STScI Logo

Hubble Space Telescope
Phase II Overheads

Q.: What are the overhead times for switching between Cameras?
A.: Camera 3 has a different focus position from Cameras 1 and 2, and the overhead times to switch from one to another are the following:

from Camera 3 to Camera 1 (and vice versa) 280 seconds

from Camera 3 to Camera 2 (and vice versa) 280 seconds

from Camera 2 to Camera 1 (and vice versa) 90seconds

These values will change slightly if the nominal best-focus positions of each camera are changed, as this overhead contains the time required to move the PAM (focus mechanism) from one camera to the other.

Q.: If I want to chop between a target and the background, what is the overhead I have to account for?
A.: If the background is located less than 2 arcmin from the target, then the slew time is (x+20) seconds, EACH way (target to background and background to target) and for EACH chop slew, where x is the distance between target and background positions in arcseconds. For such small slews, the same guide stars can be retained. If the background is located more than 2 arcmin from the target, the slew time is (x+31) seconds, EACH way and for EACH chop slew, where, again, x is the distance between the target and the background in arcseconds. If long on-target exposures are necessary, then after slewing back on the target, a guide star re-acquisition (6 minutes) should be requested. For short exposures, on-target guide star re-acquisition may not be necessary, and the observations following the first acquisition will be carried out on gyros, if a pointing uncertainty of about 1 milliarcsec/second due to telescope drift is acceptable. For large slews (e.g., the background is located more than a few arcmin away from the target), the user may consider the use of Type 2 Slews. The overhead for a Type 2 slew is 2.5 minutes for a move of less than 1 degree + 9 minutes for guide star acquisition. In this case, the background is treated as another target. The downside is that the observations obtained in this way will be not treated as "associations" by the calibration pipeline, and will be regarded as individual images.