5.5 Special Background Emission Requirements

5.5.1 Low-Sky (LOW) Observations

The continuum background for HST observations is a function of when and how a given target is observed. If your observations would be adversely affected by scattered light (e.g., zodiacal light and earthshine), you may request the special LOW scheduling requirement. Then your observations will be scheduled such that the sky background is within 30% of its yearly minimum for the given target, which is done by restricting the observations to times that minimize both zodiacal light and earthshine scattered by the OTA. To minimize the zodiacal light, the scheduling algorithm places seasonal restrictions on the observations; to reduce the earthshine, the scheduling system reduces the amount of time data is taken within an orbit by approximately 15% (see Section 6.3 ). The former complicates scheduling, while the latter reduces the observing efficiency of HST. Therefore, using the LOW restriction must have adequate scientific justification included in a Phase I proposal. With this restriction, the zodiacal background light for low-ecliptic latitude targets can be reduced by as much as a factor of 4. Avoiding the earthshine at the standard earth-limb avoidance angle (see Section 2.4 ) can make a similar difference.

5.5.2 Shadow (SHD) Observations

A second special scheduling requirement, SHD, is available to restrict observing to times when HST is in Earth shadow. This can be useful for reducing the geocoronal Lyman-alpha background emission. This special requirement complicates scheduling and reduces the HST observing efficiency, and must therefore have adequate scientific justification in a Phase I proposal.

(Note: The SHD requirement should not be used if low continuum background is required; in that case use LOW instead.)