Optolong OIII 6.5nm Narrowband Filter 36mm Unmounted (OPT-OIII-65NM-36MM) Manual Mount The Bahtinov mask is used

The Bahtinov mask is used focus sma telescopes accurately

put M-13 at the edge of the field and enjoy fully resolving its beauty as it drifts across your view

2") Secondary Mirror Obstruction by Diameter: 28% Secondary Mirror Obstruction by Area: 7

even at the edges of the field of view

For a bit more than generic brands

Optolong OIII 6.5nm Narrowband Filter 36mm Unmounted (OPT-OIII-65NM-36MM) Manual Mount The Bahtinov mask is usedOptolong improved extra Narrowband OIII CCD 6. 5nm filter is designed for nebula observation allowing 6. 5nm bandwidth of light centered on a wavelength of 500nm through, which corresponds to OIII emission lines, and reducing the transmission of certain wavelengths of light, specifically those produced by artificial light including mercury vapour, and both high and low pressure sodium vapour lights and the unwanted natural light caused by neutral