Timing and ORIENT Requirements
Timing and/or ORIENT requirements may be essential for the successful design of some observations, but they are also limited resource requirements since they can greatly limit the times when the observations may be made.
- Any Timing and/or ORIENT requirements (as well as relative Timing and/or ORIENT requirements) must both be specified at least to the degree of detail required in Phase I proposals in order to be used in Phase II. (Phase I documentation specifies the level of detail required for Phase I proposals.) Desired numerical U3 ORIENT range values must be specified and use of relative ORIENTs must be specified to the degree required in the Phase I documentation.
- More detailed information on the exact requirements for this can be found in the relevant Phase I documents such as the Call for Proposals, the Phase I Roadmap, and/or the HST Primer.
- Primary drivers for specifying timing requirements are observations of variable objects, observations of solar system objects, or any type of temporally variable source monitoring.
- ORIENT requirements may arise for various reasons. They may often be related to fitting some inherent source geometry on the sky cleanly into the field of view.
- ORIENTs may also be used to avoid the bad effects of scattered light and artifacts of telescope optics and detector characteristics from bright stars near or on the field of view – things such as (1) avoiding diffraction spikes or CCD column bleeding from bright stars spilling onto the position of a target of interest in the field of view, (2) avoiding so-called “dragon’s breath” from stars just off the field of view (Stankiewicz et al. 2008, HLA/ACS ISR 2008-01 and references therein; Porterfield et al. 2016, ACS ISR 2016-06, and references therein), (3) avoiding ORIENTs where a problematic star might fall on the edge of the inter-chip gap and cause a glint (Hartig 2002; Stankiewicz et al. 2008, HLA/ACS ISR 2008-01), and/or (4) avoiding ghosts shaped like the number “8” and other similar ghosts going diagonally across the field of view from bright stars being placed on the Amp D quadrant of the ACS/WFC detectors (Hartig et al. 2002). (The Amp D quadrant is the quadrant of the ACS/WFC which is farthest from the location of the WFC3 in HST’s focal plane.)
There is a web page regarding the many possible various ACS image anomalies at https://www.stsci.edu/hst/instrumentation/acs/performance/anomalies-and-artifacts
- ORIENTs are also required for attempts to match the exact pointing and field of view coverage of some earlier observations, as is the use of an aperture location that centers the coordinates on exactly the same spot as in the original observations.
- Note especially that if attempting to duplicate exactly the field of view of an earlier observation (including field corners) which had no ORIENT specification originally (whether ACS/WFC or ACS/SBC), one will need to find out the exact ORIENT at which that data was taken and the named aperture and coordinates which were specified for the primary observations. A look at the formatted version of the Phase II version of the program on the Program Information web page should show that information, including whether there were any dithers or offsets from the reference aperture location such as POS TARGs. (POS TARGs – short for “POSition TARGet” - are manually-specified Phase II target offset requirements on Exposure lines and are used for displacing the target position with respect to the reference aperture location. They are given for the x and y detector axes, and are specified in units of arcseconds.)
- If very precise identical coverage is required, one may need to take care that unless the original aperture was either WFCENTER or one of the “-FIX” apertures, it is even possible that the reference aperture location such as “WFC” or “WFC1” or “WFC2” may have been shifted a bit over time by STScI to account for the location of any subsequent chip or detector defects which may have appeared later. (Similar considerations apply to the WFC3 detectors.)
- The ACS WFCENTER aperture is simply the location of the intersection of lines drawn from the point of the 4 corners of the ACS/WFC detector, and the “-FIX” apertures are at invariant locations on the detectors and are not moved by STScI.
- In attempting to match the U3 ORIENT (the value which goes into APT) of earlier data which was taken when no ORIENT was specified in the original program, one will need to find the PA_V3 angle in the data header for the original observations and then use the angle 180 degrees off from that in a 360-degree circle for the new U3 ORIENT angle in APT. Also, in APT/Aladin, fairly shortly after the execution of a visit, once the visit status shows as “executed”, the actual position and ORIENT angle on the sky as executed is shown in APT/Aladin for earlier observations and should confirm what is seen as the PA_V3 in the data headers for those executed observations, and therefore the U3 angle 180 degrees off from that which would be required to add in the new APT proposal in order to exactly repeat the earlier observations.
- ORIENTs may also be important in observations such as polarimetry and in techniques such as using multiple roll angles (ORIENT angles) to disentangle PSF effects when searching for evidence of extrasolar planets around other stars.
- One may specify a single fixed ORIENT range with both ends the same, or it may be opened up some or even very widely if only wanting to exclude a very small ORIENT range.
- Multiple ORIENT ranges may also be defined which exclude regions where something bad or undesirable may happen (scattered light effects from stars, as above and elsewhere in this article), or where there are a number of alternative acceptable ORIENT ranges.
- Either way, the ORIENT ranges that are specified by the observer are the range(s) of acceptable ORIENTs. Please see more about the combined use of ORIENTs and apertures in the text in “Aperture Choice and using ORIENTs” in the phase II portion of this document, particularly in the beginning of that section and in the parts dealing with the various ACS/WFC apertures.