WFPC2 Instrument Handbook for Cycle 11

Observing with Linear Ramp Filters

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The Linear Ramp Filters (LRFs) 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 or Table 3.8, 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 sometimes be accommodated by placing the target's 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 Linear Ramp Filters 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 7.9. 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 7.9 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 Optical Distortion, 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.

We note that analysis of FR533N VISFLAT images has revealed an apparently randomly occurring offset in the filter position (Gonzaga et al. 2001, WFPC2 ISR 01-04). This anomalous offset corresponds to one step in the filter wheel rotation, or about 0.5 degrees. We expect no significant impact on point-source observations; any photometric effect is less than 1%. But caution needs to be exercised for extended sources greater than about 5 arcseconds. (A cursory check of several other filters on other filter wheels shows no similar problems.) Figure 7.12 shows throughput plotted against CCD pixels in the direction of the anomalous offset/rotation. The two curves in each plot show the throughput effect of the filter offset. Several points in the wavelength mapping (from actual GRW+70D5824 observations in proposals 6939, 8054, and 8454) are indicated for illustrative purposes. At this time, the source of this anomaly, whether it is mechanical or due to a software error, is not known. There are plans to further investigate this anomaly as part of the WFPC2 Cycle 10 Calibration Plan.

Figure 7.12: Linear Ramp Filter Anomaly.