Shiny Spacer rings…

[JonHigh]

Hi All
Just out of curiosity, I have an assortment of M48 and M42 spacer rings but they are a little too shiny on the inside for my liking. What paint would you guys and gals recommend to get a flat or flatter matt black in there? 
 

[Steve Ward]

What are they made from ?

If aluminium then you could have them anodised , or anodise them yourself.

If steel then there a number of gunsmithing 'blueing' or 'blacking' methods/solutions that would work .

Both use oxide layers that are much more resistant to wear and tear than any paint method.

[Oddsocks]

Unfortunately black anodising inside the optical path is very common in mechanical-optical components aimed at the amateur-consumer end of the telescope market, and, at low ray-grazing angles totally useless because the black dye lies at the bottom of pits in the aluminium created by the anodising process.

For anodised aluminium there always remains a sizeable un-pitted surface that carries no dye at all, although we can’t easily see that when looking directly at the surface, it only becomes apparent when looking at the surface close to a parallel angle.

The other problem is that light at the red / Infrared end of the spectrum is barely absorbed by the black dyes used in anodising and with a small percentage of IR light leaking through even the best blocking or band pass filters you will begin to see uncorrected flat-field errors creeping in when calibrating images taken through telescopes with inappropriately blackened optical path components.

IMO there’s no real need to spend £££’s on the special black coatings aimed at professional optical telescopes but a good mineral based mat-black paint such as Rustoleum High-Temperature Barbecue or Stove Paint is more than adequate.

It usually sells in an aerosol can but is best applied when sprayed into another container first and then painted onto the components with a brush.

You will need to prep the surface before painting because wax is often applied to black anodised aluminium to seal in the dye and if the surface is baffled that will make de-waxing harder, a tooth brush dipped in isopropyl alcohol and used liberally as a scrub-wash will remove the wax.

If you want to read more on the deleterious affects that plain black anodising has on astro-imaging have a look at this document from the manufacturer of SBIG cameras:

https://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/
 

HTH

William.