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.