Under conditions of high humidity, the relatively cold window of the ESI dewar can start to fog up. The condensation will scatter light locally, causing halos around emission lines, and broad, fuzzy increases in images and flat fields.

Some diagnostic characteristics of this:

1. Roughly circular or elliptical scattered light, without sharp edges (but see below for sharp edged features).

2. Prominent near the center of the detector.

3. Localized halos around lamp lines near the detector center. (See image.)

4. Ice: If the condensation starts to freeze and ice forms on the dewar window, then sharper-edged features will form. In fact you can see crystalline structure at some level (albeit very out of focus). The LRIS Web page on condensation shows this quite clearly; we have not seen it with ESI.

To cure this problem requires a knowledgeable technician to attach a bottle of dry nitrogen to a hose and purge the area between the dewar window and the back element of the CCD camera until the halos go away.