Narrow-band imaging in the CN band at 388.33 nm

National Solar Observatory/Sacramento Peak (Operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for the National Science Foundation.) , PO Box 62, Sunspot, NM 88349, USA e-mail: [huitenbroek;atritschler]@nso.edu

Received: 22 August 2006
Accepted: 5 November 2006

Abstract

Aims.We promote the use of narrow-band (0.05–0.20 nm FWHM) imaging in the molecular CN band head at 388.33 nm as an effective method for monitoring small-scale magnetic field elements because it renders them with exceptionally high contrast.

Methods.We create synthetic narrow-band CN filtergrams from spectra computed from a three-dimensional snapshot of a magnetohydrodynamic simulation of the solar convection to illustrate the expected high contrast and explain its nature. In addition, we performed observations with the horizontal slit spectrograph at the Dunn Solar Tower at 388.3 nm to experimentally confirm the high bright-point contrast, and to characterize and optimize the transmission profile of a narrow-band (0.04 FWHM) Lyot filter, which was built by Lyot and tailored to the CN band at Sacramento Peak in the early 70's.

Results.The presented theoretical computations predict that bright-point contrast in narrow-band (0.04 FWHM) CN filtergrams is more than 3 times higher than in CN filtergrams taken with 1 nm FWHM wide filters, and in typical G-band filtergrams. Images taken through the Lyot filter after optimizing its passband confirm that the filter is capable of rendering small-scale magnetic elements with contrasts that are much higher than in traditional G-band imaging. The filter will be available as an user instrument at the Dunn Solar Tower.