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Field Journal from William Hathaway - 2/20/96

PROGRAMMING THE HST

This is my first attempt to provide a journal for the Live From HST readers.

When I first heard of the observations that were intended, I asked Alex Storrs for the proposal number and the anticipated execution times. Every proposal gets a five digit number AND a three character Program ID. In general we use the proposal number during preparation and planning while the program number gets associated with the data as they are received after observation. I wanted to know if the observations would occur during a week I would be building. We all work in teams here and (thankfully) take turns with building calendars, so each one of us gets to work with a particular week in detail, while having only slight or no input on other weeks. It turns out that these observations will likely be completed before I start my next calendar. So I can just watch as other capable hands make them happen, yet I can supply some background on just how these things occur.

My other excitement is that the week I _am_ scheduled to build covers the week that the newly discovered comet Hyakutake passes closest to Earth. While it is still being determined if HST will be able to observe it during this time - while it will be moving very fast! - it will be an interesting experience one way or the other.

We have (as you can imagine) quite a number of software tools for scheduling observations and analyzing the calendar. In this and later journals, I hope to show you some of them.

One that can be seen directly is the Calendar display. This shows activities that must occur for an observation to succeed and the times they start and finish. Here is an example from today (20-FEB-1996, day 051 of the year. (Thirty-one days in January plus the 20 in Feb. equals 51.) This one shows what was executing on the spacecraft as I looked this afternoon.

Following this display I will translate some of the odd "words" so everyone should be able to follow the progress.

                        CALENDAR TIMELINE

   C&C List ID:   960507B3;1                  Generated On:  20-FEB-1996 18:32
    Orbit File:   OR96022WA;1                Baseline Name:  GLOBAL 
     TDRS East:   TDRSTDE                        TDRS West:  TDRSTDW
Displayed From:   1996.051:19:00:00         Calendar Start:  1996.050:00:00:00
            To:   1996.052:00:00:00                    End:  1996.057:00:00:00
-------------------------------------------------------------------------------
    START         END           DESCRIPTION                PROP-PROG:OB:AL VER
-------------------------------------------------------------------------------

050:00:00:00 ********        Calendar Start     *******************************


051:20:46:56  051:21:26:36      FGS_AVD    (ENT,L=  15.8)       06095:11:01 01
051:21:03:55  051:21:19:13   M  Slew (AN=112,RA=279,DE=-33,PA= 79,OR=  0,SN=55)
051:21:19:13  051:21:24:43   M  FHST Updte (FULL ,MAN,E= 185,1,2,3)

051:21:24:43 ********        Main SU 0609511    *******************************

051:21:24:43  051:21:33:43   M  PCS AQ(FGS ,E= 54,BASE1   ) 06095-2VO:11:01
051:21:26:36  051:22:23:27      FGS_AVD    (EXT,L=   7.8)       06095:11:01 01
051:21:33:42  051:21:33:43   SI UP   WFII      OPER         06095-2VO:11:01 01
051:21:33:43  051:22:23:43   M  Sci  WFII UPC1FIX        1  06095-2VO:11:01 01
051:22:23:27  051:23:03:04      FGS_AVD    (ENT,L=  15.8)       06095:11:01 01
051:22:23:43  051:22:24:43   SI DOWN WFII      READY        06095-2VO:11:01 01

051:22:23:43 ********    End Main SU 0609511 **********************************

051:22:24:43  051:22:25:43   SI DOWN WFII      STANDBY      06095-2VO:11:01 01
051:22:48:58  051:23:00:15      SAA 02     (ENTRY)
051:23:00:15  052:00:27:07      SAA 02     (EXIT)

051:23:03:14  051:23:19:48   M  Slew (AN=128,RA=167,DE= 73,PA= 12,OR= 2,SN=118)
051:23:19:48  051:23:25:18   M  FHST Updte (FULL ,MAN,E= 213,1,3, )

057:00:00:00 ********        Calendar End       *******************************

Here is the translation:

C&C List ID:   960507B3;1

Each week of observations gets a name. The "96" means the year is 1996; the "50" is the first day of the week (19-FEB-1996 equals day 050), 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 "B3" 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

051:20:46:56  051:21:26:36      FGS_AVD    (ENT,L=  15.8)       06095:11:01 01
                                ^^^

FGS is the Fine Guidance Sensor which looks for the guide stars needed for holding the telescope steady for the observations. The "AVD" means the AVoiDance region where the Earth blocks our view. The "ENT" means that the spacecraft is ENTering that AVoiDance region. Notice that this lasts for 39 minutes and 40 seconds. Subtracting hours and minutes and seconds 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. The target that is blocked is indicated by the 06095:11:01. This translates to Proposal 06095, Observation Set 11 and Alignment 01.

051:21:03:55  051:21:19:13   M  Slew (AN=112,RA=279,DE=-33,PA= 79,OR=  0,SN= 55)
                                ^^^^

This uses 15 minutes and 18 seconds to "Slew" (turn the telescope) to the desired pointing. The target is at Right Ascension 279 degrees and DEclination -33 degrees. This points to the middle of the Milky Way in the constellation Sagittarius. The telescope turned 112 degrees (AN) from the previous target. The pointing is 55 degrees (SN) from the position of the Sun for that day. The PA and OR show the roll of the spacecraft. We cannot point the telescope any closer to the Sun than 50 degrees, but 55 degrees is fine.

051:21:19:13  051:21:24:43   M  FHST Updte (FULL ,MAN,E= 185,1,2,3)
                                ^^^^

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.)

051:21:24:43 ********        Main SU 0609511    *******************************

This 0609511 is the name of the "SU" (Scheduling Unit) which starts running at
this time. 

051:21:24:43  051:21:33:43   M  PCS AQ(FGS ,E= 54,BASE1   ) 06095-2VO:11:01
051:21:26:36  051:22:23:27      FGS_AVD    (EXT,L=   7.8)       06095:11:01 01

Again, the FGS_AVD shows the AVoiDance region, but this time we EXiT the region at 051:21:26:36 - but on the dark limb of the earth, which is why we can start looking at only 7.8 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 both limbs as bright?

Since we can now see the sky, the PCS - Pointing Control System will start to AQ (AcQuire) the guide stars for the observation 06095:11. Notice the Program number is 2VO. 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.

During the nine minutes of PCS AQ, ( 21:33:43 minus 21:24:43 ) 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.

051:21:33:42  051:21:33:43   SI UP   WFII      OPER         06095-2VO:11:01 01

This simply gives the time it takes for the WFII instrument to turn on to an OPERating condition to make it ready to observe the target. This SI (Scientific Instrument) only takes a second to get ready, but we must account for every second of activity. Other SIs take longer times to get ready. Since we can get them ready to turn on during the slew to the target or during the PCS AQ (as we did here) no time gets wasted.

051:21:33:43  051:22:23:43   M  Sci  WFII UPC1FIX        1  06095-2VO:11:01 01

Finally, after all the preparation, we can expose the WFII to light from the target for 50 minutes. The "Sci" means Science data is being collected here. The UPC1FIX shows which of the (many) apertures the target is centered on. In this case the PC1 shows it is in the Planetary Camera CCD chip - the smallest one and the one with the sharpest view.

051:22:23:27  051:23:03:04      FGS_AVD    (ENT,L=  15.8)       06095:11:01 01

After 56 minutes and 51 seconds the spacecraft has flown half-way around the world and at 051:22:23:27 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. Again you might notice that the WFII observation does not end until 051:22:23:43, 16 seconds later. But we do know that the exposure has already ended and this is part of the time the WFII needs to read out the data so we are safe - and again squeezing out a few more seconds of valuable time that can be used for the next observation.

051:22:23:43 ********    End Main SU 0609511 **********************************

The end of observations for this SU.

051:22:23:43  051:22:24:43   SI DOWN WFII      READY        06095-2VO:11:01 01

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

051:22:24:43  051:22:25:43   SI DOWN WFII      STANDBY      06095-2VO:11:01 01

Plus one minute to go into STANDBY.

051:22:48:58  051:23:00:15      SAA 02     (ENTRY)
051:23:00:15  052:00:27:07      SAA 02     (EXIT)

Just a few minutes later the spacecraft crosses a region over the South Atlantic that has a large amount of high energy radiation. During these eleven minutes and 17 seconds, we want to have all cameras turned off and all high voltage turned down. Luckily this happened when the target had already been blocked by the Earth anyway. Actually it is not just luck - we try to schedule things to match up this way.

051:23:03:14  051:23:19:48   M  Slew (AN=128,RA=167,DE= 73,PA= 12,OR= 2,SN=118)

This shows the maneuver of the telescope to the next target on the list, located 128 degrees away in the sky at Right Ascension 167, Declination 73. This is in the northern area of Ursa Major, just beyond the "Pointer Stars" in the bowl of the Big Dipper. Why did we not slew immediately to the next target after the end of the previous exposure? Hint: There is no simple way to answer this with just the information contained in this calendar display. Maybe you will have some good guesses.

051:23:19:48  051:23:25:18   M  FHST Updte (FULL ,MAN,E= 213,1,3, )

Another Fixed Head Star Tracker Update to check the position before the next observation.

Well, you have just seen a fairly standard sequence of events of a scheduled observation. Different programs could use one or more of the other Scientific Instruments on the spacecraft; some might take a shorter exposure; some might need a longer one. We might point at a target for another orbit - waiting for the earth to clear out of the way, then the PCS would find the same guide stars again, and the camera could take another exposure. Sometimes we may stay at the same pointing for many hours. The target might be so faint that many exposures are needed, or perhaps the astronomer might request the same exposure through several different filters.

A target that is moving (like a planet or comet) is a little more complicated. We not only have to know where it is, but at just what time it will be located there. Once we are pointed there though, we have to make the telescope follow its motion exactly throughout the exposure. And if the planet is rotating, we would want to look at it at the time the side we want is facing us. All these make scheduling a little more fun.

In future journals I hope to show some other computer tools we use to show the motion of the HST and its view of the sky. I'll use this same example so readers can compare the position and motion of the telescope as it moves and the positions of the stars in the sky.