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3.1 Skymaps

As a planning aid the IUE Observatory provides sky maps which illustrate the celestial sphere as viewed as a rectangular grid in Beta and Yaw coordinates (Figure 3.1). Superimposed on the equatorial coordinate grid are a number of symbols representing various features and constraints to be kept in mind when planning your observing shift. These are

Figure 3.1: Sample IUE skymap.

skymap date
- This is the date for which the map was generated. Positions of lines, Beta angles, etc. will noticeably change over a period of days. Targets near regions of spacecraft-pointing constraints may be observable at the beginning of a run, but not at its end.
top dark shaded region
- The boundary of the forbidden region around the sun. The telescope may not point within a region of 45 degrees in radius, centered on the apparent position of the sun (i.e. for Beta >= 135 degrees).
light shaded region
- Zone of power constraints. Project approval is required for all observations.
small ovals
- The warning zone around the Earth for each hour of the day. Roman numerals inside the ovals give the UT time at which the Earth's center passes that point. The width of the ovals is approximately the Earth's apparent diameter(~ 17°). It provides an indication of where operational problems may develop due to scattered light and weak telemetry signal strength.
dashed lines
- A warning zone around the Earth. The width of this zone is 40 degrees larger than the Earth's apparent diameter. It provides an indication of where operational problems may develop due to scattered light and weak telemetry signal strength.
bottom dark shaded region
- The anti-solar region (Beta < 28 degrees). The telescope may not point within 28 degrees of the anti-solar position.
M & L w/ short line between
- Apparent path of the Moon as seen by IUE for that day.

  Lower Upper    Lower Upper
Month Limit Limit   Month Limit Limit
January 65.0° 85.0°   July - -
February 70.0° 79.0°   August - -
March- -   September - -
April - -   October - -
May - -   November 70.0° 79.0°
June-- December 65.0° 85.0°
Table 3.1: Monthly Beta Angle Regions with OBC Temperature Constraints

The warning zones about the Earth and Moon may be freely encroached upon. However, occultations and/or significant scattered light within the telescope optics may hamper observations and limit the period of time at a particular target on a given shift. Moreover, there is no transmitting antenna on the telescope-end of the spacecraft, so the quality of the received signal may be poor when the telescope is pointing near the Earth. Near the time of sunrise or sunset (Eastern Standard Time) when the spacecraft has a solar panel pointed at the Earth, telemetry may be of poor quality as far as 110 degrees from the Earth. While observations may be made with a poor signal, the cameras can not be read down (i.e. the spectral image transmitted to the ground) without adequate signal strength in order to avoid permanent loss of data. Targets at poor signal locations thus require extra overhead time to slew the spacecraft to a location of good signal strength for reading down the cameras.

The Moon and lunar scattered light actually occupy a very small part of the lunar warning zone. In practice, observations of targets near the position of the Moon are seldom a problem, though the potential exists and should be kept in mind for time-critical observations or for single-target programs.

Skymaps can help in making efficient use of your observing time. You can use them to plan around the Earth and Moon. In addition, they can help you judge the environment affecting the spacecraft during your observations. The great-circle distance between the anti-solar point and the target, called the Beta angle (Beta) (see Figure 2.2) is the primary quantity which sets many spacecraft operational constraints. Along with the skymap, you will receive a listing of your targets. The last column of this list gives the Beta for each target on the date for which the skymap was generated. (Beta angles may be calculated for any date; see Section 3.8). As of September 1986 the following limitations apply as a function of Beta:

Beta >= 135°
Solar avoidance zone; no observations allowed.
112° =< Beta < 135°
Severe power constraints; battery discharge expected. Power drain on batteries may not permit observations longer than 1 hour. Project approval is required for all observations. (See Section 3.3). Some increase in overhead time may be required to avoid significant levels of battery discharge.
Beta(U) < Beta < 112°
Cool Beta region. OBC is at a cool temperature and battery power is good. (Beta(U) refers to the upper Beta angle limit for the OBC temperature constraint zone. See below and Table 3.1)
Beta(L) =< Beta =< Beta(U)
"Hot Betas"; OBC temperature will rise, possibly forcing a move to cooler Betas after a few hours. Maximum power to solar arrays. Table 3.1 gives the monthly upper (Beta(U)) and lower (Beta(L)) Beta angle limits within which observations can be restricted if the OBC temperature gets too hot. Note that there are normally no OBC temperature-related observing constraints (Teays 1991) except during the Winter months (see Sonneborn 1985). If the OBC is already at its high-temperature limit, a target in this zone may not be observed until the OBC temperature decreases.
30° =< Beta < Beta(L)
Cool Betas. Battery power is good.
28° =< Beta < 30°
Power Constraints. OBC temperature will decrease. Telescope focus may degrade over several hours. Battery discharge will occur. Observations may normally be obtained, but with somewhat lower efficiency in order to conserve power. Maneuvering within the low beta region may be time-consuming.
Beta =< 28°
Anti-solar region; no observations allowed.
For long exposures the target should generally be located at a "cool Beta angle," as defined above, to insure that power or OBC temperature constraints do not necessitate an interruption of the exposure. Short exposures (less than 30 minutes) may usually be performed at any Beta between 112 and 28 degrees. If the OBC is very hot, observing at cold Betas for an hour or two can sufficiently cool the OBC to allow a long exposure of a target at a hot OBC Beta.

NOTE, since January 21, 1991 we have not had to cool the spacecraft and in general observations at ANY Beta are allowed. However, within the Beta constraints given there is always some chance that cooling the spacecraft will be required.

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Next: 3.2 Choice of Targets Up: 3 Pre-Observation Preparations Previous: Pre-Observation Preparations

Last updated: 11 June 1997