Detailed Instructions for viz

This tool allows the user to calculate windows of visibility (availability) for a target list, over a given time range. Target availability is subject to the traditional constraints of beta (anti-sun), ram, and moon avoidance, as well as the torque authority limitations imposed by the new magnetic control mode. Additionally, slit position angle and ephemeris constraints may be applied. The intervals returned give the start and end of a period of time during which the target can be safely scheduled, subject to all the specified constraints.

The following is a detailed description of all the major elements of the input form.

Time range

The Start time and End time fields allow the user to specify the time range between which target availability should be calculated, and (by clicking on the verbose button), to request extra information regarding beta, orbit pole, and position angle. Valid format for both start and end times are:

dd-mon-year, e.g. 31-july-2002
year:doy:hh:mm:ss, e.g. 2002:212:00:00
JD, e.g. 2452486.50000

Target information

The user is allowed to request visibility intervals for up to three targets, by filling in relevant information in the target information field:

The Name field is just a string to identify the target in the output. The RA and Dec fields give the J2000 coordinates of the target, and may either be in sexagecimal ("04:03:23.00 +62:11:59.00") or decimal degrees ("60.84583 62.19972") format (NB: the RA argument must be decimal degrees, not decimal hours). These two formats can not be mixed between RA and Dec arguments -- use one or the other for both fields.

Aperture PA and PA tolerance fields allow the user to apply an additional position angle constraint on top of visibility windows. When both these fields are empty, no PA constraint is applied. The position angle increases by rotating the slits from north through east. A position angle of 0 degrees is defined as the orientation in which the 30" LWRS (large square) aperture is due north of the 1.25" HIRS aperture. Thus, the position angle provides information on the angle of the long dimension of the FUSE rectangular apertures on the sky.

Similarly, the Phase zero, Period, Phase desired, and Delta fields allow the user to apply an ephemeris constraint; when left blank, no ephemeris constraint is applied. Phase zero provides the Julian day (note: NOT Modified Julian day) of the assumed zero point in the periodic phenomenon of interest. Only the day number ABOVE 2,400,000 SHOULD BE PROVIDED, and no commas should be entered. (For instance, June 1, 1997, was JD2,450,600.5, and would be specified 50600.5) The Period keyword should provide the period of the variation in decimal days, to whatever accuracy is required.

If position angle and ephemeris fields are populated, these constraints will both be simultaneously applied on top of the visibility intervals.

Time format

The user can request availability window start and end times in several different formats, using the Time format field.

The output formats are:

Day of year - YEAR:DOY:hh:mm:ss, e.g. 2002:212:00:00:00
Year/month/day - YEAR:MM:DD:hh:mm:ss, e.g. 2002:07:31:00:00:00
JD - julian day, e.g. 2452486.50000

Action

Here's where all the fun starts:

Please click Generate Report once, and wait for the results. Because calculating torque authority is very computationally intensive, it may take a minute or two for the results to come back.

Parameters

These fields allow the user to experiment with non-default constraint parameters in order to solve problem visibility cases. HOWEVER, these parameters should not be changed until after reviewing and understanding the viz tool introduction document. Furthermore, any use of non-default parameters must be cited and justified in your proposal. The parameters are

min beta - minimum allowed angle between the target and the antisun. Lowering this value makes the target less constrained.
max beta - maximum allowed angle between the target and the antisun. Raising this value makes the target less constrained.
min ram - minimum allowed angle between the target and the ram direction (instantaneous velocity vector). Lowering this value makes the target less constrained.
min moon - minimum allowed angle between the target and the moon. Lowering this angle makes the target less constrained, but in all likelihood, moon avoidance will not be the dominant constraint for most targets.
torque control - Starting in October 2002, the FUSE satellite will use an optimized momentum wheel unloading algorithm which increases the torque authority available for pointing control. This significantly increases availability of most parts of the sky, and Cycle 4 proposers can expect that this will be the mode of operation by the time their targets are scheduled. Therefore, the default option is Optimized unloading (Cycle 4) For targets being scheduled before October 2002, a more simple unloading algorithm is currently in use. Select the Simple unloading (Cycle 3) option if you are trying to find visibilities before that date. The last option, No torque constraint calculates windows of target availability subject only to beta, ram, and moon avoidance constraints.

Results

The results appear as follows:

  viz start         viz end           duration start  end  start   end start   end
  YEAR:DOY:HH:MM:SS YEAR:DOY:HH:MM:SS     ksec  beta  beta  pole  pole    pa    pa
  ----------------- ----------------- -------- ----- ----- ----- ----- ----- -----
  2002:001:00:00:00 2002:001:07:23:20     26.6  37.0  37.0  38.0  38.7 268.0 267.5
  2002:001:20:10:00 2002:002:05:03:20     32.0  37.1  37.1  39.8  40.5 266.6 266.0
  2002:002:21:03:20 2002:003:04:20:00     26.2  37.2  37.3  41.8  42.4 264.8 264.3

The columns listed are:

interval start time
interval end time
duration of interval, in ksec (NB: NOT exposure time; the available exposure time is of the order 30% of the visibility window duration).

The following extra fields will be printed if the verbose option is selected in the Time range entry field.

beta angle (also known as anti-sun angle) at interval start (degrees)
beta angle at interval end (degrees)
pole angle at interval start (degrees)
pole angle at interval end (degrees)
position angle at interval start (degrees East of North)
position angle at interval end (degrees East of North)