

068:02:12:54 ********    End Main SU 0684602 ***********************************

068:02:15:30  068:02:56:09      FGS_AVD    (ENT,L=  15.8)       06846:02:01 01
068:02:24:50  068:02:30:20   M  FHST Updte (FULL ,MAN,E= 152,1,3, )             

... (intervening observations and activities deleted) ... 

068:08:41:29  068:09:22:12      FGS_AVD    (ENT,L=  15.8)       06846:02:01 01
068:08:59:10  068:09:14:49   M  Slew (AN=116,RA=299,DE=-20,PA= 79,O=   0,SN= 51)
068:09:14:49  068:09:20:19   M  FHST Updte (FULL ,MAN,E= 193,2,3, )             

068:09:20:19 ********        Main SU 0684603     Link Set L0684602        ******

068:09:20:19  068:09:29:19   M  PCS AQ(FGS ,E= 54,BASE1   ) 06846-34U:03:01(  3)
068:09:22:12  068:10:17:58      FGS_AVD    (EXT,L=   7.7)       06846:02:01 01
068:09:29:19  068:09:50:24   M  TRK51  (ANGLE=   1.3240",RATE=0.0008,MODE=FGS ) 
068:09:29:52  068:09:29:53   SI UP   WFII      OPER         06846-34U:03:01 01  
068:09:29:53  068:09:52:53   M  Sci  WFII UFQCH4N15      1  06846-34U:03:01 01  
068:09:50:24  068:09:50:51   M  SAM    (ANGLE=   5.0793",ROLL=   0.00)          
068:09:52:53  068:10:12:58   M  TRK51  (ANGLE=   1.2575",RATE=0.0008,MODE=FGS ) 
068:09:53:27  068:10:15:27   M  Sci  WFII UFQCH4P15      2  06846-34U:03:02 01  
068:10:12:58  068:10:25:32   M  Slew (AN= 75,RA=219,DE=-15,PA=108,O=   0,SN=126)
068:10:15:27  068:10:16:27   SI DOWN WFII      READY        06846-34U:03:02 01  

068:10:15:27 ********    End Main SU 0684603 ***********************************

068:10:17:58  068:10:58:42      FGS_AVD    (ENT,L=  15.9)       06846:02:01 01
068:10:25:32  068:10:31:02   M  FHST Updte (FULL ,MAN,E= 119,1,2,3)             

------------------------------- end of calendar display -----------------------

Translation: 

   C&C List ID:   960647D1;1                   

Each week of observations gets a name.  The "96" means the year is 1996; 
the "64" is the first day of the week (04-MAR-1996 equals day 064), the 
"7" means the calendar does indeed cover seven days (it is possible to use 
a shorter or longer calendar but we generally stick to a standard week); 
and the "D1" shows the version we finally ended up with.  For example, 
the first attempt at building a calendar might be "A1".  

    START         END           DESCRIPTION                 PROP-PROG:OB:AL VER 

068:00:39:00  068:01:19:39      FGS_AVD    (ENT,L=  15.8)       06846:02:01 01
                                ^^^
                                FGS is the Fine Guidance Sensor which looks 
for the guide stars needed for holding the telescope steady for observations.  
The "AVD" means the AVoiDance region where the Earth blocks our view of the 
target for observation 06846:02:01 (which is Neptune.)  This translates 
to Proposal 06846, Observation Set 02 and Alignment 01.  

The "ENT" means that the spacecraft is ENTering that AVoiDance region, so 
we cannot observe the target until we leave the Avoidance region.  Notice 
that this avoidance lasts for 40 minutes and 39 seconds.  Subtracting 
hours and minutes and seconds from one time to the next in your head is one 
of the uses of arithmetic!  We can also make the computer do this, but 
sometimes it is faster to do it yourself.  And it also is useful to check 
occasionally to see that the computer has really done what you want it to do.  
Blind faith in computer generated numbers will leave you in the dark sometimes. 

The "L" means that we are actually avoiding the bright Limb of the earth by 
15.8 degrees, which is our practical limit for scattered light from the Earth. 

068:00:59:47  068:01:12:16   M  Slew (AN= 74,RA=299,DE=-20,PA= 79,O=   0,SN= 51)
                                ^^^^
This uses 12 minutes and 29 seconds to "Slew" (turn the telescope) to the 
desired pointing.  The target (Neptune) was at Right Ascension 299 degrees and 
Declination -20 degrees on the day we observed it.  This is on the ecliptic 
(the path of the Zodiac) in the constellation Capricornus.  The telescope 
turned 74 degrees (AN) from the previous target.  The pointing is 51 degrees 
(SN) from the position of the Sun for that day.  We cannot point the telescope 
any closer to the Sun than 50 degrees, but 51 degrees is fine.  A few days 
earlier we could not look at Neptune, as it was still too close to the Sun.  
We also could not look at it from day 074:12:57:32 to 075:18:27:14, because 
the Moon, in its monthly passage around the sky, was too close to Neptune's 
position at that time.  The computer software helping us schedule observations 
keeps track of all these details.  Finally, the PA shows the Position Angle 
of the spacecraft and the O shows how far Off from the normal roll position 
we are (zero, which is common.)  
 
068:01:12:16  068:01:17:46   M  FHST Updte (FULL ,MAN,E= 118,2,3, )             
                                ^^^^
As soon as the Slew completed, we schedule a Fixed Head Star Tracker Update.  
The FHSTs are small telescopes pointed out the bottom end of the spacecraft to 
help us be sure the movement of the telescope took us to the right place.  
(Those round holes you might see on a picture or model of HST.) 
The stars that it sees are compared to a catalog and if all match up, we 
have a confirmation.  (If the check shows we are a little off, the 
controllers can nudge the pointing to put us back where we should be.)  

068:01:17:46 ********        Main SU 0684602     Link Set L0684602        ******

This 0684602 is the name of the "SU" (Scheduling Unit) which starts running at 
this time.  

068:01:19:39  068:02:15:30      FGS_AVD    (EXT,L=   7.7)       06846:02:01 01

Again, the FGS_AVD shows the AVoiDance region, but this time we EXiT the 
region at 068:01:19:39 - but on the dark limb of the earth, which is 
why we can start looking at only 7.7 degrees from the edge.  The dark edge 
gives less scattered light for us to avoid than the bright limb does.  Most 
times if we enter from the bright limb, we will exit from the dark limb and 
if we enter from the dark limb, we will exit from the bright limb.  
Can you visualize why this is so?  Why would we rarely see the dark limb 
both on entry and exit?  When would we see the bright limb at both ends?  

068:01:17:46  068:01:26:46   M  PCS AQ(FGS ,E= 54,BASE1   ) 06846-34U:02:01(  3)

Since we can now see the sky, the PCS - Pointing Control System will start 
to AQ (AcQuire) the guide stars for the observation 06846:02.  During the 
nine minutes of PCS AQ, ( 01:26:46 minus 01:17:46 ) the FGS looks for the 
guide stars, locks onto their light, confirms that they have the correct 
brightness and positions, nudges the pointing of the telescope to put the 
target star into the light-gathering aperture of the Scientific Instrument, 
in this case the Wide Field Planetary Camera II.  Only then can the 
observations proceed.  Notice the Program number is 34U.  

A sharp reader might notice that this sequence actually starts 1 minute 53 
second BEFORE the EXiT from AVoiDance.  We are able to do this because we know 
that it takes that long for the FGS mechanisms and detector to turn on and get 
in position to see any light.  Rather than let this time go to waste (at 
estimates of spacecraft time of $8.00 per second) we use this time to start 
the sequence running. 

068:01:26:19  068:01:27:19   SI UP   WFII      READY        06846-34U:02:01 01  
068:01:27:19  068:01:27:20   SI UP   WFII      OPER         06846-34U:02:01 01  

These simply give the time it takes for the WFII instrument to turn on to 
its READY condition (one minute) and to get into its OPERating condition 
(one second) to make the SI (Scientific Instrument) set to make the 
observation.  The one second might seem trivial, but we must account for every 
second of activity.  Other SIs take longer times to get ready.  We were able 
to save time by doing some of the READY during the PCS Guide Star Acquisition. 
We will overlap activities such as this whenever possible.  

068:01:26:46  068:01:47:51   M  TRK51  (ANGLE=   1.3308",RATE=0.0009,MODE=FGS ) 

This odd-looking TRK51 is used only for targets that are moving on the sky, 
such as the planet Neptune.  This command tells the spacecraft to move the 
pointing of the telescope to follow (TRacK) the predicted motion of the 
planet (at 0.0009 arcsecs per second of time - Neptune moves slowly, but 
if we did not track it, the pictures would be blurred.)  

068:01:27:20  068:01:50:20   M  Sci  WFII UFQCH4N15      1  06846-34U:02:01 01  

Finally, after all the preparation, we can expose the WFII to light from the 
target for 13 minutes.  The "Sci" means Science data is being collected here. 
The UFQCH4N15 shows which of the (many available) apertures the target is 
centered on.  

068:01:47:51  068:01:48:18   M  SAM    (ANGLE=   5.0786",ROLL=   0.00)          

Here is a Small Angle Maneuver needed to keep the planet centered in the 
camera for the next set of exposures.  

068:01:50:20  068:02:10:25   M  TRK51  (ANGLE=   1.2640",RATE=0.0008,MODE=FGS ) 

Again we need the telescope to track the planet, at a slightly different rate 
and direction.  The planets do not move in straight lines, but we have these 
means to compensate.  

068:01:50:54  068:02:12:54   M  Sci  WFII UFQCH4P15      2  06846-34U:02:02 01  

This takes the second set of exposures, using a different set of filters.  If 
you saw the TV presentation by Heidel Hammel, you saw the effect of looking 
at different colors.  

068:02:10:56  068:02:24:50   M  Slew (AN= 93,RA=193,DE=-30,PA=143,O=   1,SN=138)

Once the exposure is completed, we move (Slew) the telescope off to the 
pointing of the next target for HST to observe.  

068:02:12:54  068:02:13:54   SI DOWN WFII      READY        06846-34U:02:02 01  

It takes one minute for the WFII to stop operating and be READY for the 
next use.  

068:02:12:54 ********    End Main SU 0684602 ***********************************
The end of observations for this SU. 

Here you might notice that the WFII observation did not end until 068:02:12:54,
118 seconds after the telescope started moving away!  But we do know from the 
way the camera works that the exposure has already ended and these couple of 
minutes is during the time the WFII needs to read out the data so we are safe 
- and again we have squeezed out a few more moments of valuable time that can 
be used for the next observation.  

068:02:15:30  068:02:56:09      FGS_AVD    (ENT,L=  15.8)       06846:02:01 01

After 55 minutes and 51 seconds (from 01:19:39 to 02:15:30) the spacecraft has 
flown half-way around the world and at 068:02:15:30 the Bright edge of the 
earth comes around and blocks our view of that piece of the sky.  So we 
ENTer the FGS AVoiDance region again.  By this time however we have completed 
the observations and the telescope is turning away to the next pointing.  If, 
for example, the target were fainter and we needed to take a longer exposure, 
we could have used another two minutes and 36 seconds to expose, but since 
we did not, we used the time to move off to the next target.  It is a 
delicate balance for the astronomer to ask for as much time as possible 
while not asking for more time in an orbit than may be available.  Ask 
for too much and you'll never find an orbit during the year with enough 
clear viewing time.  Ask for too little and you waste resources and get 
underexposures!  

068:02:10:56  068:02:24:50   M  Slew (AN= 93,RA=193,DE=-30,PA=143,O=   1,SN=138)

This shows the maneuver of the telescope to the next target on the list, 
located 138 degrees away in the sky at Right Ascension 193, Declination -30.  
Can you find this location on a sky chart?  

I have left out the intervening activities, but a few hours later, we see 
we are returning to Neptune to take the second orbit's worth of observations.  

068:08:59:10  068:09:14:49   M  Slew (AN=116,RA=299,DE=-20,PA= 79,O=   0,SN= 51)

Can you follow the activities for observation 0684603?  

------------------------------------------------------------------------------

The second question on how the image gets back to earth is a little simpler. 
All communications to and from the HST are conveyed by radio signals and 
go through an elaborate system of communications satellites.  The cameras 
that take the pictures send their images to one of three tape recorders 
on the HST.  This is similar to recording a TV program on a VCR (except 
the images are stored as digits rather than analog as VCRs use.)  
Sometime later, usually several times each day, the ground controllers 
at the Goddard Space Flight Center in Greenbelt Maryland send commands to 
play back the tape recorder which sends the image through a radio transmitter 
on the HST to one of the Tracking and Data Relay Satellites used by NASA 
to communicate with most of their other satellites including the Space 
Shuttle.  The TDRS then forwards the radio signal to a ground station in 
White Sands New Mexico, which then sends the image back up to another 
communications satellite which relays the image to the GSFC where it 
is captured by computers which then sends the image by ground lines on 
to the Space Telescope Science Institute in Baltimore where it is 
received, processed, carefully stored in an archive and a copy given 
to the astronomer who requested the observation.  From the time the 
controllers ask the tape recorder to play back to the time the data are 
received in Baltimore is usually just a few minutes and the data can be 
processed and made ready for the astronomer in 48 hours - less time if they 
have a real pressing need for it right away.  

Hope this answers your questions and leads you to think of more.  Keep 
asking.