Black paint for telescopes is one of most important factors of making telescopes. Typical frosting black has a 4% reflection even though it seems to be black completely. Glass surface has the same reflection, so probably black paint has a same or similar optical index.
Many amateur astronomer use a black paper because it seems more black than paint. I measured the reflection of the paper, and it has a only 1% reflection in visual. However, reflection in IR is so bad, 20-30%. CCD has a high sensitivity in IR. if you use this black paper for your telescope and take astro photograph by CCD camera, spray the black paint on it. The paper has a 1% reflection in visual, and 4% reflection in IR. I think that this is one of the best and cheapest way for telescopes.
Next images are RGB and IGB. The center patch is the black paper painted black spray. Pay attention to black alminized objects. They have a high reflection in IR too.
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