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4.1 Typical Target Acquisition and Guest Observer Activities during an Observing Shift

The Guest Observer (GO) is assisted in using the IUE by a Resident Astronomer (RA) and Telescope Operator (TO) who are responsible for implementing the GO's observational requirements. By reviewing the observing plans with the RA and TO prior to the start of the shift, the GO can modify the target and exposure sequence to make efficient use of telescope time, taking into account up-to-date information on spacecraft and camera status (e.g. current telescope pointing, OBC temperature, and overexposures), radiation background, and any operational constraints which may be anticipated. New GOs may want to arrive at GSFC a day before the first shift to familiarize themselves with the IUE observing facilities. Experienced GOs should plan on arriving 30 to 60 minutes before the shift begins. At this time, the TO will need to see your first observing script (see Section 4.2). An early look at the scripts allows the staff to plan out the most efficient sequence of maneuvering and camera preparation before acquiring the first target, thus minimizing operational overhead.

Special requirements (e.g. offset slews or time-critical exposures), as well as anticipated problems (e.g. earth occultation or planned observations at a Beta angle likely to heat the OBC), should be indicated in advance. Spacecraft constraints (Section 4.4 and Section 3.3) may limit the length of time spent at a particular target and need to be taken into account by the RA and TO when planning the observing sequence. In general, it is advantageous to plan the target and exposure sequence at the beginning of the shift. This allows the staff to minimize overhead and anticipate any potential spacecraft constraints which might necessitate a change in the observing plans. Last-minute changes to enhance the scientific value of your observations can be made, but may require additional time for proper execution. Observing overhead times for some of the more commonly performed activities are given in Table 4.1.

Activity Time Required (Minutes) Comments
SPREP on LWP 18 Standard Camera Prep. Only one camera
LWR 14 can be commanded at a time. Camera
SWP 13 preps can be done during a slew or
    while exposing the other camera.
XSPREP on LWP 33 Overexposed Camera Prep.
LWR 19  
SWP 17  
READ (All Cameras) 10 Camera reads are not performed
    while the spacecraft is slewing.
Target acquisition (Excluding time to identify target)
FES image 2 Default size with limiting visual
    magnitude of 11.5.
deep FES image 8 Default size with limiting visual
    magnitude of 13.5.
wheel unload 3-5 Normally required after and sometimes
    before a maneuver.
gyro trim 3-7  
position target in aperture 5  
and start exposure    
Blind-offset acquisition 15-20 Additional time required over a
    normal acquisition.
Switching target from one 5  
aperture/camera to another    
and starting exposure    
Spacecraft ranging 5-10 Normally performed once per hour for 24
    hours twice each month.
Slewing 20-30 Longer slews ( > 120 degrees) may
    require up to 40-50 minutes.
Table 4.1: Characteristic Overhead Times For Typical Observing Activities

The TO will enter the coordinates of the first target or offset star into the computer and calculate the maneuver (see Section 4.5). At this time, a wheel unload may be required before the slew can be made (see Section 2.4). Once the spacecraft has been configured for the maneuver, the commands to begin the slew are sent. The average maneuver time is about 30 minutes.

Cameras can usually be prepared during maneuvers. There are two camera preparation sequences: the standard "prep" (SPREP) and the XSPREP, which is used following significant overexposure levels (see Section 4.6). The SPREP is used for the vast majority of cases.

When the maneuver is completed, the TO will collect a FES image of the field so that you can identify the target or offset star. If the stars in the field are expected to be relatively faint (fainter than 11th magnitude), you may request a deeper FES image with an appropriately longer integration time. Following most slews, the target should fall within a few arc minutes of the center of the field. However on some occasions, there may be pointing errors as large as 12 arc minutes. Thus it is best to have a finding chart slightly larger than the approximate 11 x 11 arc minute field provided by the default FES image (see Section 3.5). Section 3.6 discusses techniques for acquiring objects with visual magnitudes fainter than 13.5. A wheel unload (see Section 2.4) is usually performed by the Operations Control Center staff while you are identifying your target in the FES image. The wheel unload usually takes from 3 to 5 minutes.

Once you have identified your target, the TO will initiate a series of slews to position it at a standard FES location called the reference point. From the reference point, calibrated slews are used to accurately position the target in the desired aperture. While the target is at the reference point, the TO checks for any measurable drift in the gyro position reference. An accurate gyro "trim" is needed to prevent target motion away from the aperture center before offset-guiding is initiated.

During the acquisition process, the FES provides visual brightness measurements for targets brighter than 14th magnitude. There is a visual magnitude calibration for the FES measurements (Pérez 1991, and references therein). Note that the FES photometric calibration is preliminary and has not yet been officially adopted by the Project. Since the FES was not designed for photometric work, the FES photometric calibration is accurate to no better than 0.1 magnitudes. For the time dependent behavior of the FES for comparison to previous observations see Huber and Pérez (1991). If accurate relative magnitudes are desired, please inform the TO that you want FES measurements to be taken exactly at the reference point. This usually requires an additional 1 to 2 minutes per measurement.

If the exposure is longer than a three or four minutes the TO normally will use a star in the field for automatic on-board guiding to insure that the spacecraft pointing remains stable. Such a star is often found in the FES image taken to identify the target. If the field is sparse, however, additional time may be needed to locate a guide star. This time may be reduced by using guide star information from a previous GSFC observation of the target (the observing script, see Section 4.2) or its measured equatorial (1950) coordinates. The guide star should be within 7 arc minutes of the target and brighter than 13th or 14th magnitude. For short exposures, or if no guide star can be found, the spacecraft pointing is maintained by the gyros. Some drift will occur, typically at a rate of 0.06 arc seconds per minute or less.

Even with a guide star, circumstances exist where recentering of a target should be done during long exposures. On the two-gyro/FSS control system, the roll axis of the spacecraft is normally controlled by data from a sun sensor. However, since the sun sensor effectively tracks the sun, the changing apparent solar position due to Earth's orbital motion results in the spacecraft roll orientation changing slowly with time. For targets near the ecliptic this roll motion is negligible, but at the ecliptic poles the motion can be up to 4 arc minutes per hour. For low-Beta targets (Beta < 35), the motion may be larger. If the guide star is near the edge of the field, this motion can change the position of the target in the aperture during long exposures. The staff can calculate the expected motion of the target due to the changing spacecraft roll angle. If it is more than 1.5 arc seconds, you may wish to take the exposure in several sections to improve the centering.

If no guide star is available and a long exposure is required, the exposure will be done in 15 to 30 minute segments. At the end of each segment the target must be returned to the reference point, where the target is recentered and gyro trim corrected. This introduces 5 to 7 minutes of overhead per exposure segment. For blind offsets without guide stars, slews to the offset star between segments are necessary, introducing a large increase in overhead. Blind offsets to fields lacking guide stars are therefore not recommended.

During moderate to long exposures, especially during the US2 shift, you will need to monitor the trapped particle radiation levels (see Section 4.9) to avoid an excessive accumulated background on the image.

If the next exposure is to be taken at a different target, the TO will calculate the maneuver toward the end of the first exposure. At the end of the exposure, the TO will prepare to read the image, verifying that the telemetry signal from the spacecraft is good. (Note: If you have taken an LWR spectrum, you may wish to specify a special read sequence which reduces the likelihood of microphonic noise contamination (see Section 4.11)). Once the camera read is started, any data lost in transmission may not be recoverable. Telemetry dropouts ( i.e. momentary loss of spacecraft telemetry) due to very weak signals can not be recovered. Dropouts for other reasons (e.g. problems with ground system hardware or data transmission from the Wallops Island tracking station) can normally be recovered by replaying an analog recording of the spacecraft telemetry made at the tracking station. Such a "history replay" is usually done within 24-48 hours of the observation. Consequently, you would not able to see the complete image at the TOCS console during your shift (see Section 4.14 for a discussion of the image header).

The spacecraft maneuver is typically started at the end of the read. The camera may be prepared for the next exposure during the slew. The spectrograph data collected by the ground computer is stripped from other spacecraft telemetry, and reconstructed into a two-dimensional image. The image and science header are archived onto magnetic tape and disk. Finally, the image is transmitted from the ground computer to the color display console for the observer's inspection.

The console display allows some interactive, quick-look analysis of the raw image. Various contrasts and expansions may be used. Exposure levels in DN and approximate wavelengths may be determined. Quick-look plots of the raw data may be generated; it is not, however, possible to do any data processing at this point.

It is most efficient to alternate long- and short-wavelength cameras when obtaining a sequence of exposures. If an exposure is 30 minutes or longer, it is possible to read down an image obtained on the first camera and prepare it while the second camera is exposing. If an overexposed spectrum is expected, you should warn the TO, so that the correct camera preparation procedure is preformed. Otherwise, observing time may be lost while a second "prep" is performed (see Section 4.6). Additional sources of time loss are discussed in Section 4.12.

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Next: 4.2 Observing Scripts Up: 4 Observing at GSFC Previous: 4 Observing at GSFC

Last updated: 23 July 1997