1. STScI
2. HST
3. Two Gyro Mode Handbook

	HST Two-Gyro Handbook

Chapter 2:
Planning Observations in Two-Gyro Mode

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2.1 All-Sky Availability of Fixed Targets

2.1.1 Overview

The schedulability of an HST observation depends upon many factors and differs considerably between three-gyro and two-gyro operations. To briefly highlight some of these differences, it is useful to compare the accessible regions of the sky in the two modes on a given day of the year.

Figure 2.1: Sky Availability on 5 December 2005

Caption: Sky availability for 5 December 2005 in both three-gyro mode (top panel) and two-gyro mode (bottom panel).
 

Figure 2.1 shows the sky availability for a single day assuming attitude control with either three gyros (top panel) or two gyros (bottom panel). Blue regions indicate areas of the sky that can be observed on the date shown, while grey areas of the sky are not observable at that time. The unobservable region of sky is much larger in two-gyro mode than in three-gyro mode because of constraints imposed to achieve guide star acquisitions and to ensure the safety of the observatory. On any given day there is a region of the sky that cannot be observed in either two-gyro or three-gyro mode because of solar avoidance constraints. In three-gyro mode, all regions of the sky outside the solar avoidance zone (50 degree radius) are accessible on any day. The solar avoidance zone for two-gyro mode is larger, with a 60 degree radius. A large region of the sky ahead of the solar avoidance zone (at larger right ascensions than the Sun) is also unobservable in two-gyro mode because of constraints imposed by the process of correcting slew errors and achieving fine guiding lock. Thus, in two-gyro mode most targets can only be observed when they are on the trailing side of the Sun as it moves along the ecliptic. Over the course of the year, all areas of the sky are available in two-gyro mode, but the total time available in any given direction is less than in three-gyro mode.

2.1.2 All-sky Availability Movie

A short narrated movie showing the sky availability during the course of a year can be found on the web at:

http://www.stsci.edu/hst/HST_overview/TwoGyroMode/2GyroMovi
es/2gyro.html  

This movie compares the three-gyro and two-gyro availabilities in one-week increments in a format similar to the figure above. It shows the variable nature of the sky availability in two-gyro mode and the features discussed previously. It also shows that the availability near the equatorial poles is periodic and alternating. This availability pattern at high declinations is tied to the precession of the HST orbit.

2.1.3 Number of Available Days During the Course of a Year

It is useful to consider how many days per year a target of fixed position can be observed by HST. The top panel of Figure 2.2 shows the number of days in a cycle that any position in the sky is observable by HST with three-gyro pointing capabilities. This plot is essentially an encapsulation of the contents of the sky availability movie described above. The color-coding of this figure indicates the number of days for which at least one orbit (defined here to be a contiguous time block of at least 30 minutes) is available to observe a fixed target. In this figure, the allowable Sun angle range is 50-180 degrees. The fewest number of schedulable days occurs over a small swath of sky near the ecliptic, with availability increasing toward the equatorial poles. The minimum number of days available is approximately 260. A large portion of the sky at greater than 50 degrees ecliptic latitude has at least one schedulable orbit over the entire cycle duration. Be aware that this plot does not convey the information necessary to judge uninterrupted availability as occurs in the continuous viewing zones (CVZs). CVZ opportunities depend on a variety of additional factors that are described elsewhere (see the HST Primer and Section 2.5).

Figure 2.2: All-Sky Target Schedulability

The bottom panel of Figure 2.2 shows the number of days in a cycle that any position in the sky is observable with HST operating in two-gyro mode. Here, the allowable Sun angle range is restricted to 60-180 degrees. Note that the absolute level of the color scaling is different than it is in the three-gyro case shown in the top panel. There are several things worth noting about this panel when comparing it to the three-gyro results. First, and most importantly, the total number of schedulable days at all positions in the sky decreases substantially in two-gyro mode. Second, the smooth progression in availability seen in the top panel becomes slightly less regular, with pockets of reduced availability occurring across the sky. The overall trend for greater availability increasing toward the equatorial poles remains, but even some high declination pointings have fewer than half as many schedulable days as in three-gyro mode.