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Q: What filter should I use? A: Due to hardware limitations, the Clear (F583W) filter should not be used for any target brighter than visual magnitude m = 8.1.For Position Mode observations, Clear (F583W) is the recommended filter since the Optical Field Angle Distortion (OFAD) calibration has been performed only for this filter. For bright targets m < 8.0, the Neutral Density filter (F5ND) may be used as this filter has had a preliminary cross-filter calibration performed with respect to the Clear filter. If an OFAD correction is necessary to the reduction of the data, no other filter should be used. For Transfer Mode only the PUPIL, Clear (F583W) and Neutral Density (F5ND) are currently calibrated. The user is, of course, always free to obtain his own calibration scan with the filter of his choice. The Neutral Density (F5ND) must be used for targets brighter than m = 7.5. the Clear filter (F583W) cannot be used for targets brighter than m = 8.1. The PUPIL filter yields a Transfer Function with a higher peak-to-peak but the data is intrinsically noisier than data taken through the Clear filter since the object's effective magnitude is about 1 mag fainter with the PUPIL in place. At the center of the field of view, a good "rule of thumb" is: targets fainter than m = 13 should be observed with the Clear (F583W) filter, brighter objects with PUPIL. In fields of high background, the PUPIL should be used. |
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