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7.10 Observing with Linear Ramp Filters

Filters provide a narrow band ( ) imaging capability which is continuously tunable from 3710Å to 9762Å. These are essentially a collection of narrow band interference filters whose central wavelength varies with position on the filter glass. The filter and aperture should be specified as LRF on the Phase II proposal, and the desired central wavelength should also be specified. The HST scheduling software will then select the target position so as to provide the desired wavelength.

Note that it is not possible to choose between PC1 and WFC for the LRFs; one must use whatever CCD is automatically assigned by the scheduling software. If it is necessary to know which CCD will be used, observers can consult Table 3.7 on page 48 or Table 3.8 on page 49, or use the on-line LRF calculator tool on the WFPC2 WWW pages.

It is possible to use POS TARGs with LRF observations; the offsets are made from the default pointing for the specified wavelength. Observers should be mindful that the unvignetted field-of-view has a minimum size of ~10" in diameter, so that only small POS TARGs (<4" ) should be used.

While it is recommended that observers assume a 10" diameter field-of-view when using the LRFs, larger elongated (e.g. 15" x 10" ) targets can be sometimes be accommodated by placing the target major axis along the direction of the wavelength variation on the filter. This will result in a small reduction in throughput (i.e. small central wavelength offset) at the outer edges of the target. However, placing targets outside the central 10" of each ramp is strongly discouraged; outside the central 10" width the light will pass through more than one ramp segment, hence mixing light from different wavelengths, and making the data very difficult to calibrate. (See page 40 for further details on LRFs.)

A common situation is one in which observers desire to make observations through an LRF filter, and then repeat the observation in a standard broad or narrow band filter at the same position on the CCD. The LRF Calculator Tool, available on the WFPC2 WWW pages, will tell observers the aperture (PC1-FIX, WF2-FIX, etc.) and POS TARG for any wavelength setting of the LRFs. Observers merely need to use this same aperture and POS TARG for the exposure through the other filter. If it is necessary to calculate the POS TARG manually, one can do this using the information in Table 3.7, Table 3.8, Table 3.14, and Figure 7.8. For example an LRF observation at 5034Å would be made on WF2 at pixel (673.4, 235.7) (from interpolation by wavelength between X1 and X2, and between Y1 and Y2 in Table 3.7). These offsets are referred to the WF2-FIX aperture which is located (Table 3.14) at pixel (423.5,414). From Figure 7.8 we can deduce that pixel X direction is parallel to POS TARG "+Y" on WF2, and that pixel Y direction runs in the POS TARG "-X" direction. Using the pixel scale in section 5.10, "Optical Distortion", on page 103, we have

POS TARG "X" = -0.09961 (235.7-414) = 17.76" , and

POS TARG "Y" = 0.09961 (673.4-423.5) = 24.89" ,

hence POS TARG=+17.76,+24.89 would be requested for the non-LRF exposure.