The SV550 122mm telescope adopts a FPL-51 triplet apochromatic lens structure which contains one piece of ED glass. The triplet lens provides better correction of aberration and significantly reduces chromatic aberration and enhance the clarity and sharpness.SV209 0.8x reducer/field flattener can correct the defect of field curvature of the user's shooting target, improve the imaging quality and shooting efficiency of the edge of the field of view.UHC filter is perfect for observation of faint and low-contrast deep-sky objects, significantly reduces light pollution and increase contrast. This filter could cause a halo, if you turn down the exposure it does go away or weakens up enough not to notice.