Is there such a thing as flocking paint?

[Garyh]

Hi everyone,
With the building of my 8" scope slowly happening, it will be time soon to repaint the inside of the OTA. I am wondering if there is such a thing as a flocking paint or similar? I know you can get the Protostar flocking but does anyone make a paint for this purpose..If not any suggestions to whats the best paint for this?
cheers and thanks for any info....
Gary

[Starkler (Geoff)]

The issue is that even the blackest of commercially available paints are still no better than dark grey when light is glancing off at a shallow angle.

Materials like flock paper and velvet work well by changing the nature of the surface into a light trap instead of just a coating on a flat surface.

The next best thing is to coat the inside of the ota with somethign to give it a texture eg fine sawdust, before painting. This reduces the surface area which will reflect light from shallow angles toward your optics.

[Garyh]

Thanks Geoff, was thinking of putting something in the paint to thicken it up to give it a cause texture..maybe sawdust or similar? and dab it on thick. My 6" newt has baffles in it but the 8" tube isn`t quiet large enough for them.
I have read on the net that Krylon ultra flat black is about the best you can get...
Can one buy it still in Auzz?
cheers Gary

[gbeal]

Baffles mate, baffles/
OK, so the 8" perhaps isn't quite big enough, but still try some.
If you think about flocking paper, in essence it really only is baffles, and lots of them. What is the size of the tube inside?
Gary

[Garyh]

Hi Gary, The tube is around 230mm inside so there is around 15mm around the mirror. Maybe I could put a few at the bottom end without introducing vignetting?
My 6" has a good 25mm around the mirror and have baffles around 6mm high about 6-7 of them made from black self adhesive rubber strips..Works a treat!
But still need a good black paint..
cheers Gary

[gbeal]

Hmmmm, never thought of that rubber foam stick on stuff, good idea. Anything will be better than nothing I believe. Do as many as your sanity can stand, something like you suggest 6-7. In between, stick flocking paper, or even felt. I baffle every scope I can now, did the 10" ages ago, and even the home-brew 8" imaging scope has them. Maybe psychological of course, but I reckon it helps.
Gary

[skwinty (Steve)]

When I cut my ota down I repainted the inside with matt black enamel mixed with fine saw dust. Works very well IMO.
Was going to use black velvet but realised it would be hard to keep dust and fluff from sticking to the velvet. At least with the paint you can blow any dust and fluff off. Not so sure blowing would clean stuff off velvet.

[sjastro]

Gary,

My BRC-250 uses a thick matt black paint. It has a high filler content and has a gritty feel. I suspect it's an epoxy type paint designed for low IR reflectance.

There is a military spec on such paints.
http://www.dstan.mod.uk/data/80/216/00000100.pdf

BTW if you use stick on paper, felt etc, these materials can act as thermal insulators and increase cool down times. They may still reflect IR which could be a problem if you are imaging with a CCD or DSLR without IR filtering.

Regards

Steven
http://users.westconnect.com.au/~sjastro/small

[Starkler (Geoff)]

I wonder about the effectiveness of baffles in newtonians.

On a refractor, you are viewing from the non-illuminated side of the baffles, With a newtonian the face of the baffles illuminated by external light is plainly visible from the focal plane given that you can see greater than the width of the primary reflected in the secondary mirror.

The other downside I can see is the impedement to tube currents skimming the tube wall and being kept out of the optical path.

[Garyh]

Thanks Gary, Steven and Geoff for the input.
I think the baffles do work to a degree but just how effective they are I don`t know. But I do know it affects the thermal air flow in my 6" and things don`t look good till the scope cools right down.
Actually thinking of using a heap of charcoal from the fireplace and crush it and put it through a sieve! mis it into the black paint...Sounds like a good idea anyway!
cheers Gary

[anthony.tony]

John Trudgeon Did his 12inch Dob with Black Velvet to kill the staylight. Tony

[Garyh]

Thanks Tony, I shall check out spotlight and see what they have!
cheers Gary

[davewaldo]

Hi Gary,

I did a 10" Dob a little while back, everyone seems to think that Krylon Ultra Flat Black is the darkest, flatest paint available. True flocking and baffles are still probably better, but if you want to use paint thats the one to use. It was a big improvment for me.

Cheers,

Dave.

[anthony.tony]

I think I have heard of someone saying Black Terry Towling was used to kill stray light in a scope.

[tnott]

Knife edge baffles down the length of the tube as in a refractor (which are designed to stop light grazing off the sides and getting into the eyepiece at the bottom) do not work in the case of a newtonian which has the eyepiece up the top. A baffle just in front of the focuser can block extreme off axis light, one on the opposite side of the tube can block light from that direction and baffling light from coming around the primary (but don't block the flow of air) can also help.

Covering anything the eyepiece sees, especially at the top end, with something like flocking (eg. Protostar) can cut down on the more secondary, reflective stray light. Some people put an extension tube around the top to stop light from illuminating the secondary assembly and wall of the tube opposite the focuser.



For more info try Mel Bartels' site or search through the ATM archives and put in key words like baffle, shroud and newtonian.

Good luck.

[Ian Robinson]

Actually , baffling the inside of a newtonian is a good idea , and there is a proper way of doing it. It is a very good way of increasing contrast and stopping stray light (which will fog long exposures).

The method involves a graphical design method from what I remember. If I can locate the link or the article (I think it was in S&T several years ago) I'll post the link or the citation.

Parallax Telescopes does this in their premium newtonians (last I heard).

Your mass manufactured newtonians and dobs don't because it is labor intensive and time consuming fabricating the baffles and installing them == $

Been thinking of doing this on my next newtonian revamp.

As to "flocking paint" , you will be looking at getting the same kind of paint the military uses to make their stuff hard to see at night , it's most likely top secret.
Essentially you need the paint to turn the surface into a near perfect black body (that absorbs all light that falls onto it) and does not reradiate or reflect or scatter any light.
Easiest and cheapest fix is to use a matt black paint that has fine sand mixed with it , or just to get some flocking paper.

[Ian Robinson]

found it : Sky and Telescope, volume 101, number 4, page 128 (04/2001) for a description of newtonian (and dob) baffling.

[tnott]

If you search the ATM archives you will find that this has been debated before. http://astro.umsystem.edu/atm/search.html

A few people have, mistakenly I believe, used programs like Newt which will give you the necessary positions to put knife-edge baffles along the tube but this is supposed to be for a refractor.

But think about the light path in a newtonian.
It may stop light from grazing along the tube of the scope on its descent to the primary. But what's the point. It will not stop light from directly shining into the eyepiece from the opposite side of the top of the tube, or from light reflecting off the secondary assembly. If you do a flashlight test you will see that this makes much more difference - the closer a source of stray light is to the eyepiece the more difference it makes. Direct light is also much more obvious than reflected light - esp. when the surfaces the eyepice can see are flocked.

Other programs will give you the correct positions and sizes of the baffles just below the focuser and behind the secondary.

This is why Pletstone, Mel Bartels, Dan Gray, Tom Kraji, Bruce Sayre and my 16" Tridob (based on Mel's) scopes work so well without shrouds. The baffles block all direct stray light and covering everything the eyepiece sees in flocking dampens the less obvious reflected stray light. Of course having a shroud, tube, observatory wall or extension tube out the top of the scope will block light from hitting the scope in the first place but there are diminishing returns. Light travels in straight lines, not diffuse around corners like a bad smell.

[tnott]

BTW I got Krylon ultra flat black in cans from an auto supply place but the Protostar flock is much more absorbent.

[Garyh]

Thanks everyone for all this info!
I shall follow the links and do some more reading..
I have sometime before I start flocking or painting!
cheers Gary