Each ramp filter consists of three segments that can be rotated across the WFC field of view as indicated in Figure 7.4. The IRAMP filter can only be placed on WFC1 in a location which will define the aperture WFC1-IRAMP and the ORAMP only on WFC2 creating the aperture WFC2-ORAMP. The MRAMP filter can lie on WFC1 or WFC2 with corresponding apertures WFC1-MRAMP and WFC2-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.
Figure 7.4: Schematic WFC apertures and ramp filters.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 arcseconds. 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 7.5 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 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 chip or just the quadrant on which the ramp filter lies. This second choice is 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 listed apertures will be supported and may also be used with non-ramp filters. A target may thereby be put at the same position using a ramp and a non-ramp filter.
Figure 7.5: Monochromatic patches in ground calibration data showing actual aperture sizes through ramp filters (superimposed on photo of ramp filters).
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