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We apply sticky backed velvet sheet to all the internal surfaces of the Spectra-L200 spectroscopes. Works very well and doesn't re-radiate IR like anodising.
You can normally find suitable material in hardware/ hobby shops.
For longer larger tubes, to get a good bond, it's better to use long strips axially on the inside surface.....
As an alternative on larger Newtonian tubes I've often used a mix of blackboard paint and fine sawdust. This gives a really matt, low reflective outcome. Applied by hand.
Hope this helps.
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