Ion chambers having a polished MgF2window and filled with nitric oxide gas have been developed for the measurements of absolute intensities of solar and geocoronal hydrogen Lyman‐&agr; radiation. The area sensitivities of the detectors and the effects of moisture on the transmittances of MgF2single crystals were measured and compared with those in the cleaved LiF cases. The results indicate that an ion chamber with a polished MgF2window has a superior suitability for the reliable measurement in rocket and satellite experiments as well as in laboratory work.