We define apertures named WFC1
which represent the two CCDs, with their reference points near the geometric center of each chip. The positions have been moved about 50 pixels from the center line to avoid a discontinuity at the amplifier readout boundary. However, we keep two other apertures named WFC1-FIX
at the original central locations (2072,1024). For extended sources, choosing new positions may not be of any advantage and it may be more effective to use these fixed positions.
The aperture WFC
encompasses both detectors, and has its reference point near the overall center but about 10 arcseconds away from the inter-CCD gap. This reference point is (2124,200) on the WFC1 CCD. Again, this point has been moved away from the center line, but the reference point for WFC-FIX
remains at (2073,200). Selection of WFC1
only changes the pixel where the target will be positioned. In all three cases data is normally delivered in a file containing two science image extensions, one for each detector. See the ACS Data Handbook
for details of the ACS data format. Reading out a subarray, which consists of part of only one of the CCDs, is done only if requested.
is similar to WFC
, but is placed at the center of the combined WFC full field. The center is defined as the average of the four corners in the distortion corrected space. Because of the scale variation, this does not appear at the center in pixel space, but rather is on WFC2 about 20 pixels from the edge. Selection of WFCENTER
can be of use in obtaining observations with maximum overlap at unique orientations and for mosaics.
For sets of observations that take place over a substantial part of a year, the telescope roll limitations will require measurements to be taken over most of the angular range. On sky, the WFC
aperture is roughly square, and it is natural to design observations in steps of 90°
to consistently cover the same area. There will be some region at the edges not covered at all four orientations. However, a square area of side 194.8 arcseconds centered on WFCENTER
, and with edges parallel to the V2 and V3 axes, is overlapped at all four positions. In designing a mosaic that combines observations at 90°
steps, a translation of about 190 arcseconds between pointings would provide continuous coverage.
There are five ramp filters. Each ramp filter consists of three segments (inner, middle, outer) that can be rotated across the WFC field of view as indicated in Figure 7.4
. The IRAMP filters can only be placed on WFC1
in a location that will define the aperture WFC1-IRAMP
and the ORAMP filters only on WFC2 creating the aperture WFC2-ORAMP
. The MRAMP filters can lie on WFC1 or WFC2 with corresponding apertures WFC1-MRAMP
. The approximate aperture locations are indicated in Figure 7.4
, while actual data obtained during ground calibrations are overlayed on an image of a ramp filter in Figure 7.5
. Operationally, a fixed reference point will be defined for each detector and filter combination. Then the ramp filter will be rotated to place the required wavelength at the reference position.
The reference positions for all defined apertures are given in Table 7.6
in pixels, and in the telescope (V2,V3) reference frame where values are measured in arc seconds. The values given here are based on in-flight calibration results. The x and y axis angles are measured in degrees from the V3 axis towards the V2 axis. This is in the same sense as measuring from North to East on the sky. The “extent” of the ramp filter apertures given in Table 7.6
are the FWHM of the monochromatic patches (visible in Figure 7.4
) measured from a small sample of ground calibration data. To use a ramp filter in a Phase II program, specify the filter name, the central wavelength, and the aperture. The scheduling software will then automatically rotate the filter to the appropriate wavelength, and point at the reference point of the aperture chosen.
The aperture chosen may either be the full CCD or just the quadrant on which the ramp filter lies. This second choice was new as of Cycle 15, and requires that the aperture be specified. The apertures matching each filter are given in Table 7.6
. Those with names ending in Q are the quadrants. It will normally be preferable to choose the quadrant aperture to save data volume and buffer dumping time. All apertures may be used with non-ramp filters. A target may thereby be put at the same position using a ramp and a non-ramp filter. Please see the footnotes to Table 7.6
for a complete list of available but unsupported apertures.
Only the middle segments of the five ramp filters could be used with the HRC. They are FR914M, FR459M, FR505N, FR388N and FR656N. All five middle segments could be used with any of the HRC apertures listed in Table 7.7
(see Table 7.8
for the aperture reference positions). There were no special ramp apertures with the HRC because when a ramp filter is used with the HRC it covered the entire HRC CCD. This region was defined by the HRC aperture in Table 7.8
. To use the HRC-512 or HRC-SUB1.8 subarray apertures with a ramp filter, POS TARGs had to be used to align the aperture correctly.
When a filter designed for the HRC is used with the WFC, it only covers a small area on either WFC1 or WFC2. The projected filter position can be placed on either chip by selection of the filter wheel setting. Figure 7.4
shows how the filter projection may be placed so as to avoid the borders of the CCDs. When a WFC observation is proposed using a HRC filter spacecraft commanding software automatically uses internal built-in apertures designed for these observing scenarios, called WFC1-SMFL and WFC2-SMFL. Reference positions at or near the center of these apertures are defined so that a target may be placed in the region covered by the chosen filter. Note that WFC2 SMFL is available but not supported.
The axis angles given in Table 7.6
do not refer to the edges of the apertures as drawn, but rather to the orientation of the x and y axes at the WFC reference pixel. These angles vary slightly with position due to geometric distortion.
Apertures have been provided for use with the polarizer sets similar to the SMFL
apertures. These apertures are selected automatically when a polarizing spectral element is used, and a single WFC chip quadrant readout is obtained. The aperture parameters given in Table 7.6
are valid for all three polarizing filters in each polarizer set, UV or visible, to the stated significant figures.
The HRC was equipped with two coronagraphic spots, nominally 1.8 and 3.0 arcseconds in diameter and a coronagraphic finger, 0.8 arcseconds in width. Apertures HRC-CORON1.8
were defined to correspond to these features. The coronagraphic spots were only in the optical train and thus in the data if HRC-CORON1.8 or HRC-CORON 3.0 was specified. Their locations are shown in Figure 7.6
(left panel). In addition we defined a target acquisition aperture, HRC-ACQ
designed for acquiring targets that were subsequently automatically placed behind a coronagraphic spot or the occultation finger. The positions of the coronagraphic spots had been found to fluc
tuate. Observations needed to incorporate a USE OFFSET
special requirement to allow current values to be inserted at the time of the observation.
A substantial region masked out by the occulting finger was present in the HRC data (Figure 7.6
, right panel). The occulting finger was not retractable. However as with any other detector feature or artifact, the “lost” data could be recovered by combining exposures which were suitably shifted with respect to each other. A dither pattern, ACS-HRC-DITHER-LINE had been defined for this purpose and spanned the area flagged for the HRC occulting finger (~1.6 arcseconds or ~56 pixels wide), with an extra ~0.3 arcseconds or ~10 pixels of overlap.
aperture is 1024 pixels square. There are no overscan pixels to consider. The x and y scales are 0.034 and 0.030 arcseconds/pixel leading to a coverage on the sky of 35 by 31 arcseconds. The reference point has been moved to (512,400) to place targets further from a bad anode that disables several rows of the detector near y = 600. As with the CCDs we maintain an SBC-FIX
aperture that will always have position (512,512). MAMA detectors slowly lose efficiency with each exposure, therefore the SBC reference point may be shifted again if the chosen position exhibits significant loss of efficiency.
Although the SBC direct imaging and prism apertures have the same name in APT, they are actually distinct, and HST
executes a small angle maneuver between observations of a given target with them, to compensate for the positional deflection by the prisms. One consequence is that the Special Requirement SAME POS AS
cannot be used among mixed direct and prism exposures. Figure 7.7
shows a direct and prism observation of the same field. The prism now is vignetted on the positive x side. The reference points and the small angle maneuver place the same target near the reference point of each view.