1) What is Estimated Orbital Visibility?
It is the estimated orbital visibility period per orbit. This number is used to estimate how many orbits of HST time will be needed for a particular observation. Estimated orbital visibility is based on the target declination as well as flags and scheduling requirements that affect the visibility period.
2) What flags and scheduling requirements affect Estimated Orbital Visibility?
The program level "Large Program" flag as well as the observation level "Increase Scheduling Flexibility" will both cause the estimated orbital visibility to be short enough that the observation(s) will never be unschedulable due to the orbital visibility. (For those familiar with HST Phase II terminology this is called Schedulability 100%.)
The scheduling requirements CVZ, Low Sky, and Shadow also affect the estimated orbital visibility. CVZ lengthens it to the entire 96 minutes of the orbit while Low Sky and Shadow may shorten it considerably.
3) What value of declination is assumed for solar system targets?
Solar system targets are assumed to be near the ecliptic plane (declination -30 to +30).
4) Is this different than the values in Table 6.1 of the HST Primer?
You may notice difference between the estimated orbital visibility listed in APT and that listed in the HST Primer. The visibilities in the Primer may be slightly lower due to the courser declination bands used in the determination compared to APT. The Primer also lists lower values of Low Sky visibilities. In both cases, the values in APT are more accurate than those in the Primer.