Time Series Observations in Two Gyro Mode

The requirement for use of Fixed Head Star Trackers (FHSTs) to remove pointing error after slews reduces the fraction of the sky that can be viewed in any week in Two Gyro mode. This effect is made visible in the Narrated Movie on the Two Gyro Science website. The net result is that any given target will be available for fewer days per year in Two Gyro mode than in Three Gyro mode. The impact is different for different targets, but in Two Gyro mode the range of available time per year is ~110 to 230 days, while in Three Gyro mode the range is 260 to 365 days. The scheduling aids on the website can be used to determine the availability for any specific target.

The reduced target availability can affect the design of programs that collect time series data. As a general rule, targets at low latitude will be available for one contiguous interval for the bulk of their availability time. Targets at high latitude will be available in episodes spread over the year, but not continuously.

The above plot provides a rough indication of the duration of continuous observing intervals that are possible in Two Gyro mode. For each declination, we show the range of the longest window duration that can be achieved, in days. The most important thing to take from this plot is that there is a large range in possible window durations. If you are designing a time series program that requires observations over more than two weeks, use the tools on the website to determine the windows for your specific target. Targets at low latitude generally have one long availability period per year, targets at high latitude tend to have multiple availability periods spread around the year.

The above plot provides a rough indication of the sizes of gaps in very long time series that are forced by the scheduling requirements. For each declination, we show the range of the longest gap, in days, between availability windows. At low declinations, targets are lost for about two thirds of the year. At higher declinations, it may be possible to return to targets regularly on time-scales of 50 to 100 days. If you are designing a time series program that requires observations spread over an extended period, use the tools on the website to determine the availability windows for your specific target.

The Two Gyro Science Mode web site can be found at: http://www.stsci.edu/hst/HST_overview/TwoGyroMode