The Measurement and Analysis of the Wavelength Shift of the FOV of Narrow-Band High-Resolution Imaging Channel at NVST

The working wavelength of the High-Resolution Imaging System operated at 1-meter New Vacuum Solar Telescope (NVST) includes 3 narrow-band wavelength:Ha (6562.8Å), Ca Ⅱ (3933Å) and He I (10830Å). The monochromatic observation of these 3 wavelength is implemented by tunable Lyot filters. This paper analyses the phenomenon of the spacial non-uniform intensity distribution of solar images at different off-bands based on observed data at the wavelength of 6562.8Å (Ha channel). This non-uniform distribution is different to the stray light, for its spatial distribution and variation changes at different off-bands. The analysis of multiple sets of observation data shows the FOV effect of the Lyot filter results in this phenomenon, that the working wavelength at the edge of the FOV deviates from that at the centre of FOV and the degree of the wavelength shift depends on the accuracy of the optical installation. The wavelength shift at different part of the FOV of Ha channel is obtained and compared by the data before the optical adjustment in March 2017, and the result shows that within the 2.2' FOV of the Ha channel, the maximum wavelength shift is 0.05Å, which is less than the passband of this Lyot filter (0.25Å), and is located at the left corner of the FOV.