How black are black things?

[Sporadic Dobstronomer]

Here is an experiment I did comparing different black things:

The light was placed to get the worst shine.

Clockwise from the top left they are:

-Sticky flocking material from Wilko as recommended by John.  Clearly the darkest.

-A piece of polycarbonate sprayed with Krylon Flat Black.  The lower right portion was given a light overcoat to make it rough.

-Some sandpaper sprayed with Krylon (this is strictly an experiment; you do not want sandpaper in your telescope )

-Some plastic painted with Ronseal so-called "blackboard paint".  The right end has flour mixed into the paint to make it duller.  This is clearly not a good paint to use.

-Some plastic painted with old and rather congealed Rustins blackboard paint.  The far end has poppyseed mixed in.

Not shown are the blackest velvet I could find, which is slightly lighter than the flocking material, and artist's foam board which is very similar to the smooth Krylon.

Based on some earlier fiddling around, I think the Krylon scatters about 4% of the light.  I would guess the flocking does about 0.5%.  You can see it is almost invisible against the true black in this picture.  It looks a lot more visible in real life.

[andrew s]

Nice experiment. One thing you need to watch out for if you have imaging in mind is that some "blacks" in the visual reflect strongly in the IR where CCD cameras can be quite sensitive.

Regards Andrew

[PeterW]

If you want black then this stuff is probably the best toucan buy easily… Not cheap and you want to keep it clean… Dust will just make it scatter more light!

http://www.edmundoptics.com/lab-production/general-tools/acktar-light-absorbent-foil/3634

Remember that flock is not black in the near infra red… One for imagers to remember.. Don't black anodise things either, it's as bad on the near infrared. http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=190

Think where the light is going, sometimes it might be better to use a shiny black material as you then know where the reflection is going, rather than see materials which scatter the non absorbed light. Baffle design is a “black” art;-)

Good luck,

PeterW

[Gina]

Most adapters, extension tubes etc. are black anodised and we use flock paper in our scopes - what on earth are we supposed to do?

Surely, anodising alumium is better than leaving it bright and shiny

[Cath]

It doesn't look to good for black anodizing Gina ..

[Sporadic Dobstronomer]

Thanks PeterW; interesting links!

While I am mostly visual type, the infrared performance may be very relevant to my Green Machine...

[Sporadic Dobstronomer]

Anyone who is inconvenienced by infrared can probably fix it with an IR blocking filter

[PeterW]

Green machine.... Do tell?! "Gloss traps" made of 2 angles bits of black glass/Perspex are pretty good black objects for normally incident light. There are lots of different super blacks..... Nanorods from Surrey being the latest as seen on stargazing live...

Cheers

PeterW

[Sporadic Dobstronomer]

Green machine.... Do tell?!

There is a picture of it here:

http://stargazerslounge.com/topic/232069-rosette-nebula-at-11x-through-light-pollution/

Somewhere I have seen a picture of yours which looks more this-century (big eyepieces posting?)

[PeterW]

Aha... Enjoy! H-alpha filtering will avoid the flocking IR reflection. Mine is a bit lighter than yours, but still larger than a conventional eyepiece, though lighter as it isnt full of glass.

Milkyway is mostly gone till the autumn.... Pesky clock changes don't help either... Time for planning and scope dreaming to take over.... Or observe the sun instead!

Cheers

Peter