Two of the IUE cameras experience some form of periodic microphonic noise during the read of an image. The noise is most noticeable on the LWR camera, where it is often strong (sometimes exceeding 100 DN) and localized in a half-dozen or so lines in the image (a "ping"). Microphonics also occur on the SWP camera but at a much lower level, typically with a maximum of 3 DN, and cannot easily be discerned by inspecting the image under normal contrast. A ping on an LWR image, on the other hand, can severely affect portions of the long-wavelength spectrum. The microphonics usually occurs in the lower half of the LWR image, missing a low-dispersion spectrum. In high dispersion, however, its placement frequently puts it near the Mg II lines. The LWP camera has not experienced any microphonic noise.
A technique has been developed to displace the LWR ping to the top of the image, or off it entirely. A camera warm-up period of 4 minutes precedes the read of the camera. The ping still occurs in about one-quarter of the LWR images but generally falls near the top of the image, above the useful spectrum. The warm-up is known to cause a marginal increase in raw DN levels, but there does not seem to be any effect on extracted spectra at reasonable exposure levels. A report describing the photometric consequences of the "ping avoidance" read technique is given by Holm and Panek (1982). (Please note that the material on pages 56 and 57 of this report were printed in inverted order.)
The "ping avoidance" read may be requested on the standard observing script. The technique is used most often for high dispersion LWR images and for long exposures in low dispersion LWR which cannot easily be repeated, should a ping affect an important part of the spectrum. The standard read may be preferred for short exposures in low dispersion for which the extra 4 minutes of overhead is costly, or for exposures requiring the highest photometric accuracy.